Accelerated life testing (ALT) is used to determine the reliability of products by subjecting them to higher levels of stress than anticipated during normal operation. Traditional ALT is a well-known method that uses a high, constant stress to induce specimen failure. Another method of ALT is step-stress ALT (SSALT), which places components under progressively increasing levels of stress. SSALT can be more efficient than traditional ALT. If test subjects are not failing fast enough, the stress level can be increased to shorten the time to failure. This has the potential to reduce testing time and costs.
This project uses SSALT to develop a life prediction model of nitrile rubber. By aging rubber o-rings in the Systems Health Lab temperature oven, the effect of temperature stress on the tensile elongation behaviour of rubber is observed. The rate of o-ring tensile failure over time was observed and fitted to an Arrhenius-Weibull distribution model using maximum likelihood estimation.
A function of time and temperature stress f(t;V) was derived, allowing prediction of rubber life at any temperature of interest. The experiment was run on two batches of o-rings. The resultant models predict a 20% reduction in ultimate elongation at 100°C after: 74.3 hours for batch 1; and 98.5 hours for batch 2. The average lifespan of o-rings in storage at 25°C was predicted to be roughly 7.2 years.