Stress-Strain Response of Wood Under Radial Compression. Part I. Test Method and Influences of Cellular Properties

Abstract

A new test system was developed for real-time microscopic observation of wood cell-wall deformation and stress-strain relationship under transverse compression. The system consists of a small compression device, a stereo-microscope, a video microscaler, a videocassette recorder and a computer-based data logger. The significance of this system is that it allows the influence of cellular structure of wood on its stress-strain behavior and the cell-wall collapse mechanism to be studied. This test system was used in a research program aimed at generating some basic understanding of microstructural behavior of wood under transverse compression. Tests were conducted on white spruce specimens to evaluate the proposed test procedure and system, and the influences of some microscopic and macroscopic features such as wood density, cell-wall thickness, and earlywood/latewood ratio. End-matched specimens were tested using the test system at three levels of magnification. Gross and individual ring behaviors were observed and measured by testing at a low magnification (12X). Earlywood and latewood behaviors were measured separately using a medium level of magnification (32X). Finally the mechanism of cell-wall collapse was observed using the highest magnification (160X). Test results show that earlywood and latewood have varying degrees of influence on the various segments of the gross stress-strain curve in radial compression. First collapse of cellular structure occurs at a location with minimum cell-wall thickness and density. Initiation of cell-wall collapse and its gradual progression are clearly visible using the apparatus, thereby verifying the capability of the proposed method.