Dian-sen Li

Bend properties and failure mechanism of a carbon/carbon composite with a 3D needle-punched preform at room and high temperatures

Abstract A 3D needle-punched C/C composite with a high density was fabricated and its bend properties were investigated at room and high temperatures. Macro-fracture and SEM micrographs were examined to understand the deformation and failure mechanism. Results show that the load-deflection curves below 400 °C exhibit a linear elastic and brittle fracture failure, while the curves at temperatures above 500 °C show an obvious tough and plastic failure. The bend strength and modulus decrease significantly with increasing temperature due to severe carbon oxidation. Below 500 °C, the main damage to the composite is in the form of matrix cracking, 90° fiber/matrix debonding, local twisting and fracture of the 0° fibers. Above 500 °C, the oxidation of the composite is significant and the interfacial adhesion between fibers and matrix is decreased significantly.

High-strain-rate compression behavior and failure mechanism of 3D MWK carbon/epoxy composites

The high-strain-rate compression experiments were performed on the three-dimensional multiaxial warp knitted (3D MWK) carbon/epoxy composites with different fiber architectures at room and elevated temperature using a split Hopkinson pressure bar apparatus. Macro-fracture and scanning electron microscope (SEM) micrographs were examined to understand the deformation and failure mechanism. The results show that 3D MWK carbon/epoxy composites have excellent high-strain-rate compression properties. The dynamic properties increase significantly with the strain rate and the composites show a high-strain-rate sensitivity. Meanwhile, composites with multidirectional symmetric fiber architecture have higher dynamic properties. Moreover, the composites show temperature sensitivity at high strain rates and the dynamic strength decreases significantly. The results also indicate the composites take on more serious damage and failure with the increase of strain rate. The failure of material with [0°/0°/0°/0°] behaves as binding fibers fracture in shear, 0° fibers tearing, overall expansion, and multiple 45° angle shear fracture, while the material with [0°/90°/+45°/−45°] is mainly interlaminar delaminating, and the local fibers tearing and shear on different fiber layers. In addition, with the increase of temperature, the composite shows less fracture and becomes more softened and plastic. The damages of matrix falling off, plasticity, and fiber/matrix interface debonding increase significantly.

Static and dynamic mechanical behavior of 3D integrated woven spacer composites with thickened face sheets

Abstract3D integrated woven spacer composites with thickened face sheets are fabricated successfully. Static compression and impact behavior are analyzed to evaluate the effect of thickened face sheets. The results show that thickened face sheet is an important influence parameter, warp compression and impact properties have been improved significantly than those of composite without thickened face sheets. Moreover, the damage and failure mechanism is significantly different. The main failure mode under flat compression is an abrupt rigid breakage of core fiber bundles. However, the thickened face sheets reduce the shear stress that transfer to core fibers. For warp compression, there is no face sheets fracture or dislocation, the local shear fracture occurs on the thickened face sheets. In regard to low-velocity impact, the strength of thickened face sheets dominates the failure, the damage is mainly manifested as the penetration of the top and bottom face sheets, matrix cracking, interface debonding, micro-buckling, as well as tearing and breakage of core fibers.

Temperature effects on the compression behavior and failure of 3-D MWK glass fabric-reinforced epoxy composites:

This article reports the temperature effects on the in-plane and out-of-plane compression behavior and failure of 3-D multiaxial warp-knitted glass fabric-reinforced epoxy composites. The damage and fracture morphology are observed from macroscopic and microscopic views, and the failure mechanism is demonstrated. The results show that the temperature has significant effect on in-plane and out-of-plane compression properties, the stress versus strain curves decline, and the properties decrease significantly with increasing the temperature. The temperature of 75°C is a key point, at which change in compression properties occurs, and at 150°C, the materials become plastic. Moreover, fiber architecture and loading modes are also important factors on compression properties of composites. The results also show that the damage and failure patterns vary with temperature, fiber architecture, and loading modes. Under in-plane compression, material A shows local 0° fiber layers delaminating and becomes softening and plasticity with increasing temperature. Material B shows delaminating between 0°, 90°, +45°, and −45° fiber layers along 45° angle and exhibits multiple delaminating at elevated temperatures. Under out-of-plane compression, material A shows multiple local shear fracture with 45° angle and experiences softening, roughness, and expansion at elevated temperatures. Material B exhibits shear brittle failure clearly, and delaminating dominates the main failure with increasing the temperature.