Significance and Use
Relaxation test data are necessary when designing most mechanically fastened joints to assure the permanent tightness of bolted or riveted assemblies, press or shrink-fit components, rolled-in tubes, etc. Other applications include predicting the decrease in the tightness of gaskets, in the hoop stress of solderless wrapped connections, in the constraining force of springs, and the stability of wire tendons in prestressed concrete.
The ability of a material to relax at high-stress concentrations such as are present at notches, inclusions, cracks, holes, fillets, etc., may be predicted from stress relaxation data. Such test data are also useful to judge the heat-treatment condition necessary for the thermal relief of residual internal stresses in forgings, castings, weldments, machined or cold-worked surfaces, etc. The tests outlined in these methods are limited to conditions of approximately constant constraint and environment.
The test results are highly sensitive to small changes in environmental conditions and thus require precise control of test conditions and methods.
The reproducibility of data will depend on the manner with which all test conditions are controlled. The effects of aging or residual stress may significantly affect results, as may variations in material composition.
Note 1—The method of testing for the stress relaxation of plastics has been withdrawn from this standard, and the responsibility has been transferred to Practice D 2991.
1.1 These test methods cover the determination of the time dependence of stress (stress relaxation) in materials and structures under conditions of approximately constant constraint, constant environment, and negligible vibration. In the procedures recommended, the material or structure is initially constrained by externally applied forces, and the change in the external force necessary to maintain this constraint is determined as a function of time.
1.2 Specific methods for conducting stress relaxation tests on materials subjected to tension, compression, bending and torsion stresses are described in Parts A, B, C, and D, respectively. These test methods also include recommendations for the necessary testing equipment and for the analysis of the test data.
1.3 It is recognized that the long time periods required for these types of tests are often unsuited for routine testing or for specification in the purchase of material. However, these tests are valuable tools in obtaining practical design information on the stress relaxation of materials subjected to the conditions enumerated, and in investigations of the fundamental behavior of materials.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.