Nondestructive tests are extremely useful in failure investigations. These tests can be divided into nine distinct areas:
- Flaw detection
- Leak detection
- Location determination
- Microstructure characterization
- Estimation of mechanical properties
- Stress (strain) and dynamic response determination
- Signature analysis
- Chemical composition determination
Flaw Detection and Evaluation
When performing flaw detection, it must be specified which types of flaws that are rejectable, the size and orientation of flaws that are rejectable, and the locations of flaws that can cause the object to become rejectable. The type, size, orientation and location of flaws that will cause a rejection should be determined, if possible, using stress analysis and/or fracture mechanics calculations.
Ultrasonic Testing (UT)
This method uses a transducer to introduce high frequency sound waves into a material to detect imperfections. In the pulse echo technique, reflections (echoes) from internal imperfections or geometrical surfaces are returned to a receiver, providing images of a defect.
The advantages of ultrasonic test are:
– High sensitivity, which permits the detection of minute cracks
– Great penetrating power, which allows the examination of extremely thick sections
Limitations of Ultrasonic tests include:
– Size-contour complexity and unfavorable discontinuity orientation can pose problems in interpretation of the echo pattern
– Undesirable internal structure (for example – grain size, structure, , inclusion content, or fine, dispersed precipitates) can similarly hinder interpretation
Magnetic Particle Testing (MT)
This method induces a magnetic field in a ferromagnetic material and then iron particles are applied to the surface of the test piece. When a surface or subsurface discontinuity interrupts the induced magnetic field, a leakage field forms above the discontinuity. Iron particles will form an outline of the leakage field revealing its location, size and shape.
Among the advantages of magnetic particle testing are:
– It is the best and most reliable method available for detecting surface cracks – especially very fine and shallow cracks and cracks filled with foreign matter
– There is little or no limitation on size or shape of the part to be tested
Limitations of magnetic particle tests are:
– It is not completely reliable for locating discontinuities that lie entirely below the surface
– The magnetic field must be in a direction that will intercept the principal plane of the discontinuity
Eddy Current Testing (ET)
Using this method, electrical currents (Eddy Currents) are generated in a conductive material by a changing magnetic field. Relevant indications are noted when the flow of eddy currents is interrupted by a defect.
Some of the advantages of ultrasonic inspection include:
– It is sensitive to both surface and subsurface discontinuities.
– The depth of penetration for flaw detection or measurement is superior to other NDT methods.
Some of its limitations include:
– Surface must be accessible to transmit ultrasound.
– Materials that are rough, irregular in shape, very small, exceptionally thin or not homogeneous are difficult to inspect.
Penetrant Testing (PT)
This method involves coating the test specimen with a visible or fluorescent dye. A developer is then applied to draw out the penetrant left behind in surface defects. With fluorescent dyes, ultraviolet lights are used to find the surface imperfections.
The main advantages are:
– Ability to be used on nonmagnetic materials
– Portability and ease with which results can be interpreted
The limitations are:
– Discontinuities must be open to the surface
– Surface films may prevent detection of the discontinuities
“ASM Metals Handbook” Eighth Edition, Volume 10, Failure Analysis and Prevention, American Society for Metals, 1975
“ASM Metals Handbook” Ninth Edition, Volume 17, Nondestructive Evaluation and Quality Control, American Society for Metals, 1975
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