8.
Assessing Product Reliability
8.1. Introduction


Acceleration models predict time to fail as a function of stress 
Acceleration factors show how timetofail at a particular operating stress level (for one failure mode or mechanism) can be used to predict the equivalent time to fail at a different operating stress level. A model that predicts timetofail as a function of stress would be even better than a collection of acceleration factors. If we write \(t_f = G(S)\), with \(G(S)\) denoting the model equation for an arbitrary stress level \(S\), then the acceleration factor between two stress levels \(S_1\) and \(S_2\) can be evaluated simply by \(AF = G(S_1) / G(S_2)\). Now we can test at the higher stress \(S_2\), obtain a sufficient number of failures to fit life distribution models and evaluate failure rates, and use the Linear Acceleration Relationships Table to predict what will occur at the lower use stress \(S_1\). A model that predicts timetofail as a function of operating stresses is known as an acceleration model. 

Acceleration models are often derived from physics or kinetics models related to the failure mechanism  Acceleration models are usually based on the physics or chemistry underlying a particular failure mechanism. Successful empirical models often turn out to be approximations of complicated physics or kinetics models, when the theory of the failure mechanism is better understood. The following sections will consider a variety of powerful and useful models: 