Modeling small fatigue crack growth in cast aluminum alloys

The fatigue life of cast aluminum automotive engine components is, in some cases, controlled by the growth of small crack from pores rather than fatigue crack initiation. A method has been devised to efficiently measure the growth rates of small cracks initiating from micronotches produced by femtosecond pulsed laser machining. Two cast aluminum alloys, W319 with an overaged (T6) heat treatment and A356 with a peak aged (T6) heat treatment, were examined. The use of femtosecond pulsed lasers results in essentially no damage to the microstructure surrounding the notch. Both ultrasonic (20 kHz) and conventional (30 Hz) loading frequencies were applied to study the growth rate of small cracks at different nominal stress levels in the temperature range 20-250°C. Increasing either the temperature or maximum stress led to increased growth rates at equivalent stress intensity factor ranges. The growth rates at 20 kHz was, however, found to be lower than the growth rates at 30 Hz. The growth rate variations with temperature, applied stress, frequency and heat-treatment can be modeled with a large-scale crack-tip plasticity based growth parameter.