NON-DESTRUCTIVE TESTING:

Radiography

Radiography is the use of X-rays to view a non-uniformly composed material (i.e. of varying density and composition) such as the human body. A heterogeneous beam of X-rays is produced by an X-ray generator and is projected toward an object. The density and composition of each area determines how much of the ray is absorbed. The X-rays that pass through are captured behind the object by a detector (either photographic film or a digital detector). The detector gives a 2D representation of all the structures superimposed on each other.

Sources

A number of sources of X-ray photons have been used; these include X-ray generators, betatrons, and linear accelerators (linacs). Today, the most powerful and brilliant sources of X-rays (from soft to hard X-rays) are synchrotron sources. For rays, radioactive sources such as 192Ir, 60Co or 137Cs are used.

Detectors

A range of detectors including photographic film, scintillator and semiconductor diode arrays have been used to collect images.

Ultrasonic testing:

In ultrasonic testing (UT), very short ultrasonic pulse-waves with center frequencies ranging from 0.1-15 MHz and occasionally up to 50 MHz are transmitted into materials to detect internal flaws or to characterize materials. A common example is ultrasonic thickness measurement, which tests the thickness of the test object, for example, to monitor pipework corrosion.

Ultrasonic testing is often performed on steel and other metals and alloys, though it can also be used on concrete, wood and composites, albeit with less resolution. It is a form of non-destructive testing used in many industries including aerospace, automotive and other transportation sectors.

Advantages

1. High penetrating power, which allows the detection of flaws deep in the part.

2. High sensitivity, permitting the detection of extremely small flaws.

3. Only one surface needs to be accessible.

4. Greater accuracy than other non-destructive methods in determining the depth of internal flaws and the thickness of parts with parallel surfaces.

5. Some capability of estimating the size, orientation, shape and nature of defects.

6. Non-hazardous to operations or to nearby personnel and has no effect on equipment and materials in the vicinity.

7. Capable of portable or highly automated operation.

Disadvantages

1. Manual operation requires careful attention by experienced technicians. The transducers alert to both normal structure of some materials, tolerable anomalies of other specimens (both termed “noise”) and to faults therein severe enough to compromise specimen integrity. These signals must be distinguished by a skilled technician, possibly, after follow up with other non-destructive testing methods.

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

3. Parts that is rough, irregular in shape, very small or thin, or not homogeneous are difficult to inspect.

4. Surface must be prepared by cleaning and removing loose scale, paint, etc., although paint that is properly bonded to a surface need not be removed.

5. Couplants are needed to provide effective transfer of ultrasonic wave energy between transducers and parts being inspected unless a non-contact technique is used. Non-contact techniques include Laser and Electro Magnetic Acoustic Transducers (EMAT).

6. Inspected items must be water resistant, when using water based couplants that do not contain rust inhibitors.

Magnetic particle Inspection:

Magnetic particle Inspection (MPI) is a non-destructive testing (NDT) process for detecting surface and slightly subsurface discontinuities in ferromagnetic materials such asiron, nickel, cobalt, and some of their alloys and must be performed to worldwide standards such as EN473 and ISO9712 by qualified personnel. The process puts a magnetic field into the part. The piece can be magnetized by direct or indirect magnetization. Direct magnetization occurs when the electric current is passed through the test object and a magnetic field is formed in the material. Indirect magnetization occurs when no electric current is passed through the test object, but a magnetic field is applied from an outside source. The magnetic lines of force are perpendicular to the direction of the electric current which may be either alternating current (AC) or some form of direct current (DC) (rectified AC).

The presence of a surface or subsurface discontinuity in the material allows the magnetic flux to leak, since air cannot support as much magnetic field per unit volume as metals. Ferrous iron particles are then applied to the part. The particles may be dry or in a wet suspension. If an area of flux leakage is present, the particles will be attracted to this area. The particles will build up at the area of leakage and form what is known as an indication. The indication can then be evaluated to determine what it is, what may have caused it, and what action should be taken, if any.

A popular name for magnetic particle inspection is or used to be magnafluxing.

Inspection:

The following are general steps for inspecting on a wet horizontal machine:

1. Part is cleaned of oil and other contaminants

2. Necessary calculations done to know the amount of current required to magnetize the part.

3. The magnetizing pulse is applied for 0.5 seconds during which the operator washes the part with the particle, stopping before the magnetic pulse is completed. Failure to stop prior to end of the magnetic pulse will wash away indications.

4. UV light is applied while the operator looks for indications of defects that are 0 to +/- 45 degrees from path the current flowed through the part. Indications only appear 45 to 90 degrees of the magnetic field applied. The easiest way to quickly figure out which way the magnetic field is running is grab the part with either hand between the head stocks laying your thumb against the part (do not wrap your thumb around the part) this is called either left or right thumb rule or right hand grip rule. The direction the thumb points tell us the direction current is flowing, the Magnetic field will be running 90 degrees from the current path. On complex geometry like an engine crank the operator needs to visualize the changing direction of the current and magnetic field created. The current starts at 0 degrees then 45 degrees to 90 degree back to 45 degrees to 0 then -45 to -90 to -45 to 0 and repeat this for crankpin. So inspection can be time consuming to carefully look for indications that are only 45 to 90 degrees from the magnetic field.

5. The part is either accepted or rejected based on pre-defined accept and reject criteria

6. The part is demagnetized

7. Depending on requirements the orientation of the magnetic field may need to be changed 90 degrees to inspect for indications that cannot be detected from steps 3 to 5. The most common way to change magnetic field orientation is to use a Coil Shot.

Fluorescent penetrant inspection:

Fluorescent penetrant inspection (FPI) is a type of dye penetrant inspection in which a fluorescent dye is applied to the surface of a non-porous material in order to detect defects that may compromise the integrity or quality of the part in question. Noted for its low cost and simple process, FPI is used widely in a variety of industries.

Inspection Steps:

See the following main steps in a Fluorescent Penetrant Inspection Process:

1. Initial Cleaning:

Before the penetrant can be applied to the surface of the material in question one must ensure that the surface is free of any contamination such as paint, oil, dirt, or scale that may fill a defect or falsely indicate a flaw. Chemical etching can be used to rid the surface of undesired contaminates and ensure good penetration when the penetrant is applied. Sandblasting to remove paint from a surface prior to the FPI process may mask (smear material over) cracks making the penetrant not effective. Even if the part has already been through a previous FPI operation it is imperative that it is cleaned again. Most penetrants are not compatible and therefore will thwart any attempt to identify defects that are already penetrated by any other penetrant. This process of cleaning is critical because if the surface of the part is not properly prepared to receive the penetrant, defective product may be moved on for further processing. This can cause lost time and money in reworking, over processing, or even scrapping a finished part at final inspection.

2. Penetrant Application:

The fluorescent penetrant is applied to the surface and allowed time to seep into flaws or defects in the material. The process of waiting for the penetrant to seep into flaws is called Dwell Time. Dwell time varies by material and the size of the indications that are intended to be identified but is generally around 30 minutes. It requires much less time to penetrate larger flaws because the penetrant is able to soak in much faster. The opposite is true for smaller flaws.

3. Excess Penetrant Removal:

After the identified dwell time has passed, penetrant on the outer surface of the material is then removed. This highly controlled process is necessary in order to ensure that the penetrant is removed only from the surface of the material and not from inside any identified flaws. Various chemicals can be used for such a process and vary by specific penetrant types. Typically, the cleaner is applied to a lint-free cloth that is used to carefully clean the surface.

4. Developer Application:

Having removed excess penetrant a contrasting developer may be applied to the surface. This serves as a background against which flaws can more readily be detected. The developer also causes penetrant that is still in any defects to surface and bleed. These two attributes allow defects to be easily detected upon inspection. Dwell time is then allowed for the developer to achieve desired results before inspection.

5. Inspection:

In the case of fluorescent inspection, the inspector will use ultraviolet radiation with intensity appropriate to the intent of the inspection operation. This must take place in a dark room to ensure good contrast between the glow emitted by the penetrant in the defected areas and the unlit surface of the material. The inspector carefully examines all surfaces in question and records any concerns. Areas in question may be marked so that location of indications can be identified easily without the use of the UV lighting. The inspection should occur at a given point in time after the application of the developer. Too short a time and the flaws may not be fully blotted, too long and the blotting may make proper interpretation difficult.

6. Final Cleaning:

Upon successful inspection of the product, it is returned for a final cleaning before it is shipped, moved on to another process, or deemed defective and reworked or scrapped. Note that a flawed part may not go through the final cleaning process if it is considered not to be cost effective.

Advantages

· Highly sensitive fluorescent penetrant is ideal for even the smallest imperfections

· Low cost and potentially high volume

Potential Disadvantage:

· Requires adequate cleaning (neglect of this step can have costly repercussions)

· Test materials can be damaged if compatibility is not ensured

· Penetrant stains clothes and skin and must be treated with care

· Limited to surface defects

· Extensive training is required for the Inspector

Penetrants stain cloth, skin and other porous surfaces brought into contact. One should verify compatibility on the test material, especially when considering the testing of plastic components. Further information on inspection steps may be found in industry standards (e.g. the American Welding Society, American Society for Testing and Materials, the British Standards Institute, and the Society for Automotive Engineers).

Eddy-current testing:

Eddy-current testing uses electromagnetic induction to detect flaws in conductive materials. There are several limitations, among them: only conductive materials can be tested, the surface of the material must be accessible, the finish of the material may cause bad readings, the depth of penetration into the material is limited by the materials’ conductivity, and flaws that lie parallel to the probe may be undetectable.

In a standard eddy current testing a circular coil carrying current is placed in proximity to the test specimen (which must be electrically conductive).The alternating current in the coil generates changing magnetic field which interacts with test specimen and generates eddy current. Variations in the phase and magnitude of these eddy currents can be monitored using a second ‘receiver’ coil, or by measuring changes to the current flowing in the primary ‘excitation’ coil. Variations in the electrical conductivity or magnetic permeability of the test object, or the presence of any flaws, will cause a change in eddy current and a corresponding change in the phase and amplitude of the measured current. This is the basis of standard (flat coil) eddy current inspection, the most widely used eddy current technique.

However, eddy-current testing can detect very small cracks in or near the surface of the material, the surfaces need minimal preparation, and physically complex geometries can be investigated. It is also useful for making electrical conductivity and coating thickness measurements.

The testing devices are portable, provide immediate feedback, and do not need to contact the item in question. Recently tomographic notion of ECT has been explored see for example:

Another eddy-current testing technique is pulsed eddy-current testing. A major advantage of this type of testing is that there is no need for direct contact with the tested object. The measurement can be performed through coatings, weather sheeting’s, corrosion products and insulation materials. This way even high temperature inspections are possible. Compared to the conventional eddy-current testing, pulsed eddy-current testing allows multi-frequency operation.

Acoustic emission Testing:

Acoustic emission (AE) is the sound waves produced when a material undergoes stress (internal change), as a result of an external force. AE is a phenomenon occurring in for instance mechanical loading generating sources of elastic waves. This occurrence is the result of a small surface displacement of a material produced due to stress waves generated when the energy in a material or on its surface is released rapidly. The wave generated by the source is of practical interest in methods used to stimulate and capture AE in a controlled fashion, for study and/or use in inspection, quality control, system feedback, process monitoring and others.

The application of acoustic emission to non-destructive testing of materials, typically takes place between 100 kHz and 1 MHz. Unlike conventional ultrasonic testing, AE tools are designed for monitoring acoustic emissions produced within the material during failure or stress, rather than actively transmitting waves, then collecting them after they have travelled through the material. Part failure can be documented during unattended monitoring. The monitoring of the level of AE activity during multiple load cycles forms the basis for many AE safety inspection methods that allow the parts undergoing inspection to remain in service.

The technique is used, for example, to study the formation of cracks during the welding process, as opposed to locating them after the weld has been formed with the more familiar ultrasonic testing technique. In a material under active stress, such as some components of an airplane during flight, transducers mounted in an area can detect the formation of a crack at the moment it begins propagating. A group of transducers can be used to record signals, and then locate the precise area of their origin by measuring the time for the sound to reach different transducers. The technique is also valuable for detecting cracks forming in pressure vessels and pipelines transporting liquids under high pressures. Also, this technique is used for estimation of corrosion in reinforced concrete structures.

In addition to non-destructive testing, acoustic emission monitoring has applications in process monitoring. Applications where acoustic emission monitoring has successfully been used include detecting anomalies in fluidized beds, and end points in batch granulation.

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