Effect of freezing and heating on the screw withdrawal capacity of Norway spruce and European larch wood

Abstract

Wood and wooden joints are exposed to fluctuating environmental condition during its use. Temperature is one of the most important environmental factors besides humidity that affects the behavior of wood and wooden joints in use. Though, seasoned wood is considered as a good thermal insulator, nevertheless, the performance of metal screws may get affected by the fluctuating temperature. This article deals with the effect of alternating lower (freezing) and higher (heating) temperatures on the screw holding capacity of Norway spruce (Picea abies (L.) H. Karst.) and European larch (Larix decidua Mill.) wood. Two thermal loading regimes, (freezing followed by heating) were carried out at temperatures −15 °C/70 °C and −25 °C/70 °C. Three types of wood screws such as partial double thread construction screw (A), full shank single thread universal screw (B) and two intertwined threaded construction screw (C) were used for the experiment. Withdrawal capacity was determined on radial and tangential surfaces. Freezing did not cause any significant change in the density and moisture content of wood. Although anatomical directions caused variations in the withdrawal parameter in both of the species, these variations were not statistically significant. Larch wood exhibited significantly higher (64–80%) screw withdrawal parameter as compared to spruce wood and the reason being the higher wood density of larch. Reference wood (without thermal loading) with universal (B) type screws exhibited the highest withdrawal parameters (23.7 N/mm2) followed by construction (C) types (22.6 N/mm2) and construction (A) types (19.7 N/mm2) when data for both the species are pooled together. The trend was similar for thermally loaded wood also. The average screw withdrawal parameter values of universal (B) and construction (C) type screws were comparable, while these values were significantly higher as compared to the value exhibited by construction (A). Thermal loading did not cause any detrimental effect on the screw withdrawal parameter; rather, it marginally improved the screw withdrawal parameters.