Compression perpendicular-to-grain behaviour of wood

The major objective of this study was to provide a better understanding on compression perpendicular-to-grain behaviour of wood and to examine the capability of mechanical models for predicting this behaviour. A new test procedure for testing wood behaviour in compression was developed which enabled the collection of load-deformation data and recording of cellular structure images in real time. To understand the relation between cellular structure deformation and load-deformation behaviour of wood in transverse compression, specimens of four species which have different structure (two softwoods: white spruce and jack pine, two hardwoods: white ash and aspen) were subjected to radial compression. Results showed that, in softwoods under radial compression, bending of cell walls occur. Bending deformation was distributed non-uniformly in a growth ring. The largest deformation occurred in the weakest zone in earlywood which led to the first collapse in this zone. Collapse then developed toward latewood. At the load level used in this study, latewood layer did not collapse. Corresponding images recorded in real time and stress-strain curve showed that the plateau region is a reflection of cell lumen removal in earlywood and the upward part at the end of stress-strain curve is related to elastic behaviour of latewood layer. In hardwoods, the mechanisms were different. In ring porous white ash, first collapse occurred in large vessels located in earlywood, but in diffuse porous aspen, first collapse occurred in vessels aligned near each other. In tangential compression of softwood, it was found that the mechanism of deformation in tangential compression was dominated by buckling of latewood layers.; It was concluded that different mechanical models should be developed for wood behaviour in tangential compression based on the mechanism of latewood layer buckling. In this study the applicability of some of the mechanical models developed by Gibson and Ashby (1982) were successfully examined for softwood behaviour in radial compression. Cell wall properties (cell wall modulus and yield point) of a fast grown white spruce were calculated using these mechanical models. Gross properties of a slow grown white spruce and jack pine were predicted using calculated cell wall properties and cell dimensions of these species. To predict entire stress-strain in radial compression, a parameter called compression factor (CF) was introduced in this study. CF relates the gross strain to the number of collapsed cells and hence the location of collapse in growth ring. The plateau region in stress-strain curve was predicted using the CF parameter and last upward part of stress-strain curve was predicted using calculated latewood modulus. Predicted stress-strain curve was verified by experimental results. The effects of temperature on cell wall properties were found through experiments. Empirical models were developed to describe these effects. The empirical models were successfully incorporated in the mechanical models. The extended models can potentially be used to generate stress-strain curves of softwoods at any given temperature within the range studied in this project based on cellular structure geometry and dimensions.