Issue 33

G.P. Nikishkov et alii, Frattura ed Integrità Strutturale, 33 (2015) 73-79; DOI: 10.3221/IGF-ESIS.33.10 73 Focussed on characterization of crack tip fields Specimen thickness effect on elastic-plastic constraint parameter A G.P. Nikishkov, Yu.G. Matvienko Mechanical Engineering Research Institute Russian Academy of Sciences, Moscow, RUSSIA nikishkov@imash.ru , ygmatvienko@gmail.com A BSTRACT . Three-dimensional elastic-plastic problems for a power-law hardening material are solved using the finite element method. Distributions of the J -integral and constraint parameter A along the crack front for varying specimen thickness and crack depth are determined for edge cracked plate, center cracked plate, three point bend and compact tension specimens. The constraint parameter A is a measure of stress field deviation from the HRR field. Higher A values signify lower specimen constraint. Results of finite element analyses show that the constraint parameter A significantly decreases when specimen thickness changes from 0.1 to 0.5 of the specimen width. Then it has more or less stable value. Among four specimen the highest constraint is demonstrated by the compact tension specimen which has the constraint parameter A lower than its small scale yielding value. K EYWORDS . Elastic-plastic crack tip field; Three-term elastic-plastic asymptotic expansion; Constraint parameter; Finite element method. I NTRODUCTION he J -integral [1, 2] is the most used fracture mechanics parameter for structural integrity analysis of elastic-plastic cracked structures. However, fracture criterion based on the J -integral alone correctly works when the near crack tip stress fields are described by the one-term HRR asymptotic solution [3, 4]. In many cases (for example, short and inner cracks) a one-parameter approach is not suitable for fracture prediction. Finite element modeling shows that the one-term asymptotic expansion is unable to produce satisfactory description of near-tip stress fields in the microstructurally significant region. Even for the small scale yielding conditions the deviation of actual stress field from HRR-field is noticeable. It is natural to assume that fracture in a structure occurs when a stress field in some region near the crack tip approaches the same value as in a test specimen under fracture load. Since the J -integral that controls the HRR-field cannot describe stresses in the crack-tip region under different load conditions it is necessary to utilize additional parameters and construct better equations for stress fields. Betegon and Hancock [5] used elastic T -stress for studying effects on crack-tip triaxiality. While the T -stress can distinguish states with high and low constraint it cannot serve as a constraint parameter for elastic-plastic bodies because of its elastic nature. O’Dowd and Shih [6] introduced a second fracture parameter in the form of a dimensionless stress Q which is defined as the difference between normal stresses in the near-tip region determined by a numerical analyses and the HRR stress field. T