考虑环境与荷载长期共同作用的木材本构模型研究

Study on wooden constitutive model considering long-term effects of environment and loads

针对木材的变形易受周围环境温湿度变化影响的问题,提出了一种能描述木材在外界环境和荷载长期共同作用下的本构模型和力学性能研究方法。首先基于Tsai-Wu屈服准则、Drucker公设和一致性理论提出了考虑蠕变、机械吸附蠕变的木材本构模型;然后采用Fortran语言将推导的本构模型编写成UMAT子程序模块并嵌入到ABAQUS软件中,实现了在荷载及环境共同作用下木材本构的有限元分析;最后,选取老旧木材进行了单轴材性试验以及机械吸附蠕变试验,验证了所提出本构模型的有效性,并采用Hankinson理论公式验证了木材在斜纹受力状态下模拟的有效性。研究结果表明,有限元计算的木材顺纹、横纹径向以及横纹弦向的屈服应力与实测屈服强度相对误差分别为19.5%、4.6%和8.7%,所推导的木材模型与所编写的用户子程序(UMAT)可以用于研究木材在单向和斜纹受力状态下的力学特性,机械吸附蠕变的试验结果与有限元结果吻合较好,能够较为准确地预测木材在荷载和环境共同作用下的力学行为。

Given that the deformation of the timber is susceptible to the change of ambient temperature and the humidity, a timber constitutive model that can accurately describe the interaction with load and long-term ambient environment was proposed. Firstly, based on the Tsai-Wu yield criterion, the Drucker postulation and the consistency rule, a timber constitute model considering the normal creep and the mechanical absorption creep was proposed. Then, the deduced constitutive model was compiled into the UMAT subroutine and was embedded in the ABAQUS software by using the Fortran language, which realized the simulation analysis of the timber characteristics under the combined action of load and ambient environment. Finally, uniaxial material tests and mechanical absorption creep tests were conducted on old timber to verify the validity of the proposed constitutive model. Besides, the Hankinson theorem formula was also employed to verify the validity of the constitutive model when the timber was under the stress in perpendicular-to-wood-grain direction. The results show that the errors of the prediction of the yield stress when the timber is subjected to longitudinal, radial and chordwise compressions are 19.5%, 4.6% and 8.7%, respectively, which indicates that the deduced constitutive model and the user subroutine(UMAT) are effective to simulate mechanical properties of timber under the uniaxial and the perpendicular-to-wood-grain direction loading. Moreover, the results from the mechanical absorption creep experiments are in good agreement with the simulation results, which means that the deduced constitutive model can better predict the timber mechanical behavior under the combined action of load and ambient environment.