FRACTURE MORPHOLOGY OF HICKORY (CARYA SPP, JUGLANDACEAE) UNDER SINGLE-BLOW IMPACT LOADING

Wood specimens of the true hickory group were tested for their shock r esistance with the single blow impact method. From this test series 10 pieces with a uniform fracture mode, 'brash' and 'tough', were select ed for electron microscopy to examine the fracture mode of individual fibres in the tensile zone. 'Brash' specimens are characterized by wel l defined transverse or slightly oblique fracture lines across the ent ire cell wall, or a stepped-up course preferably within the secondary wall. Stepping-up mainly occurs along the interface between S-1 and S- 2 layer or between primary wall and S-1 layer. The smooth fracture of fibres in 'brash' specimens appears to be caused by compression-induce d 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 fibril lar bundles. In addition, 'tough' specimens are characterised by the r egular development of microcracks at a distance of up to several milli meters from the fracture plane. Such microcracks were not observed in 'brash' specimens. The described micro morphological differences betwe en both fracture modes help to explain the considerable variation in s hock resistance between specimens of similar density and gross wood st ructure.