22-11-2012, 04:15 PM
Failure Analysis: Nondestructive Testing Methods
Failure Analysis_ Nondestructive Testing Methods - Feature Articles - Industrial Heating.pdf (Size: 831.83 KB / Downloads: 32)
Visual Inspection
Visual inspection is the most basic and widely used method for the examination of parts. It is used to detect surface
abnormalities (cracks and imperfections). Since it can be performed by virtually anyone and is subjective in nature, it is a
good idea to develop standards or use comparative aids. It is useful in pinpointing areas that require closer inspection
(perhaps by another NDT method) and is the place to start before performing any other type of NDT test.
Always begin by preparing an area where the visual inspection will take place. It should be well lit and allow easy
manipulation of the part so that all surfaces can be easily viewed. Start by inspecting all surfaces in the “as received”
condition. As the examination proceeds, the surface may need to be cleaned or treated in some manner to highlight a
defect in more detail. All observations should be documented and adequate time allowed to do a thorough inspection.
Magnetic-Particle Inspection
Magnetic-particle inspection involves the basic principles of magnetism and can be thought of
as a combination of two nondestructive testing methods: magnetic-flux-leakage testing and
visual testing.
The first step in a magnetic-particle inspection is to magnetize the component that is to be
inspected. If any defects on or near the surface are present, the defects will create a leakage
field (Fig. 3). After the component has been magnetized, iron particles either in a dry or wet
suspended form are applied to the surface of the magnetized part. The particles will be
attracted and cluster at the flux-leakage fields, thus forming a visible indicator that the
inspector can detect.
The basic components that make up magnetic-particle equipment are: yokes (either permanent magnets or
electromagnets), coils (multi-loop windings) and prods (to pass current directly to the part).
Radiographic Testing
This nondestructive method can be used to detect internal defects in a wide variety of
components such as castings, forgings or weldments by exposure to X-ray or gamma-ray
radiation. Skilled radiographers can produce high-quality images using X-ray tubes or gammaray
isotopes such as Iridium 192 and C obalt 60. Defects are detected by differences in
radiation absorption in the material as seen on a “shadow graph” displayed on photographic
film or a fluorescent screen.
In radiography, the process to produce an image is quite different from that of photography.
The camera is actually a radiation source, while the film is not placed inside the camera but
instead is placed on the opposite side of the object being imaged. The radiation is not reflected
to the film but rather passes through the object and then strikes the film. The image on the
film is dependent upon how much of the radiation makes it through the object and to the film.
Differences in the type of material and the amount of material that the X-rays must penetrate
are responsible for the details in the image[3].
C racks can be detected in a radiograph only if the crack is propagating in a direction that produced a change in thickness –
parallel to the X-ray beam (Fig. 4). C racks will appear as jagged and often very faint irregular lines. C racks can sometimes
appear as "tails" on inclusions or porosity.
Eddy Current
Eddy-current testing is an indirect method and involves the principle of electromagnetic
induction. Eddy current is simply electric current induced by an alternating magnetic field.
Parts to be inspected are placed within or adjacent to an electric coil. A high-frequency electric
current is applied, and the primary field around the coil causes eddy currents to flow in the
part [1].
Two principle purposes for eddy-current testing are to investigate material properties and to
detect surface flaws.
Material-properties testing using the eddy-current method involves the relative permeability
property of metals (and sometimes conductivity) to verify that a component is the correct alloy and/or that it has achieved
the desired properties from heat treatment, including surface hardness, case hardness and case depth. Testing actually
verifies correct microstructure because it is the microstructure that determines the relative permeability and conductivity.
Mixed structures (Fig. 5A, 5B) from incorrect heat treatments can also be reliably detected and sorted.