SEM STUDIES ON TENSION-FRACTURE SURFACES OF SPRUCE SAMPLES

The influence of temperature, wood moisture content, and duration of l oad on the ultrastructure of tension-fracture surfaces of bending-load ed spruce samples has been investigated by means of a high-resolution SEM with a field-emission cathode (FE-SEM). The main results are: The influence of the loading conditions - especially the load duration - o n the tension strength and deformation of the microscopic fracture-sur face of the cell tissue and the cell wall is clearly visible. Thus, br ittle fractures of the cell wall are significantly distinguishable fro m ductile fractures. Heat and moisture increase the ductile character of the fracture surface. Within the cell wall, delaminations between t he compound middle lamella and the secondary wall layer S1 or between S1 and S2, respectively, have frequently been observed. They result fr om transverse extension due to the longitudinal tension. Other weak el ements of the tension-loaded wood structure are the ray cells and the fringe of the bordered pits. The FE-SEM micrographs of the fracture su rfaces provide new information on the fibril/matrix structure of the s pruce cell wall.