Creep Tests

CREEP TESTS

✓ Creep defined: The continuous deformation of a metal under a steady load is known as creep.

✓ The purpose of creep tests is to determine the creep limit. The creep limit or the limiting creep stress is defined as the stress that will not break the specimen when applied for an infinite period at a specific constant temperature.

✓ The creep tests require the measurement of four variables- stress, strain, temperature, and time.

✓ The creep tests are simply tension tests run at constant load and constant temperature. Then the value of strain of the test piece is noted as a function of time.

1. Arrangement

✔ A creep testing installation is illustrated in Fig.5.30.

✔ The specimens for creep testing are usually the same as for conventional tensile tests.

✔ The basic requirements for a creep testing machine are:

(a) It must possess means for applying and maintaining constant tensile load.

(b) There must be a furnace capable of keeping the temperature of the test-piece at the desired value to within very close limits.

(c) There should be means for the accurate measurement of test- piece extension.

2. Test Procedure

(i) The specimen to be tested is placed in an electric furnace. At the electric furnace, the specimen is heated to the given temperature under the constant load.

(ii) The strain in the specimen is measured by a strain gauge or an optical extensometer as a function of time.

(iii) The above test is repeated for four to five specimens at each temperature under different loads.

(iv) Now the creep curve i.e., the elongation versus time curves are plotted for each specimen, as shown in Fig.5.31.