Gradual transition zone between cell wall layers and its influence on wood elastic modulus

Aiming at bringing the modeled cell wall modulus to a more accurate level, for the first time, the gradual transition zone between the S1 and S2 layers was considered as an independent interlayer in the mechanic model (the interlayer model). In this study, the fibril angle and polymer contents in the interlayer were assumed to change gradually, continuously, and simultaneously as functions of the distance to the interface between the interlayer and S2. It was found that the location and thickness of the interlayer significantly influenced the results of the interlayer model. For the cell wall modulus, the excellent longitudinal predictions of replacing the outer S2 with the interlayer were coupled with the poor ones in the transverse direction. For the replacement of the inner S1, the situation reversed. Considering these coupling effects, the best configuration of location and thickness for the interlayer was obtained through a simple evaluation process. The results show the possibility to diminish the discrepancy between the measured and modeled longitudinal modulus by introducing the interlayer, while in the transverse direction, the discrepancy is reduced to a distinct extent but remains present. It was also highlighted that the transition zone located in the inner part of S1 has a critical impact on the performance of the interlayer model. Moreover, different functions for describing the transition were compared.