Residual strength and time to failure of bonded wooden lap joints at high temperatures and superimposed constant loading

Abstract

ABSTRACT The study focused on the effect of high temperatures on wooden bond lines of single lap tensile shear specimens according to EN 302-1 made from beech wood at superimposed sustained mechanical loading. The quasi-duration of load (DOL) tests encompassed temperature levels of 160–232°C with specimens loaded at 30% of their mean shear strength at ambient temperature for 40 min. The investigations comprised one phenol resorcinol formaldehyde (PRF), two melamine urea formaldehyde (MUF) and three one-component polyurethane (1C-PUR) adhesives as well as solid wood reference specimens. Complementary ramp load tests were performed with specimens heated in unloaded state up to 250°C. The investigations revealed three highly differentiated adhesive DOL behaviours although showing a well comparable pure temperature-bound strength reduction effect at 200°C. PRF and one MUF showed little to no additional influence of applied load, while for two 1C-PUR brands a drastic impact on survival times and residual strength was revealed. One MUF and a special 1C-PUR forwarded results between both extremes. The investigations provide evidence that the assessment of the temperature behaviour of structural adhesives can be biased when relying exclusively on ramp load tests with specimens heated in unloaded state, and thus necessitates a superimposed load as present in realistic fire scenarios.