ASTM E328 Stress Relaxation for Materials and Structures

Get Certified ASTM E328 Testing for Reliable Stress Relaxation Evaluation

ASTM E328 testing is a standard procedure for determining stress relaxation under essentially constant constraint, a constant test environment, and minimal vibration. The test begins by applying a (restraining) force to the specimen from the outside and then determines how much force is needed over time to keep the specimen constrained. Thus, engineers are able to assess the ability of materials to retain stress in long service lives. The results can be used to assess mechanically fastened joints, pressed or shrink-fit components, gaskets, springs, wrapped connections, and wire tendons for prestressed concrete.

• Material type: Materials and structures suitable for stress-relaxation evaluation.
• Purpose: Determination of the time-dependent decrease in stress under approximately constant constraint.
• Primary property: Stress relaxation and the corresponding change in external force.
• Loading method: Tension, compression, bending, or torsion.
• Test condition: Approximately constant constraint throughout the test.
• Environment: A constant and controlled test environment.
• Vibration condition: Negligible vibration during testing.
• Testing duration: Long-duration testing to characterize time-dependent stress relaxation.
• Data evaluation: Remaining stress, relaxed stress, applied force, and fraction of initial stress relaxed can be evaluated.
• Plastics limitation: Plastics stress-relaxation testing falls under ASTM D2991 rather than ASTM E328.
• Bending limitation: Ring-shaped specimens machined from bulk material fall outside the 

What are the Uses of ASTM E328 Testing?

ASTM E328 testing assesses the loss of stress or force when materials or structures are subjected to nearly constant restraint. Thus, the outcomes can be useful for engineers to comprehend how materials cope with stress for long service times. The ASTM E328 standard:

• Measures relaxation of stresses at nearly constant constraint.
• Assesses the stress-relaxation properties under tension, compression, bending, and twisting forces.
• Identifies changes in stress or force during a test period.
• Assesses the capacity of materials to hold stress when they are continually and slowly deforming.
• Helps in designing mechanically fastened joints.
• Assists in the analysis of permanent tightness of bolted or riveted joints.
• Helps assess press- or shrink-fit parts.
• Assists in determining the loss of gasket tightness.
• Allows the evaluation of hoop stress in solderless wrapped connections.
• Helps with the analysis of the force of springs.

Which materials can be tested under ASTM E328?

ASTM E328 is applicable to materials and structures from which suitable specimens for stress-relaxation tests in tension, compression, bending, or torsion can be made. When the geometry of wire, plate, sheet, strip, bar, and rod materials is appropriate to the applicable method, it can be used as a suitable specimen. The specimen preparation should minimize changes in the residual stress, since stress-relaxation characteristics may be influenced by changes in the residual stresses. Thus, the technician should control the machined, handled, aligned, and surface condition before testing.

Why is ASTM E328 Important?

ASTM E328 is important because stress relaxation can reduce the force or stress maintained by a material while its constraint remains approximately constant. Mechanically fastened connections, press- or shrink-fit fittings, gaskets, springs, wrapped connections, and prestressed structural components can all be affected by this behavior. The standard offers controlled procedures for measuring this time-dependent behavior. Thus, the results can be used by engineers to achieve practical design data and to explore the fundamental behavior of materials under prolonged constraint. But small changes in constraint can make a big difference in the stress-relaxation results. Measured response can also be affected by the temperature, residual stress, material composition, geometry of the specimen, vibration, and loading conditions.

ASTM E328 Testing Process and Data Collection

ASTM E328 testing starts with selecting and preparing a specimen appropriate for the loading mode being tested. The specimen is then set up in the desired apparatus and a desired strain, displacement, or other constraint is applied. The laboratory makes the specimen thermally equilibrated if it is required. The initial force or stress is applied without a shock, and zero time is defined when the desired initial condition is met. The testing system then records the results of the measurements of changes in force or stress over time, while keeping the constraint approximately constant. The resulting data characterize the relaxation of the material or structure under the chosen conditions; stress-retention characteristics of materials can be compared when evaluated under similar temperature, loading, and environmental conditions.

Common Challenges and Troubleshooting

Accurate ASTM E328 results require careful control of constraint, specimen alignment, temperature, loading, and vibration. It can be challenging to maintain constant constraint, and slight differences in constraint can impact the observed stress-relaxation behavior. The presence of residual stress should also be considered, as machining may alter the stress-relaxation behavior of the material. Therefore, it is necessary for technicians to use proper machining practices and not cause unnecessary deformation when preparing specimens. Another significant issue is temperature, as stress-relaxation behavior is dependent on the test environment. So it is important for the laboratories to keep the mentioned temperature and reduce the vibrations during the test period.

• The laboratory records stress, force, strain, and test time to establish the stress-relaxation response.
• The analyst compares stress or force reduction to assess the amount of stress relaxation with time.
• The laboratory provides results in MPa or ksi, depending on the applicable test, and in h or min, and records the test temperature in °C.
• Engineers compare stress-relaxation behavior among materials tested under equivalent loading, temperature, and environmental conditions.
• Analysts use the recorded results to support material evaluation, component design, quality control, and prediction of long-term stress retention.

Link to ASTM E328