INFLUENCE OF WELDING TIME ON MECHANICAL FRICTION WELDING OF BEECH

The optimum welding time for welded beech specimens is assessed with a compression shear test method and the softening of the wood polymers during friction welding is evaluated by dynamic mechanical analysis (DMA). A welding time of 2-3 sec yields the highest mechanical performance (13.6-14.7 MPa) that is equivalent to the shear strength of glued-laminated beech. A shear strength reduction of up to 56% is observed when specimens are loaded perpendicular to the grain, compared with parallel-to-grain loading. An increase in welding time to 5 and 7 secs resulted in a sudden drop in shear strength capacity and smooth failure interfaces within the weld-line. This is attributed to expelled cellulose fibres and thermal degradation of cellulose with increasing temperature at longer friction welding times. The softening of beech wood to form the weld-line is postulated to occur at temperatures greater than 65ºC due to intermolecular debonding and depolymerisation of low molecular weight lignin. Drops in storage modulus via DMA were also observed at 125 and 175ºC attributed to softening of lignin and hemicellulose.