Issue 33

F.V. Antunes et alii, Frattura ed Integrità Strutturale, 33 (2015) 199-208; DOI: 10.3221/IGF-ESIS.33.25 199 Focussed on characterization of crack tip fields A numerical study of non-linear crack tip parameters F.V. Antunes CEMUC, Department of Mechanical Engineering, University of Coimbra, 3030-129 Coimbra, Portugal fernando.ventura@dem.uc.pt R. Branco CEMUC, Depart. of Mechanical Engineering, Polytechnic Institute of Coimbra rbranco@isec.pt L. Correia, AL Ramalho CEMUC, Escola Superior de Tecnologia do Inst. Politécnico de Castelo Branco, Av. do Empresário, 6000 - 767 Castelo Branco lcorreia@ipcb.pt, aramalho@ipcb.pt A BSTRACT . Crack closure concept has been widely used to explain different issues of fatigue crack propagation. However, different authors have questioned the relevance of crack closure and have proposed alternative concepts. The main objective here is to check the effectiveness of crack closure concept by linking the contact of crack flanks with non-linear crack tip parameters. Accordingly, 3D-FE numerical models with and without contact were developed for a wide range of loading scenarios and the crack tip parameters usually linked to fatigue crack growth, namely range of cyclic plastic strain, crack tip opening displacement, size of reversed plastic zone and total plastic dissipation per cycle, were investigated. It was demonstrated that: i) LEFM concepts are applicable to the problem under study; ii) the crack closure phenomenon has a great influence on crack tip parameters decreasing their values; iii) the  K eff concept is able to explain the variations of crack tip parameters produced by the contact of crack flanks; iv) the analysis of remote compliance is the best numerical parameter to quantify the crack opening level; v) without contact there is no effect of stress ratio on crack tip parameters. Therefore it is proved that the crack closure concept is valid. K EYWORDS . Fatigue crack propagation; Plasticity induced crack closure; Non-linear crack tip parameters;  K eff I NTRODUCTION odern design methodologies consider that inherent defects are always present in components. Fatigue life is therefore defined as the number of load cycles required to propagate these defects up to a critical size. Engineering analysis of fatigue crack propagation is usually performed by relating the crack advance per unit cycle, da/dN, to the stress intensity factor range,  K. Initially it was surprising that this linear-elastic parameter could successfully describe the rate of plastic processes at the crack tip. Rice  1  showed that the small-scale cyclic plasticity at the crack tip is, indeed, controlled by  K. According to Paris law, da/dN is uniquely determined by one loading M