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When is metallic gear too old ? 20-Jun-2008 At 11:19:12 AM AlanD

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I'm also a metallurgist, with some experience in NDT.

While typically cracks do start in the surface, they may also start at other locations such as grain boundaries, inclusions and voids. With the case of voids, these maybe growing within the metal from the interaction of dislocations. The issue is that these point defects may not detected through routine testing as these tests only likely to discover things within the size range they are looking at and even then they are not 100% accurate. If the point defect is smaller or even sometimes bigger, the tests are significantly less likely to detect them.

See if your father has a copy of Honeycombe's "The plastic deformation of metals", all this is going into some fairly heavy dislocation theory.