Experimental study on rate-dependent uniaxial whole-life ratchetting and fatigue behavior of polyamide 6

Abstract Rate-dependent whole-life ratchetting and fatigue failure of polyamide 6 were explored by conducting a series of stress-controlled uniaxial fatigue tests at room temperature. Meanwhile, the temperature rises on the surface of specimens were recorded to reflect the self-heating occurred during the fatigue tests. The experimental results demonstrate that ratchetting strain rate is sensitive to the variation of prescribed stress rate, and the whole-life ratchetting of PA6 presents a tri-staged evolution feature at high stress rate but a four-staged one at low stress rate. Two failure modes (i.e., the necking controlled by unstable self-heating and resultant large ratchetting strain; and the fracture controlled by the low-cycle fatigue damage) are experimentally observed in the tests at different stress rates. By considering the relationship between ratchetting evolution and temperature rise, it is found that whether increasing the stress rate retards or promotes the ratchetting is dependent on the magnitude of self-heating produced at each fatigue stage, and whether it will be beneficial or detrimental to the total fatigue life is determined by the competition of resultant strain hardening and self-heating softening.