Issue 35

V. Shlyannikov et alii, Frattura ed Integrità Strutturale, 35 (2016) 114-124; DOI: 10.3221/IGF-ESIS.35.14 118 Stress triaxiality As a secondary fracture parameter, a local parameter of the crack-tip constraint was proposed by the authors [10] because the validity of some of the above-mentioned concepts depends on the chosen reference field. This stress triaxiality parameter is described as follows:   3 , , 3 2          kk ij ij h r z s s (2) where  kk and s ij are the hydrostatic and deviatoric stresses, respectively. Being a function of both the first invariant of the stress tensor and the second invariant of the stress deviator, the stress triaxiality parameter is a local measure of the in- plane and out-of-plane constraint that is independent of any reference field. Plastic stress intensity factor Here, our primary interests are to obtain an accurate description for the distribution along the crack front of the governing parameter for the elastic-plastic solution in the form of an I n -integral and to determine the accuracy that this type of calculation, which will later be used for the general 3D problem, provides for the plastic stress intensity factor (SIF). The method developed here for combining the knowledge of the dominant singular solution with the finite element technique to obtain accurate solutions in the neighborhood of a crack tip is also applicable to the treatment of problems involving cracks in finite bodies. The plastic stress intensity factor P K in pure Mode I can be expressed directly in terms of the corresponding elastic stress intensity factor using Rice’s J -integral. That is             ; 1 1 2 1 2 0 1 1 2 0 2 1                         n n n n P I waY I K K           waY K 1 1     (3) where w K K 1 1  is normalized by a characteristic size of cracked body elastic stress intensity factor and '  E E for plane stress and   2 ' 1    E E for plane strain. In the above equations,  and n are the hardening parameters, wa   is the dimensionless crack length, w is characteristic size of specimen (for our case that is specimen width),  is the nominal stress, and  0 is the yield stress. The numerical constant    n I is obtained from the singularity analysis by means of the conjugation solutions for the far and near fields. In the classical first-term singular HRR-solution [11], the numerical parameter I n is a function of only the material strain hardening exponent n . Shlyannikov and Tumanov [5] reconsidered the HRR-solution for both plane strain and plane stress and supposed that under small-scale yielding, the expression for I n depends implicitly on the dimensionless crack length and the specimen configuration. In this section, we extend the analysis to the I n -integral behavior in an infinitely sized cracked body [11] to treat the test specimen’s specified geometries. The use of the Hutchinson’s theoretical definition for the In -factor directly adopted in the numerical finite element analyses leads to [5]     1 cos sin 1 , , , 1 cos 1                                                                             FEM FEM FEM n FEM FEM FEM FEM r e rr r r FEM n FEM FEM FEM FEM rr r r du du n u u c a n d d I n d w t u u n            (4) In this case, the numerical integral of the crack tip field I n changes not only with the strain hardening exponent n but also with the relative crack length c/w and the relative crack depth a/t . More details to determine the I n factor for different test specimen configurations are given by Refs. [5-7].