Planning Inspection Times for Degradation Tests

Degradation tests are often accompanied by pre-defined product inspection plans because real-time monitoring of product performance is difficult, if not impossible, in most cases. If the purpose of an inspection plan is to predict a failure time before an actual failure happens, it is reasonable to increase inspection frequency gradually over time as the test unit deteriorates over time. Curiously, this strategy has not been discussed much in the degradation test (DT) planning literature. In this paper, we propose a risk-based inspection (RBI) method to determine the final inspection time. By balancing the probability of failure and the consequence of failure, we place the optimal inspection time at the moment of highest failure risk, thus it is closer to the soft failure time that engineers actually care about. Furthermore, we test three inspection scheduling strategies - the equal-distance inspection time strategy, the equal-proportion inspection time strategy, and the middle inspection time strategy. It is found that progressively adding middle inspection times can enhance the prediction property of the degradation model. Through simulation, we show that this inspection plan performs better than other plans.