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D. Nowell et alii, Frattura ed Integrità Strutturale, 25 (2013) 1-6 ; DOI: 10.3221/IGF-ESIS.25.01 1 Special Issue: Characterization of Crack Tip Stress Field Characterisation of crack tip fields under non-uniform fatigue loading D. Nowell, M.E. Kartal University of Oxford, Department of Engineering Science, Parks Road, Oxford, OX1 3PJ, UK. P.F.P. de Matos U3Is, Unidade de Investigação e Internacionalização do ISVOUGA, Santa Maria da Feira, Portugal. A BSTRACT . The paper analyses previously reported work, which uses digital image correlation to measure fatigue crack closure. As well as determining crack opening loads, the information on crack shape may be used to estimate the stress intensity factor, as well as other parameters in more complex models of crack tip fields. A number of specimens were subjected to single overload cycles, which produced a significant retardation in crack growth rate. The method previously applied to the analysis of constant amplitude loading is here used to analyse the single overload case. The stress intensity factor history is found to be very different in the two cases and the consequences of this observation for analysis of fatigue crack propagation are discussed. K EYWORDS . Digital image correlation; Displacement measurement; Crack-tip fields; Elastic-plastic fatigue crack. I NTRODUCTION he study of crack tip stress, strain, and displacement fields has a long history. One hundred years ago, Inglis recognized that a sharp elastic crack would exhibit a stress (and strain) singularity [1]. Twelve years later Westergaard [2] produced a stress function solution for a crack in a biaxial stress field which formed the basis for the later concept of the stress intensity factor. An excellent compilation of seminal papers in the field has been provided by Sanford [3,4]. The stress intensity factor has proved to be an extremely useful concept for the assessment of cracks and it has been widely used in industry. Paul Paris’ extension of the concept to its use in fatigue crack propagation [5] is particularly worthy of mention. However, it is clear that the stress intensity approach has many limitations. In particular, all fatigue cracks must exhibit some irreversibility at or close to the crack tip, otherwise they would not propagate. The stress intensity approach attempts to characterize this by a similitude argument, which depends on the assumption of small scale yielding (i.e. that the process zone of the crack tip is contained within a wider ‘K-dominant’ region ) . Use of the stress intensity approach has particular limitations for the case of non-uniform loading, since load history effects are difficult to capture. Hence, there has been increasing interest in improved characterization of crack tip fields. In parallel, new experimental techniques, such as digital image or volume correlation (DIC/DVC); X-ray tomography; and large scale synchrotron facilities have provide tools which allow various models of crack tip behavior to be tested. A significant number of papers were presented at the First I.J. Fatigue & FFEMS Joint Workshop on the Characterisation of Crack Tip Stress Fields held in Forni di Sopra in 2011. This resulted in the publication of two special issues of the journals [6, 7], but the workshop showcased only a small proportion of the work in the field. On the experimental side, other recent papers of note include tomographic studies of cracked specimens under live loading [8] and in-situ SEM T