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  • Author: E. K. Tschegg x
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Summary

Tests under mode I and mode III loading were performed on side grooved Compact-Tension specimens of larch and beech under steady state crack propagation to study the damage and fracture behaviour and the influence of two fibre orientations. From the complete load-displacement diagram, all important damage and fracture mechanical values (stiffness/compliance, microstructural damage, crack initiation energy, specific fracture energy, etc.) have been determined. Crack initiation energy and specific fracture energy are approximately ten times higher for mode III loading than for mode I loading in both wood species. Crack initiation occurs in mode III under external mode III loading, crack propagation, however, takes place under mode I, owing to crack surface interference. The influence of fibre orientation on the (fracture) mechanical properties of beech and larch is different. This difference may be explained mainly by the high number of rays in beech.

Summary

Crack propagation in wood is strongly influenced by the microscopic structure of the material. The relationship between structure and function with regard to damage and fracture behaviour can only be understood with a sufficiently fine level of examination. An experimental approach to perform micro-wedge splitting tests on spruce and beech inside the chamber of an Environmental Scanning Electron Microscope and under atmospheric conditions is presented. The specimens are loaded in mode I in the TR crack propagation system. Based on the load-displacement diagram, the characteristic parameters of fracture energy, critical load and initial elasticity are determined. The load and displacement data for the in situ experiments are related to the obtained ESEM images and allow a discussion of the fracture process on the cellular level. Density was found to be an important factor for fracture mode and several crack arresting phenomena depending on the variation of elasticity could be identified.

Summary

In situ tensile tests parallel to the grain were carried out in an Environmental Scanning Electron Microscope (ESEM) chamber on Norway spruce (Picea abies [L.] Karst.) samples. The ESEM-mode combined with a cooling device allowed examination of the specimens at a moisture content of 12% with unsputtered surfaces. By recording load-displacement curves and observing crack propagation simultaneously, a detailed image of fracture progress and tissue interaction could be described. Since these experiments required a sufficient specimen size and geometry, focus was concentrated on the methodology.