Fracture morphology of hickory ( Caryaenspeciesspp. . Juglandaceae) under single-blow impact loading)

Wood specimens of the true hickory group were tested for their shock resistance with the single blow impact method. From this test series 10 pieces with a uniform fracture mode, 'brash' and 'tough' , were selected for electron microscopy to examine the fracture mode of individual fibres in the tensile zone. 'Brash' specimens are characterized by well defined transverse or slightly oblique fracture lines across the entire cell wall, or a stepped-up course preferably within the secondary wall. Stepping-up mainly occurs along the interface between S1 and S2 layer or between primary wall and S1 layer. The smooth fracture of fibres in 'brash' specimens appears to be caused by compression-induced pre-slip planes which do not occur in 'tough' specimens. Individual fibres of 'tough' specimens mostly display a conspicuous and irregular zigzag course of the fracture line due to the tearing apart of fibrillar bundles . In addition, 'tough' specimens are characterised by the regular development of microcracks at a distance of up to several millimeters from the fracture plane. Such microcracks were not observed in 'brash' specimens. The described micro morphological differences between both fracture modes help to explain the considerable variation in shock resistance between specimens of similar density and gross wood structure.