Mechanical behavior of hybrid 3D woven composites

Resumen

Unidirectional fiber-reinforced composites are widely used in the aerospace industry due to their excellent in-plane mechanical properties, high corrosion resistance, dimensional stability and fatigue life. Nevertheless, they exhibit poor delamination resistance and damage tolerance, particularly under impact. The lack of reinforcement in the through-thickness direction makes them particularly vulnerable to out-of-plane threats caused by foreign objects, such as ice slabs or open-rotor blade fragments impacting on skin fuselages. A cost-effective alternative is the use of 3D woven orthogonal reinforcements, in which delamination resistance and damage tolerance are improved by weaving a yarn in the through-thickness direction. This technique allows the combination of several fiber types (hybridization) and enables the optimization of the composite properties by varying the fiber content. Preforms may be infused by using out-of-autoclave processing techniques, such as Vacuum Assisted Resin Transfer Molding (VARTM), leading to considerable cost savings, as opposed to autoclave consolidation. Despite of the potential of these materials, the use of hybrid 3D woven composites is limited by the lack of experimental data and reliable models able to predict the mechanical response of the material. This work analyzes the mechanical behavior of a hybrid 3D woven orthogonal composite made up of a thermoset polymeric matrix (epoxy-vinylester) reinforced with carbon and glass fibers, as well as with polyethylene z-yarns in the through-thickness direction. The mechanical behavior of the material was studied under tension, compression and shear, as well as under high- and low-velocity impact. The mechanical behavior of the yarns, the notch-sensitivity of the composite and its residual strength after impact were also measured. The study includes an extensive inspection campaign carried out by means of X-ray computer tomography, optical and electron microscopy, as well as ultrasounds. These results provide a critical information about the failure micromechanisms involved in the damage process, which helps to explain the macroscopic properties of the composite. The influence of hybridization was also discussed under out-of-plane loading, such as drop-weight tests, ballistic impacts and short beam tests. To this end, the hybrid 3D composite was alternatively impacted on the carbon or the glass faces. Regarding the short beam tests, the influence of the z-yarns was also discussed in detail. A set of analytical models was also included to predict the notch-sensitivity in tension and compression, the delamination load threshold and the ballistic curve of the composite material. Finally, two mesoscale finite element models were formulated within the continuum damage mechanics framework to simulate the response of the material under high- and low-velocity impact. The first one shows a good correlation with experimental results, especially during low-velocity impact, whereas the second one is suited to predict the ballistic curve and the failure mechanisms during high-velocity impact. The latter is based on the combination of cohesive elements and a mesh superposition technique called embedded element.