Creep tests can be performed using a variety of loading, but they are generally conducted using tensile loads. In a standard test, the specimen is held under constant tensile load while held at an elevated temperature by a sleeve furnace. At pre-determined intervals, data are collected and recorded concerning the condition of the specimen, the change in gauge length, and the elapsed time. Often, several specimens are tested under differing conditions simultaneously. This provides a broad picture of the creep properties of the material. Additionally, these tests tend to take a long time.
Based on the applied load and deformation data for each test specimen, a graph can be constructed which illustrates the resultant stress per unit of time for a given temperature. Higher temperatures tend to accelerate creep rates. Expectedly, greater loads also increase the creep rates.
Creep rate is the slope of the creep curve once the initial elasticity has been overcome.
As indicated in the accompanying diagram, the creep of a material can be divided into three stages. First stage, or primary creep, starts at a rapid rate and slows with time. Second stage (secondary) creep has a relatively uniform rate. Third stage (tertiary) creep has an accelerating creep rate and terminates by failure of material at time for rupture.