Study On Rate-dependent Constitutive Model Of Uniaxial Compression Of Wood Under Low Strain Rate

The constitutive model of timber is an important basis for mechanical response analysis of wood structure.Traditional Wood Structure not only bears the static load,but also inevitably suffers the influence of earthquake.Wood is a rate-sensitive material.There is a non-negligible difference between the dynamic and the static response.Therefore,conducting research on wood dynamic performance is a very important and practical work.Considering that wood is in the low strain rate range(10^-3s^-110^-1s^-1)under earthquake,the main research work and conclusions of this paper are as follows:（1）The mechanical properties parallel to grain of spruce,Larch and Pinus sylvestris var and that perpendicular to grain of Larch and Pinus sylvestris var under uniaxial compression and cyclic loading were studied at strain rates 10^-4s^-1、10^-3s^-1、10^-2s^-11 and10^-1s^-1.The stress-strain curves and cyclic responses at different loading rates were obtained.The evolution of stress-strain curves,elastic modulus,strength,peak strain,yield stress and energy absorption capacity under different loading rates are analyzed,and the mechanical properties such as load-unload modulus are also analyzed.The results showed that the mechanical properties of wood along and across grain showed a certain rate sensitivity.（2）Based on the rate-sensitive characteristics and non-linear behavior of timber,a phenomenological non-linear rate-dependent constitutive model for timber was established by considering the mechanical behavior of uniaxial compression and cyclic loading parallel to grain and perpendicular to grain.The model can simultaneously reflect the mechanical behavior of wood under uniaxial compression and cyclic loading parallel to grain and perpendicular to grain,and the rate-sensitive characteristics of timber.On this basis,the finite element formulation of the constitutive model is deduced for the two-dimensional plane stress problem,and the corresponding wood subroutine is developed with the material subroutine interface（User-defined Material Mechanical Behavior,UMAT）in ABAQUS/Standard solver.（3）Based on the viscoplastic unified constitutive theory,the Bodner-Partom model widely used in the metal field was introduced into the wood dynamic analysis,and the model parameters were determined.The model does not require the yield surface,which can simultaneously reflect the rate sensitive properties and the cyclic loading behaviors.Based on this,the finite element formulation of Bodner-Partom model is derived for three-dimensional problems.Finally,the corresponding timber subroutine is developed with the material subroutine interface（Vectorized User-Material Subroutine,VUMAT）in ABAQUS/Explicit.（4）Taking the mechanical properties of larch as a typical example,the validity of the two subroutines was verified by the test results.The results show that the two wood subroutines can reflect the rate-sensitive characteristics of wood.And the mechanical response of wood column under dynamic compression is analyzed simply by the developed VUMAT subroutine.