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Failure Rate Estimation from Field Data under Time‐Varying Stress

A model to assess the failure rate of equipment under use conditions is proposed. This model links noncontrolled variables to a piecewise failure rate combined with a proportional hazard model. Two influential variables are considered. One is the temperature characterizing the outdoor climate, and the other one is moisture—as an intrinsic variable. The maximum likelihood estimates of the model parameters are obtained. The efficiency of the method is evaluated through simulated data. Results on data from the field are provided. Copyright © 2012 John Wiley & Sons, Ltd.