Dynamic crushing characteristics of spruce wood under large deformations
An extensive series of large deformation crushing tests with spruce wood specimens was conducted. Material orientation, lateral constraint and loading rate were varied. Regarding material orientation, a reduction in the softening effect and the general force level was observed with a higher fiber-load angle. A comparison with characteristics gained by application of Hankinson’s formula showed discrepancies in compression strength and the beginning of the hardening effect. Lateral constraint of the specimens caused a multiaxial stress state in the specimens, which was quantified with the applied measuring method. Further, a higher force level compared to specimens without lateral constraint and significant hardening effect at large deformations resulted. Thus, the influence of a multiaxial stress state on the force level could be determined. An increase in the loading rate led to higher force levels at any displacement value and material orientation.
A simple size effect model for tension perpendicular to the grain
The strength in tension perpendicular to the grain is known to decrease with an increase in the stressed volume. Usually this size effect is explained on a stochastic basis, that is, an explanation relying on the increased probability of encountering a strength reducing flaw when the volume of the material under stress is increased. This paper presents an experimental investigation on specimens with a well-defined structural orientation of the material. The experiments exhibit a large size effect and the nature of the failures encountered suggests that the size effect can be explained on a deterministic basis. Arguments for such a simple deterministic explanation of size effect is found in finite element modelling, using the orthotropic stiffness characteristics in the transverse plane of wood.
Stress-Strain Response of Wood Under Radial Compression. Part I. Test Method and Influences of Cellular Properties
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.