Influence of anatomical, physical, and mechanical properties of diffuse-porous hardwoods on moisture durability of bonded assemblies

Abstract

Studying wood adhesive bond durability is challenging because wood is highly variable and heterogeneous at all length scales. In this study, three North American diffuse-porous hardwoods (hard maple, soft maple, and basswood) and their adhesively bonded as-semblies were exposed to wet and dry cyclic tests. Then, their den-sity differences were related to bond durability. Physical and me-chanical properties (compression-shear) of the solid wood were measured along with mechanical properties of epoxy-bonded as-semblies. Specific gravity tests gave the expected result (hard maple > soft maple > basswood), but volumetric swelling–shrinkage values for basswood were as high as those for hard maple. Tangen-tial swelling–shrinkage values for basswood were approximately equal, suggesting that basswood can withstand swelling stresses with minute deformation. The compression-shear test showed that solid wood lost half its strength after vacuum-pressure soaking but recovered well after redrying. The epoxy-bonded assemblies also lost substantial strength after vacuum-pressure soaking, but soft maple and basswood assemblies completely recovered their strength after redrying. The goal of this study was to further the understanding of how density affects wood adhesive bond durabil-ity under a single wet–dry cycle in which swelling and shrinkage stresses are greatest. This study found that epoxy bond durability begins to deteriorate from the swelling–shrinkage stress in the range of densities between that of hard maple and soft maple. For species that have low density and high volumetric swelling–shrink-age (such as basswood), density is a controlling factor for moisture durability of bonded assemblies.

Citation

Yelle, Daniel J.; Stirgus, Ashley M. 2016. Influence of anatomical, physical, and mechanical properties of diffuse-porous hardwoods on moisture durability of bonded assemblies. FPL-GTR-244. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 8 p.