Abstract: 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.

Key words: PMMA, fatigue crack pr opag ation ( FCP), fatigue cr ack propagation t hr eshold, str ess intensityfactor, fracture toughness