Advanced placed ply laminate architectures for improved impact tolerance

The aviation industry accounts for approximately 2.1% of global carbon dioxide emissions. Through the use of increasing quantities of advanced composites it is possible to reduce aircraft weight, improving fuel efficiency and reducing greenhouse gas emissions. Currently, the further adoption of composite materials in structural applications is hampered by their susceptibility to damage from out-of-plane loading. Existing methods to address this shortcoming, e.g. 3D weaving, have undesirable effects on a composite’s in-plane strength and stiffness. Recently, an automated fiber placement based preforming method, known as Advanced Placed Ply (AP-PLY), has been developed, which harnesses the flexibility and precision of automated fiber placement machinery to produce laminates with a quasi-woven internal architecture. The through thickness fiber connectivity present in these laminates has been shown to improve their impact tolerance while having little to no effect on their undamaged in-plane strength and stiffness. This thesis presents a comprehensive study of the mechanical response of AP-PLY laminates. Experimental techniques and numerical models have been employed to determine the influence of the 3D reinforcements present in AP-PLY laminates on their quasi-static and dynamic behavior. Macroscopic mechanical testing (quasi-static and dynamic), optical and scanning electron microscopy, as well as X-ray tomography, were used to characterize the deformation and energy dissipation mechanisms of AP-PLY laminates at different length scales. The AP-PLY laminates exhibit outstanding toughness when subjected to dynamic loads, while their undamaged in-plane response is improved in comparison with conventional angle-ply laminates. It was found that in the quasi-static regime the tensile moduli of AP-PLY laminates were comparable to those of their conventional non AP-PLY counterparts – there was no reduction in laminate stiffness despite the presence of fiber crimp. The effect of the AP-PLY preforming process on laminate strength was found to be dependent on each laminate’s layup. Strain concentrations at through thickness undulations were observed using digital image correlation. Specimen failure was found to occur along planes aligned with these strain concentrations. The dynamic tensile response of AP-PLY laminates was studied using a split-Hopkinson bar. Due to the large size of the AP-PLY specimens, the experiments were conducted at the European Commission’s Joint Research Center in Italy, who operate the world’s largest split Hopkinson bar. At strain rates of 30s-1, the AP-PLY specimens proved to be approximately 10\% stiffer than their conventional equivalents, while strengths were similar for both configurations. The short duration of the dynamic experiments limited the realignment of undulating tows with the loading direction, reducing strain concentrations and mitigating the effect of the AP-PLY architecture on laminate performance. Finally, low velocity impact and compression after impact experiments were conducted to characterize the impact resistance and toughness of AP-PLY composites. Cross-ply, triaxial, and quasi-isotropic AP-PLY laminates were subjected to 30J and 50J impacts, and subsequently loaded in compression to evaluate the effect of the impact induced damage on their strength. While the high energy impacts resulted in significantly larger delamination footprints, the corresponding reductions in residual compressive strength were marginal. This was attributed to formation of stable sub laminates in the AP-PLY laminates as a consequence of their through thickness fiber connectivity. In addition to the experimental characterization of AP-PLY composites, a multiscale numerical modelling framework was developed to efficiently and accurately capture the behavior of AP-PLY laminates. The multiscale framework divided each AP-PLY laminate into simple prismatic regions consisting of varying quantities of fibers and resin. The constituents of each region were rotated in-plane and out-of-plane to account for the orientations of the fibers at through thickness undulations. The use of a multiscale framework to represent through thickness fiber undulations allowed the complex damage mechanisms occurring in AP-PLY composites to be captured using coarse meshes. A continuum damage mechanics approach was used to capture the initiation and propagation of intralaminar damage, and a cohesive zone model was implemented to account for delamination at ply interfaces. The multiscale framework was found to be in good agreement with the experimental results, correctly predicting laminate stiffness, strength, and strain-to-failure in both the quasi-static and dynamic regime. The strain concentrations at tow undulations were well captured by the numerical modeling approach, and there was good agreement between the experimentally observed and numerically predicted specimen failure modes. Furthermore, the numerical models were able to provide additional insights into the deformation mechanisms occurring within AP-PLY laminates, including the formation and propagation of fiber and matrix cracks, as well as delamination. Lastly, the model facilitated the investigation of AP-PLY design parameters – e.g. the number of tows width gaps between tows placed in a single pass – which would have been prohibitively expensive and time consuming to study experimentally.