Question

Describe the five different techniques demonstrated.

Background

Non-Destructive Testing During the design process great care must be taken to match the design to the application. This process involves, among others, material selection, material processing, and service condition analysis. As we have found through the material outlined in the text a great number of decisions have to be made during the design process to insure that the product meets all of the needs of the application. Once a product is produced we must evaluate its quality to determineio9 if the design is successful. Destructive testing will give us some information about component properties, but requires the loss of the product. Some types of destructive testing can also be impractical for large components. In some instances only a small number of components are actually produced and destruction of any of the parts may be too costly. Nondestructive techniques are used to evaluate the quality of components without the sacrifice of the part. These methods can be used together with fracture toughness information to evaluate whether flaws will cause crack propagation or can be used to monitor the growth of cracks that could result in fatigue failure. The simplest nondestructive test method, abbreviated NDT, is a visual inspection. Often this visual inspection can reveal cracks or other flaws such as in welds or castings or other production problems.

Many quality problems at the surface may be invisible to the naked eye or may exist hidden in the interior of the component. For these hidden or microscopic flaws other techniques are used.

These techniques include:

Magnetic-Particle inspection               Inductive (eddy current) inspection

Fluorescent-Penetrant inspection         Radiographic inspection

Dye Penetrant inspection                     Acoustic-Emission inspection

Ultrasonic inspection

Answer 1

Describe the five different techniques demonstrated.

Background

Non-Destructive Testing During the design process great care must be taken to match the design to the application. This process involves, among others, material selection, material processing, and service condition analysis. As we have found through the material outlined in the text a great number of decisions have to be made during the design process to insure that the product meets all of the needs of the application. Once a product is produced we must evaluate its quality to determineio9 if the design is successful. Destructive testing will give us some information about component properties, but requires the loss of the product. Some types of destructive testing can also be impractical for large

components. In some instances only a small number of components are actually produced and destruction of any of the parts may be too costly. Nondestructive techniques are used to evaluate the quality of components without the sacrifice of the part. These methods can be used together with fracture toughness information to evaluate whether flaws will cause crack propagation or can be used to monitor the growth of cracks that could result in fatigue failure. The simplest nondestructive test method, abbreviated NDT, is a visual inspection. Often this visual inspection can reveal cracks or other flaws such as in welds or castings or other production problems.

Many quality problems at the surface may be invisible to the naked eye or may exist hidden in the interior of the component. For these hidden or microscopic flaws other techniques are used.

These techniques include:

Fluorescent-Penetrant inspection     

Dye Penetrant inspection                     Acoustic-Emission inspection

1. Magnetic-Particle inspection:

Magnetic particle Inspection (MPI) is a non-destructive process for detecting surface and shallow subsurface discontinuities in ferromagnetic materials such as iron, nickel, cobalt, and some of their alloys. The process puts a magnetic field into the part.

The principle of the method is that the specimen is magnetised to produce magnetic lines of force, or flux, in the material. The most versatile technique is using a 110v AC hand held electromagnetic yoke magnet, a white strippable paint as contrast background and a magnetic "ink" composed of iron powder particles in a liquid carrier base.

The area is magnetised with the yoke magnet. In the event of a surface or slightly sub surface defect being present, the lines of magnetic force will deform around the defect.

The magnetic ink is applied and the iron powder particles will bridge the gap caused by the defect and give a visible indication against the white contrast background.

MAGNETIC PARTICLE INSPECTION IS PERFORMED IN FOUR STEPS:

1. Induce a magnetic field in the specimen
2. Apply magnetic particles to the specimen's surface
3. View the surface, looking for particle groupings that are caused by defects
4. Demagnetize and clean the specimen

ADVANTAGES OF MAGNETIC PARTICLE INSPECTION

• Can find both surface and near sub-surface defects
• Some inspection formats are extremely portable and low cost
• Rapid inspection with immediate results
• Indications are visible to the inspector directly on the specimen surface
• Can detect defects that have been smeared over
• Can inspect parts with irregular shapes (external splines, crankshafts, connecting rods, etc.)

LIMITATIONS OF MAGNETIC PARTICLE INSPECTION

• The specimen must be ferromagnetic (e.g. steel, cast iron)
• Paint thicker than about 0.005" must be removed before inspection
• Post cleaning and post demagnetization is often necessary
• Maximum depth sensitivity is typically quoted as 0.100" (deeper under perfect conditions)
• Alignment between magnetic flux and defect is important

2. Ultrasonic inspection

ULTRASONIC INSPECTION is non-destructive method (NDT) in which beams of high-frequency mechanical waves are introduced into materials, using transducers, for the detection and characterization of both surface and subsurface anomalies and flaws in the material.

The mechanical waves travel through the material with some attendant loss of energy (attenuation, including both scattering and absorption) and interact with interfaces (reflection, transmission) and discontinuities, including flaws and other anomalies. Response signals are detected, displayed, and then analyzed to give signatures that are used to define the presence, location, and characteristics of flaws or other discontinuities

Advantages and Disadvantages

The principal advantages of ultrasonic inspection, as compared to other methods used for non-destructive inspection of parts, are:

· Superior penetrating power, which allows the detection of flaws deep in the part

· High sensitivity, permitting the detection of extremely small flaws

· Greater accuracy than other non-destructive methods in determining the position of internal flaws, estimating their size, and characterizing their orientation, shape, and nature

· Only one surface needs to be accessible.

· Operation is electronic, which provides almost instantaneous indication of flaws.

· With most systems, a permanent digital record of inspection data can be made for subsequent off-line review and for future reference.

· Volumetric scanning ability, enabling the inspection of a volume of metal extending

from front surface to back surface of a part.

· It is nonhazardous to operators or to nearby personnel and has no effect on equipment and materials in the vicinity.

· Portability (in many implementations)

· It provides an output that can be processed digitally in the test unit or by an external

computer to characterize defects and to determine material properties.

The disadvantages of ultrasonic inspection

include:

· Manual implementations require careful attention by experienced technicians.

· Extensive technical knowledge is required for the development of inspection procedures.

· Parts that are rough, irregular in shape, very small or thin, or not homogeneous are more difficult to reliably inspect.

· Discontinuities that are present in a shallow layer immediately beneath the surface (dead zone) may not be detectable.

· For many types of piezoelectric-based transducers, couplants are needed to provide effective transfer of ultrasonic wave energy between transducers and parts being inspected.

· Reference standards are needed in many applications, both for calibrating the equipment and for characterizing flaws.

3. Inductive current inspection:

Eddy-current testing (also commonly seen as eddy current testing and ECT) is one of many electromagnetic testing methods used in nondestructive testing (NDT) making use of electromagnetic induction to detect and characterize surface and sub-surface flaws in conductive materials.

Eddy current testing uses the principle of electromagnetic induction to detect flaws in conductive materials. An excitation coil carrying current is placed in proximity to the component to be inspected. Pulsed Eddy Current (PEC) is a technique used to detect flaws or corrosion in ferromagnetic materials. ... Long Range Ultrasonic Testing (LRUT) is an ultrasonic testing technique that is used to inspect pipelines for corrosion. This technique is done by strapping a series of rings around a pipe

Disadvantages of Eddy Currents:

There is a major heat loss during cycling eddy currents due to friction in the magnetic circuit, especially where the core is saturated. Thus there is the loss of useful electrical energy in the form of heat. There is magnetic flux leakage.

Eddy currents in conductors of non-zero resistivity generate heat as well as electromagnetic forces. The heat can be used for induction heating. Theelectromagnetic forces can be used for levitation, creating movement, or to give a strong braking effect.

Common applications

Eddy current instruments can be used in a wide variety of tests. Some of the most common are listed below.

Weld Inspection - Many weld inspections employ ultrasonic NDT for subsurface testing and a complimentary eddy current method to scan the surface for open surface cracks on weld caps and in heat affected zones.

Conductivity Testing - Eddy current testing's ability to measure conductivity can be used to identify and sort ferrous and nonferrous alloys, and to verify heat treatment.

Surface Inspection - Surface cracks in machined parts and metal stock can be readily identified with eddy current. This includes inspection of the area around fasteners in aircraft and other critical applications.

Corrosion Detection - Eddy current instruments can be used to detect and quantify corrosion on the inside of thin metal such as aluminum aircraft skin. Low frequency probes can be used to locate corrosion on second and third layers of metal that cannot be inspected ultrasonically.

Bolt Hole Inspection - Cracking inside bolt holes can be detected using bolt hole probes, often with automated rotary scanners.

Tubing inspection - Both in-line inspection of tubing at the manufacturing stage and field inspection of tubing like heat exchangers are common eddy current applications. Both cracking and thickness variations can be detected.

4. Radiographic inspection:

The history of radiographic testing actually involves two beginnings. The first commenced with the discovery of x-Rays by Wilhelm Conrad Röntgen in 1895 and the second with the announcement by Marie Curie, in December of 1898, that the demonstrated the existence of a new radioactive material called "Radium".

Radiographic Testing (RT or X-ray or Gamma ray) is a non-destructive testing (NDT) method that examines the volume of a specimen. Radiography (X-ray) uses X-rays and gamma-rays to produce a radiograph of a specimen, showing any changes in thickness, defects (internal and external), and assembly details to ensure optimum quality in your operation.

RT usually is suitable for testing welded joints that can be accessed from both sides, with the exception of double-wall signal image techniques used on some pipe. Although this is a slow and expensive NDT method, it is a dependable way to detect porosity, inclusions, cracks, and voids in weld interiors.

RT makes use of X-rays or gamma rays. X-rays are produced by an X-ray tube, and gamma rays are produced by a radioactive isotope.

5. Dye penetration inspection:

Dye penetrant inspection (DP), also called liquid penetrate inspection (LPI) or penetrant testing (PT), is a widely applied and low-cost inspection method used to check surface-breaking defects in all non-porous materials (metals, plastics, or ceramics). The penetrant may be applied to all non-ferrous materials and ferrous materials, although for ferrous components magnetic-particle inspection is often used instead for its subsurface detection capability. LPI is used to detect casting, forging and welding surface defects such as hairline cracks, surface porosity, leaks in new products, and fatigue cracks on in-service components.

DPI is based upon capillary action, where low surface tension fluid penetrates into clean and dry surface-breaking discontinuities. Penetrant may be applied to the test component by dipping, spraying, or brushing. After adequate penetration time has been allowed, the excess penetrant is removed and a developer is applied. The developer helps to draw penetrant out of the flaw so that an invisible indication becomes visible to the inspector. Inspection is performed under ultraviolet or white light, depending on the type of dye used - fluorescent or nonfluorescent (visible).

Below are the main steps of Liquid Penetrant Inspection:

1. Pre-cleaning:

The test surface is cleaned to remove any dirt, paint, oil, grease or any loose scale that could either keep penetrant out of a defect, or cause irrelevant or false indications. Cleaning methods may include solvents, alkaline cleaning steps, vapor degreasing, or media blasting. The end goal of this step is a clean surface where any defects present are open to the surface, dry, and free of contamination. Note that if media blasting is used, it may "work over" small discontinuities in the part, and an etching bath is recommended as a post-blasting treatment.

Application of the penetrant to a part in a ventilated test area.

2. Application of Penetrant:

The penetrant is then applied to the surface of the item being tested. The penetrant is usually a brilliant coloured mobile fluid with high wetting capability.[1] The penetrant is allowed "dwell time" to soak into any flaws (generally 5 to 30 minutes). The dwell time mainly depends upon the penetrant being used, material being tested and the size of flaws sought. As expected, smaller flaws require a longer penetration time. Due to their incompatible nature one must be careful not to apply solvent-based penetrant to a surface which is to be inspected with a water-washable penetrant.

3. Excess Penetrant Removal:

The excess penetrant is then removed from the surface. The removal method is controlled by the type of penetrant used. Water-washable, solvent-removable, lipophilic post-emulsifiable, or hydrophilic post-emulsifiable are the common choices. Emulsifiers represent the highest sensitivity level, and chemically interact with the oily penetrant to make it removable with a water spray. When using solvent remover and lint-free cloth it is important to not spray the solvent on the test surface directly, because this can remove the penetrant from the flaws. If excess penetrant is not properly removed, once the developer is applied, it may leave a background in the developed area that can mask indications or defects. In addition, this may also produce false indications severely hindering the ability to do a proper inspection. Also, the removal of excessive penetrant is done towards one direction either vertically or horizontally as the case may be.

4. Application of Developer:

After excess penetrant has been removed, a white developer is applied to the sample. Several developer types are available, including: non-aqueous wet developer, dry powder, water-suspendable, and water-soluble. Choice of developer is governed by penetrant compatibility (one can't use water-soluble or -suspendable developer with water-washable penetrant), and by inspection conditions. When using non-aqueous wet developer (NAWD) or dry powder, the sample must be dried prior to application, while soluble and suspendable developers are applied with the part still wet from the previous step. NAWD is commercially available in aerosol spray cans, and may employ acetone, isopropyl alcohol, or a propellant that is a combination of the two. Developer should form a semi-transparent, even coating on the surface.

The developer draws penetrant from defects out onto the surface to form a visible indication, commonly known as bleed-out. Any areas that bleed out can indicate the location, orientation and possible types of defects on the surface. Interpreting the results and characterizing defects from the indications found may require some training and/or experience [the indication size is not the actual size of the defect].

5. Inspection:

The inspector will use visible light with adequate intensity (100 foot-candles or 1100 lux is typical) for visible dye penetrant. Ultraviolet (UV-A) radiation of adequate intensity (1,000 micro-watts per centimeter squared is common), along with low ambient light levels (less than 2 foot-candles) for fluorescent penetrant examinations. Inspection of the test surface should take place after 10- to 30-minute development time, and is dependent on the penetrant and developer used. This time delay allows the blotting action to occur. The inspector may observe the sample for indication formation when using visible dye. It is also good practice to observe indications as they form because the characteristics of the bleed out are a significant part of interpretation characterization of flaws.

6. Post Cleaning:

The test surface is often cleaned after inspection and recording of defects, especially if post-inspection coating processes are scheduled.

Advantages and disadvantages[edit]

The main advantages of DPI are the speed of the test and the low cost. Disadvantages include the detection of only surface flaws, skin irritation, and the inspection should be on a smooth clean surface where excessive penetrant can be removed prior to being developed. Conducting the test on rough surfaces, such as "as-welded" welds, will make it difficult to remove any excessive penetrant and could result in false indications. Water-washable penetrant should be considered here if no other option is available. Also, on certain surfaces a great enough color contrast cannot be achieved or the dye will stain the workpiece.[2]

Limited training is required for the operator — although experience is quite valuable. Proper cleaning is necessary to assure that surface contaminants have been removed and any defects present are clean and dry. Some cleaning methods have been shown to be detrimental to test sensitivity, so acid etching to remove metal smearing and re-open the defect may be necessary.[3]