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Creep is 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 boilers, gas turbines, jet engines, ovens, or any application that involves high temperatures under load. Understanding high temperature behavior of metals is useful in designing failure resistant systems.
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 occurs in three stages: Primary, or Stage I; Secondary, or Stage II: and Tertiary, or Stage III. Stage I, or Primary creep occurs at the beginning of the tests, and creep is mostly transiently, not at a steady rate. Resistance to creep increases until Stage II is reached. In Stage II, or Secondary creep, The rate of creep becomes roughly steady. This stage is often referred to as steady state creep. In Stage III, or tertiary creep, the creep rate begins to accelerate as the cross sectional area of the specimen decreases due to necking or internal voiding decreases the effective area of the specimen. If stage III is allowed to proceed, fracture will occur.
The creep test is usually employed to determine the minimum creep rate in Stage II. Engineers need to account for this expected deformation when designing systems.
Like the Creep Test, Stress Rupture Testing involves a tensile specimen under a constant load at a constant temperature. Stress rupture testing is like creep testing aside from the stresses are being higher than those utilized within a creep testing. Stress rupture tests are utilized to find out the time it takes for failure so stress rupture testing is always continued until failure of the material occurs. Data is plotted similar to the graph above. A straight line or best fit bend is normally obtained at every temperature of interest. The Stress Rupture test is used to determine the time to failure and elongation.