PSI - Issue 60

Amardeepa KCS et al. / Procedia Structural Integrity 60 (2024) 60–74 Amardeepa KCS/ Structural Integrity Procedia 00 (2023) 000–000

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2. LITERATURE REVIEW Lekhnitskii [1] introduced a beam theory-based calculation method to predict the transverse tension state and curved beam strength in an L-shaped beam with unidirectional composite plies under four-point bending. These loads were valid only for pure bending or edge loading because they cannot sustain the circumferential force without radial restraints. Baba [2] proposed a simplified post-processing approach to predict the folding-unfolding interlaminar stress components created at the symmetrically balanced curved composite laminate thickness under the combined action of shear forces, axial forces, and bending moments applied in the curvature plane. This approach is based on the extraction of forces and moments in the elements at the baseline curvature region. It provides a comparison between 3-D layered solid elements and layered shell elements. Thurnherr. C et al. [3] investigated the delamination failure generated in the curved laminates from applied bending moments, which induces tensile stresses in the radial direction using a higher-order beam model, which gives an idea about the mechanics of failure initiation for design purposes. In thin laminates, in-plane failure is initiated by the hoop stress (critical stress component). However, the transverse shear stress (critical stress component) initiates delamination cracks (de-bonding failure), which are predominant rather than in-plane failure, as the thickness increases in the laminates. As an extension of this theory, two clamped ends and one non-clamped end case were studied, making interlaminar shear stress and radial stress critical. J.S.Charrier et al. [4] investigated the out-of-plane tensile strength of an L shaped specimen with a constant radius-to thickness ratio (different thicknesses) and various carbon /epoxy laminates such as highly oriented, highly disoriented, quasi-isotropic, and unidirectional were used. At experimental failure load, maximum out-of-plane tensile stress is approximated by a 3D linear elastic finite model to predict the Curved Beam Strength (CBS). They concluded that the thick laminates were sufficient to avoid arms bending in four-point bending simultaneously; stacking sequence may not cause significant changes in observed strength. Hao et al. [5] investigated carbon-epoxy prepreg L-shaped specimens with 20, 40, and 60 plies. The delamination starts at once at various interfaces for the thinnest specimens, whereas for thicker specimens, delamination was started close by the center of the arc at the mid-thickness and then occurs in upper and lower interfaces. Polagangu James et al. [6] carried out a two-stage FE analysis for analytically determining the strength of composite co-cured T-joint against internal fuel pressure. In the first stage, an analytical study was carried out on a wing /wing box by applying an internal fuel pressure of 12.50 PSI to identify the high out of-plane stress region along the co-cured region of both bottom skin and spars. In the second stage, he created a 1D 2D FE model of a composite co-cured T-joint through an innovative, simplified approach developed based on a physical phenomenon inspired by nature named Bubbles in the Bermuda Triangle. The 1D elements represented the resin-bonded roving fragments filled in that triangle region precisely captured the stress distribution in that failure region. Polagangu James et al. [7] used a similar FE modelling approach and created adhesively bonded regions in various adhesively bonded composite (ABC) repaired joint configurations, explaining the structural failure mechanics. He also proposed a novel failure criterion using mechanical energy principles, named the Bond Energy Method. This modelling approach and the novel failure criterion precisely estimated the ultimate failure load of various third-party experiments. By using this approach, areas of bonded regions can be identified that are subjected to tensile, compression, and shear stresses, and the failure of bonded joints was explained through six modes of failures. Martin. R. H [8] studied delamination failure in a unidirectional curved composite laminate. In the curved region, at the location of the highest radial stress, delamination was supposed to be created, and this delamination caused the unstable failure of curved laminates. The interlaminar tension failure was predicted by strength-based failure criterion, and 2D FEA and a closed-form curved beam elasticity solution found excessive radial stress location. The present paper discusses details of the analytical study carried out on the standalone FE model of a composite L angle with a stringer cut-out. The structural integrity is analytically assessed when subjected to internal fuel pressure. The static stress analysis was carried out to understand the failure load of the L angle when subjected to out-of-plane loads through a novel FE modelling approach and failure criterion [6]. 3. FE ANALYSIS OF WING / WINGBOX The experimentally validated FE modelling approach and novel failure criteria [6,7] are the baseline methodology adopted in the present study for creating an FE model of an L angle joint and estimating the ultimate failure load or margin of safety values. Static stress and buckling analysis are carried out for critical air load cases generated as per