Punch Shear Properties And Failure Mechanism Of 3-D Braided Composites

Three-dimensional (3-D) braided composites have attracted significant attention in aerospace, industry and sports applications due to their excellent mechanical properties.3-D braided composites shows mechanical advantage along their thickness direction because of the integrality structure and high delamination resistance, as well as high specific strength, high damage tolerance, and impact resistance. This paper aims to obtain the punch shear properties and failure mechanism of 3-D braided composites through experimental and finite element analysis, which could provide significant knowledge for their structural design and applications.The dominant research of this work is:(1) The punch shear properties of 3-D braided composite are investigated under quasi-static and high strain rate loadings by Instron 810.23 and split Hopkinson pressure bar (SHPB) testers. By comparing the mechanical response and failure morphology under various strain rate loadings, the failure mechanism and energy absorption capacity of 3-D braided composites are obtained. In addition, the influences of braiding angle and specimen thickness on their mechanical response are also analyzed.(2) Based on the real braided structural and geometry features of 3-D braided composites, a full size meso-structure finite element model (FEM) is established, which simulates their punch shear response and progressive failure process.(3) To study the internal damage characteristic and its distribution of 3-D braided composite under high strain rate by X-ray micro-computed tomography (micro-CT). The punch shear failure mechanism and validation of FEM are further investigated.The main conclusions are:(1) The punch shear behaviors of 3-D braided composite shows significant strain-rate sensitivity. The strain-stress response of 3-D braided composite presents elastic-plastic behavior at quasi-static loading. While the plastic behavior is not obvious under high strain rates due to the harden effects of the composites. The failure modes of 3-D braided composite are complex in which the dominant failure mode is transited from resin cracking into fiber fracture. Furthermore, the failure morphology becomes more severe at higher strain rate loading.(2) The braiding angle and specimen thickness show significant effects on the punch shear properties of 3-D braided composites. The punch shear behaviors of 3-D braided composite with braiding angles of 25°, 35° and 45° are studied. The selected thicknesses are 3 mm,5 mm and 8 mm, respectively. The failure stress and shear modulus of 3-D braided composites increase with increase in braiding angle. The enhancement in punch shear performance of 3-D braided composites is attributed to more compact braid structure with larger braiding angle, which in return improves the punch shear resistance. Furthermore, the punch shear performance is improved with increase in specimen thickness. The higher specimen thickness contributes to higher volume fracture of inner unit cell of 3-D braided composite. As a result, the punch shear resistance of composites is enhanced. Besides, the failure morphology shows great variations between 3-D braided composites with different braiding angle and specimen thickness.(3) The punch shear response and progressive failure process of 3-D braided composite under various strain rates loading are obtained by finite element analysis (FEA). The stress of surface unit cells and corner unit cells in braided preform is high and cannot be ignored. The failure of 3-D braided composite was determined by the tensile stress on shear back surface and compression stress on shear front surface. When braid parameters are constant, the stress propagates more rapidly along the thickness direction with increasing strain rate, which results in a smooth shear fractured surface. When strain rate is constant, the propagation resistance of stress wave along the thickness direction is greater with increasing specimen thickness, while the stress wave tends to spread into yarn direction with increasing braiding angle. Furthermore, the braiding yarn in 3-D braided composite with thickness of 5 mm exhibits more bending points than composite with thickness of 3 mm or 8 mm, which leads to a tough and stripped fracture surface. Moreover, the propagated resistance of stress wave along yarn direction is greater with increasing braiding angle. And the damages concentrate on the composite surface. The typical curve damage distribution is showed on the shear back surface of composite with braiding angle of 35°. While a "three straight line" damage type is exhibited for composite with braiding angle of 45°. Furthermore, it shows a low stress zone between two shear bands which reduces with increasing loading time. The low stress zone of composite with braiding angle of 25° exhibits a "circle" type. While a "cross" type is showed for both composites with braiding angle of 35° and 45°(4) Internal damage characteristics and distribution of 3-D braided composite under punch shear loading are inspected through micro-CT examinations. The FEA results and micro-CT images show good agreement which validates the FEA model. During punch shear loading, the dominated failure modes of 3-D braided composite are resin cracking, debonding, delamination and fiber breakage. The internal damages concentrate on the middle thickness region of composite at strain rate of 1500 s"1. The resin cracking and pulled-out fiber breakage increased with increase in strain rate. Only resin cracking occurred on the surface of 3-D braided composite with braiding angle of 35° and no damage is observed in the interior. While the composite with smaller specimen thickness shows more brittle fracture mode in which severe delamination is observed.(5) Further investigation of the energy absorption features and mechanism of 3-D braided composite is completed under punch shear loading through FEM and micro-CT analysis. The energy absorption capacity of 3-D braided composites is closely related to their failure characteristics. The 3-D braided composite with severe failure morphology could absorb more energy, which mainly resulted from damage energy absorption and plastic energy absorption. According to progressive failure process of 3-D braided composite, the energy absorbing process could be divided into three stages:linear absorbing stage, first plastic absorbing stage (before reaching failure stress) and second plastic absorbing stage (after reaching failure stress).