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A study was conducted to investigate the effect of solidification rate on the growth behavior of small fatigue cracks in a 319-type aluminum alloy, a common Al-Si-Cu alloy used in automotive castings. Fatigue specimens were taken from cast material that underwent a hot isostatic pressing (HIP) process in order to eliminate shrinkage pores and to facilitate the observation of surface-initiated cracks by replication. Naturally initiated surface cracks ranging in length from 17 m to 2 mm were measured using a replication technique. Growth rates of the small cracks were calculated as a function of the elastic stress-intensity-factor range (⌬K ). Long-crack growth-rate data (10 mm Յ length Յ 25 mm) were obtained from compact-tension (CT) specimens, and comparison to the small-crack data indicates the existence of a significant small-crack effect in this alloy. The solidification rate is shown to have a significant influence on small-crack growth behavior, with faster solidification rates resulting in slower growth rates at equivalent ⌬K levels. A stress-level effect is also observed for both solidification rates, with faster growth rates occurring at higher applied-stress amplitudes at a given ⌬K. A crackgrowth relation proposed by Nisitani and others is modified to give reasonable correlation of smallcrack growth data to different solidification rates and stress levels.

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An investigation of the fatigue properties of cast W319-T7, an Al-Si-Cu alloy used in automotive engine components, was conducted using ultrasonic testing equipment with operating frequencies of 20 kHz. The stress-life (S-N) behavior at room temperature was determined for three solidification conditions of this alloy, where stresses for fatigue lives ranging from 10 5 to 10 9 cycles were determined. The results are compared to fatigue data acquired using servohydraulic equipment operating at 40 Hz. No influence of loading frequency has been observed. A discrete endurance limit is indicated for each of the three solidification conditions of W319-T7. The scientific and practical implications of this result are discussed. A material model presented previously is modified by introducing a crack growth threshold condition in order to predict the observed endurance limits. The model is shown to effectively predict the influence of solidification time on the fatigue properties of W319-T7.

The mean and the life-limiting behavior under fatigue of the nickel-based superalloy, IN100, separated (or converged) as a function of stress level and dwell loading. This behavior was related to the control of the life-limiting behavior by the smallcrack growth regime, producing its much slower response to stress level and dwell-time, relative to the mean-lifetime behavior. The lifetime probability density is therefore, modeled as a superposition of the crack growth lifetime density and a meanlifetime density. The crack growth density is calculated with the help of small-crack growth data and the distribution in the crack initiation size. The mean-lifetime density is estimated from a relatively small number of total lifetime fatigue tests. In IN100, we apply this approach to predict the effects of stress level and dwell time on the lifetime distribution and the B0.1 (1 in 1000 probability of failure) lifetime limit.