Investigation on anisotropic mechanical behavior of spruce subjected to impact compression loading

Wide plastic stress plateau region is a typical characteristic for wood large deformation compression. Its microstructure is made up of regular array polymeric constituent cells. The cellular cell array mode and its multicomponent lead to wood anisotropic mechanical properties. Medium strain rate 10⁰/s∼10²/s compression experiment of spruce is performed by high-speed loading equipment INSTRON. Loading is parallel to axial, radial, tangential and included angle direction respectively. Angles of 15°, 30°, 45°, 60°, 75° between loading direction and grain are taken into account in radial and tangential sections. Loading direction has effect on spruce failure modes. For axial loading condition, specimen middle part becomes bulgy, folding and fiber breaking. With include angle of loading and axial direction increasing, 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. Spatial yield surface is an elliptic cylinder surface for different strain rate loading condition. Yield surface radius increases with strain rate. Experimental and theory analysis results indicate spruce mechanical property is spatial anisotropic. Yield strength is relative with strain rate and loading direction. Loading direction mainly determines failure mode in compression condition. (authors)