Fatigue crack propagation in E319 cast aluminum alloy at ultrasonic and conventional frequencies

The effect of cyclic frequency on the fatigue crack propagation behavior of E319 cast aluminum alloy was investigated at 20 and 250 degrees C at I Hz, 30 Hz and 20 kHz. At 20 degrees C the influence of environment on fatigue crack growth behavior was also examined. Fatigue cracks grew faster at 30 Hz than at 20 kHz in air at both 20 and 250 degrees C, which was attributed to an environmental effect. At ambient temperature, fatigue crack growth rates at a given Delta K increased with increasing water exposure (characterized by the ratio of water pressure over test frequency, P/f) and saturation of the environment effect on crack growth occurred at a critical value of P/f. Fatigue crack growth rates substantially increased with temperature, which primarily resulted from the change of Young's modulus and yield strength with temperature. The environmental and frequency effect at all temperatures can be characterized by an environmental superposition model for fatigue crack growth. Based on this model, fatigue crack growth rates over the entire range of Delta K in various environments can be predicted and the predictions generally agreed well with the experimental observations.