SYMPOSIA PAPER Published: 01 January 1976
STP33361S

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

Source

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.

Author Information

Stephens, RI
The University of Iowa, Iowa City, Iowa
Chen, DK
The University of Iowa, Iowa City, Iowa
Hom, BW
The University of Iowa, Iowa City, Iowa
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Details
Developed by Committee: E08
Pages: 27–40
DOI: 10.1520/STP33361S
ISBN-EB: 978-0-8031-4672-3
ISBN-13: 978-0-8031-0344-3