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Theory Of Stage II Fatigue Crack Propagation

Report Number: AFML TR 64-415
Author: Grosskreutz, J. C.
Corporate Author: Midwest Research Inst Kansas City MO
Laboratory: AF Materials Laboratory
Date of Publication: 1965-03
Pages: 21
Contract: AF 33(657)-10883
Project: 7353 - Characterization of Solid Phase and Interphase Phenomena in Crystalline Structures
Task: 735301
AD Number: AD0464816

A continuum model of crack extension is used to derive a crack propagation law for the case of constant plastic strain amplitude fatigue. The rate of crack growth is found to be proportional to the square root of the crack length. Integration over the total number of cycles to failure, N sub f, yields an expression of the form N sub f(delta epsilon sub p bar to the n+1 power = f(delta epsilon sub p bar), where delta epsilon sub p bar is the applied plastic strain range, and f is a function which varies rapidly in the region of large strains, but approaches a constant as delta epsilon sub p bar becomes small. The strain hardening coefficient, n, and the fracture strain enter as material constants. Comparison with experimental data gives good agreement for delta epsilon sub p bar > or = 0.01, which is consistent with the assumptions used in the theory. A discussion is given which interprets the well known power law N sub f to the 1/2 power times delta epsilon sub p bar = const. in terms of crack propagation.

Lockheed Martin Missiles & Fire Control
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