DIC and X-ray Diffraction Analysis of Cyclic Creep in Semi-Crystalline Peek Under Block Loading

The effect of creep development has been studied to clarify the nature of fatigue failure in neat polyetheretherketone (PEEK). The cyclic creep parameter was utilized to quantitatively characterize the structure rearrangement. The magnitude of cyclic creep was calculated taking into account the parameters of mechanical hysteresis loops. To reveal the dependence of the cyclic creep magnitude on the external cyclic stress, block loading was implemented with a stepwise increase in the amplitude. It has been shown that the mechanism of cyclic creep exhibited a two-stage pattern (under the block loading). The first stage is responsible for the polymer adaptation to an increase in the applied load, and the second stage corresponds to the steady state in which the rate of creep development slows down. It has been found that despite the cyclic loading induced structure conformation (rearrangement) and the strain hardening, the degree of crystallinity did not change a lot and amounted to ~20%. At the same time, the microstresses decreased from 22 MPa to 14 MPa. It is shown that cyclic strain hardening with a block load increase can lead to enlarging the level of withstand cyclic stresses up to 90% of the neat PEEK ultimate strength.