Issue 38

S. Hörrmann et alii, Frattura ed Integrità Strutturale, 38 (2016) 76-81; DOI: 10.3221/IGF-ESIS.38.10 81 The results for the T-C fatigue loading are presented in Fig. 5b. Here, it can be noted that the specimens with lower c values still fall into the scatter band of the specimens without defect, but a reduced fatigue life is recorded for the specimens with a higher distance c . This is because of the fact that the T-C specimens have to be short (in order to avoid compression buckling) and for high c values the folds are located into the tabs effects area of the specimens. An interaction of stress concentrations at fold and tabs occurs, which reduces the fatigue life of specimens. The second damage mechanism is compression failure, corresponding to static compression and fatigue C-C. In transverse compression the fracture surface is inclined, forming a wedge. By optical microscopy it was observed that, for the material with defect, compression fracture always happened through the triangular resin pockets at the fold, as shown in Fig. 4c. In a linear static FE analysis the influence of the fold was modelled by stiffness variation in resin and folded volume, which showed a local increase of the von Mises stress in the plies near the resin pockets, and orientation of the higher stress areas corresponding to the fracture lines in experiments. An influence of the distance c could also be observed for C-C fatigue loading (Fig. 5c): for smaller distance c the material withstands a reduced fatigue life. C ONCLUSIONS unidirectional carbon fiber reinforced polymer material featuring folds as manufacturing defect has been studied by means of experimental tests and simple numerical FE simulations. The influence on the fatigue life has been investigated under constant amplitude fatigue loading. In fiber direction a dependency on the distance c between the folds was found, where smaller distances lead to higher stress concentrations and lower fatigue life, while in transverse to fibers direction failure usually occurred in resin rich areas nearby the fold without an influence of c . All in all, the studied folds had a minor influence on the fatigue performance and are considered to be allowable in structural parts. However, for multidirectional laminates, waviness might be introduced by the folds in the off-axis plies, which lead to a strength and fatigue life reduction of about 50 % in compression, as was found in a previous study [1]. A combination of these two defects could have a higher influence on the fatigue life, and should be investigated in future work. A CKNOWLEDGEMENTS he financial support by the Austrian Federal Ministry of Economy, Family and Youth and the National Foundation for Research, Technology and Development is gratefully acknowledged. The authors are grateful to Mr. Erich Humer for his technical support during the experiments. REFERENCES [1] Hörrmann, S., Adumitroaie, A., Viechtbauer, C., Schagerl M., The effect of fiber waviness on the fatigue life of CFRP materials, Int. J. Fatigue (submitted 2015). [2] Li, X., Hallett, S.R., Wisnom, M.R. Modelling the effect of gaps and overlaps in automated fibre placement (AFP)- manufactured laminates, Sci. Eng. Compos. Mater. 22(2) (2015) 115–129. http://dx.doi.org/10.1515/secm-2013- 0322 [3] Croft, K., Lessard, L., Pasini, D., Hojjati, M., Chen, J.H., Yousefpour, A., Experimental study of the effect of automated fiber placement induced defects on performance of composite laminates Composites Part A, 42 (2011) 484–491. http://dx.doi.org/10.1016/j.compositesa.2011.01.007 [4] Jensen, E.M., Leonhardt, D.A., Fertig III, R.S., Effects of thickness and fiber volume fraction variations on strain field inhomogeneity, Composites Part A 69 (2015) 178-185. http://dx.doi.org/10.1016/j.compositesa.2014.11.019 [5] DIN EN ISO 527-4:1997-07, Plastics - Determination of tensile properties - Part 4: Test conditions for isotropic and anisotropic fibre-reinforced plastic composites, (ISO 527-4:1997). [6] ASTM D3410 / D3410M-03, Standard Test Method for Compressive Properties of Polymer Matrix Composite Materials with Unsupported Gage Section by Shear Loading, ASTM International, West Conshohocken, PA, 2008. http://dx.doi.org/10.1520/D3410_D3410M-03R08 A T