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8. Assessing Product Reliability
8.1. Introduction


What are some common acceleration models?

Acceleration models predict time to fail as a function of stress

Acceleration factors show how time-to-fail 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 time-to-fail 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 time-to-fail 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: 
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