Development and Implementation of an Advanced User Material Model for UHMWPE

Ultra-high molecular weight polyethylene (UHMWPE) is a semi-crystalline polymer with excellent strength, impact resistance, and abrasion resistance. These mechanical properties have led to extensive use of UHMWPE as a bearing material in total joint replacements. In order to accurately capture the experimentally observed response of this important polymer we have developed a new advanced material model—the Hybrid Model (HM). This constitutive model is physically motivated and based on a decomposition of the deformation gradient into elastic and viscoplastic components. The elastic response of the underlying molecular network is captured using the eight- chain hyperelastic model, and the viscoplastic response is represented using energy activated flow driven by the molecular reorganization that occurs during large deformations. The constitutive theory for the HM has been implemented as a user-material model (UMAT) for ls971 and is available for both explicit and implicit simulations. For optimal accuracy and numerical efficiency the UMAT uses a forward Euler integration scheme for explicit simulations, and a higher order backward differentiation formula (BDF) method for implicit simulations. The HM was calibrated to data from uniaxial tension experiments performed at different strain rates and with different loading-unloading segments. For validation, the calibrated HM was then used to simulate a small punch test. A direct comparison between the experimental data and model predictions of the calibration and validation data demonstrate that the HM accurately captures the non-linear response of UHMWPE. The ability to simulate large-scale contact problems was examined by simulating the deformation behavior of a total knee replacement component and the CharitéTM artificial discs.