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Experimental off-axis tensile tests aimed at characterising the in-plane shear behaviour of biaxial non-crimp fabric (NCF) composite laminates obtained from different directions within the same panel (warp and weft) reveal noteworthy variations. The in-plane shear modulus, shear strength, and shear strain at failure are significantly higher in the weft direction compared to the warp direction. To gain insights into the underlying reasons for these discrepancies, a parametric study is performed. This numerical study utilises mesoscopic 3D finite element (FE) models, representing the unit cell of a [+45,-45]2S NCF laminate. The analysis indicates that neither the presence of stitching yarns nor the out-of-plane fibre crimp induced by the yarns account for the substantial differences observed in the experimental findings. The maximum initial tangent in-plane shear modulus (Gxy), due to out-of-plane fibre crimp is only 2%. However, this difference increases to approximately 6.1% when stitching yarns are also included in the FE model. Moreover, it is found that fibre volume fraction of the tows and the non-linear behaviour negligibly impact the in-plane shear performance of the NCF laminate in both directions. In contrast, the study reveals that the primary factor contributing to the differences between the warp and weft directions is related to misalignment between the nominal 45° tows within the NCF composite laminate panels. The misalignment observed in the experimentally tested specimens is approximately 6°, leading to an 8.5% decrease in the Gxy of SP laminate compared to ST laminate.

Composite sandwich structures have gained significant attention in engineering applications due to their exceptional strength-to-weight ratio, stiffness, and energy absorption capabilities. One promising core material is cork, which possesses desirable characteristics such as low density, high compressive strength, and good energy absorption capacity. This study aimed to compare the impact performance of composite shells with composite sandwich shells featuring an agglomerated cork core, while investigating the influence of semicylindrical shell diameters. The results underscore the significant impact of specimen diameter on the performance of these structures. Larger diameter specimens (106 mm) exhibited notably higher maximum force, albeit with lower maximum displacement, energy absorption, and contact time compared to those with smaller diameter (82 mm). Similar trends were observed in composite sandwich shells. Furthermore, the inclusion of cork in composite sandwich shells marginally reduced strength of the specimens. For identical diameters, both configurations exhibited comparable maximum force and displacement. However, composite sandwich shells with a cork core absorbed less energy than those without a cork core. This behaviour persisted across both shell geometries, with slightly higher percentage differences observed in the larger diameter specimens.

This study investigates the effect of damage interference resulting from impact loads on the mechanical response and impact fatigue life of E-glass/epoxy composite laminate shells. The analysis involves single impacts at different points to evaluate the impact strength's dependency on boundary conditions. Notably, there is a gradual reduction in both maximum impact force and absorbed energy beyond 10 mm from the centre, towards unconstrained edges, accompanied by an increase in displacement. Specifically, at a distance of 30 mm, the maximum force and absorbed energy register a decrease of 9.4% and 7.9%, respectively, while the displacement rises by 14.5%. The findings reveal that the damage severity decreases as the impact point is closer to the unconstrained edge of the specimen, which can be attributed to a stiffness reduction, which can reach up to 22%. Symmetrically induced pre-damages exhibit no discernible effect on the subsequent impact response. It is observed that beyond 10 mm from the shell's centre, the number of impacts required to reach puncture increases by 10.5% and 21.1% for distances of 20 and 30 mm, respectively. Furthermore, it is also found that alternating impacts between symmetrical points show no inclination towards a preferential point for puncture.

Based on numerical modelling, this study investigates asymmetric semicylindrical composite laminate shells' damage characteristics under low-velocity impact loads. For this purpose, several asymmetric stacking sequences were subjected to low-velocity impact and the results were analysed in terms of force, displacement, contact time, and absorbed energy. It is concluded that the maximum impact force decreases with an increase in the number of layers oriented at 0º, particularly in the upper half of the laminate. The laminates with a 45º orientation in the upper layers present the lowest displacement values, whereas the laminates with the upper layers oriented at 0º exhibit longer contact times. It is also observed that intralaminar damage is responsible for almost half of the total impact energy, followed by delaminations and friction. Stacking sequences with upper layers at 45º exhibit slightly higher energy dissipation due to intralaminar damage (fibre failure) and interlaminar damage (delamination).

The existing literature extensively explores the influence of stacking sequences on symmetric flat laminates subjected to low-velocity impacts. However, this is not true in the case of curved laminates. Therefore, the main goal of this study is to analyse the effect of stacking sequences on the impact response of semicylindrical woven composite shells. For this purpose laminates with [0]8 , [45]8 , [＋15,−15]2S , [＋22.5,−22.5] 2S, and [＋30,−30]2S , were considered in order to evaluate the impact response of layer orientations. Additionally, laminates with [0 2 , 45 2 ]S , [0,45]2S , [0,45 2 , 0]S , and [45 2, 0 2 ] S were employed to analyse the effects of changing the positioning of the layers in relation to the laminate mid-plane. The FE model was validated with the experimental results obtained for [0] laminates, where good numerical–experimental correlation was obtained. The findings suggest that an increase in impact bending stiffness (IBS) leads to a rise in maximum force, accompanied by a decrease in maximum displacement, contact time, and dissipated energy. The primary mechanism responsible for dissipating the majority of impact energy is intralaminar damage, followed by delaminations and friction. Importantly, changing the layer positions has a significant impact on how each layer within the semicylindrical quasi-isotropic composite shell distributes and dissipates energy during the impact event.