Elastic-Plastic Fracture Mechanics for Two-Dimensional Stable Crack Growth and Instability Problems

Abstract

An elastic-plastic fracture mechanics methodology for treating two-dimensional stable crack growth and instability problems is described. The paper draws on “generation-phase” analyses in which the experimentally observed applied-load (or displacement) stable crack growth behavior is reproduced in a finite-element model. In these calculations a number of candidate stable crack growth parameters are calculated for the material tested. The quality of the predictions that can be made with these parameters is tested with “application-phase” analyses. Here, the finite-element model is used to predict stable crack growth and instability for a different geometry, with a previously evaluated parameter serving as the criterion for stable growth. These analyses are applied to and compared with measurements of crack growth and instability in center-cracked panels and compact tension specimens of the 2219-T87 aluminum alloy and the A533-B grade of steel. The work shows that the crack growth parameters (COA)c, Jc, dJc/da, and the linear elastic fracture mechanics (LEFM)-R, which sample large portions of the elastic-plastic strain field, vary monotonically with stable crack extension. However, the parameters (CTOA)c, R, Go, and Fc, which reflect the state of the crack tip process zone, are essentially independent of the amount of stable growth when the mode of fracture does not change. Useful, stable growth criteria can therefore be evaluated from the crack tip state at the onset of crack extension and do not have to be continuously measured during stable crack growth. The possibility of making accurate predictions for the extent of stable crack growth and the load level at instability is demonstrated using only the value of J c at the onset of crack extension.

Characterization of stable crack extension in aluminium sheet material using the crack tip opening angle determined optically and by the δ5 clip gauge technique

“…(5) From these results it follows that the resistance to stable crack extension of the material investigated is characterized by two distinct regions with significantly different CTOAbehavior. Following initiation of crack extension, CTOA opt decreases strongly until it reaches steady state conditions characterized by a constant value, thus confirming the earlier findings mentioned in the introduction [3]. This crack propagation phase is terminated by ∆a maxCTOA.…”

“…It was shown by experimental work and finite element analyses [3] that after a short initiation phase the crack tip opening angle attains a steady state condition which means that cracks propagate with a nearly constant opening angle over a wide range of crack extension. This has stimulated the hope that CTOA can be used for an efficient characterization of large amounts of ductile crack extension in large structures.…”

Review of fracture toughness (G, K, J, CTOD, CTOA) testing and standardization

“…The CTOA fracture mechanics method was developed primarily to characterize the stable crack extension behavior for thin-walled materials in low constraint conditions. The early experimental work and FEA results of Kanninen et al [219] showed that after a short initiation phase the CTOA attains a steady state condition which means that the crack propagates with a nearly constant operation angle over a wide range of crack extension. Most applications of the CTOA criterion are made for thin-walled or sheet metals as used in aircraft engineering and pipeline industry.…”

A review of the CTOA/CTOD fracture criterion - Why it works

“…[1][2][3][4][5][6]. These studies were conducted to develop efficient techniques to simulate crack extension and to study various local and global fracture criteria.…”

“…Of these, the critical cracktip-opening angle (CTOA, y,) or displacement (CTOD) at a specified distance from the crack tip was shown to be the most suited for modeling stable crack growth and instability during the fracture process (e.g., Refs. [3][4][5][6][7][8]. However, some discrepancies among the various analyses have been observed at initiation of stable crack * Senior Scientist, Mechanics and Durability Branch t National Research Council Associate, Mechanics and Durability Branch growth.…”