A proof test is intended to be a nondestructive test that demonstrates the acceptability of a load-carrying component to survive design loads. As such, proof loads often equal the design load and are sometimes above the design load in order to show margin. Overloads of 1.25-1.5 are common; where safety is required for lift equipment, a factor of 2.0 is used. Proof tests may be performed not only before the component enters service, but also after a period of time, to ensure that no hidden or other kind of damage has occurred during that period of use.
Since proof loads are high, there is always the risk of component failure during the test. For production units, this screens out those units with design issues prior to service. For one-of-kind units, of course, one needs to ascertain the benefits and drawbacks of proof testing.
For glasses and ceramics, an overload proof test may be desirable to ensure design adequacy over their lifetimes in the presence of moisture, residual stress, and applied tensile stress. If expected lifetimes are long, as they usually are, one does not have the luxury to test at the applied stress levels for such an extended period. Thus, an overload test of short duration can be applied, ensuring the lifetime over the required long duration.
In this case, we cannot choose an arbitrary overload factor of 1.25, 1.5, or any other number, as we can for static loading. Stress corrosion of glass and ceramics is anything but static, with gradual (slow) crack growth occurring over the component life. However, we already have the tools to compute the required overload factor.
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