Creep is defined as high temperature progressive deformation at constant stress. "High temperature" is a relative term dependent upon the materials involved. Creep rates are used in evaluating materials for, gas turbines, jet engines, boilers, ovens, or any application that involves high temperatures under load. Understanding high temperature behavior of metals is useful in designing failure resistant systems.
The rate at which a sample material is deformed to stress when subjected to a constant temperature is known as the creep rate. Creep usually occurs at higher temperatures therefore it is common for this type of testing to be performed with an environmental chamber for accurate heating and cooling control. Temperature control is vital to lessen the effects of thermal expansion on the sample.
A creep test involves a tensile specimen under a constant load maintained at a constant temperature. Measurements of strain are then recorded over a period of time. Creep testing should be conducted using a material tester, since speed control is critical to measuring the deformation over time. This test method is used to determine a sample's creep properties when subjected to a prolonged tensile or compressive load at a constant temperature.
For example, plastics, including those that are reinforced have a tendency to creep under stress. The amount of creep is dependent on the material, the load, the temperature and the time under stress. Stress can be revealed in the flexural, compression and tensile modes. Tensile creep is the favored method for rupture tests because some ductile plastics cannot rupture in flexural or compression mode.
Creep is generally divided into three stages:
the primary creep starts at a rapid rate and slows with time;
the secondary creep has a relatively uniform rate;
the tertiary creep has an accelerated creep rate and terminates when the material breaks or ruptures
If creep recovery is measured, the test will determine the stress-relaxation - the rate of decrease in deformation that takes place when the load is removed. Creep is sometimes referred to as Stress-Relaxation testing.
ASTM E139 - 11 Standard Test Methods for Conducting Creep, Creep-Rupture, and Stress-Rupture Tests of Metallic Materials
ASTM D2990 - 09 Standard Test Methods for Tensile, Compressive, and Flexural Creep and Creep-Rupture of Plastics
ASTM D6992 - 03(2009) Standard Test Method for Accelerated Tensile Creep and Creep-Rupture of Geosynthetic Materials Based on Time-Temperature Superposition Using the Stepped Isothermal Method
ASTM F1140 - 07(2012) Standard Test Methods for Internal Pressurization Failure Resistance of Unrestrained Packages