Relationships Between Ultrasonic-Based Elastic Modulus Loss, Mass Loss and Strength Loss in Two Hardwoods Commonly Used in Northern Chinese Timber Heritage

Assessing decay-induced mechanical deterioration in hardwood components is essential for the conservation of northern Chinese timber heritage, where structural members such as the Dou and Gong have been exposed to complex environments for centuries. Within a unified experimental framework, this study systematically investigated the mechanical degradation behavior of two hardwood species commonly used in traditional timber buildings in northern China—elm (Ulmus pumila L.) and Chinese scholar tree (Styphnolobium japonicum (L.) Schott)—subjected to controlled brown-rot fungal decay (Gloeophyllum trabeum) over decay durations of 0–6 months. Four mechanical loading configurations were considered: tension, bending, compression parallel to grain and compression perpendicular to grain. Decay progression was quantitatively characterized using mass loss rate (MLR), ultrasonic elastic modulus loss rate (ELR) and strength loss ratio (SLR). The two hardwoods exhibited distinct material- and loading-dependent deterioration patterns. Elm showed faster and more variable degradation, with clearer time-dependent strength loss under tension and bending, whereas Chinese scholar tree displayed slower and more scattered strength deterioration. For both species, elastic modulus reduction generally preceded measurable mass loss, indicating that modulus-based indicators are more sensitive to decay progression under the tested conditions. Correlation analyses further indicate that ELR tends to show more stable and consistent associations with strength loss than MLR across most loading modes. Overall, the results suggest that elastic modulus–based ultrasonic indicators have potential advantages for characterizing mechanical deterioration under controlled decay conditions. However, the findings are limited to the tested materials, decay scenarios and loading configurations, and further validation on aged or naturally decayed components is required before in situ application to heritage structures can be established.