A STUDY OF THE STRAIN-HARDENING AND CUMULATIVE DAMAGE BEHAVIOR OF 2024-T4 ALUMINUM ALLOY IN THE LOW-CYCLE FATIGUE RANGE

摘要

An experimental fatigue investigation in the range of life between 1 and 10000 cycles to failure was conducted with a series of round tapered specimens of 2024-T4 aluminum alloy. The specimens had a minimum diameter of 0.356 inch and a maximum diameter of 3/4 inch with taper radii of 1 1/2, 0.6 and 0.3 inch. Cycling tests were carried out between limits of local strain based upon the change in diameter at the minimum section or strain across the specimen based upon the change in length of the tapered section. It was found that the fatigue data for each of the three specimen types could be represented by two straight line segments when range of strain was plotted against cycles to failure on log-log scales. One line segment covered the life range 4 to 1000 cycles and the other above 1000 cycles. When the data for each of the configurations was normalized with respect to its local strain at failure in a static tensile test, all of the data was represented by the same two straight line segments. Using data from the literature for the endurance range above 10000 cycles to failure, the entire fatigue range from 1 to 10 cycles to failure was discussed. The fatigue behavior at zero mean strain of the 0.6 inch radius specimens in terms of range of local strain was found to be directly related to that determined by cycling the specimens between fully reversed limits of strain across the specimen. The strain-hardening behavior of the three configurations was compared and effects of mean strain and rate of straining were considered. The relationship between the appearance of the fracture surfaces and the endurance of the specimens was discussed. Excellent agreement was found between the lives observed during the cumulative damage tests and the lives predicted by ∑(n/N)i = 1.0 when compared on the basis of equivalent strain range.