ASTM E 328-13 Standard test method for Stress Relaxation for Materials and Structures

Scope:

This test method is used to determine stress relaxation of materials and structures under conditions of Constant environment, Constraint, and negligible vibration. In this test, the testing material or structure is initially constrained by extremely applied forces and changes in external forces to keep constraint are determined as a function of time. Specific methods for conducting stress relaxation tests on materials subjected to tension, compression, bending and torsion stresses are described in sections A, B, C, and D, respectively.

Figure 1: Tension Stress-Relaxation Test Using Periodic Force Measurement

Figure 2: Compression Stress-Relaxation Test

Result:

For the method for conducting stress relaxation torsion test 

Strain will be taken as equal to the angle of twist per unit length, multiplied by the radius of the specimen.

1. The initial maximum torsional stress may be calculated as follows, Obtain a torque versus angle-of-twist-per-unit-length curve up to the desired angle of twist, If the curve is linear, use the following equations.

1.1 For a cylindrical specimen

Where,

τ₀ = initial maximum torsional stress,

T₀ = initial torque,

d = specimen diameter.

1.2. for a tubular specimen,

Where,

d₀ = specimen outside diameter,

d_i = specimen inside diameter.

2. If the torque-twist curve is nonlinear, the initial maximum torsional stress can be estimated for a cylindrical specimen or for a thin-walled tubular specimen.

2.1 For a cylindrical specimen,

Where

θ = angle of twist per unit length at torque T0, and

α = slope of the torque-twist curve at torque τ0.

3. The accuracy with which the remaining torsional stress may be determined, at any instant, depends upon the nature of the specimen.

3.1For a thin-walled tubular specimen, the remaining torsional stress may be approximated by,

Where

τ = remaining maximum torsional stress, and

T = remaining torque.