As a supplement to the Accelerated Life Testing Data Analysis Basics quick subject guide, this document presents an overview of some common stress loading schemes and stress profiles used in accelerated life testing. This subject is covered in more detail in ReliaSoft's Accelerated Life Testing On-Line Reference on weibull.com. See Stress Loading. 

Stress is Time-Independent (Constant Stress)
When the stress is time-independent, the stress applied to a sample of units does not vary with time. For example, if temperature is the stress type, each unit will be tested under the same accelerated temperature for the duration of the test.

 Constant Stress Profile

Stress is Time-Dependent (Time-Varying Stress)
When the stress is time-dependent, the stress applied to a sample of units varies with time. For example, if temperature is the stress type, each unit may be tested at 100K for 10 hours then increased to 150K for 10 hours then increased to 200K for 10 hours over the duration of the test. Time-dependent stresses can be applied in a variety of ways, including step-stress, ramp-stress etc. Some common stress profiles are shown next.

Analyzing Data from Time-Dependent Stress Conditions
ReliaSoft's ALTA PRO software is capable of analyzing accelerated life data when the accelerated stress level(s) and or the use stress level(s) vary with time. You can use the Stress Profile Explorer to define the profile by time segments (start and end) and a stress for each time segment. The stress can be a constant value (e.g. 200K) or an equation which describes the stress behavior as a function of time, S(t).

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