Issue 38

S. Hörrmann et alii, Frattura ed Integrità Strutturale, 38 (2016) 76-81; DOI: 10.3221/IGF-ESIS.38.10 77 when complex parts are manufactured by HP-RTM. These defects can cause a reduction in static strength or fatigue life. This influence has to be known in order to define if a composite part featuring manufacturing imperfections can be accepted or not. In previous research work the effect of out-of-plane ply waviness in the same material was studied and a reduction in compressive static and fatigue properties was found depending on a defect severity parameter [1]. In this work the influence of ply folding along fiber direction on the fatigue life of a unidirectional laminate is investigated. A ply folding along fiber direction causes a local increase in fiber volume fraction and by this a local increase in the stiffness in fiber direction. Effects of local stiffness changes due to gaps and overlaps were already studied in CFRP laminates manufactured through automated fiber placement. A meshing tool for numerical investigation of the effect of defects and their interaction has been developed [2]. However the model has not been validated by test data. An experimental study on the same topic was performed in [3] by static testing of unidirectional and multidirectional material with introduced gaps and overlaps along fiber direction. The conclusion was that ultimate static strength is reduced less than 5% at the lamina level, while at the laminate level the reduction is up to 13 %. This is mainly due to ply waviness induced into the plies surrounding the defect ply. A study on the effect of thickness and fiber volume fraction variations on strain field inhomogeneity under transverse load was done in [4]. In this study it was found that a strain inhomogeneity of up to 10 % is possible for fiber volume fraction variations in a flat part. This is expected to be important for reliability and fatigue behavior. In this work, the effect of ply folding on the fatigue performance of unidirectional CFRP laminates is experimentally assessed. The fold is induced along fiber direction into the extreme top and bottom plies of the laminate, as this is the most common case encountered during manufacturing. The specimens are loaded axially under constant amplitude loading. Progressive damage and failure due to the defect are investigated. The results are assessed using SN curves. Additionally, the stress states leading to failure for selected load cases are numerically modelled. A defect metric is defined in order to assess the influence of the detailed defect geometry. EXPERIMENTAL METHODS Material and specimen configuration est specimens provided by the industry partner were water jet cut out of [0] 6 carbon/epoxy plates with a constant thickness of t = 2.23 mm. The specimens feature artificially introduced manufacturing defects. The composite material is manufactured out of a unidirectional automotive non-crimp fabric (NCF) with an areal weight of one ply of m A = 300 g/m². The matrix constituent is Epoxy and the manufacturing method is HP-RTM. For production reasons the unidirectional NCF is assembled using transverse glass fiber tows with a spacing of about 2 mm. The average fiber volume fraction of the material without defect is V f,n=6 = 0.45, which is calculated using the formula: f,n A f V =(n m ) / (ρ t) (1) where n = 6 is the number of plies and ρ f = 1.8 g/mm³ is the density of the carbon fibers. Tensile and compressive specimens were cut out of the plates along and transverse to fiber direction and aluminum end tabs were bonded on each specimen. The specimen geometries are based on DIN EN ISO 527-4 for pure tensile and ASTM D 3410 for static compression loading; the same specimen configurations are also used for the fatigue load cases [5, 6]. The specimen configurations including the fold position (the black strips) are shown in Fig. 1. Figure 1 : Specimen configuration for different loading cases: (a) longitudinal tension; (b) longitudinal compression; (c) transverse tension; (d) transverse compression. T (a) (b) (c) (d) 0° 0°