Achieving Reliability Growth by Probabilistic Design of Components Considering Strength Degradation of Materials and Stochastic Nature of Design Variables

In real time applications, the geometry of components and loads that act upon them are stochastic in nature. In such situation, designing a product, assuming a factor of safety, using deterministic design procedures often misguides the designer to deliver a unreliable product; or in over designing the product. The factor of safety provided in design is totally a factor of ignorance. Probabilistic design could aid the designer in designing a product for a desired ppm level. Accounting for the series/parallel system configuration of components in the product, reliability allocation for each individual components can be done. Often, one parameter, that keeps the designer in dark is accounting for degradation of materials in design. As the product performs in the field, strength degradation of components occurs due to failure mechanisms. This paper discusses a method to account for strength deterioration of materials with respect to time in the probabilistic design procedure for a component. Ten products were tested in the laboratory and failure times were noted. Eight failures were due to breakage of journal shaft. Probabilistic design, including strength deterioration, was done for journal shaft and dimensions finalized. Upon continuing lab testing it was found that reliable life of the product has considerably improved.