ASTM E2368-10 Standard Practice for Stress-Controlled Thermomechanical Fatigue Testing

The American Society for Testing and Materials (ASTM) standard “ASTM E2368-10 Standard Practice for Stress-Controlled Thermomechanical Fatigue Testing” covers the determination of thermomechanical fatigue properties of materials under uniaxially loaded stress-controlled conditions. Here, thermomechanical fatigue cycling is defined as a condition in which uniform temperature and stress fields on the specimen gauge section are simultaneously varied and independently controlled. This practice is intended to address thermomechanical fatigue testing performed to support activities such as materials research and development, mechanical design, process and quality control, product performance, and failure analysis. Although this practice is specific to stress-controlled testing, many sections provide useful information for force-controlled or stress-controlled thermomechanical fatigue testing.

This application allows for maximum and minimum values ​​of temperature and mechanical stress and temperature-mechanical stress phasing, with the restriction that these parameters remain cyclically constant throughout the test period. No restrictions are placed on environmental factors such as pressure, humidity, ambient environment and others, provided that they are controlled throughout the test, do not cause loss or change in specimen dimensions over time and are detailed in the data report.

The use of this application is limited to samples and does not include testing of full-scale components, structures, or consumer products.

For the purposes of this standard, the following terms and definitions apply:

• Stress is defined here as engineering stress, which is the ratio of force to the original cross-sectional area of ​​the specimen. The area is that measured in an unloaded condition at room temperature.
• The coefficient of thermal expansion expresses the fractional change in free expansion strain for a unit temperature change measured in the test specimen.
• Total strain is the strain component measured on the test specimen and is the sum of thermal strain and mechanical strain.
• Thermal strain is the component of strain resulting from a change in temperature under free expansion conditions (as measured on a test specimen). For some materials, it may be nonlinear over the temperature range of interest.
• Mechanical strain is the component of strain that results when the free expansion thermal strain (as measured on the test specimen) is subtracted from the total strain.
• Elastic strain is the strain component that results when the stress is divided by the temperature-dependent Young's modulus (in accordance with "ASTM E111-17 Standard test method for Young's modulus, tangent modulus, and chord modulus").
• Inelastic strain refers to the strain component that results when the elastic strain is subtracted from the mechanical strain.
• Strain ratio is the ratio of the minimum mechanical strain to the maximum mechanical strain in a strain cycle.
• The mechanical stress/temperature true phase angle, for the purpose of evaluating phasing accuracy, is defined as the waveform shift between the maximum temperature response measured at the sample and the maximum mechanical stress response. For reference purposes, if the temperature response maximum leads the mechanical stress response maximum by 180 degrees or less, the angle is considered positive, otherwise the phase angle is considered negative.
• In-phase thermomechanical fatigue refers to a cycle in which the maximum value of temperature and the maximum value of mechanical stress occur simultaneously.
• Out-of-phase (anti-phase) thermomechanical fatigue refers to a cycle in which the maximum value of temperature precedes the maximum value of mechanical strain by a time value equal to 1⁄2 of the cycle period.

In the use of structural materials in high-temperature environments, components susceptible to fatigue damage may experience a combination of thermal and mechanical forces that change simultaneously over a given cycle. These conditions are often of critical concern because they combine temperature-induced and cyclic (fatigue) damage mechanisms with varying severities related to the phase relationship between cyclic temperature and cyclic mechanical strain.

Such effects can be seen to affect the evolution of the microstructure, the micromechanisms of degradation, and various other phenomenological processes that ultimately affect the cyclic life. Stress-controlled thermomechanical fatigue testing is often used to investigate the effects of simultaneously varying thermal and mechanical loading under idealized conditions, where cyclic theoretically uniform temperature and stress fields are applied and controlled externally across the gauge section of the sample.

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