The importance of wood relaxation when studying nail withdrawal capacity

Nails are among the most widely utilised fasteners in timber construction. Nail withdrawal capacity (NWC) has been extensively studied; however, the specific mechanisms by which moisture affects NWC remain unclear. In addition, the effects of wood relaxation on NWC in durability studies are often overlooked, introducing potential errors. This study assessed the impacts of wood moisture content (MC) and relaxation on NWC, with investigations on potential affecting mechanisms. The key innovation of this study lies in the mechanistic explanation of NWC changes using Coulomb’s Law of Friction, supported by microscopy-based characterisation of the wood–nail interface. Smooth-shank nails driven into radiata pine conditioned to 9%, 12%, and 18% MC were tested up to 28 days after driving. Affecting mechanisms were investigated using stress relaxation tests, confocal microscopy, and scanning electron microscopy (SEM). NWC was significantly reduced by 41% and 44% 28 days after driving at 9% and 12% MC (n = 30), respectively. In contrast, NWC increased by 11% at 18% MC. The time required to stabilise NWC in all conditions was 14 days. Stress relaxation tests, confocal microscopy, and SEM results suggest the NWC losses at 9% and 12% MC were caused by wood relaxation, while the increase at 18% MC might be attributed to softer wood fibres, suspected metal oxidation or corrosion, or trapped wood fibres on the nail surfaces. These findings not only clarify the mechanisms driving NWC changes but also highlight the importance of accounting for wood relaxation in experimental design and durability assessment. • Nail withdrawal capacity decreased by 41% and 44% after 28 days in wood conditioned at 9% and 12% moisture content, respectively. • A modest NWC increase (~ 10%) was observed at wood MC 18% after 28 days • Moisture-dependent changes in NWC were linked to wood relaxation and variations in friction coefficients at the wood–nail interface. • A minimum 14-day waiting period is recommended to stabilise NWC measurements in smooth-shank nails driven into radiata pine.