Aircraft technologies and materials have been developing and improving drastically over the last hundred years. Over the last three decades, an interest in the use of composites for external structures has become prominent. For this to be possible, thorough research on the performance of composite materials, specifically the impact performance, have been carried out. For example, research of impact testing for pristine carbon-reinforced epoxy composites mentions matrix cracks, fibre fracture, and delamination as the failure modes that require monitoring. In addition, thorough testing has been carried out on composites repaired with an adhesive bond to observe the effects of conditioning on the adhesively bonded repair. The results suggest there are no major changes in the adhesive under the testing condition. By reviewing the impact testing on the pristine and repaired composite materials for aerostructures, this paper aims to illustrate the main findings and also explore the potential future work in this research scope.

In recent decades, the application of composite materials in aerostructures has significantly increased, with modern commercial aircraft progressively replacing aluminum alloys with composite components. This shift is exemplified by comparing the material compositions of the Boeing 777 and the Boeing 787 (Dreamliner). The Boeing 777 incorporates approximately 50% aluminum alloy and 12% composite materials, whereas the Dreamliner reverses this ratio, utilizing around 50% composites and 12% aluminum alloy. While metals remain advantageous due to their availability and ease of machining, composites offer greater potential for property tailoring to meet specific performance requirements. They also provide superior strength-to-weight ratios and enhanced resistance to corrosion and fatigue. To ensure the reliability of composites in aerospace applications, comprehensive testing under various loading conditions, particularly impact, is essential. Impacts were performed on quasi-isotropic (QIT) carbon-fiber reinforced epoxy panels with stainless steel, round-nosed and flat-ended impactors with rubber discs of 1-, 1.5- and 2 mm thickness, adhered to the flat-ended impactor to simulate the transition between hard and soft impact loading conditions. QIT composite panels were tested in this research employing similar lay-ups often being implemented in aircraft wings and other structures. The rubber discs were applied in the flat-ended impactor case but not for the round-nosed impactor due to the limited adhesion between the rubber and the rounded stainless-steel surface. Impact energies of 7.5, 15 and 30 J were investigated, and the performance of the panels was evaluated using force-time and force-displacement data alongside post-impact ultrasonic C-scan imaging to assess the damaged area. Damage was observed at all three energy values for the round-nosed impacts but only at the highest impact energy when using the flat-ended impactor, leading to the hardness study with adhered rubber discs being performed at 30 J. The most noticeable difference with the addition of rubber discs was the reduction in the damage in the plies nearest the top (impacted) surface. This suggests that the rubber reduces the severity of the impact, but increasing the thickness of the rubber from 1 to 2 mm does not notably increase this effect. Indentation clearly plays a significant role in promoting delamination at low-impact energies for the round-nosed impactors.