Fatigue Crack Growth with Negative Stress Ratio Following Single Overloads in 2024-T3 and 7075-T6 Aluminum Alloys

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Abstract
Modified pre-cracked compact specimens of 2024-T3 and 7075-T6 aluminum alloys were subjected to four different overload patterns followed by subsequent constant-amplitude steady-state loading with R=Pℓmin/ℓmax equal to 0, -1/2, -1, and -2. The overload patterns were tension, compression-tension, tension-compression, and compression. Cyclic loading with negative stress ratio, R, drastically reduced crack-growth retardation. The higher the negative R ratio the greater the reduction in retardation. Overload ratios, OLR =Phmax/Pℓmax, ranging from 2.0 to 3.0 were used. For compression overloads, the OLR ranged from -2.0 to -4.0. High compression overloading was detrimental and dependent upon subsequent R ratio loading. Substantial fracture surface abrasion near the mid-thickness occurred for higher negative R ratios. Striations were not readily found in this region, however, they were quite evident near the edges, which indicated crack closure was greater near the mid-thickness. The 2024-T3 gave better crack growth life than 7075-T6 in some loading conditions, while the opposite was true for other loadings. The results indicate negative R ratio must be considered in retardation models and that retardation life cannot be modeled based solely on overload plastic zone sizes.

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