On the anelasticity and fatigue fracture entropy in high-cycle metal fatigue

• Phase lag between stress and strain causes anelastic energy in high-cycle fatigue.
• A procedure is presented for the estimation of anelastic energy in metal fatigue.
• Uniaxial fatigue tests are performed to assess the validity of the procedure.
• Accumulation of thermodynamic entropy is used to characterize metal fatigue.
• Fatigue fracture entropy is found to be fairly constant in high-cycle fatigue.

The concept of thermodynamic entropy generation in a degradation process is utilized to study the high-cycle fatigue of medium carbon steel 1018. Uniaxial tension–compression fatigue tests are carried out with tubular dogbone specimens at different stress levels and loading frequencies. It is shown that a phase lag between the stress and the strain caused by the internal friction includes a considerable amount of non-damaging anelastic energy in a hysteresis loop when the amplitude of cyclic load is substantially smaller than the yield strength of the material. A methodology is proposed to determine the anelastic energy associated with metal fatigue at a stress level lower than the yield strength of a material. Finite element simulations are carried out with a 3-D model of the specimen to determine the validity of the proposed methodology. The evolutions of the plastic strain energy and temperature are discussed and utilized to calculate the entropy accumulation. It is shown that the accumulation of entropy generation in the HCF of the material—beginning with a pristine specimen and ending at fatigue fracture—is nearly constant within the experimental and loading conditions considered. The concept of tallying entropy is useful for the prediction of the fatigue life evolution of a material undergoing cyclic loading.

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