中应变率加载下云杉各向异性力学行为研究

采用高速加载INSTRON设备对云杉开展100 s-1~102 s-1中应变率压缩实验,研究了材料沿顺纹、横纹径向、弦向、以及径(弦)切面内与顺纹呈15°、30°、45°、60°和75°夹角方向的力学性能。实验表明随着加载方向由顺纹向横纹径(弦)向变化,材料屈服强度逐渐减小,应力-应变曲线塑性流动段由"塑性软化"向"塑性硬化"转变;试件沿不同方向压缩屈服强度表现出较强应变率敏感性。冲击压缩下云杉宏观破坏模式与加载方向相关,沿顺纹方向加载时,试件中部向外膨胀,产生褶皱、纤维屈曲折断;当载荷方向与顺纹夹角逐渐增大时,材料失效模式体现为木材纤维分层滑移、撕裂。采用简化Hill强度理论对中低应变率下云杉空间屈服行为进行了理论描述,不同应变率下云杉空间屈服面为椭圆柱曲面,屈服曲面半径长度随应变率提高而增加。实验与理论分析表明,云杉沿空间不同方向具有各向异性力学特性,屈服强度受应变率和加载方向影响较大,而破坏模式则主要依赖于载荷方向。

Investigation on anisotropic behavior of spruce mechanical properties under medium strain rate loading conditions

Medium strain rate (100 s-1~102 s-1) compression experiments were performed on spruce using high-speed loading equipment (INSTRON). Loading was conducted in the axial, radial, tangential and included angle directions, respectively. Angles of 15°, 30°, 45°, 60°, 75° between loading direction and grain are taken into account in radial and tangential sections. The experimental results indicate spruce yield strength decreases when loading varies from axial to radial or tangential direction. The plastic phase of stress versus strain curve changes from ‘plastic softening’ to ‘plastic hardening’. Spruce compression yield strength in different directions is sensitive to strain rate. The loading direction has effects on the spruce failure modes. For axial loading conditions, the middle part of the specimen bulges, folds, and finally fiber breaks. When the angle between loading and axial direction increases, grain delamination and tears occur when loading is near to radial or tangential direction. Hill strength theory is adopted to simplified describe spruce spatial yield behavior. The spatial yield surface is an elliptic cylindrical surface for all strain rate loading conditions. Yield surface radius increases with strain rate. Experimental and theoretical analysis results indicate spruce mechanical properties are spatially anisotropic. Yield strength is sensitive to strain rate and loading direction. Loading direction is the primary determining factor of failure mode under compression.