有机玻璃疲劳裂纹扩展表达式及控制参量

FORMULAE AND GOVERNING PARAMETERS OF FATIGUE CRACK PROPAGATION IN POLYMETHYL METHACRYLATE

将用于建立金属材料疲劳裂纹扩展关系式的静态断裂模型引入有机玻璃裂纹扩展行为的研究中 ,得到了在近门槛区、中部区和快速扩展区都能较好地与实验结果相吻合的疲劳裂纹扩展速率表达式。该关系式揭示了疲劳裂纹扩展速率和应力强度因子与弹性模量、门槛值和断裂韧性之间的内在关系 ,式中不含无明确物理意义的待定系数 ,是较为完善的表达式。认为有机玻璃疲劳裂纹扩展的控制参量为 :有效应力场强度因子、门槛值与断裂韧性之比值及最大应力场强度因子与断裂韧性之差。

A modified static fracture model developed for fatigue crack propagation (FCP) of metals was introduced to describe the FCP of polymethyl methacrylate (PMMA) because of the similar FCP behavior between metal and PMMA. The formula for cyclic FCP of PMMA was derived from the above model and modified to include the critical condition of FCP. It can be used to represent the test results of cyclic FCP of four types of PMMA in near threshold , intermediate and rapid crack propagation regions well. The correlation between the FCP rate and the stress intensity factor (Δ K ), the FCP coefficient ( B ), the FCP threshold (Δ K th ) and the fracture toughness ( K 1c ) was revealed in the formula. The value of FCP coefficient can be calculated by Young's modulus as B=15.9/E 2 . So the coefficient and parameters involved in the formula all have definite physical meaning. Therefore, the above mentioned formulae could be thought as the almost perfect formulae for cyclic FCP of PMMA. The governing parameters of FCP of PMMA are, respectively, the effective stress intensity factor (Δ K- Δ K th ) in the near threshold region and intermediate region, and the difference between the fracture toughness and the maximum value of stress intensity factor ( K 1c - K max ) in the rapid crack propagation region. The FCP coefficient ( B ) and the ratio of FCP threshold to fracture toughness (Δ K th / K 1c ) are the governing parameters in the intermediate region. Compared with those of the normal PMMA, the lower FCP rate of oriented PMMA during FCP experiments mainly results from the higher values of FCP threshold, higher fracture toughness and the smaller ratio value of FCP threshold to fracture toughness.