A Practical Methodology for Elastic-Plastic Fatigue Crack Growth

Abstract

A practical methodology is being developed to characterize elastic-plastic fatigue crack growth (EPFCG) behavior. The methodology will be implemented in engineering software for crack growth analysis and life prediction of advanced reusable aerospace propulsion systems. The correlating parameter upon which the methodology is based is the range of the J-integral, ΔJ. Existing J solutions are summarized, and robust methods for developing new J solutions under various loading configurations are introduced and validated. Some practical crack growth algorithms required to translate a J calculation into a quantitative prediction of EPFCG life are highlighted. Crack closure plays a significant role in the engineering characterization of EPFCG rates, and simple algorithms to estimate closure stresses are described. Other algorithms address the tearing-fatigue interaction near final instability and the estimation of required material properties. Early results from experimental verification tests are reported.

Effect of cyclic plastic strain on fatigue crack growth

“…The overview on the development of crack driving force parameters has been well documented by McClung et al [1]. A first obvious attempt to move from small to large scale yielding was to replace stress by strain quantities, see for example Boettner et al [2], McEvily [3], or El-Haddad et al [4].…”

Determination of fracture mechanics parameters J and C∗ by finite element and reference stress methods for a semi-elliptical flaw in a plate

“…Other calculations have been presented in the literature [11][12][13][14][15] for a wider range of loading conditions including combined tension and bending. They have also been made for plate geometries with W/c = 4-2 and crack sizes, shapes and depths covering a/t = 0.2-0.8, a/c = 0.2-1.0 and n = 5, 10 and 15.…”

“…Typically creep strain rate can be described by a power-law relation of the form for the case when J FE is determined for n having the same value as m. Consequently finite element solutions for J FE obtained when plastic strains dominate can be used for estimating C* from eq (17). Equation (17) is particularly valuable because there are more solutions available in the literature for J than for C* (see, for example [11][12][13][14][15] The normalising stress of 2σ b /3σ Y has been chosen for bending because it corresponds with the reference stress for an uncracked plate in bending. Included in the figures are estimates of normalized J p determined from the reference stress procedure outlined, the current finite element calculations and additional finite element results taken from the literature [11,13,15].…”

“…Equation (17) is particularly valuable because there are more solutions available in the literature for J than for C* (see, for example [11][12][13][14][15] The normalising stress of 2σ b /3σ Y has been chosen for bending because it corresponds with the reference stress for an uncracked plate in bending. Included in the figures are estimates of normalized J p determined from the reference stress procedure outlined, the current finite element calculations and additional finite element results taken from the literature [11,13,15]. It is evident that there are significant differences in some instances between the individual finite element results.…”

“…Initially expressions for J and C* are derived in terms of reference stress. Comparisons are then made with numerical solutions taken from the literature [11][12][13][14][15] and with additional calculations made by ABAQUS [16].…”