Fatigue Crack Growth Analysis of Polymonochlorotrifluoroethylene (CTFE)

Fatigue crack growth behavior of a polymonochlorotrifluoroethylene (CTFE) was investigated. Tension-tension fatigue propagation tests on geometrically identical specimens were performed at different frequencies. Specimens with different notch to width ratios(a/w)were also tested. The applied stress level in all tests was kept the same. The maximum stress was 16.0 MPa and the ratio of minimum stress to maximum stress was kept at 25%. Fatigue test data such as the number of loading cycles, the crack length, and the hysteresis loops were recorded. The Modified Crack Layer (MCL) theory was employed to analyze the FCP behavior of this CTFE material. Also the results were analyzed using the Paris equation for comparison. It has been found that the fatigue crack growth behavior of this material cannot be described by the Paris power law due to the three distinct stages of the FCP kinetics. The MCL model was successfully used for extracting material parameters; (gamma)ʹ, the specific energy of damage, and (beta)ʹ, the coefficient of energy dissipation. The specific energy of damage for this CTFE was 855 kJ/m^3 and the value of the coefficient of energy dissipation was 6.2* 10^-6 It was found that the values of (gamma)ʹ and (beta)ʹ are independent of a/w and the frequency (f), though the FCP kinetics and lifetime were dependent on a/w and f. The variation in (gamma)ʹ and (beta)ʹ when a/w changed from 0.04 to 0.1 were only ±0.9% and ± 6.8%, respectively. For different frequencies (from 0.5 Hz to 5 Hz), the variation of these two parameters were ±0.6% and ± 1.4%, respectively. In situ and post failure analysis of the fatigue fractured samples revealed a clear damage zone adjacent to the main crack. The larger the volume of the damage zone, the longer the fatigue lifetime. It was also found that a higher micro-crack density associated with a larger damage zone. A larger damage zone with higher micro-crack density consumes more energy and thus reduces the portion of energy required to fuel the main crack growth.