A cumulative damage experiment, also called a varying-stress experiment, is an accelerated life test where the stress levels can change over time. The stress can be applied by many different forces, such as load, temperature, or pressure. A typical cumulative damage experiment consists of multiple test units. Each unit has an initial stress level, and the stress level can be changed throughout the experiment.
The most common cumulative damage experiment is a step-stress experiment. A step-stress experiment uses multiple units with varying levels of stress applied. Stress can be applied using factors such as temperature, pressure, or voltage. For each unit, there is an initial stress level. At specified time points, the stress levels are adjusted based on different patterns of stress levels. Between stress level changes, the stress level remains constant.
The Cumulative Damage platform also includes three other varying-stress pattern models:
• In a ramp-stress experiment, the stress levels start at an initial value and then increase linearly over time at a specified slope.
• In a sinusoid-stress experiment, the stress levels fluctuate in a periodic fashion that is defined by a sine wave.
• In a piecewise ramp-stress experiment, the stress levels are defined at specified time points similar to the step-stress case. However, the stress level is not required to stay constant between time points. Rather, it changes linearly from a starting stress level to an ending stress level between time points. If a pair of starting and ending stress levels are equal, the interval is equivalent to a step-stress interval.
For more information about varying-stress and step-stress models, see Nelson (2004, ch. 10).