Creep Test

Method for determining creep or stress relaxation behavior. To determine creep properties, material is subjected to prolonged constant tension or compression loading at constant temperature. Deformation is recorded at specified time intervals and a creep vs. time diagram is plotted. Slope of curve at any point is creep rate. If failure occurs, it terminates test and time for rupture is recorded. If specimen does not fracture within test period, creep recovery may be measured. To determine stress relaxation of material, specimen is deformed a given amount and decrease in stress over prolonged period of exposure at constant temperature is recorded. Standard creep testing procedures are detailed in ASTM E-139, ASTM D-2990 and D-2991 (plastics) and ASTM D-2294 (adhesives).

A creep test, sometimes referred to as stress-relaxation, is used to determine the amount of deformation a material experiences over time while under a continuous tensile or compressive load at a constant temperature. Maintaining a constant temperature during a creep test is a critical factor due to the possible thermal expansion or shrinkage of the material.
While testing, the materials deformation is recorded at specific time intervals and overall data is plotted on a creep vs time diagram. The slope at any point on this curve is known as the creep rate, of which the units are in/in/hr or % Elongation/hr. Throughout a creep test, there are three stages a specimen will undergo. The first being primary creep.

For the primary creep stage, the creep rate begins by rising quickly and then slows down and decreases for the remaining of this stage. Next stage is the Secondary creep stage, where the creep rate is steady. Last is the Tertiary creep stage. The Tertiary creep stage is when the specimen is expected to reach its breaking point, during which the creep rate is much steeper than the secondary stage. If a specimen does not fracture within the creep test period, creep recovery may be measured.
Creep tests are fundamental for materials that are needed to withstand certain operational temperatures under load. For materials such as metals or alloys, their material properties significantly change at higher or lower temperatures. By examining the results from a creep test, engineers can determine a material’s expected deformation and avoid failure when designing new systems for different environmental conditions.
Some examples of standards that require creep testing are ASTM E-139, ASTM D-2990, ASTM D-2991, and ASTM D-2294. For more details on testing procedures, please refer to these standards.