Issue 41

D. Nowell et alii, Frattura ed Integrità Strutturale, 41 (2017) 197-202; DOI: 10.3221/IGF-ESIS.41.27 197 Focused on Crack Tip Fields Measurement and analysis of fatigue crack deformation at the micro-scale D. Nowell, K.I. Dragnevski, S.J. O’Connor University of Oxford, UK david.nowell@eng.ox.ac.uk, http://orcid.org/0000-0001-9997-8364 A BSTRACT . This paper introduces the use of digital image correlation for the measurement of surface displacements in the neighbourhood of a crack tip, both at the macro- and micro- scale. Various methods of interpreting the measured data and producing a crack driving force are then discussed, including the use of the full CJP model. A reduced set of parameters are then proposed, corresponding to the three principal interaction forces between the plastic enclave and the surrounding elastic material. Our own results, and those of Vasco Olmo, previously reported in the literature are then re- analysed using this new framework, and excellent agreement between two independent experiments is obtained. Implications for the analysis of further data sets are then discussed. K EYWORDS . Fatigue; Crack Driving Force; Digital Image Correlation; CJP model. Citation: Nowell, D., Dragnevski, K.I., O’Connor, S.J., Measurement and analysis of fatigue crack deformation at the micro-scale, Frattura ed Integrità Strutturale, 41 (2017) 197-202. Received: 28.02.2017 Accepted: 15.04.2017 Published: 01.07.2017 Copyright: © 2017 This is an open access article under the terms of the CC-BY 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. I NTRODUCTION nderstanding the mechanics of fatigue crack propagation is important if we are to safely operate a wide range of engineering systems. Damage tolerant life prediction methods are increasingly popular and give a number of advantages over safe life approaches. Nevertheless, most of the methods are based on the Paris law relation between da/dN and delta K. Although this ‘law’ is justifiably popular, it is in essence a simple empirical curve fit to laboratory data obtained at constant remote load amplitude. Load histories in real structures can differ considerably, and may include variable amplitude cycles, hence it is important to understand how crack tips behave in these circumstances. There is an increasing realisation that simple elastic crack models may have limited applicability, particularly when there is significant plastic deformation ahead of the crack and a corresponding plastic wake. Our developing work at Oxford in the area described above has been presented previously at the Forni di Sopra [1], Malaga [2], and Urbino [3] IJ Fatigue/FFEMS workshops. It has employed digital image correlation on the macro and micro scale to measure and analyse near tip displacements fields. In particular, at Urbino [3], we reported measurements taken during in-situ loading of a fatigue crack in a scanning electron microscope (Fig.1.). U