Characterisation of thermally treated beech and birch by means of quasi-static tests and ultrasonic waves

Wood, being renewable and highly abundant material, with excellent high specific strength and stiffness, has received increasing attention to be used in high performance applications such as the structural element of a battery case in an electric vehicle. For a successful implementation of wood in the automotive sector, it is, therefore, crucial to understand the behaviour of wood during and after temperature exposure and in the event of fire with the presence/absence of oxygen. In this study, the mechanical properties of thermally modified and unmodified European beech and birch in air and nitrogen environments at six different treatment intensities were characterised using compression tests, tensile tests, shear tests and Poisson's ratio tests. Further, the elastic properties of these wood species were quantified using the ultrasound measurements. The obtained strength and stiffness exhibited mild improvement upon moderate temperature treatment (200 degrees C), followed by a decrease at elevated temperature levels. This improvement was somewhat more pronounced under nitrogen treatment than under air treatment conditions. Nevertheless, a more noticeable decrease in the material performance was observed in beech compared to birch, occurring at earlier stages of modifications. This study confirms the tension-compression asymmetry of beech and birch where higher Young's moduli were obtained from tensile than from compression tests for reference and thermally treated beech and birch. The shear moduli obtained from ultrasound for birch were comparable to those obtained from quasi-static tests, whereas there was an overestimation of approximately 11-59% for the shear modulus of beech compared to quasi-static tests. Poisson's ratios from ultrasound tests corresponded well with those from quasi-static tests for untreated beech and birch, but not for thermally modified samples. The Saint-Venant model can satisfactorily predict the shear moduli of untreated and treated beech wood.

Automated summary

Wood has potential to be used in high-performance applications like electric vehicle battery cases due to its renewable nature and excellent mechanical properties. This study characterized the mechanical and elastic properties of thermally modified and unmodified beech and birch wood under different treatment conditions. The results showed that moderate temperature treatment improved the strength and stiffness of the wood, but higher temperatures caused a decrease in these properties. Nitrogen treatment was found to have a more pronounced effect compared to air treatment, and beech wood exhibited a greater decrease in performance compared to birch wood. Tension-compression asymmetry was observed in both wood species, with higher Young's moduli obtained from tensile tests. The shear moduli obtained from ultrasound measurements were comparable to quasi-static tests for birch wood, but there was an overestimation for beech wood. The Saint-Venant model successfully predicted the shear moduli of beech wood.