Study the effects of Thermal Modification on the energy absorption of Western Hemlock wood

Abstract

Wood is increasingly being considered for structural applications because of its sustainability, availability, cost effectiveness, and favorable mechanical performance relative to its weight.Thermal modification is commonly used to improve the durability, dimensional stability and moisture resistance of wood; however, its effect on the response of wood under impact loading is not yet fully understood. This is particularly important for applications involving dynamic loading, such as timber-based military shelter systems. This study investigates the low-velocity impact behavior of unmodified and thermally modified Western Hemlock, with particular emphasis on the influence of modification temperature and natural wood topology on energy absorption and failure behavior. Specimens were tested under three-point bending using an Instron 9250HV drop-weight impact system. The resulting force–displacement response was used to evaluate energy absorption, while high-speed imaging and Digital Image Correlation (DIC) were used to examine thedevelopment of strain and damage during impact. Post-impact fracture surfaces were further characterized using Scanning Electron Microscopy (SEM), and thermal analyses using Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) were performed to investigate changes associated with thermal modification. Three dominant failure mechanisms were identified: tensile fiber failure, inclined intergranular failure associated with local deviations in fiber orientation, and grain shear slip involving interfacial delamination along the wood structure. The occurrence of these mechanisms was observed to be influenced by the local fiber and growth-ring orientations relative to the impact direction, demonstrating that the natural topology of wood plays an important role in governing its impact response. The failure mechanisms also exhibited distinctly different energy absorption capacities, with inclined inter-granular failure generally dissipating the least energy, followed by tensile fiber failure, while grain shear slip resulted in the greatest energy dissipation. Within a given failure mechanism, specimens modified near 180 ◦C exhibited the highest energy absorption, followed by a reduction in energy absorption at higher modification temperatures. Thermal analysis further indicated evidence of limited degradation of the fundamental molecular components of wood over the investigated modification range, while temperature-dependent changes in molecular interactions provided additional insight into the observed mechanical response. Overall, the results demonstrate that the low-velocity impact behavior of thermally modified Western Hemlock cannot be described by modification temperature alone. Both thermal modification and the underlying wood topology must be considered when evaluating failure and energy absorption. These findings provide the mechanical behavior understanding of the impact response of thermally modified Western Hemlock and highlight the importance of controlling material topology and modification conditions when designing wood-based structures for applications subjected to dynamic loading.