High Cycle Fatigue of Cast Aluminum Alloys at Ultrasonic Frequency

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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.

Section: B S-n Curve and Fatigue Strength At 10 8 Cyclesmentioning

confidence: 97%

A Probabilistic Model of Fatigue Strength Controlled by Porosity Population in a 319-Type Cast Aluminum Alloy: Part I. Model Development

Zhu

Jones

et al. 2007

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“…In Engler-Pinto, Jr. et al, [16,35] it was reported that the higher the yield strength, the greater the influence of the water on crack propagation for some cast Al-Si alloys. This is possibly because the higher-yield strength is obtained from peak-aged (T6) heat treatment, and the peak-aged aluminum alloy was found to be more sensitive to environmental effect than the overaged alloy.…”

Section: ½19mentioning

confidence: 99%

“…By incorporating the Paris law (Eqs. [16] and [17]) and (P/f) s (Eq. [10]) into the modified superposition model, the effect of environment on fatigue-crack growth rates at all levels of stress intensity range, DK, can be estimated as follows:…”

Section: Mechanisms and Modeling Of Environmental Effect On Fatigumentioning

confidence: 99%

“…It has been observed that the fatigue-crack growth threshold decreased and fatigue-crack growth rate increased in the presence of water vapor in atmospheric air for aluminum alloys. [10][11][12][13][14][15][16][17] Because the duration of crack-tip opening under ultrasonic-frequency loading at 20 kHz is an order of magnitude shorter for each cycle than for conventional fatigue experiments, any environmentally assisted increase in fatigue-crack growth rate is generally presumed to be less pronounced at 20 kHz, leading to lower fatigue-crack growth rate at this frequency. Holper et al [9] studied the influence of frequency on fatiguecrack growth of aluminum alloys and reported that fatigue cracks propagated at lower growth rates at 20 kHz than at 20 Hz in ambient air if cycled above threshold; however, the test frequency had no influence on the fatigue-crack growth threshold itself.…”

Section: Introductionmentioning

confidence: 99%

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Effect of Frequency, Environment, and Temperature on Fatigue Behavior of E319 Cast-Aluminum Alloy: Small-Crack Propagation

Zhu

Jones

Allison

2008

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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.

“…THE study of very-high-cycle fatigue (VHCF) behavior is attracting increased interest from industry because many components in structural applications, such as automobile cylinder heads, engine blocks, [1] and turbine engines, will accumulate 10 8 to 10 10 cycles in service. The conventional approach of designing components to a fatigue limit is not applicable in VHCF because fatigue failures have been observed [2] below the conventional fatigue limit.…”

Section: Introductionmentioning

confidence: 99%

Microstructural Influences on Very-High-Cycle Fatigue-Crack Initiation in Ti-6246

Szczepanski

Jha

Larsen

et al. 2008

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152

The fatigue behavior of an alpha + beta titanium alloy, Ti-6Al-2Sn-4Zr-6Mo, has been characterized in the very-high-cycle fatigue (VHCF) regime using ultrasonic-fatigue (20 kHz) techniques. Stress levels (r max ) of 40 to 60 pct of the yield strength of this alloy have been examined. Fatigue lifetimes in the range of 10 6 to 10 9 cycles are observed, and fatigue cracks initiate from both surface and subsurface sites. This study examines the mechanisms of fatiguecrack formation by quantifying critical microstructural features observed in the fatigue-crack initiation region. The fracture surface near the fatigue-crack-initiation site was crystallographic in nature. Facets, which result from the fracture of primary alpha (a p ) grains, are associated with the crack-initiation process. The a p grains that form facets are typically larger in size than average. The spatial distribution of a p grains relative to each other observed near the initiation site did not correlate with fatigue life. Furthermore, the spatial distribution of a p grains did not provide a suitable means for discerning crack-initiation sites from randomly selected nominal areas. Stereofractography measurements have shown that the facets observed at or near the initiation sites are oriented for high shear stress; i.e., they are oriented close to 45 deg with respect to the loading axis. Furthermore, a large majority of the grains and laths near the site of crack initiation are preferentially oriented for either basal or prism slip, suggesting that regions where a p grains and a laths have similar crystallographic orientations favor crack initiation. Microtextured regions with favorable and similar orientations of a p grains and the lath a are believed to promote cyclic-strain accumulation by basal and prism slip. Orientation imaging microscopy (OIM) indicates that these facets form on the basal plane of a p grains. The absence of a significant role of spatial clustering of a p grains, coupled with the observation of regions of microtexture on the order of 300 to 500 lm supports the idea that variability in fatigue life in the very-high-cycle fatigue regime results from the variability in the nature (intensity, coherence, and size) of these microtextured regions.