Improvements in current design approaches require further studies of the damage interaction effects of composite materials subjected to repeated out-of-plane concentrated loads. To that end, a combined simulation and experimental investigation on composite laminate under repeated indentations is reported. The repeated indentations consist of seven identical peak-force indentations that are separately applied to the centre of the laminate. The results show that delaminations grow in all seven indentations, which can be interpreted as a continuous degradation of the effective delamination growth threshold with each subsequent indentation. More specifically, the second indentation effective delamination growth threshold is 62.4 MPa, which is about 19 % lower compared to the first one (77.2 MPa). Subsequently, the delamination growth threshold degraded approximately linearly with indentation. This effective delamination growth threshold reduction can be associated with the occurrence and evolution of the crack-rich zone preceding the delamination front.

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To improve current design approaches for composite structures, it is required to further investigate the damage interaction effects of composite materials under multiple out-of-plane concentrated loads. It is first necessary to comprehend the dependence of the effective delamination threshold, which describes whether a delamination grows on pre-existing delamination damage in composite laminates. A combined experimental and numerical study is presented, in which two sequential out-of-plane quasi-static indentations are applied to fully clamped composite laminated panels, with changing distances between the two indentation locations changing. The results show that the second indentation delamination is more likely to propagate, particularly in the straight-line direction from the second indentation site to the first one, which can be interpreted as a decrease in the effective delamination threshold associated with microcracks ahead of the delamination front. The relevant percentage reduction is 37% and is independent of the imposed indentation load. As a crucial take-away, designers should be mindful that the damage interaction effects could result in greater damage than the sum of the individual cases.@en

Fibre-reinforced polymer composite laminates are used extensively nowadays in various applications due to their superior mechanical properties. However, a great weakness of composite laminates is their susceptibility to out-of-plane concentrated loading (which can be broadly divided into quasi-static indentation and impact) owing to their weak interlaminar strength. Delamination is a typical damage type for composite laminates under such loading conditions. The occurrence of delamination in composite laminates could significantly degrade their load-carrying capabilities, especially the compressive performance.

Principles of strength of materials and fracture mechanics can be adopted to apply specific delamination initiation and propagation prediction methods to composite laminates. It is known that fracture mechanics methods have advantages in addressing delamination growth problems. On the other hand, it is also shown in the literature that strength of materials methods are generally best suited for quasi-static delamination growth. This implies that the growth of low-velocity impact or quasi-static indentation delaminations in composite laminates could also be predicted with an appropriate strength of materials approach. It is therefore necessary to comprehensively assess the ability of strength of material approaches to predict delamination of a composite laminate under out-of-plane concentrated loading. Since a stable stress field is the basic condition for the application of the strength of materials methods, the out-of-plane quasi-static indentation loading condition is first considered in this thesis.@en

Full impact damage tolerance assessment requires the ability to properly mimic the repeated impact response and damage behaviour of composite materials using quasi-static approximations. To this aim, this paper reports an experimental investigation evaluating two quasi-static methods for mimicking repeated impact response and damage behaviour of Carbon Fibre Reinforced Polymer (CFRP) composite laminates. In this study, an 8.45-J single impact was repeated 225 times and mimicked with 225 times 6.51-J quasi-static (energy equivalent) indentations and with 225 quasi-static (force equivalent) indentations following the recorded impact peak force variation. Results show that the loading rate and the inertial effect are the two major factors affecting the responses of the composite laminates under out-of-plane concentrated loading. Both the energy- and force-equivalent quasi-static indentations failed to reproduce the impact responses greatly associated with high loading rate and inertial effect. The force-equivalent quasi-static indentations were performed in a semi-automatic way and induced damage states more similar to those of the repeated impacts than those of the energy-equivalent quasi-static indentations, whereas the latter can be better automated and has better reproducibility compared to that of the repeated impact responses, as it is less dependent on high loading rate and inertial effect.

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To develop a simplified and physically sound delamination-failure criterion, this paper reports a joint experimental and numerical study in which out-of-plane quasi-static indentations with different indentation forces were applied to the centre of fully clamped rectangular carbon fibre-reinforced polymer (CFRP) laminates. The results show that two basic micro-delamination growth modes, which are separately dominated by opening and sliding microcracks, can be distinguished according to the fracture morphologies of the delaminations along the 0° fibre direction. Both are driven by the out-of-plane shear stress σ13, and the corresponding delamination failure threshold is the out-of-plane shear strength S13. Based on these results, a concise Hashin-type delamination-propagation criterion containing only a single stress component and single strength parameter is proposed, which, by incorporating it into the finite element model, the delamination growth along the fibre directions of multidirectional CFRP laminates can be precisely predicted; the prediction error is less than 10 %.@en

To further investigate the effects of in-plane and out-of-plane stresses on the delamination initiation for composite laminates under out-of-plane loading, this paper reports a joint experimental and numerical study, in which the fully clamped rectangular CFRP composite laminates were subjected to out-of-plane quasi-static indentations. The results show that the combination of the out-of-plane shear and in-plane tensile stresses together determined the initiation of delamination, whereas the influences of the out-of-plane compressive stress on the delamination initiation can be neglected. For the purpose of understanding the effects of deformations on the out-of-plane shear and compressive stress distributions, a concise analytical model was developed, which was validated against the numerical and experimental results. As a key take-away, this study reveals that the common impact tests at the geometric centre of the panel may not resemble sufficient similitude with stiffened panels where panel flexure is suppressed by various geometrical stiffening concepts.@en