The range of strengths of a set of identically manufactured ceramic windows is usually quite large. For example, in Table 3.1 the weakest member of a set of 13 zinc sulfide disks had about half the strength of the strongest member. The median strength of ZnS windows with the Weibull distribution in Fig. 3.18 is 94 MPa. If you were asked to design a system with 99.9% reliability, you would have to limit the stress on the window to â¤28 MPa. Such a limitation severely inhibits the use of ceramic components. A proof test removes the weakest members of a population of components so that the remaining population will function reliably at a higher level of stress.
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