Standard Test Methods for Stress Relaxation for Materials and Structures

Importancia y uso:

5.1 Stress-relaxation test data are necessary when designing most mechanically fastened joints to ensure 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 in the stability of wire tendons in prestressed concrete.

5.2 The ability of a material to relax at high-stress concentrations such as are present at notches, inclusions, cracks, holes, and fillets can 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 test environment.

5.3 The general stress-relaxation test is performed by isothermally applying a force to a specimen with fixed value of constraint. The constraint is maintained constant, and the constraining force is determined as a function of time. The major problem in the stress-relaxation test is that constant constraint can be very difficult to maintain. The effects on test results are very significant, and considerable attention shall be given to minimize the constraint variation. Also, experimenters should determine and report the extent of variation in each stress-relaxation test so that this factor can be taken into consideration.

5.4 There are many methods of performing the stress-relaxation test, each with a different starting procedure. However, the constraint is usually obtained initially by the application of an external force at either a specific force-application rate or a specific strain rate. The two methods will produce the characteristic behavior shown in Fig. 1 when the initial stress, σ₀, exceeds the proportional limit. Some testing machines, while reaching the constraint value, do not produce either a constant force-application rate or constant strain rate, but something in between. However, the general characteristics of the data will be similar to those indicated. The stress-application rate in either case should be reasonably rapid, but without impact or vibration, so that any relaxation during the stress-application period will be small.

5.5 The stress-relaxation test starts at zero time, t₀, in Fig. 1.

Note 2: This zero time is the reference time from which the observed reduction in force to maintain constant constraint is based. Selection of this time does not imply that the force-application procedure and period are not significant test parameters which are important in the application of the data.

Subcomité:

E28.04

Referida por:

A0648-18, E0633-21A, E1450-24, A0911_A0911M-21, A0421_A0421M-21, A1061_A1061M-20AE01, F2902-16E01, D6048-07R23, A0421_A0421M-21, F2789-10R20, A0421_A0421M-21, A0193_A0193M-24

Volúmen:

03.01

Número ICS:

19.020 (Test conditions and procedures in general)

Palabras clave:

bending stress relaxation; bolting; compression stress relaxation; hoop stresses; riveting; springs; stress relaxation; tension stress relaxation; torsion stress relaxation ;