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The fatigue stress-life (S-N) behavior of E319 cast aluminum alloy was studied by using both ultrasonic and conventional fatigue techniques in order to understand the potential effect of frequency on fatigue behavior of cast aluminum alloys. It was observed that, at the investigated temperature (20°C, 150°C, and 250°C), fatigue life in air at 20 kHz is 5 to 10 times longer than that at 75 Hz. The difference in fatigue life between 20 kHz and 75 Hz is attributable to an environmental effect on fatigue crack growth rate. The effect of frequency, environment, and temperature on S-N behavior of E319 cast aluminum alloy can be predicted by use of a general version of a modified environmental superposition model. Environmental effects need to be considered when ultrasonic fatigue is used for estimating fatigue lives of aluminum alloys that are under cyclic loading at lower frequencies in service. It is possible to extrapolate ultrasonic fatigue data to conventional fatigue behavior for an E319 cast aluminum alloy based on the environmental superposition model.

The very high-cycle fatigue behavior of a 319-type cast aluminum alloy was investigated using ultrasonic fatigue instrumentation operating at 20 kHz. An endurance limit was demonstrated in the lifetime regime beyond 10 7 cycles. Accordingly, the fatigue strength at 10 8 cycles was determined using the staircase test method. Large pores at or close to the specimen surface or in the specimen interior were responsible for crack initiation in all specimens, and the staircase results were associated with both size and location of the initiating pores through a critical stress intensity factor for fatigue crack growth. Based on the experimental observations, a probabilistic model was developed to establish the relationship between the porosity population and the fatigue strength of the alloy. Good agreement was obtained between the modeling results and experiments.

The influence of test frequency on fatigue-crack propagation behavior of small cracks in E319 cast-aluminum alloy was studied using ultrasonic and conventional test techniques. It was observed that fatigue cracks grow faster at 30 Hz than at 20 kHz in air at both 20°C and 250°C. The effect of frequency on the fatigue-crack growth rates was attributed to an environmental effect. For E319 cast-aluminum alloy, fatigue-crack growth rate increases with increasing water exposure (characterized by the ratio of water partial pressure over test frequency, P/f), and this behavior can be estimated using a modified superposition model. The effect of temperature on fatigue-crack growth behavior was primarily attributed to the effect of temperature on Young's modulus and yield strength. The environmental contribution to fatigue-crack growth rates modestly decreases with increasing temperature.

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