Department of Mechanical and Industrial Engineering, University of Brescia, via Branze, 38 – 25123 Brescia, Italy.
J Mech Behav Biomed Mater. 2011 Oct;4(7):1242-56. doi: 10.1016/j.jmbbm.2011.04.010. Epub 2011 Apr 16.
Ultra High Molecular Weight Polyethylene (UHMWPE) is a polymeric material employed in critical biomedical applications. Knowledge of its mechanical behavior is essential in order to obtain accurate prediction of stresses and deformations in real components, in particular when cyclic loading is considered. In the present research the effects of alternating and pulsating cyclic strain on the mechanical response of UHMWPE were studied by means of an experimental procedure based on tests carried out in strain control at different mean cyclic strain levels. During the tests the temperature increase due to hysteretic heating was controlled by means of a compressed air cooling apparatus specifically devised. By taking advantage of the possibility to control and stabilize temperature, cyclic steady-state mechanical response was investigated at room temperature and at 37 and 50 °C, comparing the effects of alternating and pulsating loading cycles. A transient thermal analysis using the finite element method (FEM) was also carried out to analyze temperature distribution within the specimen. UHMWPE exhibited cyclic softening as a result of a thermal contribution due to temperature increase and of a mechanical contribution related to the effects of applied load on the microstructure. The material exhibited different peak stress percent reductions for pulsating and alternating loading and during tensile and compressive loading phases. For pulsating tests significant cyclic mean stress relaxation was also observed. Based on the experimental procedure described the cyclic curve was determined as a function of temperature and fitted with a Ramberg-Osgood type constitutive equation, in which material parameters are temperature dependent. In this way the combined effects of temperature rises, such as those that might occur in biological environments or due to frictional heating, and mechanical loads could effectively be taken into account for constitutive modeling purposes of cyclic mechanical behavior of UHMWPE.
超高分子量聚乙烯（UHMWPE）是一种在生物医学领域具有重要应用的高分子材料。为了对实际部件中的应力和变形进行准确预测，尤其是在考虑循环载荷的情况下，了解其力学行为是至关重要的。在本研究中，通过一种基于应变控制的实验程序，研究了交变和脉动循环应变对 UHMWPE 力学响应的影响。在测试过程中，通过专门设计的压缩空气冷却装置控制由于滞后加热引起的温度升高。利用控制和稳定温度的可能性，在室温以及 37 和 50°C 下研究了循环稳态力学响应，比较了交变和脉动加载循环的影响。还利用有限元法（FEM）进行了瞬态热分析，以分析试件内的温度分布。由于温度升高导致的热贡献和施加的负载对微观结构的影响导致的机械贡献，UHMWPE 表现出循环软化。材料在脉动和交变加载以及拉伸和压缩加载阶段表现出不同的峰值应力百分比降低。对于脉动测试，还观察到显著的循环平均应力松弛。基于所描述的实验程序，将循环曲线作为温度的函数确定，并拟合了 Ramberg-Osgood 型本构方程，其中材料参数随温度变化。通过这种方式，可以有效地考虑温度升高的综合影响，例如在生物环境中或由于摩擦加热可能发生的温度升高，以及机械载荷，从而有效地为 UHMWPE 的循环力学行为的本构建模目的考虑。
