Finite element analysis of failure at the aluminium-carbon fiber composite

The aluminium alloy is used for fabricating structural components in aeroplanes. The development of cracks and their propagation is a cause of concern because it may lead to failure of the structure if the crack undergoes an uncontrolled growth. The composite patching has been successfully used for repairing cracks in secondary load carrying structures of aeroplanes. A composite patch when bonded to the cracked region increases the strength of the structure and prevents the growth of the crack front. But the adhered patch separates at the bonding surface because of external loads. Mechanism by which this separation occurs is still not well understood and an investigation of the same is carried out in this research. The experiments were performed by subjecting the patched specimen to external tensile loads. But the experiments only give the load at which the specimen fails. For understanding the separation process, numerical simulation was performed in Ansys program by employing finite element method (FEM) and cohesive zone model (CZM). Instantaneous values of the loads, separation distance and the separation areas were obtained at different stages of loading and they were compared with their critical values. The composite patch begins to separate at low loads because of simultaneously acting normal (peel) and perpendicular (shear) stresses. Speed at which separation occurs increases once the yield stress is exceeded and at high values of external loads, the patch separates completely from the substrate.