Discrete Element Analysis Of Dynamic Mechanical Behavior Of Brittle Materials

The dynamic mechanical behavior of brittle materials under external force was simulated by using the discrete element method with the particle flow software PFC.The dynamic mechanical behavior of concrete and inorganic glass is studied,and concrete is the main research area.The two-dimensional concrete model was constructed by using discrete element software in the middle and later stages.The meso-parameters of concrete were calibrated by simulation experiments such as uniaxial compression,splitting tension,three-point bending and three-point bending of notched beams.And on the basis of this model:1.The Brazilian splitting experiment of concrete is simulated and reproduced.The simulation results show that:1)the force chain distribution is most concentrated in the numerical direction of the middle part of concrete,and gradually decreases to both sides.Macroscopic cracks are first generated at the bonding surface between aggregate and mortar,and expand along the loading line to the upper and lower sides.In the process of high-speed growth,accompanied by the release of high strain energy,cracks pull the aggregate apart,but the cracks mainly expand along the bonding surface between aggregate and mortar,as well as between mortar.2)The splitting tensile strength of concrete shows obvious strain rate effect,which increases with the increase of strain rate.3)The SHPB system is further established.It is found that under low strain rate,concrete can well satisfy the central cracking phenomenon,but not at high strain rate,and the critical strain rate is about 8s^-1.Because at higher strain rate,the internal stress of concrete can not be uniform in a short time,but the stress concentration on both sides of concrete is first produced to reach the compressive limit,which results in this kind of phenomenon.2.The experiment of pulling out concrete with round bars is simulated.The simulation results show that:1)There will be an obvious taper of contact force in concrete during pulling out of bars.2)The influence of strain rate on bond strength during pull-out process was studied.It was found that the strength slip curve presented a multi-segment form,and the strength increased with the increase of strain rate,and the peak value moved back with the increase of strain rate.The results were compared with the experiments.3)Under different strain rates,concrete presents different failure modes,including direct pull-out and splitting,and crack propagation mainly propagates along the interface between mortar and aggregate.At low strain rates,aggregate will not penetrate,while at high strain rates,aggregate will be pulled apart with crack propagation.3.The experiments of concrete beams impacted by drop hammer are simulated,and the failure modes of concrete beams impacted by different velocities are studied and compared with the experiments.The simulation results show that the different failure modes of concrete beams are the result of the interaction between the impact force of two kinds of indenters and the tension force caused by deformation.As a brittle material,the dynamic mechanical behavior of inorganic glass,which is also a brittle material,was studied on the basis of studying the dynamic mechanical behavior of reinforced concrete in the later stage.4.Based on the discrete element algorithm（DEM）,a numerical Split Hopkinson Pressure Bar（SHPB）platform was established by means of particle flow code software(PFC^2D)and the feasibility of the system was verified.The failure mode and the dynamic compressive strength of an inorganic glass specimen at different strain rates were investigated.The numerical simulation shows that the inorganic glass exhibits typical brittle characteristics during dynamic compression,and its compressive strength is significantly affected by the strain rate.The Young’s modulus,however,is strain rate insensitive.The failure mode of the specimen is affected by the boundary friction as well as the Poisson ratio.In the case of frictional contact,the initial micro-cracks within the specimen are distributed in a triangular zone due to the combined effect of longitudinal pressure and frictional force.With the increase of the longitudinal stress,the transverse tensile stress creates the longitudinal cracks,resulting in the axial splitting.The failure mode in the case of frictionless contact differs from the frictional case,in which no triangular crack zone exists.Moreover,the value of Poisson ratio affects the failure mode as it results in the transverse tensile stress during dynamic loading.Numerical simulations of dynamic Brazilian compression are also conducted to support future experimental works.It shows that Brazilian disk starts failure at the center in the moderate strain rate and the macroscopic splitting tensile strength is strain rate dependent.