• cohesive zone model
• composite laminates
• damage initialization
• delaminations
• finite element simulations
• Low-velocity impacts
• non-linear shear

Composite materials are assuming a predominant role in structural applications, in particular in aeronautics where high specific strength and stiffness, advancements in manufacturing and assembly technologies and the possibility of adapting mechanical properties to structural requirements have shown how significant weight savings, lower maintenance costs and innovative design philosophies are already within reach.
In aeronautics, safety is a primary concern, thus detrimental conditions and events to which composites are particularly sensitive must be carefully taken into account. Among these, low-velocity impacts, due to a combination of difficulties in damage detection and presence of significant damage within the structure, can reduce strength and stiffness of the structure. To satisfy the structural requirements, safety factors are introduced to cope with detrimental effects and, as a consequence, the real structural potential of composites is not completely exploited.
This thesis deals with numerical simulations of low-velocity impacts for which, in particular, the mechanisms that generate the permanent indentation left after the impacts and the onset and propagation of delaminations are numerically investigated in the context of traditional composite laminates.
Intra-laminar and inter-laminar numerical damage models have been implemented into user-defined material routines to be used within the Finite Element software Abaqus in which meso-scale composite laminate models are created and analyzed.
An heuristic non-linear shear damage model has been developed, in the context of the Continuum Damage Mechanics, to characterize the out-of-plane shear lamina constitutive behavior which, once tuned on the considered material system, is able to reproduce the shape and depth of the dent caused by the impactor.
Due to the fact that the experimental characterization of the lamina out-of-plane shear behavior is quite challenging, sensitivity studies of the main parameters that define the constitutive law are performed.
Inter-laminar damage (delaminations), approached through the Cohesive Zone Model, is modeled via a bi-linear traction-separation constitutive law which characterizes pure and mixed-mode behavior of Abaqus cohesive elements.
Both the accuracy and the computational performances of the simulations results obtained for different combinations of cohesive parameters are investigated and compared with experimental references.
Since delaminations are, typically, the most common, extended and threatening type of damage in low-velocity impacts on composite laminates, procedures were developed to create damage scenarios (inter-laminar damage is here considered but also intra-laminar damage can be used) on composite structures in order to evaluate their residual mechanical properties. The damage to be initialized can be user-designed, extracted by other numerical simulations or based on real data obtained, for example, through non-destructive inspections.
An initialization technique is, then, used to investigate the mechanical response of damaged composite laminates in simulations of compression after impact tests. In these simulations, the delaminations scenario is captured from previously performed low-velocity impact simulations and injected into the new Finite Element model. Thanks to this procedure, the boundary conditions and the laminate discretization can be modified as well as the cohesive parameters for which a sensitivity analysis regarding their influence on the results is performed.
Eventually, an experimental low-velocity impact campaign on composite laminates followed by ultrasonic inspections, to evaluate the damage extension, is presented. In this campaign, impacts are performed at different energy levels on thick and thin multi-directional quasi-isotropic laminates. Unexpected results have been obtained for the smallest energy levels, in which damage is essentially absent or negligible. To investigate these results, Finite Element analyses, where a detailed impactor model is used, are carried out to evaluate the role played by internal dissipation mechanisms, related to the impactor assembly, in modifying the expected force and displacement time histories. In fact, only by suitably tuning the impactor model, impact simulations to reproduce internal damage can be correctly performed.
This thesis, contributes in different ways to enhance the progressive failure analysis of composite laminates in low-velocity impact events. The permanent indentation prediction, the comprehension of the relationship between accuracy and computational costs associated to the reproduction of delaminations and the numerical capability of design damage scenarios on larger composite structures can increase the confidence in structural applications of composite materials and, at the same time, reduce the costs associated with experiments if a smart synergy bewteen numerical analysis and experimental activities is devised.