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Answer Posted / patilmalls
It is very difficult to weld or mold a solid object that
has the risk of breaking in service, so testing at
manufacture and during use is often essential. During the
process of casting a metal object, for example, the metal
may shrink as it cools, and crack or introduce voids inside
the structure. Even the best welders (and welding machines)
do not make 100% perfect welds. Some typical weld defects
that need to be found and repaired are lack of fusion of
the weld to the metal and porous bubbles inside the weld,
both of which could cause a structure to break or a
pipeline to rupture.
During their service lives, many industrial components need
regular non-destructive tests to detect damage that may be
difficult or expensive to find by everyday methods. For
example:
Aircraft skins need regular checking to detect cracks;
Underground pipelines are subject to corrosion and stress
corrosion cracking;
Pipes in industrial plants may be subject to erosion and
corrosion from the products they carry;
Reinforced concrete structures may be weakened if the inner
reinforcing steel is corroded;
Pressure vessels may develop cracks in welds;
The wire ropes in suspension bridges are subject to
weather, vibration, and high loads, so testing for broken
wires and other damage is important.
Finished machined parts, such as bearings, that have newly
been assembled can be tested for missing pieces, such as a
ball or roller bearing, or grease within the housing non-
destructively with a checkweigher. A roller motor for a
conveyor can be tested for the proper level of oil, without
disassembling the finished product. Thousand of
manufactured products can benefit from this form of testing.
Over the past centuries, swordsmiths, blacksmiths, and bell-
makers would listen to the ring of the objects they were
creating to get an indication of the soundness of the
material. The wheel-tapper would test the wheels of
locomotives for the presence of cracks, often caused by
fatigue — a function that is now carried out by
instrumentation and referred to as the acoustic impact
technique.
Use of X-rays for NDT is a common way of examining the
interior of products for voids and defects, although some
skill is needed in using radiography to examine samples and
interpret the results. Soft X-rays are needed for examining
low density material like polymers, composites and ceramics.
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