This study investigates the fatigue behavior of European ash (Fraxinus excelsior) with density range of 580-620 kg/m3 under relatively short-period cyclic compressive loading. The aim is to understand its mechanical behavior, such as: strain development, recovery, and internal damage mechanisms. Mechanical testings at varying frequencies (0.1, 1.0, and 10 Hz) were performed and the samples were scanned with micro-computed tomography (Micro-CT). Multi-scale assessment of fatigue effects was performed. Strain behavior revealed progressive increases in both viscous and viscoelastic components, demonstrating time- and rate-dependent deformation. For the same loading time, higher loading frequencies resulted in consistently lower accumulated strain; however, no significant differences in strain recovery were observed between frequency groups. Strength and stiffness showed minimal change after up to 2,000 loading cycles at 78 % stress level, highlighting relatively high fatigue resistance of wood under compression. Micro-CT imaging detected internal microcracks in samples pre-loaded with fatigue, inferring that cyclic loading induces more microstructural damage than static conditions. These findings enhance the understanding of the fatigue mechanism in European ash and highlight Micro-CT as a valuable non-destructive tool for internal damage assessment under fatigue loading. ... Read more

The strength degradation resulting from duration-of-load (DOL) effect and bacterial decay poses significant challenges to historical timber piles. Many historical European cities still heavily rely on the infrastructure supported by their original timber foundations. A reliable modelling approach on the structural performance of timber piles is needed to avoid the economic loss caused by closing down infrastructure. In this work, we consider a simplified bacterial decay model and develop a numerical framework to integrate the decay model into a standard DOL model. Two approaches are proposed and compared: one considering the homogenised effect of bacterial decay over the entire cross section, and the other taking into account the localised failure accelerated by bacterial decay and applying stiffness reduction to allow stress redistribution. Although the homogenised failure criterion is found to potentially underestimate the effect of bacterial decay, both approaches are able to capture the designated decay pattern. Ultimately, there is a potential for future extension to more intricate loading conditions and decay patterns. ... Read more

Fatigue failures pose significant challenges across various engineering disciplines. Wood, due to its low carbon emissions and high strength-to-weight ratio, has been gaining attention in engineering applications. The fatigue behavior of wood is complex due to its heterogeneous, anisotropic, and viscoelastic nature. This research explores essential insights into the fatigue behavior of wood, with a focus on S–N curves, stress–strain behavior, and failure mechanisms. Due to often varying failure criteria and test settings, direct comparison of S–N curves across different studies can be challenging and inconclusive. A closer look shows that wood in fatigue shows both irreversible and recoverable strain components that are delayed. However, there have been conflicting reports about residual stiffness changes under fatigue loading. Theoretical fatigue life models based on S–N curves or duration of load theory have shown limited applicability. Efforts to develop progressive damage model based on stress–strain behaviors have been challenging and largely unsuccessful due to the lack or inconsistency of data. Understanding the microstructural failure mechanism is crucial in order to build a more trustworthy fatigue modeling technique. Further work is suggested to monitor the microstructural deterioration during high-cycle fatigue loading. ... Read more