Issue 33

G.P. Nikishkov et alii, Frattura ed Integrità Strutturale, 33 (2015) 73-79; DOI: 10.3221/IGF-ESIS.33.10 74 Main drawback of Q is its considerable dependence on radial and angular coordinates of a point selected for its determination. A mathematical approach to the introduction of a second fracture parameter is based on higher order elastic-plastic asymptotic expansions of the stress field in the near crack tip region. Three-term asymptotic solutions have been reported by Yang, Chao and Sutton [7] and by Nikishkov [8, 9]. It was found that for Mode I plane strain crack the three terms of the asymptotic expansion are enough for representing the stresses in the crack tip region with sufficient accuracy. It appeared that the three-term expansion is controlled by just two parameters - the J -integral and an additional amplitude parameter A . Amplitude A can be used as a constraint parameter in elastic-plastic fracture. Two-parameter J - A fracture criterion has wider range of applicability than the criterion based on J -integral alone. Here we present finite element three-dimensional elastic-plastic solutions for cracked specimens. Stress fields near the crack front are used for calculation of the constraint parameter A . Distributions of A along crack front are found for specimens of different thickness. T HREE - TERM ASYMPTOTIC EXPANSION uppose that the deformation behavior of an elastic-plastic material can be described with the Ramberg-Osgood uniaxial strain-stress curve: 0 0 0 n                (1) where 0  is the yield stress, is the hardening coefficient, n is the hardening exponent ( 1 n  ), 0 / E    , E is Young's modulus. The three-term asymptotic expansion for the stress field near the tip of mode I crack in an elastic-plastic body can be presented in the following form [9]: 2 (0) (1) 2 (2) 0 0 0 ( ) ( ) ( ) ij s t t s ij ij ij A A A A                   (2) Here ij  are stress components r  ,   and r   in the polar coordinate system r  with origin at the crack tip, ( ) k ij   are dimensionless angular stress functions obtained from the solution of asymptotic problems of order (0), (1) and (2). Angular stress functions (0) ij   and (1) ij   are scaled in such a way that maximal equivalent Mises stress is equal to unity. Power t is a numerically computed eigenvalue that depends on hardening exponent n . Power s is expressed as 1/ ( 1) s n    . Dimensionless radius  is defined by formula 0 / r J    (3) where J is the energy integral computed along small contour   1 1 i ij j u J Wn n d x                (4) Here W is the density of work done by stresses on mechanical strains, ij  are stresses, i u are displacements, j n and are components of external normal to the contour. Coefficient 0 A is given by expression 0 0 ( ) s n A I   (5) where n I is a scaling integral [3, 4]. S