PSI - Issue 61

Ahmet Çevik et al. / Procedia Structural Integrity 61 (2024) 291–299

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Cevik et al. / Structural Integrity Procedia 00 (2019) 000 – 000

In the literature, curved composite laminates have been investigated in three aspects of i) assessing the effects of the process induced deformations such as wrinkles, spring-in etc. on the structure, ii) measuring the interlaminar tensile strength of a material conducting 4-pt bending test, and iii) elucidating its failure mechanism under shear, axial forces and bending moment as sub structures of ribs and spars. The unidirectional curved composite laminates have been extensively investigated regarding all of these three aspects whereas there are a limited number of studies about the fabric curved composite laminates as summarized below. Wang et al. (2021) proposed a physic-based numerical model to predict the spring-in deformation on the curved 3D woven composites and also considered the tool part interaction. They applied a compensation procedure to eliminate the spring-in procedure and showed a good agreement with their experiments and other methods present in the literature. Avalon and Donaldson (2011) and Chaurasia et al. (2023) conducted experimental studies on woven curved composite laminates using the 4pt bending fixture. Avalon and Donaldson (2011) investigated the effect of bend radius, laminate thickness and addition of carbon nanofiber to epoxy on the curved beam strength of fabric curved composite laminates. Their results show that the curved beam strength is independent of the parameters bend radius and laminate thickness. They also found that the specimens manufactured with carbon nano-fibers lose their load carrying capacity with multiple load drops rather than a single load drop observed in the experiments of the specimens without carbon nano-fiber. Chaurasia et al. (2023) conducted pull-out tests according to the ASTM 6415 as well as 4pt bending experiments with woven CFRP laminates having a stacking sequence of [90°/0°/45°/- 45°/90°/0°/45°/-45°] s . The average failure load was found to be higher in the 4pt-bending test than in the pull-out test due to matrix cracking and delamination causing load drops in the load-displacement curves. Gozluklu et al. (2015), Hu et al. (2019), and Tasdemir and Coker (2022) studied woven curved composite laminates as a sub structure of rib and spars; Hu et al. (2019) and Tasdemir and Coker (2022) used a test fixture to mimic an operational loading on the woven curved composite laminates whereas Gozluklu et al. (2015) applied pure shear loading to the structure. Hu et al. (2019) investigated the failure mechanism of 2D and 3D woven curved composite laminates under combined loading of shear and axial forces and bending moment. Their results reveal that the 2D woven composites and 3D shallow straight woven composites lose their load capacity abruptly with interlaminar delamination. 3D orthogonal woven composites and shallow-bend woven composites have higher failure load than the other types of woven composites, as a result of the delay of delamination failure mode due to the presence of Z binder yarns. Gozluklu et al. (2015) carried out experimental and numerical investigation of CFRP woven curved composite beam having a stacking sequence of [0/90] 6s . In their study, delamination is associated to a sudden load drop in the load displacement curve. Their experimental and numerical results are in a good match in terms of load-displacement data and crack tip speed where the crack tip speed is shown to reach intersonic speeds exceeding the material shear wave speed. Tasdemir and Coker (2022) experimentally investigated the fatigue behavior of fabric and unidirectional curved composite laminates with a test fixture designed to apply combined axial/moment loading to the specimens. They reported that in the fabric specimens having a lay-up of [(45/0) 7 /45/45/0/45], one major crack initiates in the 6 th ply in the curved region and grows to the arms of the specimen as intralaminar and interlaminar crack, causing a load drop. In this paper, our focus is to investigate the failure mechanism of fabric curved composite laminates having a stacking sequence of [(45/0) 7 /45/45/0/45], that is representative of stacking sequence used in commercial aircraft, subjected to pure shear loading. A loading fixture is designed to apply pure shear load to the horizontal arm while the vertical arm is held fixed and validated with finite element analysis for a model curved geometry. Using in-situ high-speed camera inspection and post-mortem fractography, experimental observations of the failure mechanisms of curved fabric composite specimen are discussed. We believe that the experimental work presented here will advance our understanding of the mechanics of failure of curved fabric materials and contribute to lighter designs. 2. Method 2.1. Material, Specimen Geometry and Preparation Fabric curved composite specimens are manufactured using Hexply AS4/8552 5 Harness Satin (HS) Fabric prepregs. Material properties and components of 3D stiffness matrix of the fabric prepregs are given in Table 1. Material wave speeds for plane strain case are calculated using the relations given in Coker and Rosakis (2001). For Hexply AS4/8552 UD 5HS prepreg the longitudinal, shear and Rayleigh wave speeds are = 6434 m/s, = 1767 m/s, = 1678 m/s . Table 1. Material Properties and 3D stiffness matrix of Hexply AS4/8552 5HS Fabric prepreg . Elastic Constants 3D Stiffness Matrix E 11 =E 22 64 GPa c 11 64.99 GPa E 33 8.5 GPa c 22 64.99 GPa ν 12 0.046 c 12 3.81 GPa ν 13 =ν 23 0.3 c 23 2.74 GPa G 12 4.9 GPa c 66 4.90 GPa G 13 =G 23 3.7 GPa c 44 3.70 GPa