Low-velocity impacts (LVI) on composite laminates pose significant safety issues since they are able to generate extended damage within the structure, mostly delaminations and matrix cracking, while being hardly detectable in visual inspections. The role of LVI tests at the coupon level is to evaluate quantities that can be useful both in the design process, such as the delamination threshold load, and in dealing with safety issues, that is correlating the internal damage with the indentation depth. This paper aims at providing a benchmark of LVIs on quasi-isotropic carbon/epoxy laminates; 2 laminates are tested, 16 and 24 plies and a total of 8 impact energies have been selected ranging from very low energy impacts up to around 30 J. Delamination threshold loads, shape and extension of delaminations as well as post-impact 3D measurements of the impacted surface have been carried out in order to characterize the behavior of the considered material system in LVIs. The analysis of test results relevant to the lowest energies pointed out that large contact force fluctuations, typically associated to delamination onset, occurred but ultrasonic scans did not reveal any significant internal damage. Due to these unexpected results, such tests were further investigated through a detailed FE model. The results of this investigation highlights the detrimental effects of the dissipative mechanisms of the impactor. A combined numerical–experimental approach is thus proposed to evaluate the effective impact energies

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This study deals with the problem of the least-weight design of a composite multilayer plate subject to constraints of dierent nature (mechanical, geometrical and technological requirements). To face this problem, a multi-scale two-level (MS2L) design methodology is proposed. This approach aims at optimising simultaneously both geometrical and mechanical parameters of the laminate at each characteristic scale (mesoscopic and macroscopic ones). In this background, at the rst level (macroscopic scale) the goal is to nd the optimum value of geometrical and mechanical design variables minimising the structure mass and satisfying the set of imposed constraints (on rst buckling load, membrane stiness and feasibility constraints). The second-level problem (mesoscopic scale) aims at nding at least one stacking sequence meeting the geometrical and material parameters provided by the rst-level problem. The MS2L optimisation approach is based on the polar formalism to describe the macroscopic behaviour of the composite (in the framework of the equivalent single layer theories) and on a special genetic algorithm to perform optimisation calculations. The optimum solutions provided by the MS2L optimisation strategy are characterised by a weight saving of about 10% with enhanced mechanical properties when compared to conventional symmetric balanced stacks. The eectiveness of the optimum solutions is also proven through an experimental campaign of buckling tests. The experimental results are in excellent agreement with those foreseen by the numerical simulations.

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