ASTM E2207-15 Standard Practice for Tension-Controlled Axial-Torsional Fatigue Testing with Thin-Walled Pipe Specimens

The standard "ASTM E2207-15 Standard Practice for Stress-Controlled Axial-Torsion Fatigue Testing of Thin-Walled Tubular Specimens" developed by the American Society for Testing and Materials (ASTM) describes the method for stress-controlled, axial, torsion and combined in-phase and out-of-phase axial torsion fatigue testing of thin-walled, circular cross-section, tubular specimens at isothermal, ambient and elevated temperatures. This standard is limited to symmetrical, completely reversed stresses (zero mean stresses) and axial and torsion waveforms at the same frequency in combined axial-torsional fatigue testing. This standard is also limited to the characterization of homogeneous materials with thin-walled tubular specimens and does not cover the testing of large-scale components or structural elements.

This standard is not intended to address all safety concerns, if any, associated with its use. The user of this standard should establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

This international standard has been developed in accordance with the internationally accepted standardization principles set out in the decision on principles for the development of international standards, guides and recommendations published by the World Trade Organization Committee on Technical Trade Barriers.

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

• Axial stress (engineering axial stress) is defined as the change in length divided by the original length.
• Shear stress refers to the engineering shear stress resulting from the application of a torsional moment to a cylindrical specimen. Such a torsional shear stress is a simple shear and the shear displacement is defined similarly to the axial stress.
• Biaxial strain amplitude ratio, in axial-torsional fatigue testing, biaxial strain amplitude ratio is defined as the ratio of shear strain amplitude to axial strain amplitude.
• Phasing between axial and shear stresses In axial torsional fatigue testing, phasing is defined as the phase angle between the axial stress waveform and the shear stress waveform. The two waveforms should be of the same type, for example, both triangular or both sinusoidal.
• Shear stress refers to the engineering shear stress acting in the orthogonal tangential and axial directions of the measurement cross-section and is a result of the torsional moment applied to the thin-walled tubular specimen. Shear stress, like shear stress, is always greatest at the outer diameter. Under elastic loading conditions, shear stress also varies linearly along the thin wall of the tubular specimen. However, under elasto-plastic loading conditions, shear stress tends to vary nonlinearly. Most stress-controlled axial-torsional fatigue tests are performed under elasto-plastic loading conditions. Therefore, it is recommended to assume a uniformly distributed shear stress.

Multiaxial forces often tend to produce deformation and failure mechanisms that are quite different and unique from those that occur under simple uniaxial loading conditions. Since most engineering components are subjected to cyclic multiaxial forces, it is necessary to characterize the deformation and fatigue behavior of materials in this mode. Such characterization allows reliable prediction of the fatigue life of many engineering components. Axial-torsional loading is one of the possible multiaxial force systems and is essentially a biaxial loading type. Thin-walled tubular specimens subjected to axial-torsional loading can be used to explore the behavior of materials in two of the four quadrants of the principal stress or strain fields. Axial-torsional loading is more suitable than in-plane biaxial loading because the stress state in thin-walled tubular specimens is constant and well known throughout the entire test section. This application is useful in generating fatigue life and cyclic deformation data on homogeneous materials under axial, torsional, and combined in-phase and out-of-phase axial-torsional loading conditions.

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