Abstract: [Objective] The purpose of this study was to investigate the characteristics of wood shear fracture, fracture orientation and shear fracture fractal curves, and calculate the fractal dimension of the shear fracture; The shear strength of the test timbers were measured, using linear regression analysis, the relationship of transverse/longitudinal shear strength of test timbers and fractal dimension were also discussed. [Method] Five tree species distributed in Heilongjiang Province, those are: Abies holophylla, Populus alba, Pinus sylvestris var. mongolica, Betula platyphylla and Tilia amurensis were selected as testing materials. Shear strength testing specimens were produced according to the sizes requirements of raw wooden material during the production of fiberboard and GB/T 7909—1999 paper chips and the sawing-testing methods of GB/T 1929—2009 physical and mechanical wood samples. Fracture morphology of wood, fracture fractal curves were gathered and the forces required to break test timbers were measured by shearing fracture of the testing specimens on mechanical testing bench. [Result] When specimens were sheared in the transverse directions: A varying fracture surface was exhibited regardless of the specimen softened or not. Unsoften-softwoods were fractured with bigger curves, hardwood specimens were fractured with flatter curves and the fracture surface were more smooth; Soften-softwood were fractured with small curves while hardwood were fractured with largish curves with avulsion in some samples. When specimens were sheared in the longitudinal direction in softened conditions: they were fractured in rectilinear line in shear directions; No obvious differences in fracture characteristics between softwood and hardwood were found. The fractal curve of transverse sheering fractures were showed the shape of serration or zigzag, fractal curves of soften-specimens were more complicated than that of the unsoftened. The longitudinal shear fractal curves of soften-samples were less complicated than that of the transverse shear fractal curves, regardless of softened or not. The fractal dimensions on the shear fracture of the testing samples for five wood species were 2.047-2.133. The transverse shear strength of soften-samples was the lowest while the longitudinal shear strength of soften-samples was still enormously stronger than that of unsoften-samples.[Conclusion] The transverse shearing soften-samples were generally shown curved fracture while linear fractures emerge along fiber direction. When soften-samples were sheared in longitudinal direction, linear fracture was presented with flatter curves or fractured in a near-gradient straight line. The fractal curves of transverse and longitudinal fracture in softened situation were more complicated with the shape of serration or zigzag. The fracture structure had irregular self-similarity and it was more complicated for the soften-samples compared to the unsoften-samples. The longitudinal fractal curves of soften-samples were simpler than that of transverse fracture, no matter softened or not. No significant correlation was found between longitudinal shear strength of soften-samples and fracture fractal dimension. While it revealed a linear correlation between the transverse shear strength and the fractal dimensions. Transverse shear strength was the lowest for soften-samples. It was suggested to soften the timber before grinding dissociation chips with transverse shearing in order to minimize energy consumption.

Key words: timber, shearing strength, fractal dimension, fracture features