Non-destructive Testing

Nondestructive Testing

The field of Nondestructive Testing (NDT) is a very broad, interdisciplinary field that plays a critical role in assuring that structural components and systems perform their function in a reliable and cost effective fashion. NDT technicians and engineers define and implement tests that locate and characterize material conditions and flaws that might otherwise cause planes to crash, reactors to fail, trains to derail, pipelines to burst, and a variety of less visible, but equally troubling events. Theses tests are performed in a manner that does not affect the future usefulness of the object or material. In other words, NDT allows parts and materials to be inspected and measured without damaging them. Because it allows inspection without interfering with a product's final use, NDT provides an excellent balance between quality control and cost-effectiveness. Generally speaking, NDT applies to industrial inspections. While technologies are used in NDT that are similar to those used in the medical industry, typically nonliving objects are the subjects of the inspections.

Reference:

Introduction to Nondestructive Testing - A Training Guide, P. E. Mix, John Wiley & Sons.NDE Handbook - Non-destructive examination methods for condition monitoring, ed. K. G. Bving, Butterworths

The use of noninvasive

techniques to determine

the integrity of a material,

component or structure

or

quantitatively measure

some characteristic of

an object.

i.e. Inspect or measure without doing harm.

Definition of NDT (NDE)

Nondestructive Evaluation

Nondestructive Evaluation (NDE) is a term that is often used interchangeably with NDT. However, technically, NDE is used to describe measurements that are more quantitative in nature. For example, a NDE method would not only locate a defect, but it would also be used to measure something about that defect such as its size, shape, and orientation. NDE may be used to determine material properties such as fracture toughness, formability, and other physical characteristics.

What are Some Uses
of NDE Methods?

Flaw Detection and EvaluationLeak Detection Location DeterminationDimensional Measurements Structure and Microstructure Characterization Estimation of Mechanical and Physical Properties Stress (Strain) and Dynamic Response Measurements Material Sorting and Chemical Composition Determination

Fluorescent penetrant indication

Why Nondestructive?

Test piece too precious to be destroyedTest piece to be reuse after inspectionTest piece is in serviceFor quality control purposeSomething you simply cannot do harm to, e.g. fetus in mothers uterus

When are NDE Methods Used?

To assist in product development

To screen or sort incoming materials

To monitor, improve or control manufacturing processes

To verify proper processing such as heat treating

To verify proper assembly

To inspect for in-service damage

There are NDE application at almost any stage in the production or life cycle of a component.

Major types of NDT

Detection of surface flaws

Visual

Magnetic Particle Inspection

Fluorescent Dye Penetrant Inspection

Detection of internal flaws

Radiography

Ultrasonic Testing

Eddy current Testing

NDT/NDE Methods

The number of NDT methods that can be used to inspect components and make measurements is large and continues to grow. Researchers continue to find new ways of applying physics and other scientific disciplines to develop better NDT methods. However, there are six NDT methods that are used most often. These methods are visual inspection, penetrant testing, magnetic particle testing, electromagnetic or eddy current testing, radiography, and ultrasonic testing. These methods and a few others are briefly described below.

Background on Nondestructive Testing (NDT) and Nondestructive Evaluation (NDE)

Nondestructive testing has been practiced for many decades. One of the earliest applications was the detection of surface cracks in railcar wheels and axles. The parts were dipped in oil, then cleaned and dusted with a powder. When a crack was present, the oil would seep from the defect and wet the oil providing visual indicating that the component was flawed. This eventually led to oils that were specifically formulated for performing these and other inspections and this inspection technique is now called penetrant testing.

X-rays were discovered in 1895 by Wilhelm Conrad Roentgen(1845-1923) who was a Professor at Wuerzburg University in Germany. Soon after his discovery, Roentgen produced the first industrial radiograph when he imaged a set of weights in a box to show his colleagues. Other electronic inspection techniques such as ultrasonic and eddy current testing started with the initial rapid developments in instrumentation spurred by the technological advances, and subsequent defense and space efforts following World War II. In the early days, the primary purpose was the detection of defects. Critical parts were produced with a "safe life" design, and were intended to be defects during their useful life. The detection of a defects was automatically a cause for removal of the component from service.

In the early 1970's, two events occurred which caused a major change in the way inspections were viewed. The continued improvement of inspection technology, in particular the ability to detect smaller and smaller flaws, led to more and more parts being rejected (even though the probability of part failure had not changed). At this time the discipline of fracture mechanics emerged, which enabled one to predict whether a crack of a given size would fail under a particular load if a particular material property or fracture toughness were known. Other laws were developed to predict the rate of growth of cracks under cyclic loading (fatigue). With the advent of these tools, it became possible to accept structures containing defects if the sizes of those defects were known. This formed the basis for a new design philosophy called "damage tolerant designs." Components having known defects could continue to be used as long as it could be established that those defects would not grow to a critical size that would result in catastrophic failure.

A new challenge was thus presented to the nondestructive testing community. Mere detection of flaws was not enough. One needed to also obtain quantitative information about flaw size to serve as an input to fracture mechanics calculations to predict the remaining life of a component. These needs, which were particularly strongly in the defense and nuclear power industries, led to the creation of a number of research programs around the world and the emergence of nondestructive evaluation (NDE) as a new discipline.

1. Visual Inspection

Most basic and common inspection method.

Tools include fiberscopes, borescopes, magnifying glasses and mirrors.

Robotic crawlers permit observation in hazardous or tight areas, such as air ducts, reactors, pipelines.

Portable video inspection unit with zoom allows inspection of large tanks and vessels, railroad tank cars, sewer lines.

Visual and Optical Testing (VT)
Visual inspection involves using an inspector's eyes to look for defects. The inspector may also use special tools such as magnifying glasses, mirrors, or borescopes to gain access and more closely inspect the subject area. Visual examiners follow procedures that range from simple to very complex.

2. Magnetic Particle Inspection (MPI)

2.1 Introduction

A nondestructive testing method used for defect detection. Fast and relatively easy to apply and part surface preparation is not as critical as for some other NDT methods. MPI one of the most widely utilized nondestructive testing methods. MPI uses magnetic fields and small magnetic particles, such as iron filings to detect flaws in components. The only requirement from an inspectability standpoint is that the component being inspected must be made of a ferromagnetic material such as iron, nickel, cobalt, or some of their alloys. Ferromagnetic materials are materials that can be magnetized to a level that will allow the inspection to be affective. The method is used to inspect a variety of product forms such as castings, forgings, and weldments. Many different industries use magnetic particle inspection for determining a component's fitness-for-use. Some examples of industries that use magnetic particle inspection are the structural steel, automotive, petrochemical, power generation, and aerospace industries. Underwater inspection is another area where magnetic particle inspection may be used to test such things as offshore structures and underwater pipelines.

Magnetic particle inspection is a nondestructive testing method used for defect detection. MPI is a fast and relatively easy to apply and part surface preparation is not as critical as it is for some other NDT methods. These characteristics make MPI one of the most widely utilized nondestructive testing methods.

MPI uses magnetic fields and small magnetic particles, such as iron filings to detect flaws in components. The only requirement from an inspectability standpoint is that the component being inspected must be made of a ferromagnetic material such iron, nickel, cobalt, or some of their alloys. Ferromagnetic materials are materials that can be magnetized to a level that will allow the inspection to be affective.

The method is used to inspect a variety of product forms such as castings, forgings, and weldments. Many different industries use magnetic particle inspection for determining a component's fitness-for-use. Some examples of industries that use magnetic particle inspection are the structural steel, automotive, petrochemical, power generation, and aerospace industries. Underwater inspection is another area where magnetic particle inspection may be used to test such things as offshore structures and underwater pipelines.

2.2 Basic Principles

In theory, magnetic particle inspection (MPI) is a relatively simple concept. It can be considered as a combination of two nondestructive testing methods: magnetic flux leakage testing and visual testing.

Consider a bar magnet. It has a magnetic field in and around the magnet. Any place that a magnetic line of force exits or enters the magnet is called a pole. A pole where a magnetic line of force exits the magnet is called a north pole and a pole where a line of force enters the magnet is called a south pole.

When a material is placed within a magnetic field, the magnetic forces of the material's electrons will be affected. This effect is known as Faraday's Law of Magnetic Induction.

However, materials can react quite differently to the presence of an external magnetic field. This reaction is dependent on a number of factors such as the atomic and molecular structure of the material, and the net magnetic field associated with the atoms. The magnetic moments associated with atoms have three origins. These are the electron orbital motion, the change in orbital motion caused by an external magnetic field, and the spin of the electrons.

Interaction of materials with an external magnetic field

Diamagnetic metals: very weak and negative susceptibility to magnetic fields. Diamagnetic materials are slightly repelled by a magnetic field and the material does not retain the magnetic properties when the external field is removed.

Paramagnetic metals: small and positive susceptibility to magnetic fields. These materials are slightly attracted by a magnetic field and the material does not retain the magnetic properties when the external field is removed.

Ferromagnetic materials: large and positive susceptibility to an external magnetic field. They exhibit a strong attraction to magnetic fields and are able to retain their magnetic properties after the external field has been removed.

Diamagnetic, Paramagnetic, and Ferromagnetic Materials

Diamagnetic materials are solids with all paired electron and, therefore, no permanent net magnetic moment per atom. Diamagnetic properties arise from the realignment of the electron orbits under the influence of an external magnetic field. Most elements in the periodic table, including copper, silver, and gold, are diamagnetic.

Paramagnetic properties are due to the presence of some unpaired electrons and from the realignment of the electron orbits caused by the external magnetic field. Paramagnetic materials include Magnesium, molybdenum, lithium, and tantalum.

Ferromagnetic materials have some unpaired electrons so their atoms have a net magnetic moment. They get their strong magnetic properties due to the presence of magnetic domains. In these domains, large numbers of atoms moments (10^12 to 10^15) are aligned parallel so that the magnetic force within the domain is strong. When a ferromagnetic material is in the unmagnitized state, the domains are nearly randomly organized and the net magnetic field for the part as a whole is zero. When a magnetizing force is applied, the domains become aligned to produce a strong magnetic field within the part. Iron, Nickel, and cobalt are examples of ferromagnetic materials. Components with these materials are commonly inspected using the magnetic particle method.

Unmagnetized material

Magnetized material

Ferromagnetic materials become magnetized when the magnetic domains within the material are aligned. This can be done by placing the material in a strong external magnetic field or by passes electrical current through the material. Some or all of the domains can become aligned. The more domains that are aligned, the stronger the magnetic field in the material. When all of the domains are aligned, the material is said to be magnetically saturated. When a material is magnetically saturated, no additional amount of external magnetization force will cause an increase in its internal level of magnetization.

Follow the path of least resistance between opposite magnetic poles. Never cross one another. All have the same strength. Their density decreases (they spread out) when they move from an area of higher permeability to an area of lower permeability. Their density decreases with increasing distance from the poles. flow from the south pole to the north pole within the material and north pole to south pole in air.

General Properties of Magnetic Lines of Force

As discussed previously a magnetic field is a change in energy within a volume of space. The magnetic field surrounding a bar magnet can be seen in the magnetograph below. A magnetograph can be created by placing a piece of paper over a magnet and sprinkling the paper with iron filings. The particles align themselves with the lines of magnetic force produced by the magnet. The magnetic lines of force show where the magnetic field exits the material at one pole and reenters the material at another pole along the length of the magnet. It should be noted that the magnetic lines of force exist in three-dimensions but are only seen in two dimensions in the image.

The magnetic field exits the north pole and reenters the at the south pole. The magnetic field spreads out when it encounter the small air gap created by the crack because the air can not support as much magnetic field per unit volume as the magnet can. When the field spreads out, it appears to leak out of the material and, thus, it is called a flux leakage field.

When a bar magnet is broken in the center of its length, two complete bar magnets with magnetic poles on each end of each piece will result. If the magnet is just cracked but not broken completely in two, a north and south pole will form at each edge of the crack.

If iron particles are sprinkled on a cracked magnet, the particles will be attracted to and cluster not only at the poles at the ends of the magnet but also at the poles at the edges of the crack. This cluster of particles is much easier to see than the actual crack and this is the basis for magnetic particle inspection.

Magnetic Particle Inspection

The magnetic flux line close to the surface of a ferromagnetic material tends to follow the surface profile of the materialDiscontinuities (cracks or voids) of the material perpendicular to the flux lines cause fringing of the magnetic flux lines, i.e. flux leakageThe leakage field can attract other ferromagnetic particles

Cracks just below the surface can also be revealed

The magnetic particles form a ridge many times wider than the crack itself, thus making the otherwise invisible crack visible

The effectiveness of MPI depends strongly on the orientation of the crack related to the flux lines

MPI is not sensitive to shallow and smooth surface defects

2.3 Testing Procedure of MPI

CleaningDemagnetizationContrast dyes (e.g. white paint for dark particles)Magnetizing the objectAddition of magnetic particlesIllumination during inspection (e.g. UV lamp)InterpretationDemagnetization - prevent accumulation of iron particles or influence to sensitive instruments

Magnetizing the object

There are a variety of methods that can be used to establish a magnetic field in a component for evaluation using magnetic particle inspection. It is common to classify the magnetizing methods as either direct or indirect.

Direct magnetization: current is passed directly through the component.

Clamping the component between two electrical contacts in a special piece of equipment

Using clams or prods, which are attached or placed in contact with the component

There are several ways that direct magnetization is commonly accomplished. One way involves clamping the component between two electrical contacts in a special piece of equipment. Current is passed through the component and a circular magnetic field is established in and around the component. When the magnetizing current is stopped, a residual magnetic field will remain within the component. The strength of the induced magnetic field is proportional to the amount of current passed through the component.

A second technique involves using clams or prods, which are attached or placed in contact with the component. Current is injected into the component as it flows from the contacts. The current sets up a circular magnetic fields around the path of the current.

Indirect magnetization: using a strong external magnetic field to establish a magnetic field within the component

(a) permanent magnets

(b) Electromagnets

(c) coil shot

The use of permanent magnets is a low cost method of establishing a magnetic field. However, their use is limited due to lack of control of the field strength and the difficulty of placing and removing strong permanent magnets from the component.

Electromagnets in the form of an adjustable horseshoe magnet (called a yoke) eliminate the problems associated with permanent magnets and are used extensively in industry. Electromagnets only exhibit a magnetic flux when electric current is flowing around the soft iron core. When the magnet is placed on the component, a magnetic field is established between the north and south poles of the magnet.

Another way of indirectly inducting a magnetic field in a material is by using the magnetic field of a current carrying conductor. A circular magnetic field can be established in cylindrical components by using a central conductors. Typically, one or more cylindrical components are hung from a solid copper bar running through the inside diameter. Current is passed through the copper bar and the resulting circular magnetic field established a magnetic field with the test components.

The use of coils and solenoids is a third method of indirect magnetization. When the length of a component is several time larger than its diameter, a longitudinal magnetic field can be established in the component. The component is placed longitudinally in the concentrated magnetic field that fills the center of a coil or solenoid. This magnetization technique is often referred to as a "coil shot."

Longitudinal magnetization: achieved by means of permanent magnet or electromagnet Circumferential magnetization:

achieved by sending an electric current through the object

When the length of a component is several time larger than its diameter, a longitudinal magnetic field can be established in the component. The component is often placed longitudinally in the concentrated magnetic field that fills the center of a coil or solenoid. This magnetization technique is often referred to as a "coil shot."

The magnetic field travels through the component from end to end with some flux loss along its length as shown in the image to the right. Keep in mind that the magnetic lines of flux occur in three dimensions and are only shown in 2D in the image. The magnetic lines of flux are much denser inside the ferromagnetic material than in air because ferromagnetic materials have much higher permeability than does air. When the concentrated flux within the material comes to the air at the end of the component, it must spread out since the air can not support as many lines of flux per unit volume. To keep from crossing as they spread out, some of the magnetic lines of flux are forced out the side of the component.

When a component is magnetized along its complete length, the flux loss is small along its length. Therefore, when a component is uniform in cross section and magnetic permeability, the flux density will be relatively uniform throughout the component. Flaws that run normal to the magnetic lines of flux will disturb the flux lines and often cause a leakage field at the surface of the component.

When a component with considerable length is magnetized using a solenoid, it is possible to magnetize only a portion of the component. Only the material within the solenoid and about the same width on each side of the solenoid will be strongly magnetized. At some distance from the solenoid, the magnetic lines of force will abandon their longitudinal direction, leave the part at a pole on one side of the solenoid and return to the part at a opposite pole on the other side of the solenoid. This occurs because the magnetizing force diminishes with increasing distance from the solenoid, and, therefore, the magnetizing force may only be strong enough to align the magnetic domains within and very near the solenoid. The unmagnetized portion of the component will not support as much magnetic flux as the magnetized portion and some of the flux will be forced out of the part as illustrated in the image below. Therefore, a long component must be magnetized and inspected at several locations along its length for complete inspection coverage.

Circular Magnetic Fields
Distribution and Intensity

As discussed previously, when current is passed through a solid conductor, a magnetic field forms in and around the conductor. The following statements can be made about the distribution and intensity of the magnetic field.

The field strength varies from zero at the center of the component to a maximum at the surface. The field strength at the surface of the conductor decreases as the radius of the conductor increases when the current strength is held constant. (However, a larger conductor is capable of carrying more current.) The field strength outside the conductor is directly proportional to the current strength. Inside the conductor the field strength is dependent on the current strength, magnetic permeability of the material, and if magnetic, the location on the B-H curve. The field strength outside the conductor decreases with distance from the conductor.

a solid conductor of a magnetic material carrying alternating current.

a nonmagnetic material carrying direct current.

a solid conductor of a magnetic material carrying direct current.

Circumferential magnetic field distribution

Either AC, DC or pulsed DC can be used

It can be seen that in a nonmagnetic conductor carrying DC, the internal field strength rises from zero at the center to a maximum value at the surface of the conductor. The external field strength decrease with distance from the surface of the conductor. When the conductor is a magnetic material, the field strength within the conductor is much greater that it was in the nonmagnetic conductor. This is due to the permeability of the magnetic material. The external field is exactly the same for the two materials provided the current level and conductor radius are the same.

When the conductor is carrying alternating current, the internal magnetic field strength rises from zero at the center to a maximum at the surface. However, the field is concentrated in a thin layer near the surface of the conductor. This is known as the "skin effect." The skin effect is evident in the field strength versus distance graph for a magnet conductor shown to the right. The external field decreases with increasing distance from the surface as it does with DC. It should be remembered that with AC the field is constantly varying in strength and direction.

Demagnetization

After conducting a magnetic particle inspection, it is usually necessary to demagnetize the component. Remanent magnetic fields can:

affect machining by causing cuttings to cling to a component. interfere with electronic equipment such as a compass. can create a condition known as "ark blow" in the welding process. Arc blow may causes the weld arc to wonder or filler metal to be repelled from the weld. cause abrasive particle to cling to bearing or faying surfaces and increase wear.

Magnetic particles

Pulverized iron oxide (Fe3O4) or carbonyl iron powder can be usedColoured or even fluorescent magnetic powder can be used to increase visibilityPowder can either be used dry or suspended in liquid

the particles that are used for magnetic particle inspection are a key ingredient as they form the indications that alert the inspector to defects. Particles start out as tiny milled (a machining process) pieces of iron or iron oxide. A pigment (somewhat like paint) is bonded to their surfaces to give the particles color. The metal used for the particles has high magnetic permeability and low retentivity. High magnetic permeability is important because it makes the particles attract easily to small magnetic leakage fields from discontinuities, such as flaws. Low retentivity is important because the particles themselves never become strongly magnetized so they do not stick to each other or the surface of the part. Particles are available in a dry mix or a wet solution.

Dry Magnetic Particles
Dry magnetic particles can typically be purchased in are red, black, gray, yellow and several other colors so that a high level of contrast between the particles and the part being inspected can be achieved.. The size of the magnetic particles is also very important. Dry magnetic particle products are produced to include a range of particle sizes. The fine particles are around 50 mm (0.002 inch) in size are about three times smaller in diameter and more than 20 times lighter than the coarse particles (150 mm or 0.006 inch), which make them more sensitive to the leakage fields from very small discontinuities. However, dry testing particles cannot be made exclusively of the fine particles. Coarser particles are needed to bridge large discontinuities and to reduce the powder's dusty nature. Additionally, small particles easily adhere to surface contamination, such as remanent dirt or moisture, and get trapped in surface roughness features producing a high level of background. It should also be recognized that finer particles will be more easily blown away by the wind and, therefore, windy conditions can reduce the sensitivity of an inspection. Also, reclaiming the dry particles is not recommended because the small particle are less likely to be recaptured and the "once used" mix will result in less sensitive inspections.

Wet Magnetic Particles
Magnetic particles are also supplied in a wet suspension such as water or oil. The wet magnetic particle testing method is generally more sensitive than the dry because the suspension provides the particles with more mobility and makes it possible for smaller particles to be used since dust and adherence to surface contamination is reduced or eliminated. The wet method also makes it easy to apply the particles uniformly to a relatively large area.

Wet method magnetic particles products differ from dry powder products in a number of ways. One way is that both visible and fluorescent particle are available. Most nonfluorescent particles are ferromagnetic iron oxides, which are either black or brown in color. Fluorescent particles are coated with pigments that fluoresce when exposed to ultraviolet light. Particles that fluoresce green-yellow are most common to take advantage of the peak color sensitivity of the eye but other fluorescent colors are also available. (For more information on the color sensitivity of the eye, see the penetrant inspection material.)

The particles used the wet method are smaller in size than those used in the dry method for the reasons mentioned above. The particles are typically 10 mm (0.0004 inch) and smaller and the synthetic iron oxides have particle diameters around 0.1 mm (0.000004 inch). This very small size is a result of the process used to form the particles and is not particularly desirable, as the particles are almost too fine to settle out of suspension. However, due to their slight residual magnetism, the oxide particles are present mostly in clusters that settle out of suspension much faster than the individual particles. This makes it possible to see and measure the concentration of the particles for process control purposes. Wet particles are also a mix of long slender and globular particles.

The carrier solutions can be water- or oil-based. Water-based carriers form quicker indications, are generally less expensive, present little or no fire hazard, give off no petrochemical fumes, and are easier to clean from the part. Water-based solutions are usually formulated with a corrosion inhibitor to offer some corrosion protection. However, oil-based carrier solutions offer superior corrosion and hydrogen embrittlement protection to those materials that are prone to attack by these mechanisms.

Some Standards for MPI Procedure

British Standards

BS M.35: Aerospace Series: Magnetic Particle Flaw Detection of Materials and Components

BS 4397: Methods for magnetic particle testing of welds

ASTM Standards

ASTM E 709-80: Standard Practice for Magnetic Particle Examination

ASTM E 125-63: Standard reference photographs for magnetic particle indications on ferrous castings

etc.

2.4 Advantages of MPI

One of the most dependable and sensitive methods for surface defectsfast, simple and inexpensivedirect, visible indication on surfaceunaffected by possible deposits, e.g. oil, grease or other metals chips, in the crackscan be used on painted objectssurface preparation not requiredresults readily documented with photo or tape impression

2.5 Limitations of MPI

Only good for ferromagnetic materialssub-surface defects will not always be indicatedrelative direction between the magnetic field and the defect line is importantobjects must be demagnetized before and after the examinationthe current magnetization may cause burn scars on the item examined

Examples of visible dry magnetic particle indications

Indication of a crack in a saw blade

Indication of cracks in a weldment

Before and after inspection pictures of cracks emanating from a hole

Indication of cracks running between attachment holes in a hinge

One of the advantages that a magnetic particle inspection has over some of the other nondestructive evaluation methods is that flaw indications generally resemble the actual flaw. This is not the case with NDT methods such as ultrasonic and eddy current inspection, where an electronic signal must be interpreted. When magnetic particle inspection is used, cracks on the surface of the part appear as sharp lines that follow the path of the crack. Flaws that exist below the surface of the part are less defined and more difficult to detect. Below are some examples of magnetic particle indications produced using dry particles.

Examples of Fluorescent Wet Magnetic Particle Indications

Magnetic particle wet fluorescent indication of a cracks in a drive shaft

Magnetic particle wet fluorescent indication of a crack in a bearing

Magnetic particle wet fluorescent indication of a cracks at a fastener hole

The indications produced using the wet magnetic particles are more sharp than dry particle indications formed on similar defects. When fluorescent particles are used, the visibility of the indications is greatly improved because the eye is drawn to the "glowing" regions in the dark setting. Below are a few examples of fluorescent wet magnetic particle indications.

3. Dye Penetrant Inspection

Liquid penetrant inspection (LPI) is one of the most widely used nondestructive evaluation (NDE) methods. Its popularity can be attributed to two main factors, which are its relative ease of use and its flexibility. LPI can be used to inspect almost any material provided that its surface is not extremely rough or porous. Materials that are commonly inspected using LPI include metals (aluminum, copper, steel, titanium, etc.), glass, many ceramic materials, rubber, and plastics.

LPI offers flexibility in performing inspections because it can be applied in a large variety of applications ranging from automotive spark plugs to critical aircraft components. Penetrant material can be applied with a spray can or a cotton swab to inspect for flaws known to occur in a specific area or it can be applied by dipping or spraying to quickly inspect large areas. At right, visible dye penetrant being locally applied to a highly loaded connecting point to check for fatigue cracking.

Liquid penetration inspection is a method that is used to reveal surface breaking flaws by bleedout of a colored or fluorescent dye from the flaw. The technique is based on the ability of a liquid to be drawn into a "clean" surface breaking flaw by capillary action. After a period of time called the "dwell," excess surface penetrant is removed and a developer applied. This acts as a "blotter." It draws the penetrant from the flaw to reveal its presence. Colored (contrast) penetrants require good white light while fluorescent penetrants need to be used in darkened conditions with an ultraviolet "black light". Unlike MPI, this method can be used in non-ferromagnetic materials and even non-metalsModern methods can reveal cracks 2m wideStandard: ASTM E165-80 Liquid Penetrant Inspection Method

3.1 Introduction

Liquid penetration inspection is a method that is used to reveal surface breaking flaws by bleedout of a colored or fluorescent dye from the flaw. The technique is based on the ability of a liquid to be drawn into a "clean" surface breaking flaw by capillary action. After a period of time called the "dwell," excess surface penetrant is removed and a developer applied. This acts as a "blotter." It draws the penetrant from the flaw to reveal its presence. Colored (contrast) penetrants require good white light while fluorescent penetrants need to be used in darkened conditions with an ultraviolet "black light".

Why Liquid Penetrant Inspection?

To improves the detectability of flaws

There are basically two ways that a penetrant inspection process makes flaws more easily seen.

LPI produces a flaw indication that is much larger and easier for the eye to detect than the flaw itself. LPI produces a flaw indication with a high level of contrast between the indication and the background.

The advantage that a liquid penetrant inspection (LPI) offers over an unaided visual inspection is that it makes defects easier to see for the inspector.

The advantage that a liquid penetrant inspection (LPI) offers over an unaided visual inspection is that it makes defects easier to see for the inspector. There are basically two ways that a penetrant inspection process makes flaws more easily seen. First, LPI produces a flaw indication that is much larger and easier for the eye to detect than the flaw itself. Many flaws are so small or narrow that they are undetectable by the unaided eye. Due to the physical features of the eye, there is a threshold below which objects cannot be resolved. This threshold of visual acuity is around 0.003 inch for a person with 20/20 vision.

The second way that LPI improves the detectability of a flaw is that it produces a flaw indication with a high level of contrast between the indication and the background which also helps to make the indication more easily seen. When a visible dye penetrant inspection is performed, the penetrant materials are formulated using a bright red dye that provides for a high level of contrast between the white developer that serves as a background as well as to pull the trapped penetrant from the flaw. When a fluorescent penetrant inspection is performed, the penetrant materials are formulated to glow brightly and to give off light at a wavelength that the eye is most sensitive to under dim lighting conditions.

415.unknown

Surface Preparation: One of the most critical steps of a liquid penetrant inspection is the surface preparation. The surface must be free of oil, grease, water, or other contaminants that may prevent penetrant from entering flaws. The sample may also require etching if mechanical operations such as machining, sanding, or grit blasting have been performed. These and other mechanical operations can smear the surface of the sample, thus closing the defects.

Penetrant Application: Once the surface has been thoroughly cleaned and dried, the penetrant material is applied by spraying, brushing, or immersing the parts in a penetrant bath.

Penetrant Dwell: The penetrant is left on the surface for a sufficient time to allow as much penetrant as possible to be drawn from or to seep into a defect. The times vary depending on the application, penetrant materials used, the material, the form of the material being inspected, and the type of defect being inspected. Generally, there is no harm in using a longer penetrant dwell time as long as the penetrant is not allowed to dry.

3.2 Basic processing steps of LPI

Surface Preparation: One of the most critical steps of a liquid penetrant inspection is the surface preparation. The surface must be free of oil, grease, water, or other contaminants that may prevent penetrant from entering flaws. The sample may also require etching if mechanical operations such as machining, sanding, or grit blasting have been performed. These and other mechanical operations can smear the surface of the sample, thus closing the defects.

Penetrant Application: Once the surface has been thoroughly cleaned and dried, the penetrant material is applied by spraying, brushing, or immersing the parts in a penetrant bath.

Penetrant Dwell: The penetrant is left on the surface for a sufficient time to allow as much penetrant as possible to be drawn from or to seep into a defect. Penetrant dwell time is the total time that the penetrant is in contact with the part surface. Dwell times are usually recommended by the penetrant producers or required by the specification being followed. The times vary depending on the application, penetrant materials used, the material, the form of the material being inspected, and the type of defect being inspected. Minimum dwell times typically range from 5 to 60 minutes. Generally, there is no harm in using a longer penetrant dwell time as long as the penetrant is not allowed to dry. The ideal dwell time is often determined by experimentation and is often very specific to a particular application.

Excess Penetrant Removal: This is the most delicate part of the inspection procedure because the excess penetrant must be removed from the surface of the sample while removing as little penetrant as possible from defects. Depending on the penetrant system used, this step may involve cleaning with a solvent, direct rinsing with water, or first treated with an emulsifier and then rinsing with water.

Developer Application: A thin layer of developer is then applied to the sample to draw penetrant trapped in flaws back to the surface where it will be visible. Developers come in a variety of forms that may be applied by dusting (dry powdered), dipping, or spraying (wet developers).

Indication Development: The developer is allowed to stand on the part surface for a period of time sufficient to permit the extraction of the trapped penetrant out of any surface flaws. This development time is usually a minimum of 10 minutes and significantly longer times may be necessary for tight cracks.

Inspection: Inspection is then performed under appropriate lighting to detect indications from any flaws which may be present.

Clean Surface: The final step in the process is to thoroughly clean the part surface to remove the developer from the parts that were found to be acceptable.

Excess Penetrant Removal: This is the most delicate part of the inspection procedure because the excess penetrant must be removed from the surface of the sample while removing as little penetrant as possible from defects. Depending on the penetrant system used, this step may involve cleaning with a solvent, direct rinsing with water, or first treated with an emulsifier and then rinsing with water.

Developer Application: A thin layer of developer is then applied to the sample to draw penetrant trapped in flaws back to the surface where it will be visible. Developers come in a variety of forms that may be applied by dusting (dry powdered), dipping, or spraying (wet developers).

Indication Development: The developer is allowed to stand on the part surface for a period of time sufficient to permit the extraction of the trapped penetrant out of any surface flaws. This development time is usually a minimum of 10 minutes and significantly longer times may be necessary for tight cracks.

Inspection: Inspection is then performed under appropriate lighting to detect indications from any flaws which may be present.

Clean Surface: The final step in the process is to thoroughly clean the part surface to remove the developer from the parts that were found to be acceptable.

Penetrant testing materials

A penetrant must possess a number of important characteristics. A penetrant must

spread easily over the surface of the material being inspected to provide complete and even coverage. be drawn into surface breaking defects by capillary action. remain in the defect but remove easily from the surface of the part. remain fluid so it can be drawn back to the surface of the part through the drying and developing steps. be highly visible or fluoresce brightly to produce easy to see indications. must not be harmful to the material being tested or the inspector.

Penetrant Types

Dye penetrants

The liquids are coloured so that they provide good contrast against the developer

Usually red liquid against white developer

Observation performed in ordinary daylight or good indoor illumination

Fluorescent penetrants

Liquid contain additives to give fluorescence under UV

Object should be shielded from visible light during inspection

Fluorescent indications are easy to see in the dark

Standard: Aerospace Material Specification (AMS) 2644.

Further classification

According to the method used to remove the excess penetrant from the part, the penetrants can be classified into:

Method A - Water Washable Method B - Post Emulsifiable, Lipophilic Method C - Solvent Removable Method D - Post Emulsifiable, Hydrophilic

Based on the strength or detectability of the indication that is produced for a number of very small and tight fatigue cracks, penetrants can be classified into five sensitivity levels are shown below:

Level - Ultra Low Sensitivity Level 1 - Low Sensitivity Level 2 - Medium Sensitivity Level 3 - High Sensitivity Level 4 - Ultra-High Sensitivity

Emulsifiers

When removal of the penetrant from the defect due to over-washing of the part is a concern, a post emulsifiable penetrant system can be used. Post emulsifiable penetrants require a separate emulsifier to break the penetrant down and make it water washable.

Lipophilic emulsification systems are oil-based materials that are supplied in ready-to-use form. Hydrophilic systems are water-based and supplied as a concentrate that must be diluted with water prior to use .

Method B - Lipophilic Emulsifier, Method D - Hydrophilic Emulsifier

The role of the developer is to pull the trapped penetrant material out of defects and to spread the developer out on the surface of the part so it can be seen by an inspector. The fine developer particles both reflect and refract the incident ultraviolet light, allowing more of it to interact with the penetrant, causing more efficient fluorescence. The developer also allows more light to be emitted through the same mechanism. This is why indications are brighter than the penetrant itself under UV light. Another function that some developers performs is to create a white background so there is a greater degree of contrast between the indication and the surrounding background.

Developer

Dry powder developer the least sensitive but inexpensive Water soluble consist of a group of chemicals that are dissolved in water and form a developer layer when the water is evaporated away. Water suspendible consist of insoluble developer particles suspended in water. Nonaqueous suspend the developer in a volatile solvent and are typically applied with a spray gun.

Developer Types

Using dye and developer from different manufacturers should be avoided.

3.3 Finding Leaks with Dye Penetrant

3.4 Primary Advantages

The method has high sensitive to small surface discontinuities. The method has few material limitations, i.e. metallic and nonmetallic, magnetic and nonmagnetic, and conductive and nonconductive materials may be inspected. Large areas and large volumes of parts/materials can be inspected rapidly and at low cost. Parts with complex geometric shapes are routinely inspected. Indications are produced directly on the surface of the part and constitute a visual representation of the flaw. Aerosol spray cans make penetrant materials very portable. Penetrant materials and associated equipment are relatively inexpensive.

3.5 Primary Disadvantages

Only surface breaking defects can be detected. Only materials with a relative nonporous surface can be inspected. Precleaning is critical as contaminants can mask defects. Metal smearing from machining, grinding, and grit or vapor blasting must be removed prior to LPI. The inspector must have direct access to the surface being inspected. Surface finish and roughness can affect inspection sensitivity. Multiple process operations must be performed and controlled. Post cleaning of acceptable parts or materials is required. Chemical handling and proper disposal is required.

4. Radiography

Radiography involves the use of penetrating gamma- or X-radiation to examine material's and product's defects and internal features. An X-ray machine or radioactive isotope is used as a source of radiation. Radiation is directed through a part and onto film or other media. The resulting shadowgraph shows the internal features and soundness of the part. Material thickness and density changes are indicated as lighter or darker areas on the film. The darker areas in the radiograph below represent internal voids in the component.

High Electrical Potential

Electrons

-

+

X-ray Generator or Radioactive Source Creates Radiation

Exposure Recording Device

Radiation

Penetrate

the Sample

4.1 Radiation sources

4.1.1 x-ray source

X-rays or gamma radiation is used

X-rays are electromagnetic radiation with very short wavelength ( 10-8 -10-12 m)The energy of the x-ray can be calculated with the equation

E = h = hc/

e.g. the x-ray photon with wavelength 1 has energy 12.5 keV

Properties and Generation of X-ray

Production of X-rays

X-rays are produced whenever high-speed electrons

collide with a metal target.

A source of electrons hot W filament, a high accelerating voltage

(30-50kV) between the cathode (W) and the anode and a metal target.

The anode is a water-cooled block of Cu containing desired target metal.

target

X-rays

W

Vacuum

X-ray Spectrum

A spectrum of x-ray is produced as a result of the interaction between the incoming electrons and the inner shell electrons of the target element.Two components of the spectrum can be identified, namely, the continuous spectrum and the characteristic spectrum.

SWL - short-wavelength limit

continuous

radiation

characteristic

radiation

k

k

I

If an incoming electron has sufficient kinetic energy for knocking out an electron of the K shell (the inner-most shell), it may excite the atom to an high-energy state (K state). One of the outer electron falls into the K-shell vacancy, emitting the excess energy as a x-ray photon -- K-shell emission Radiation. Fast moving e- will then be deflected or decelerated and EM radiation will be emitted. The energy of the radiation depends on the severity of the deceleration, which is more or less random, and thus has a continuous distribution. These radiation is called white radiation or bremsstrahlung (German word for braking radiation).

Absorption of x-ray

All x-rays are absorbed to some extent in passing through matter due to electron ejection or scattering.The absorption follows the equation

whereI is the transmitted intensity;

x is the thickness of the matter;

is the linear absorption coefficient (element dependent);

is the density of the matter;

(/) is the mass absorption coefficient (cm2/gm).

I0

I

,

x

106.unknown

4.1.2 Radio Isotope (Gamma) Sources

Emitted gamma radiation is one of the three types of natural radioactivity. It is the most energetic form of electromagnetic radiation, with a very short wavelength of less than one-tenth of a nano-meter. Gamma rays are essentially very energetic x-rays emitted by excited nuclei. They often accompany alpha or beta particles, because a nucleus emitting those particles may be left in an excited (higher-energy) state.

Man made sources are produced by introducing an extra neutron to atoms of the source material. As the material rids itself of the neutron, energy is released in the form of gamma rays. Two of the more common industrial Gamma-ray sources are Iridium-192 and Colbalt-60. These isotopes emit radiation in two or three discreet wavelengths. Cobalt 60 will emit a 1.33 and a 1.17 MeV gamma ray, and iridium-192 will emit 0.31, 0.47, and 0.60 MeV gamma rays.

Advantages of gamma ray sources include portability and the ability to penetrate thick materials in a relativity short time.

Disadvantages include shielding requirements and safety considerations.

Advantages of gamma ray sources include portability and the ability to penetrate thick materials in a relativity short time. As can be noted above cobalt will produce energies comparable to a 1.25 MeV x-ray system. Iridium will produce energies comparable to a 460 kV x-ray system. Not requiring electrical sources the gamma radiography is well adapted for use in remote locations.

Disadvantages include shielding requirements and safety considerations. Depleted uranium is used as a shielding material for sources. The storage container (camera) for iridium sources will contain 45 pounds of shielding materials. Cobalt will require 500 pounds of shielding. Cobalt cameras are often fixed to a trailer and transported to and from inspection sites. Iridium is used whenever possible, and not all companies using source material will have a cobalt source. Source materials are constantly generating very penetrating radiation and in a short time considerable damage can be done to living tissue. Technicians must be trained in potential hazards to themselves and the public associated with use of gamma radiography.

4.2 Film Radiography

Top view of developed film

X-ray film

The film darkness (density) will vary with the amount of radiation reaching the film through the test object. Defects, such as voids, cracks, inclusions, etc., can be detected.

The part is placed between the radiation source and a piece of film. The part will stop some of the radiation. Thicker and more dense area will stop more of the radiation.

= more exposure

= less exposure

The most common detector used in industrial radiography is film. The high sensitivity to ionizing radiation provides excellent detail and sensitivity to density changes when producing images of industrial materials.

Contrast and Definition

It is essential that sufficient contrast exist between the defect of interest and the surrounding area. There is no viewing technique that can extract information that does not already exist in the original radiograph

Contrast

The first subjective criteria for determining radiographic quality is radiographic contrast. Essentially, radiographic contrast is the degree of density difference between adjacent areas on a radiograph.

low kilovoltage

high kilovoltage

Definition

Radiographic definition is the abruptness of change in going from one density to another.

good

poor

High definition: the detail portrayed in the radiograph is equivalent to physical change present in the part. Hence, the imaging system produced a faithful visual reproduction.

In the example to the left, a two-step step tablet with the transition from step to step represented by Line BC is quite sharp or abrupt. Translated into a radiograph, we see that the transition from the high density to the low density is abrupt. The Edge Line BC is still a vertical line quite similar to the step tablet itself. We can say that the detail portrayed in the radiograph is equivalent to physical change present in the step tablet. Hence, we can say that the imaging system produced a faithful visual reproduction of the step table. It produced essentially all of the information present in the step tablet on the radiograph.

In the example on the right, the same two-step step tablet has been radiographed. However, here we note that, for some reason, the imaging system did not produce a faithful visual reproduction. The Edge Line BC on the step tablet is not vertical. This is evidenced by the gradual transition between the high and low density areas on the radiograph. The edge definition or detail is not present because of certain factors or conditions which exist in the imaging system.

4.3 Areas of Application

Can be used in any situation when one wishes to view the interior of an objectTo check for internal faults and construction defects, e.g. faulty weldingTo see through what is inside an objectTo perform measurements of size, e.g. thickness measurements of pipes

ASTM

ASTM E94-84a Radiographic Testing

ASTM E1032-85 Radiographic Examination of Weldments

ASTM E1030-84 Radiographic Testing of Metallic Castings

Standard:

Radiographic Images

4.4 Limitations of Radiography

There is an upper limit of thickness through which the radiation can penetrate, e.g. -ray from Co-60 can penetrate up to 150mm of steelThe operator must have access to both sides of an objectHighly skilled operator is required because of the potential health hazard of the energetic radiationsRelative expensive equipment

4.5 Examples of radiographs

Cracking can be detected in a radiograph only the crack is propagating in a direction that produced a change in thickness that is parallel to the x-ray beam. Cracks will appear as jagged and often very faint irregular lines. Cracks can sometimes appearing as "tails" on inclusions or porosity.

Burn through (icicles) results when too much heat causes excessive weld metal to penetrate the weld zone. Lumps of metal sag through the weld creating a thick globular condition on the back of the weld. On a radiograph, burn through appears as dark spots surrounded by light globular areas.

Gas porosity or blow holes are caused by accumulated gas or air which is trapped by the metal. These discontinuities are usually smooth-walled rounded cavities of a spherical, elongated or flattened shape.

Sand inclusions and dross are nonmetallic oxides, appearing on the radiograph as irregular, dark blotches.

Gas porosity or blow holes are caused by accumulated gas or air which is trapped by the metal. These discontinuities are usually smooth-walled rounded cavities of a spherical, elongated or flattened shape. If the sprue is not high enough to provide the necessary heat transfer needed to force the gas or air out of the mold, the gas or air will be trapped as the molten metal begins to solidify. Blows can also be caused by sand that is too fine, too wet, or by sand that has a low permeability so that gas can't escape. Too high a moisture content in the sand makes it difficult to carry the excessive volumes of water vapor away from the casting. Another cause of blows can be attributed to using green ladles, rusty or damp chills and chaplets.

5. Ultrasonic Testing

The most commonly used ultrasonic testing technique is pulse echo, whereby sound is introduced into a test object and reflections (echoes) from internal imperfections or the part's geometrical surfaces are returned to a receiver. The time interval between the transmission and reception of pulses give clues to the internal structure of the material.

In ultrasonic testing, high-frequency sound waves are transmitted into a material to detect imperfections or to locate changes in material properties.

5.1 Introduction

Below is an example of shear wave weld inspection. Notice the indication extending to the upper limits of the screen. This indication is produced by sound reflected from a defect within the weld.

Ultrasonic Inspection (Pulse-Echo)

High frequency sound waves are introduced into a material and they are reflected back from surfaces or flaws.

Reflected sound energy is displayed versus time, and inspector can visualize a cross section of the specimen showing the depth of features that reflect sound.

f

plate

crack

initial

pulse

crack

echo

back surface

echo

Oscilloscope, or flaw detector screen

0

2

4

6

8

10

Generation of Ultrasonic Waves

Piezoelectric transducers are used for converting electrical pulses to mechanical vibrations and vice versaCommonly used piezoelectric materials are quartz, Li2SO4, and polarized ceramics such as BaTiO3 and PbZrO3.Usually the transducers generate ultrasonic waves with frequencies in the range 2.25 to 5.0 MHz

Ultrasonic Wave Propagation

Longitudinal or compression wavesShear or transverse wavesSurface or Rayleigh wavesPlate or Lamb waves

Wave Propagation Direction

Symmetrical

Asymmetrical

In solids, sound waves can propagate in four principle modes that are based on the way the particles oscillate. Sound can propagate as longitudinal waves, shear waves, surface waves, and in thin materials as plate waves. Longitudinal and shear waves are the two modes of propagation most widely used in ultrasonic testing. The particle movement responsible for the propagation of longitudinal and shear waves is illustrated below.

Different types of waves

As the reed vibrates back and forth, the sound waves produced move the same direction (left and right). Waves that move in the same direction, or are parallel to their source are called longitudinal waves. Longitudinal sound waves are the easiest to produce and have the highest speed, however, it is possible to produce other types. Waves which move perpendicular to the way their source does are called shear waves. Shear waves travel at slower speeds than longitudinal waves, and can only be made in solids. Another type of wave is the surface wave. Surface waves travel at the surface of a material and move in elliptical orbits. They are slightly slower than shear waves but difficult to make. A final type of sound wave is the plate wave. These waves also move in elliptical orbits but are much more complex. They can only be created in very thin pieces of material.

Longitudinal waves

Similar to audible sound waves

the only type of wave which can travel through liquid

Shear waves

generated by passing the ultrasonic beam through the material at an angle

Usually a plastic wedge is used to couple the transducer to the material

In longitudinal waves, the oscillations occur in the longitudinal direction or the direction of wave propagation. Since compressional and dilational forces are active in these waves, they are also called pressure or compressional waves. They are also sometimes called density waves because their particle density fluctuates as they move. Compression waves can be generated in liquids, as well as solids because the energy travels through the atomic structure by a series of comparison and expansion (rarefaction) movements.

In the transverse or shear wave, the particles oscillate at a right angle or transverse to the direction of propagation. Shear waves require an acoustically solid material for effective propagation and, therefore, are not effectively propagated in materials such as liquids or gasses. Shear waves are relatively weak when compared to longitudinal waves In fact, shear waves are usually generated in materials using some of the energy from longitudinal waves.

Surface waves

travel with little attenuation in the direction of propagation but weaken rapidly as the wave penetrates below the material surface

particle displacement follows an elliptical orbit

Lamb waves

observed in relatively thin plates only

velocity depends on the thickness of the material and frequency

Rayleigh waves travel the surface of a relative thick solid material penetrating to a depth of one wavelength. Rayleigh waves are useful because they are very sensitive to surface defects and since they will follow the surface around, curves can also be used to inspect areas that other waves might have difficulty reaching.

Lamb waves, also known as plate waves, can be propagated only in very thin metals. Lamb waves are a complex vibrational wave that travels through the entire thickness of a material. Lamb waves provide a means for inspection of very thin materials. Propagation of Lamb waves depends on density, elastic, and material properties of a component, and they are influenced by a great deal by selected frequency and material thickness.

5.2 Equipment & Transducers

5.2.1 Piezoelectric Transducers

The active element of most acoustic transducers is piezoelectric ceramic. This ceramic is the heart of the transducer which converts electrical to acoustic energy, and vice versa.

A thin wafer vibrates with a wavelength that is twice its thickness, therefore, piezoelectric crystals are cut to a thickness that is 1/2 the desired radiated wavelength. Optimal impedance matching is achieved by a matching layer with thickness 1/4 wavelength.

Direction of wave propagation

When piezoelectric ceramics were introduced they soon became the dominant material for transducers due to their good piezoelectric properties and their ease of manufacture into a variety of shapes and sizes. The first piezoceramic in general use was barium titanate, and that was followed during the 1960's by lead zirconate titanate compositions, which are now the most commonly employed ceramic for making transducers.

In selecting a transducer the piezoelectric material is always a consideration as some materials are more efficient transmitters and some are more efficient receivers. Understanding the internal structure of the material to be inspected, as well as type, size, and probable location of defects is helpful when selecting a transducer. A transducer that performs well in one application will not always produce similar results when material properties change. For example, sensitivity to small defects is proportional to the product of the efficiency of the transducer as a transmitter and a receiver. Resolution, the ability to locate defects near surface or in close proximity in the material, requires a highly damped transducer. The backing material supporting the crystal has a great influence on damping characteristics of a transducer. Using a backing material with an impedance similar to that of the crystal will produce the most effective damping. Such a transducer will have a narrow bandwidth resulting in higher sensitivity. As the mismatch in impedance between crystal and backing material increases, transducer sensitivity is reduced and material penetration increased.

It is of importance to understand the concept of bandwidth, or range of frequencies, associated with a transducer. The frequency noted on a transducer is the central or center frequency and depends primarily on the backing material. Highly damped transducers will respond to frequencies above and below the central frequency. The broad frequency range provides a transducer with high resolving power. Less damped transducers will exhibit a narrower frequency range, poorer resolving power, but greater penetration. The central frequency will also define capabilities of a transducers. Lower frequencies (0.5Mhz-2.25Mhz) provide greater energy and penetration in a material, while high frequency crystals (15.0Mhz-25.0Mhz) provide reduced penetration but greater sensitivity to small discontinuities.

Characteristics of Piezoelectric Transducers

Immersion: do not contact the component. These transducers are designed to operate in a liquid environment and all connections are watertight. Wheel and squirter transducers are examples of such immersion applications.

Transducers are classified into groups according to the application.

Contact type

Contact: are used for direct contact inspections. Coupling materials of water, grease, oils, or commercial materials are used to smooth rough surfaces and prevent an air gap between the transducer and the component inspected.

immersion

Many factors, including material, mechanical and electrical construction, and the external mechanical and electrical load conditions, influence the behavior a transducer. Mechanical construction is the factor that influences performance, with important parameters such as radiation surface area, mechanical damping, housing, and other variables of physical construction. As of this writing, transducer manufactures are hard pressed when constructing two transducers that have identical performance characteristics. Transducer manufacture still has something of a "black art" component.

Contact Transducers have elements protected in a rugged casing to withstand direct contact with a variety of materials. These transducers have an ergonomic design so that they are easy to grip and move along a surface. They also often have replaceable wear plates to lengthen their useful life.

Immersion Transducers are designed to transmit ultrasound in situations where the test part is immersed in water. Immersion transducers are typically used inside a water tank or as part of a squirter or bubbler system in scanning applications. Immersion transducers usually have a impedance matching layer that helps to get more sound energy into the water and, in turn, into the component being inspected. Immersion transducers can be purchased with in a planner, cylindrically focused or spherically focused lens. A focused transducer can improve sensitivity and axial resolution by concentrating the sound energy to a smaller area.

Dual Element: contain two independently operating elements in a single housing. One of the elements transmits and the other receives. Dual element transducers are very useful when making thickness measurements of thin materials and when inspecting for near surface defects.

Dual element

Angle Beam: and wedges are typically used to introduce a refracted shear wave into the test material. Transducers can be purchased in a variety of fixed angles or in adjustable versions where the user determines the angles of incident and refraction. They are used to generate surface waves for use in detecting defects on the surface of a component.

Angle beam

Dual Element Transducers contain two independently operating elements in a single housing. One of the elements transmits and the other receives. Dual element transducers are especially well suited for making measurements in applications where reflectors are very near the transducer since this design eliminates the ring down effect that single-element transducers experience. (When single-element transducers are operating in pulse echo mode, the element can not start receiving reflected signals until the element has stopped ringing from it transmit function.) Dual element transducers are very useful when making thickness measurements of thin materials and when inspecting for near surface defects. The two elements are angled towards each other to create a crossed-beam sound path in the test material.

Angle Beam Transducers and wedges are typically used to introduce a refracted shear wave into the test material. Transducers can be purchased in a variety of fixed angles or in adjustable versions where the user determines the angles of incident and refraction. In the fixed angle versions, the angle of refraction that is marked on the transducer is only accurate for a particular material, which is usually steel. The angled sound path allows the sound beam to be reflected from the back wall to improve detectability of flaws in and around welded areas. They are also used to generate surface waves for use in detecting defects on the surface of a component.

Normal Incidence Shear Wave Transducers are unique because they allow introduction of shear waves directly into a test piece without the use of an angle beam wedge. Careful design has enabled manufacturing of transducers with minimal longitudinal wave contamination. The ratio of the longitudinal to shear wave components is generally below -30dB.

Delay Line Transducers provide versatility with a variety of replaceable options. Removable delay line, surface conforming membrane, and protective wear cap options can make a single transducer effective for a wide range of applications. As the name implies, the primary function of a delay line transducer is to introduce a time delay between the generation of the sound wave and the arrival of any reflected waves. This allows the transducer to complete its "sending" function before it starts it "listening" function. Delay line transducers are recommended for applications that require a contact transducer with good near surface resolution. They are designed for use in applications such as high precision thickness gauging of thin materials and delamination checks in composite materials. They are also useful in high-temperature measurement applications since the delay line provides some insulation to the piezoelectric element from the heat.

High Frequency Transducers, when used with the proper instrumentation, can improve flaw resolution and thickness measurement capabilities dramatically. Broadband transducers with frequencies between 20 MHz and 150 MHz are commercially available.

5.2.2 Electromagnetic Acoustic Transducers (EMATs)

When a wire is placed near the surface of an electrically conducting object and is driven by a current at the desired ultrasonic frequency, eddy currents will be induced in a near surface region of the object. If a static magnetic field is also present, these eddy currents will experience Lorentz forces of the form

F = J x B

F is a body force per unit volume, J is the induced dynamic current density, and B is the static magnetic induction.

EMAT: Couplant free transduction allows operation without contact at elevated temperatures and in remote locations. The coil and magnet structure can also be designed to excite complex wave patterns and polarization's that would be difficult to realize with fluid coupled piezoelectric probes (Lamb and Shear waves). In the inference of material properties from precise velocity or attenuation measurements, use of EMATs can eliminate errors associated with couplant variation, particularly in contact measurements.

5.3 Ultrasonic Test Methods

Fluid couplant or a fluid bath is needed for effective transmission of ultrasonic from the transducer to the materialStraight beam contact search unit project a beam of ultrasonic vibrations perpendicular to the surfaceAngle beam contact units send ultrasonic beam into the test material at a predetermined angle to the surface

5.3.1Normal Beam Inspection

Pulse-echo ultrasonic measurements can determine the location of a discontinuity in a part or structure by accurately measuring the time required for a short ultrasonic pulse generated by a transducer to travel through a thickness of material, reflect from the back or the surface of a discontinuity, and be returned to the transducer. In most applications, this time interval is a few microseconds or less.

d = vt/2 or v = 2d/t

where d is the distance from the surface to the discontinuity in the test piece, v is the velocity of sound waves in the material, and t is the measured round-trip transit time.

5.3.2 Angles beam inspection

Can be used for testing flat sheet and plate or pipe and tubingAngle beam units are designed to induce vibrations in Lamb, longitudinal, and shear wave modes

Angle Beam Transducers and wedges are typically used to introduce a refracted shear wave into the test material. An angled sound path allows the sound beam to come in from the side, thereby improving detectability of flaws in and around welded areas.

The geometry of the sample below allows the sound beam to be reflected from the back wall to improve detectability of flaws in and around welded areas.

Crack Tip Diffraction

When the geometry of the part is relatively uncomplicated and the orientation of a flaw is well known, the length (a) of a crack can be determined by a technique known as tip diffraction. One common application of the tip diffraction technique is to determine the length of a crack originating from on the backside of a flat plate.

When an angle beam transducer is scanned over the area of the flaw, the principle echo comes from the base of the crack to locate the position of the flaw (Image 1). A second, much weaker echo comes from the tip of the crack and since the distance traveled by the ultrasound is less, the second signal appears earlier in time on the scope (Image 2).

Crack height (a) is a function of the ultrasound velocity (v) in the material, the incident angle (2) and the difference in arrival times between the two signal (dt).

The variable dt is really the difference in time but can easily be converted to a distance by dividing the time in half (to get the one-way travel time) and multiplying this value by the velocity of the sound in the material. Using trigonometry an equation for estimating crack height from these variables can be derived.

Surface Wave Contact Units

With increased incident angle so that the refracted angle is 90 Surface waves are influenced most by defects close to the surfaceWill travel along gradual curves with little or no reflection from the curve

5.4 Data Presentation

Ultrasonic data can be collected and displayed in a number of different formats. The three most common formats are know in the NDT world as A-scan, B-scan and C-scan presentations. Each presentation mode provides a different way of looking at and evaluating the region of material being inspected. Modern computerized ultrasonic scanning systems can display data in all three presentation forms simultaneously

5.4.1 A-Scan

The A-scan presentation displays the amount of received ultrasonic energy as a function of time. The relative amount of received energy is plotted along the vertical axis and elapsed time (which may be related to the sound energy travel time within the material) is display along the horizontal axis.

Relative discontinuity size can be estimated by comparing the signal amplitude obtained from an unknown reflector to that from a known reflector. Reflector depth can be determined by the position of the signal on the horizontal sweep.

In the illustration of the A-scan presentation to the right, the initial pulse generated by the transducer is represented by the signal IP, which is near time zero. As the transducer is scanned along the surface of the part, four other signals are likely to appear at different times on the screen. When the transducer is in its far left position, only the IP signal and signal A, the sound energy reflecting from surface A, will be seen on the trace. As the transducer is scanned to the right, a signal from the backwall BW will appear latter in time showing that the sound has traveled farther to reach this surface. When the transducer is over flaw B, signal B, will appear at a point on the time scale that is approximately halfway between the IP signal and the BW signal. Since the IP signal corresponds to the front surface of the material, this indicates that flaw B is about halfway between the front and back surfaces of the sample. When the transducer is moved over flaw C, signal C will appear earlier in time since the sound travel path is shorter and signal B will disappear since sound will no longer be reflecting from it.

221.bin

The B-scan presentations is a profile (cross-sectional) view of the a test specimen. In the B-scan, the time-of-flight (travel time) of the sound energy is displayed along the vertical and the linear position of the transducer is displayed along the horizontal axis. From the B-scan, the depth of the reflector and its approximate linear dimensions in the scan direction can be determined.

5.4.2 B-Scan

The B-scan is typically produced by establishing a trigger gate on the A-scan. Whenever the signal intensity is great enough to trigger the gate, a point is produced on the B-scan. The gate is triggered by the sound reflecting from the backwall of the specimen and by smaller reflectors within the material.

The B-scan presentations is a profile (cross-sectional) view of the a test specimen. In the B-scan, the time-of-flight (travel time) of the sound energy is displayed along the vertical and the linear position of the transducer is displayed along the horizontal axis. From the B-scan, the depth of the reflector and its approximate linear dimensions in the scan direction can be determined. The B-scan is typically produced by establishing a trigger gate on the A-scan. Whenever the signal intensity is great enough to trigger the gate, a point is produced on the B-scan. The gate is triggered by the sound reflecting from the backwall of the specimen and by smaller reflectors within the material. In the B-scan image above, line A is produced as the transducer is scanned over the reduced thickness portion of the specimen. When the transducer moves to the right of this section, the backwall line BW is produced. When the transducer is over flaws B and C lines that are similar to the length of the flaws and at similar depths within the material are drawn on the B-scan. It should be noted that a limitation to this display technique is that reflectors may be masked by larger reflectors near the surface.

222.bin

5.4.3 C-Scan:

The C-scan presentation provides a plan-type view of the location and size of test specimen features. The plane of the image is parallel to the scan pattern of the transducer.

C-scan presentations are produced with an automated data acquisition system, such as a computer controlled immersion scanning system. Typically, a data collection gate is established on the A-scan and the amplitude or the time-of-flight of the signal is recorded at regular intervals as the transducer is scanned over the test piece. The relative signal amplitude or the time-of-flight is displayed as a shade of gray or a color for each of the positions where data was recorded. The C-scan presentation provides an image of the features that reflect and scatter the sound within and on the surfaces of the test piece.

Gray scale image produced using the sound reflected from the front surface of the coin

Gray scale image produced using the sound reflected from the back surface of the coin (inspected from heads side)

High resolution scan can produce very detailed images. Both images were produced using a pulse-echo techniques with the transducer scanned over the head side in an immersion scanning system.

For the C-scan image on the left, the gate was setup to capture the amplitude of the sound reflecting from the front surface of the quarter. Light areas in the image indicate area that reflected a greater amount of energy back to the transducer. In the C-scan image on the right, the gate was moved to record the intensity of the sound reflecting from the back surface of the coin. The details on the back surface are clearly visible but front surface features are also still visible since the sound energy is affected by these features as it travels through the front surface of the coin.

6. Eddy Current Testing

Eddy current testing can be used on all electrically conducting materials with a reasonably smooth surface.The test equipment consists of a generator (AC power supply), a test coil and recording equipment, e.g. a galvanometer or an oscilloscopeUsed for crack detection, material thickness measurement (corrosion detection), sorting materials, coating thickness measurement, metal detection, etc.

Electrical currents are generated in a conductive material by an induced alternating magnetic field. The electrical currents are called eddy currents because the flow in circles at and just below the surface of the material. Interruptions in the flow of eddy currents, caused by imperfections, dimensional changes, or changes in the material's conductive and permeability properties, can be detected with the proper equipment.

6.1 Principle of Eddy Current Testing (I)

When a AC passes through a test coil, a primary magnetic field is set up around the coilThe AC primary field induces eddy current in the test object held below the test coilA secondary magnetic field arises due to the eddy current

Mutual Inductance
(The Basis for Eddy Current Inspection)

The flux B through circuits as the sum of two parts.

B1 = L1i1 + i2M

B2 = L2i2 + i1M

L1 and L2 represent the self inductance of each of the coils. The constant M, called the mutual inductance of the two circuits and it is dependent on the geometrical arrangement of both circuits.

The magnetic field produced by circuit 1 will intersect the wire in circuit 2 and create current flow. The induced current flow in circuit 2 will have its own magnetic field which will interact with the magnetic field of circuit 1. At some point P on the magnetic field consists of a part due to i1 and a part due to i2. These fields are proportional to the currents producing them.

Principle of Eddy Current Testing (II)

The strength of the secondary field depends on electrical and magnetic properties, structural integrity, etc., of the test objectIf cracks or other inhomogeneities are present, the eddy current, and hence the secondary field is affected.

Principle of Eddy Current Testing (III)

The changes in the secondary field will be a feedback to the primary coil and affect the primary current.The variations of the primary current can be easily detected by a simple circuit which is zeroed properly beforehand

The bridge circuit here is known as the Maxwell-Wien bridge (often called the Maxwell bridge), and is used to measure unknown inductances in terms of calibrated resistance and capacitance.

Calibration-grade inductors are more difficult to manufacture than capacitors of similar precision, and so the use of a simple "symmetrical" inductance bridge is not always practical. Because the phase shifts of inductors and capacitors are exactly opposite each other, a capacitive impedance can balance out an inductive impedance if they are located in opposite legs of a bridge, as they are here.

Unlike this straight Wien bridge, the balance of the Maxwell-Wien bridge is independent of source frequency, and in some cases this bridge can be made to balance in the presence of mixed frequencies from the AC voltage source, the limiting factor being the inductor's stability over a wide frequency range.

In the simplest implementation, the standard capacitor (Cs) and the resistor in parallel with it are made variable, and both must be adjusted to achieve balance. However, the bridge can be made to work if the capacitor is fixed (non-variable) and more than one resistor is made variable (at least the resistor in parallel with the capacitor, and one of the other two). However, in the latter configuration it takes more trial-and-error adjustment to achieve balance as the different variable resistors interact in balancing magnitude and phase.

Another advantage of using a Maxwell bridge to measure inductance rather than a symmetrical inductance bridge is the elimination of measurement error due to mutual inductance between two inductors. Magnetic fields can be difficult to shield, and even a small amount of coupling between coils in a bridge can introduce substantial errors in certain conditions. With no second inductor to react within the Maxwell bridge, this problem is eliminated.

Conductive

material

Coil

6.2 Eddy Current Instruments

Voltmeter

Coil's

magnetic field

Eddy

currents

Eddy current's

magnetic field

The most basic eddy current testing instrument consists of an alternating current source, a coil of wire connected to this source, and a voltmeter to measure the voltage change across the coil. An ammeter could also be used to measure the current change in the circuit instead of using the voltmeter.

While it might actually be possible to detect some types of defects with this type of an equipment, most eddy current instruments are a bit more sophisticated. In the following pages, a few of the more important aspects of eddy current instrumentation will be discussed.

Eddy currents are closed loops of induced current circulating in planes perpendicular to the magnetic flux. They normally travel parallel to the coil's winding and flow is limited to the area of the inducing magnetic field. Eddy currents concentrate near the surface adjacent to an excitation coil and their strength decreases with distance from the coil as shown in the image. Eddy current density decreases exponentially with depth. This phenomenon is known as the skin effect.

Depth of Penetration

The depth at which eddy current density has decreased to 1/e, or about 37% of the surface density, is called the standard depth of penetration ().

Skin effect arises when the eddy currents flowing in the test object at any dep