When nearly identically processed materials/devices are placed under the same set of stress conditions, they will not fail exactly at the same time. An explanation for this occurrence is that slight differences can exist in the materials microstructure, even for materials/devices processed nearly identically. This means that not only are we interested in time-to-failure but, more precisely, we are interested in the distribution of times-to-failure. Once the distribution of times-to-failure is established, then one can construct a probability density function f(t) which will permit one to calculate the probability of observing a failure in any arbitrary time interval between t and t + dt, as is illustrated in Figure 6.1.
KeywordsLognormal Distribution Weibull Distribution Weibull Plot Failure Distribution Cumulative Fraction
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