PSI - Issue 8

Giuseppe Pitarresi et al. / Procedia Structural Integrity 8 (2018) 474–485 Author name / Structural Integrity Procedia 00 (2017) 000–000

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for temperature sampling. The post-processing of each block of thermograms, to filter the thermoelastic and second harmonic signals, takes just few seconds, so that TSA can be considered as a real time fast-monitoring technique in fatigue studies. In this work, a CFRP pre-preg (IM7/8552) and an UD GFRP materials have been characterized via the modified TCT test, under fatigue loading. The thermoelastic signal from the edge face of CFRPs has revealed some peculiar features of the stress field near the crack tips. Such features can be used to monitor the crack tip displacement, evaluating the crack growth rate. Furthermore, the analysis of the Thermoelastic signal on the mTCT confirms that the stress field at the crack tip allows fracture only under Mode II. The interpretation of the Thermoelastic signal has also evidenced that the thickness of the cut plies can have an influence on the nature of transverse stress components arising near the crack tips, as the block of cut plies inside the delamination acts as an internal constraint towards the lateral Poisson’s contraction of the cut plies ahead of the crack tips. In CFRP samples the Second Harmonic signal is found higher along the wake of each crack, due to some frictional effects activated by transverse compression stresses. Such trace of the second harmonic signal is very effective in showing the crack tip position and the crack path under fatigue. Furthermore, it enables to evaluate crack growths independently for each of the four delaminations present in the TCT specimen. For GFRPs, the higher thermoelastic signal allows monitoring the delamination growth from the front face of the sample, giving also indications on the curvature of the delamination fronts along the sample beam width. From the outcomes of this study it is believed that IR Thermography in general, and TSA in particular, can provide very useful insights on the fracture behavior in TCT samples under fatigue testing. Future work will attempt to run a more systematic testing campaign in order to obtain a complete fatigue characterization, and also to explore the response of mTCT samples with different dimensions. Allegri, G., Jones, M. I., Wisnom, M. R. and Hallett, S. R. (2011) ‘Anew semi-empirical model for stress ratio effect on mode II fatigue delamination growth’, Composites Part A: Applied Science and Manufacturing. Elsevier Ltd, 42(7), pp. 733–740. Bergmann, H. W. and Prinz, R. (1989) ‘Fatigue life estimation of graphite/epoxy laminates under consideration of delamination growth’, International Journal for Numerical Methods in Engineering. John Wiley & Sons, Ltd, 27(2), pp. 323–341. Brémond, P. and Potet, P. (2001) ‘Lock-in thermography: a tool to analyze and locate thermomechanical mechanisms in materials and structures’, in: Proceedings of SPIE 4360, Thermosense XXIII, Orlando, FL, USA, 23 March 2001. pp. 560–6. Brunner, A. J., Stelzer, S., Pinter, G. and Terrasi, G. P. (2013) ‘Mode II fatigue delamination resistance of advanced fiber-reinforced polymer-matrix laminates: Towards the development of a standardized test procedure’, International Journal of Fatigue, 50, pp. 57–62. Brunner, A. J., Stelzer, S., Pinter, G. and Terrasi, G. P. (2016) ‘Cyclic fatigue delamination of carbon fiber-reinforced polymer-matrix composites: Data analysis and design considerations’, International Journal of Fatigue. Elsevier Ltd, 83, pp. 293–299. Cahain, Y. M. Le, Noden, J. and Hallett, S. R. (2015) ‘Effect of insert material on artificial delamination performance in composite laminates’, Journal of Composite Materials, 49(21), pp. 2589–2597. Catalanotti, G., Furtado, C., Scalici, T., Pitarresi, G., Van Der Meer, F.P. and Camanho, P.P. (2017) ‘The effect of through-the-thickness compressive stress on the mode II interlaminar fracture toughness. Composite Structures (in Press). https://doi.org/10.1016/j.compstruct.2017.09.014. Cui, W., Wisnom, M. R. and Jones, M. (1994) ‘An Experimental and Analytical Study of Delamination of Unidirectional Specimens with Cut Central Plies’, Journal of Reinforced Plastics and Composites, 13(8), pp. 722–739 Emery, T. R., Dulieu-Barton, J. M., Earl, J. S. and Cunn-ingham, P.R. (2008) ‘A generalised approach to the calibration of orthotropic materials for thermoelastic stress analysis’, Composites Science Technology, 68, 743–752. Kawashita, L., Jones, M., Trask, R. S., Hallett, S. R. and Wisnom, M. R. (2009) ‘Static and fatigue delamination from discontinuous plies– experimental and numerical investigations’, Proceedings of the 17th International Conference on Composite Materials, ICCM-17, pp. 1–17. Pitarresi, G., Conti, A. and Galietti, U. (2005) Investigation on the influence of the surface resin rich layer on the thermoelastic signal from different composite laminate lay-ups, Applied Mechanics and Materials, Vols. 3-4, pp. 167-172. Pitarresi, G. and Galietti, U. (2010) ‘A Quantitative Analysis of the Thermoelastic Effect in CFRP Composite Materials’, Strain. Blackwell Publishing Ltd, 46(5), pp. 446–459. Acknowledgements The experimental activity has been carried out by using the IR thermal camera FLIR X6540sc, which has been purchased using funds from the project INTEP – PO FESR 2007/2013 – 4.1.2.A. References