Now that we have determined the statistical method to yield the reliable and confident material strength, we turn to methods of strength measurement. As we have seen, determination of the inert strength is an important step in determining stress corrosion effects. This must be done, of course, on test specimens that are finished in the same manner as the component being evaluated. Inert strength testing is generally accomplished by testing the specimen to failure in a tensile testing machine, which requires only a load cell and not a strain gage extensometer, as stress alone is what concerns us here, not displacement. Such tests must exercise the flaws in the finished surface under tension in a dry environment to prevent subcritical crack growth that would reduce the true strength of the surface. The dry environment can be obtained in several ways; e.g., by immersing the specimen in liquid nitrogen, containing it in dry gaseous nitrogen, or using an inert gas such as helium or another dry environment.
In order to benefit from the test, a substantial area of the specimen must be in tensile stress so that appropriate scale factors (Chapter 10) can be applied to the real article. Accordingly, methods such as the three-point bend test are not very useful, as maximum stress occurs only along a line rather than along an area. To this end, a four-point bend specimen is often used. Here, the strength is readily calculated, and stress is well distributed, as explained in the following section.
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