Product Reliability and Random Fatigue

The fatigue life of a mechanical component in a conventional ional test situation depends upon the stress amplitude. A change in the stress amplitude during a laboratory test-program involves a consideration of the damage produced by each stress cycle imposed upon the test specimen. A direct extension of this elementary concept to irregular stress waveforms provides a logical basis for a statistical analysis of product reliability due to random fatigue. The statistical behavior of the fatigue life is related to the probability distribution of the maxima and minima in a random stress waveform. The fundamental relation between the probability of survival and the maxima-minima is derived in this presentation, which contains a theoretical application to a simple random stress oscillation. The same expression may be used in the development of a computer program to evaluate product reliability from a numerical analysis of experimental data.