Research Projects

Aircraft composite components, like the skin of the wing, experience cyclic combined loads (bending and twisting). Further, discontinuities (cut-outs and notches) in these structural components act as stress raisers, resulting in sub-critical damage and loss of structural strength. A damage-tolerant design of these structures demands a detailed understanding of damage evolution and failure mechanisms under these multi-axial conditions. Extensive research is available on Carbon Fiber Reinforced Polymer (CFRP) composites under uniaxial fatigue loading. Uniaxial tension fatigue typically shows early matrix cracking (off-axis/90º plies), followed by splitting, delamination, and fiber fracture. Fibers often rotate towards the load, stiffening the material and offsetting modulus loss. Conversely, multi-axial tension-shear fatigue introduces micro-buckling in ±45º plies alongside other damage modes, leading to sudden stiffness drops and early failure. This illustrates that uniaxial data alone is insufficient, as even small shear components drastically alter dominant damage mechanisms and fatigue life. However, studies on multi-axial tension-shear fatigue are limited, and significantly, no experimental studies address compression-shear fatigue. The latter observation is more significant considering that CFRP laminates have lower compressive strength (~2/3 of tensile) and lower shear strength, significantly impacting fatigue life and damage mechanisms under combined loading. This project aims to address this gap in understanding the compression-shear fatigue behavior of CFRP laminates. The proposed research on developing an integrated experimental-numerical framework is further essential considering that existing experimental methods for multi-axial studies have limitations. For instance, tubular specimens require specialized equipment, and flat laminate biaxial testing is typically confined to tension-tension or tension-compression, resulting in incomplete failure envelopes. To overcome these shortcomings, this study employs a novel Combined Compression-Shear Modified Arcan (CCSMA) fixture to apply fatigue compression-shear loads to the specimens. Employing this experimental setup, it is proposed to explore the factors influencing the fatigue life of CFRP laminates, like layup, hole geometry, material, environment, loading frequency, and amplitude (R-ratio). These tests will be conducted with in-situ monitoring of damage initiation and propagation using advanced non-destructive evaluation techniques, digital image correlation (DIC), and acoustic emission (AE), to characterize the various damage modes realized in CFRP laminates. Additionally, high-fidelity numerical models capable of predicting the fatigue damage initiation and propagation are also proposed to be developed. Phenomenological models often fail for multi-axial compression-shear due to dramatic shifts in damage mechanisms. Therefore, this project proposes to use progressive damage modeling using LaRC05 initiation criteria, which are best suited for modeling complex damage mechanisms realized in multiaxial loading, and appropriate evolution laws to simulate damage progression accurately. Further, the complex stress states and damage mechanisms found near stress concentrations, like an open hole in CFRP laminates, can only be accurately captured using these advanced damage criteria. Finally, these numerical models are proposed to be validated with the experimental results developed in this project. This integrated framework will help advance our understanding of fatigue behavior and damage mechanisms in the CFRP laminates, especially when subject to multi-axial loads and stress raisers like open holes. Further, the results of the experimental studies can serve as a benchmark in the literature, and the numerical framework for fatigue life prediction can be used for developing damage-tolerant designs of aerospace-grade CFRP laminates.