Abstract: In order to explore the mechanical response mechanism of pre-peak unloading of filling body under true triaxial
conditions,the pre-peak unloading test of filling body under different maximum principal stress conditions was carried out by
using true triaxial rock control system and acoustic emission monitoring system. The deformation characteristics and dissipation
energy evolution law of the filling body in the whole process of loading and unloading are analyzed. Based on the overall failure
criterion of rock and the method of segmentation,a damage constitutive model which can describe the stress-strain process of
filling body is established. The results show that the essence of different maximum principal stress unloading is the degree of
damage of the filling body in the loading stage and the influence on the subsequent unloading failure. According to the characteristics
of RA-AF signal,it is judged that the failure mode of filling body is mainly tensile failure,supplemented by shear failure.
The whole process of loading and unloading is divided into four stages,namely,initial damage stage-low speed damage
stage-stable stage-unloading failure stage. With the increase of the maximum principal stress during unloading,the damage of
the filling body in the unloading failure stage is more severe,and the dissipation energy increases more rapidly. The results of
the model analysis are in good agreement with the stress-strain curve of the actual test. The damage constitutive model constructed
in the form of segments based on the overall failure criterion of rock can better reflect the stress and deformation of the filling
body during the loading and unloading process,and can effectively distinguish the two states before and after the unloading
of the filling body.
　

Key words: filling bady,true triaxial unloading,maximum principal stress,dissipated energy,failure criterion,damage
constitutive model