An experimental investigation of tensile residual strength of repaired composite laminates after low velocity impact

Low Velocity Impact (LVI) of Carbon Fibre Reinforced Polymer (CFRP) components are common during the manufacture and in-service life of aerostructures. However, the resulting effect on structural mechanical properties has not been extensively studied when the damage occurs on repaired components, which are already compromised to some degree. This research considers the relative mechanical performance of CFRP parts that are (1) pristine, (2) repaired using the stepped scarf method, and (3) repaired then impact damaged (low velocity impact at 5 J energy). For each case, subsurface damage was visually investigated and detailed measurements taken including impact load-time/energy-time behaviour, and post-impact tensile maximum force, stress and strain (and homogenised Young's modulus) before fracture. Further, three different repair patch sizes were considered. The experiments show that negligible surface impact damage was present in all repair cases but that delamination and debonding occurred for all but large repairs. For small and medium repair patches, 71% and 83% of the static strength of pristine samples were observed, with nearly full recovery for large repairs. After LVI, the strength of small repairs did not decrease whereas that of medium repairs declined to 77%. Interestingly, LVI did not have any impact on load bearing capability of large repairs. Stiffness measurements increased with repair size, even exceeding the value for pristine samples in some cases. The maximum stresses sustained were 67%, 77% and 85% that of a pristine sample for small, medium, and large repairs respectively.