The fracture toughness and properties of thermally modified beech and ash at different moisture contents

The fracture toughness of thermally modified beech (Fagus sylvatica L) and ash (Fraxinus excelsior L) wood under Mode I loading was quantified using Compact Tension (CT) specimens, loaded under steady-state crack propagation conditions. The influence of three heat-treatment levels and three moisture contents, as well as two crack propagation systems (RL and TL) was studied. Complete load-displacement records were analysed, and the initial slope, k (init), critical stress intensity factor, K (Ic), and specific fracture energy, G (f), evaluated. In the case of both species, thermal modification was found to be significantly affect the material behaviour; the more severe the thermal treatment, the lower the values of K (Ic) and G (f), with less difference being observed between the most severe treatments. Moisture content was also found to influence fracture toughness, but had a much less significant effect than the heat treatment.