Investigations into the photodegradation of wood using microtensile testing - Part 4: Tensile properties and fractography of weathered wood

This paper describes an investigation into the relationship between the changes in tensile properties and structural integrity of wood during weathering. Investigated species were Scots pine (heartwood and sapwood, and separately earlywood and latewood), and Norway spruce. The photo degradation was monitored through the changes in microtensile strength of wood during natural and artificial UV-weathering (thin strip method), and by subsequent FE SEM microscopical analysis of transverse-fracture surfaces of the strips. The tensile strength changes during exposure (initial increase and two following phases of decrease) were analysed and were shown to be consistent with fractographic evidence of the structural changes in wood. These include the breakdown of compound middle lamella (CML), the thinning of cell walls and the development of brittleness of the secondary walls. The mechanism of failure is essentially different in earlywood and latewood, and the microtensile strength is predominantly determined by the latewood portions within growth rings.