PSI - Issue 71

M Mohan Kumar et al. / Procedia Structural Integrity 71 (2025) 372–379

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was selected to achieve quasi-isotropic behavior with balanced stiffness in all directions. The symmetric lay-up minimizes residual stresses, while ±45° and 90° plies enhance shear and transverse strength, respectively. This configuration supports effective load transfer during repair assessment. A 50 mm central hole in a 300 mm wide panel results in a hole to-width ratio of 0.167, complying with ASTM D5766 for open-hole tension testing. 3. Methodology 3.1 Composite Failure Indices Failure indices are used to assess whether the stress state in a composite laminate can cause failure. A failure index (FI) greater than 1.0 indicates potential failure, while an FI less than or equal to 1.0 suggests that stresses are within the failure envelopes. Failure Index (FI) = U l At i pmpal ti ee dS tLroeandg t h (1) 3.2 Tsai-Wu Theory The Tsai-Wu failure theory, applied to orthotropic lamina under plane stress, extends Tsai and Wu's anisotropic material strength theory. Tsai-Hill criterion, which is derived from Von-Mises ’ distortion energy theory for isotropic materials, the Tsai-Wu theory incorporates both tensile and compressive strengths, offering a more comprehensive approach. While Tsai-Hill focuses on distortion energy, the Tsai-Wu theory, based on Beltrami's total strain energy approach, accounts for differing strengths in compression and tension. F 1 σ 1 +F 2 σ 2 +F 11 σ 12 +2F 12 σ 1 σ 2 +F 22 σ 22 +F 66 τ 12 2 =FI (2) 3.3 Finite Element Model Composite panels were modeled and meshed using ANSYS Composite Pre-Post (ACP), ensuring high mesh quality and accurate layup representation. A structured 3D mesh with a 5 mm element size was generated using hexahedral (brick) elements, as shown in Fig. 3. The use of hexahedral elements provided superior accuracy in capturing interlaminar stresses and modeling adhesive layers, which are critical for evaluating the performance of repaired panels. The layup configuration was defined layer-by-layer to reflect the actual stacking sequence and material properties. Mesh quality parameters such as orthogonality, skewness, and aspect ratio were closely monitored to maintain numerical stability and result fidelity. A mesh convergence study was conducted to determine the optimal mesh density. The selected 5 mm mesh size ensured a balance between computational efficiency and accuracy, with further refinement resulting in less than a 2% change in key stress and strain outputs. This level of convergence confirmed that the mesh was sufficiently fine for reliable simulation results while minimizing computational cost. The structured mesh also ensured compatibility with the ANSYS ACP layup model and contributed to reduced numerical errors, particularly in the adhesive bond regions.

Fig. 3. FE model of three different composite (CFRP) panel.

3.4 Loads and Boundary Condition The boundary conditions for the composite panels were set to simulate a standard tensile test and fully constrained edges. The composite panel was modelled in Space-claim, with one end fixed to constrain all six degrees of freedom (U, V, W, Rx, Ry, Rz = 0) while a uniformly distributed tensile load was applied to the other end from the Table 4. Fig. 4 illustrates these conditions. The same boundary conditions were applied to the damaged and repaired panels. Linear static analysis was performed on all panels to estimate failure loads and determine their strengths.