Mechanical characteristics of medical grade UHMWPE under dynamic compression.

The mechanical properties of medical grade ultrahigh molecular weight polyethylene (UHMWPE) are critical for the safety and integrity of UHMWPE implantation. Accordingly, the mechanical features of UHMWPE are tested under repeated stress-controlled and strain-controlled compression at room temperature. Some important effect factors, such as stress rate, mean stress, stress amplitude, strain rate, mean strain, strain range and multiple load steps are further considered in detail. Results indicate that the lower stress rate causes the greater accumulated plastic strain and the accumulated plastic strain rate becomes increasingly lower with increasing number of cycles. The strain range and accumulated plastic strain rate decrease rapidly in the first stage, and then become almost steady during the second stage. Especially, the accumulated plastic strain rate per cycle for each case is less than 0.01 %/cycle after the initial 100 cycles. This means that the plastic strain accumulates very slowly and the shakedown behavior always occurs. Moreover, obvious cyclic softening and stress relaxation behaviors can be observed under cyclic strain-controlled compression during the first 50 cycles. This indicates that the accumulated plastic stain in the initial 100 cycles and the cyclic stress relaxation during the first 50 cycles should be assessed for the functionality of UHMWPE implantation.