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.