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Honeycomb sandwich structures (HSSs) with carbon fibre reinforced polymer (CFRP) composite face sheets are extensively used as light-weight structures in aerospace engineering due to their high strength-to-weight ratio and energy absorption properties. However, the composite face sheets are highly vulnerable to impact loads and cause damage to the structure based on the impact energy. This study investigates the structural response of HSS with CFRP face sheets under quasi-static indentation and a wide range of impact energy: low to high velocity impact. A finite-element model was developed and numerical simulations were carried out at various impact energies, thereby providing deeper insights into the impact dynamics and understanding various damage states such as dent, front face sheet perforation, core damage and rear face sheet penetration. The numerical simulation result was compared with the experimentally tested HSS using a single-stage gas gun under 53.6 J to validate the finite-element model in terms of deformation and damage status. A quasi-static indentation test was conducted and numerically predicted force data under impact test for the complete perforation case was compared to address the dependency of rate of loading. The carbon nanotube (CNT) with various weight percentages (wt%), such as 0.2, 0.4 and 0.6, was added to the matrix system through a vacuum assisted resin transfer technique and experiments were conducted at 79, 107 and 135 J. The impact resistance increases with CNT addition and hence no perforation was recorded for all the test cases of 0.6 wt% CNT addition. The influence of CNT addition on the damage area is more on the bottom face sheet and a 57% reduction in damage area was recorded for the case of 0.6 wt% CNT addition at 135 J impact energy when compared to the neat carbon/epoxy composite.

Aerospace structures are highly vulnerable to impact loads whose damage tolerance, and its resistance vary over the range of impact velocity. Honeycomb sandwich structures are used in aerospace industries where mass efficient and impact resistant structures are needed. However, the structural integrity of these structures is reduced by impact load due to tool drop, runway debris, hailstones and improper handling of the structure. A thorough investigation of the damage behaviour of honeycomb sandwich under lowvelocity impact and the post-impact residual strength determination is required to design a crashworthy lightweight structure. This paper presents the experimental evaluation of specific energy absorption using Charpy impact, residual compressive strength by compression after impact and damage evaluation of honeycomb sandwich structures having composite face sheets. Parametric studies on composites and honeycombs are carried out by varying the cell size, cell thickness, core height, impact velocity, thickness and orientation of lamina. Densely packed thick honeycombs provide higher fracture energy. Under transverse compressive loading, the honeycomb core undergoes cell wall buckling, crushing and densification. Load-displacement history under in-plane compression and compression after impact for different impact energies is observed. The present study contributes for the understanding how various parameters affect the characteristics of face sheet indentation and plastic buckling of honeycomb sandwich structures with composite face sheets, thereby providing useful guidelines for its potential applications in impact engineering.

Composite plays a significant role in the field of aerospace due to its excellent mechanical properties, nevertheless, they are highly susceptible to out-of-plane impact load. Fibre-reinforced composite fails effortlessly under impact load and absorb energy through damage mechanics rather than deformation. The present study investigates the damage behaviour of the CNT reinforced carbon fibre-epoxy composite under high velocity impact using single stage gas gun. Composite plates were fabricated with 0 to 0.6 weight percentage content of CNT as reinforcement using vacuum assisted resin transfer moulding. A series of impact test with various impact energy was carried out on carbon/epoxy composite plate to study the impact performance. From the experimentation it was observed that the 0.3 weight percentage CNT addition provides the optimum impact performance. Damage characterization was performed for various impact velocity based on the micro and macro scale damage area. Knowledge of the damage behaviour of CNT reinforced carbon fibreepoxy composite plate under high velocity impact loads is essential for both the product development and material selection in the aerospace application.

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