PSI - Issue 61

Yogesh Kumar et al. / Procedia Structural Integrity 61 (2024) 322–330 Y. Kumar et al., / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Carbon-based fiber-reinforced composites has been extensively adopted for structural load bearing, impact resistant and other civilian applications mainly due to their high stiffness and strength with respect to weight (Nikbakt et al., 2018). Researchers have evaluated the performance of carbon-fiber composites under different loading conditions as well as in different environmental conditions (Harussani et al., 2022; Nejad et al., 2021; M. Rezasefat et al., 2021b; Wang et al., 2015).The vulnerability of such materials to impact loading restricts the design of the stacked composite laminates as failure is predominantly driven by matrix cracking, delamination, and fiber breakage (Caminero et al., 2018). Many researchers have reported the influence of the thick- or thin-ply thickness in the laminate and corresponding influence on the damage morphology (Aoki et al., 2021). Li, Yuan and Zhang, 2023 proposed a novel variable ply thickness gradient structure to demonstrate the enhancement in the damage and delamination resistance of a thin-ply laminate for peak performance in engineering related applications. They also revealed that thicker layers experience earlier delamination initiation owing to dissipated energy, leading to lesser damage in fiber mode and subsequently, thinner layers have more effectively compressive strength. Yuan et al., 2022 optimized the compression strength after impact (CAI) in the laminate by modifying the thickness and pitch angle of plies using a numerical framework. They correlated the ply thickness with global compressive strength, in situ tensile strength and the impact resistance. Thin ply laminate composites have higher strength, stiffness, damage tolerance, and fatigue performance due to refine fiber alignment and lower fault probabilities. Furthermore, their improved impact resistance, reduced delamination susceptibility, and weight reduction make them considerably desirable for aviation and defense domains (Arteiro et al., 2020; Cugnoni et al., 2018; Harussani et al., 2022). Aoki et al., 2022 studied the contribution of the 0° layers in the quasi-isotropic stacking sequence of carbon-fiber based composites through tensile loading test cases for standard and open-hole specimens. They noted that the ply thickness varies with the layer ratio. A thorough investigation on the failure processes and stress distribution was performed using finite element modeling to quantify the growing strength with the increased number of layers and the resultant higher stiffness in the laminate. Additionally, delamination or matrix cracking was not observed in the thin-ply laminates. Recently (Hu et al., 2023) extended the work to establish effect of flexural strength across the thickness of composite plies through experimental and analytical modeling. The experiments depict an 33.5 % increase in the in-plane tensile strength measured through 3-point bending test in the thinnest sample (1.48 mm). The results were also processed using the classical strength of material equations to validate the thickness dependency by characteristic length definition. However, the proposed analytical model did not incorporate the influence under compression loading over the damage morphology and global strengths. Motivated from the aforementioned literature, we studied the correlation between the thickness of the 90° plies in a cross-ply laminate with the in-plane compressive strength and the maximum failure strain. First, three different laminates (different thicknesses: 2, 3 and 4 mm) were experimentally tested under quasi-static in-plane compressive loading, as presented in Section 2 Secondly, a double cantilever beam finite element model (FEM) with zero-thickness cohesive elements was developed to estimate the Mode-I fracture energy in the composite. This numerical model was integrated into the quasi-static compression model for defining the interface between the 0° and 90° plies, as discussed in Section 3. In Section 5, the experimentally obtained results were compared against the result predicted by the FEM model developed. This research has provided insights on the optimization on the thickness of the 90° plies in the [0/90 n ] s stacking sequence, which is relevant for a variety of structural applications. 2. Material Description and Experimental Setup The carbon-fiber-reinforced- polymer (CFRP) cross-ply lay-up [0/90 n ] s composites have been used in this research. The samples were machined from flat panels of different thicknesses (~2, 3 and 4 mm) using a low-speed cutting saw. The composite samples have 0° plies with constant thickness of 0.5 mm on the outer side, as shown in Fig. 1. The general mechanical properties are listed in Table 1. The number of 90° plies was varied from 2 to 6 to obtained the final thicknesses of 2 (±0.05), 3 (±0.05) and 4 (±0.05) mm for 4 ply, 6 ply and 8 ply samples. The samples were tested under compressive loading using a servo-hydraulic Material Testing System-810 machine at the two strain rates of 4 × 10 -3 s -1 and 4 × 10 -5 s -1 . A thin layer of industrial grade grease was applied on the contacting surfaces of the samples to minimize the effect of friction in the results. The setup also includes an AOS Promon digital image correlation