Process zone size effects on naturally curving cracks

In a fuselage crack turning application, a crack may grow under stable tear ing conditions for some length, achieving a somewhat steady-state condition , then encounter a region of high tensile T-stress as it nears a stiffener, causing the crack to turn. While a T-stress related crack path instability has been described by Cotterell and Rice [Int J Fract 16 (1980) 155], rece nt experimental evidence indicates that crack turning is also influenced by the size of the process zone. In order to study this problem, the perturbed crack path solution of Cotter ell and Rice is extended to include the effects of a process zone represent ed by cohesive tractions on the crack flanks trailing the crack tip. The ch aracteristic length, r(c), over which the tractions are prescribed, represe nts a strain-localization zone which precedes the physical crack tip, and t hrough which the crack presumably must pass. The strain-localization zone i s assumed to be smaller than common measures of plastic zone size, and othe r than the strain-localization zone, plasticity is not explicitly modeled. The solution, which uses cohesive tractions analogous to the Dugdale-Barenb latt crack tip model [J Mech Phys Solids 8 (1960) 100; Advances in Applied Mechanics, vol. VII, Academic Press. 1962. p. 55], is accurate to first-ord er deviations from a straight crack path in an infinite medium for small st rain-localization zones. A correction factor is provided to the process zon e parameter to approximate the solution for larger process zones, but loses accuracy as the strain-localization zone approaches the plastic zone size. Increased process zone size is shown to result in an increase in the pertu rbation sensitivity of a crack in a positive T-stress environment, causing more rapid turning of the crack, as has also been experimentally observed. (C) 2001 Elsevier Science Ltd. All rights reserved.