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D. Nowell et alii, Frattura ed Integrità Strutturale, 25 (2013) 1-6; DOI: 10.3221/IGF-ESIS.25.01 6 crack faces. The values measured here can only be possible when a significant crack tip opening displacement is also present. This suggests that a more sophisticated description of crack tip fields is necessary, perhaps including elastic and plastic terms as suggested by Pommier and Hammam [11]. Future work will include the comparison of the measured results with strip yield and finite element models. Further experiments are also planned, which will look at the region ahead of the crack tip as well as along the crack flanks. This should allow correlation of the results obtained in the crack wake with conditions in and around the process zone itself. A CKNOWLEDGEMENT .F.P. de Matos would like to gratefully acknowledge the support of the Portuguese Fundação para a Ciência e a Tecnologia (FCT) for providing a D.Phil. scholarship (reference SFRH/BD/12989/2003, financed by POSI). R EFERENCES [1] Inglis, C.E., Stresses in a plate due to the presence of cracks and sharp corners, Trans of the Institution of Naval Architects, 55 (1913) 219-230. [2] Westergaard, H.M., Bearing pressures and cracks, Jnl Appl. Mech., 6 (1939) 49-53. [3] Sanford, R.J., Selected Papers on Foundations of Linear Elastic Fracture Mechanics, SEM/SPIE, USA (1997). [4] Sanford, R.J., Selected Papers on Crack Tip Stress Fields, SEM/SPIE, USA (1997). [5] Paris, P., Erdogan, F., A critical analysis of crack propagation laws, Jnl Basic Engineering, 85 (1963) 528-534. [6] James, M.N., Yates, J.R., Susmel, L., Iacoviello, F., Nowell, D., Lazzarin, P., Patterson, E.A., Carpinteri, A., Characterisation of Crack Tip Stress Fields, Proceedings of the 1 st Joint International Journal of Fatigue/Fatigue & Fracture of Engineering Materials & Structures conference, held in Forni di Sopra, UD, ITaly, 7-9 March, 2011, Int. Jnl Fatigue, 46 (2013) 1-1. [7] James, M.N., Yates, J.R., Susmel, L., Iacoviello, F., Guest Editorial: Special Issue on Characterisation of Crack Tip Stress Fields, Fatigue Fract. Engng Mater. Struct., 36 (2013) 1-2. [8] Limodin, N., Rethore, J., Buffiere, J.Y., Influence of closure on the 3D propagation of fatigue cracks in a nodular cast iron investigated by X-ray tomography and 3D volume correlation, Acta Materiala, 58 (2010) 2957-2967. [9] Zhang, W., Liu, Y., Investigation of incremental fatigue crack growth mechanisms using in situ SEM testing, Int. Jnl Fatigue, 42 (2012) 14-23. [10] de Matos, P.F.P., Nowell, D., Experimental and numerical investigation of thickness effects in plasticity-induced fatigue crack closure, Int Jnl Fatigue, 31 (2009) 1795-1804. [11] Pommier, S., Hamam, R., Incremental model for fatigue crack growth based on a displacement partitioning hypothesis of mode I elastic-plastic displacement fields, Fatigue Fract. Engng Mater. Struct., 30 (2006) 582-598. [12] Nowell, D., Kartal, M.E., de Matos, P.F.P., Measurement and modelling of near-tip displacement fields for fatigue cracks in 6082 T6 aluminium, Proc. First I.J. Fatigue & FFEMS Joint Workshop, Forni di Sopra, Italy, March 7-9, 2011, Gruppo Italiano Frattura, 2011. [13] Nowell, D., Kartal, M.E., de Matos, P.F.P., Digital image correlation measurement of near-tip fatigue crack displacement fields: constant amplitude loading and load history effects, Fatigue Fract. Engng Mater. Struct., 36 (2013) 3-13. [14] Eberl, C. Thompson, R., Gianola, R., Digital image correlation and tracking with Matlab, Matlab Central file exchange (2006) http://www.mathworks.co.uk/matlabcentral/fileexchange/12413-digital-image-correlation-and-tracking . P