Development of tridimensional carbon fiber/epoxy composites reinforced through the thickness and the mechanical characterization of interlaminar fracture toughness and vibration properties

Abstract

This study is focused on the assessment of the interlaminar fracture toughness in mode II and vibration mechanical properties of composites reinforced through the thickness with rectangular z-pinned manufactured by VARTM (Vacuum-assisted resin transfer molding) process. The influence of z-pinning in the mechanical properties of laminated structures is carried out and for the specimens with different z-pins sizes and pin areal densities are manufactured after Design of Experiment (DOE) matrix determination. For the composites fabricated without a polymeric mold, vibration properties z-pins reinforced composites demonstrated that the size and density of insertion of z-pins has a direct influence on the natural frequency of vibration, on the damping, or loss, and the amplitude of vibration. With the optimization made by the method of response surface (MSR), in a mono-objective analysis, it was demonstrated that it is possible to obtaining reductions in the maximum amplitude of forced vibration of 115%, and in an analysis multi objective has been shown that with a given insertion density and size of z-pins 81% reductions in maximum forced vibration amplitude and increases of 25% and 11% can be achieved damping factor and natural frequency of vibration, respectively. For the composites manufactured with polymeric mold, the fracture toughness in mode II was investigated and the results showed that pinning in composites improved the fracture resistance for all pinning proposals built. For the NPC (Non-precracked) step, the highest (GIIc)value achieved was for a 0.50 mm with a 2% pin density insertion, being 106% higher than the unpinned specimen. For the PC (Precracked) step, the thicker pins 1.00 mm and 1.10 mm acted again as a positive influence to mitigate the delamination and achieved elevated values of (GIIc), 77.5% and 78.3% higher than the unpinned specimen, respectively. The statistical results pointed that for the NPC case, the increase in density of pins always generates an increase in the fracture toughness and the contribution of the pin size to increase the fracture toughness. From there, increasing the size of the pin has little influence in NPC. For PC case, was shown that the pin size increasing decreases the fracture resistance, except for low pin density. Furthermore, the Artificial Neural Networks (ANN) trained with part of these experimental data showed excellent predictive capacity of fracture toughness. The modal responses of the laminates fabricated with a polymeric mold the experimental results indicated that, in most cases, there was an increase in the natural frequency and highlights the reduction, from approx. 60% to 70%, in the amplitude of vibration for all specimens with z-pin reinforcement in comparison to the unpinned. Furthermore, the experimental data compared the statistical results pointed that z pins had a positive influence increasing and decreasing natural frequency and forced vibration amplitude, respectively, of z-pinned composites compared to the non-reinforced and the trained ANN with the experimental data presented a very good agreement with experimental tests carried out in this investigation for predicting modal response.