Effect Of T-Stress On Dynamic Crack Initiation And Propagation Behavior

During the study of the crack penetration mechanism between adjacent holes in smooth/pre-cracking blasting,it is a common phenomenon that two symmetrically propagation mode Ⅰ cracks are deflected before they meet and finally become "hooked" in the direction of the line between the two holes when both holes are detonated simultaneously.How to explain this phenomenon,so as to achieve the prediction of blast crack propagation behavior and propagation path,will be beneficial to the design of blasting parameters and blasting quality control.Traditional fracture mechanics uses the stress intensity factor K to characterize the size of the crack tip stress field,domestic and foreign scholars found that the second term of the crack tip stress field,i.e.T-stress,has a significant effect on crack initiation and propagation behavior,resulting in a change in fracture toughness and mode Ⅰ crack propagation direction.Based on this,it is assumed that the deflection in crack extension and the "collusion" phenomenon when cracks meet under explosive loading are related to the T-stress.To this end,this thesis adopts a combination of theoretical analysis,model experiments and numerical calculations based on a simple to complex,step-by-step scientific approach to reveal the influence of T-stress on the crack initiation and propagation behavior of double-hole cracks and to explain the mechanism of the "collusion" phenomenon in the field of engineering blasting.The main research contents and results are as follows.(1)Considering that the crack propagation is mainly driven by the blast gas,and assuming that the blast gas pressure is constant,five numerical models are established,namely,collinear holes and cracks of equal length in infinite plate(Model Ⅰ),collinear holes and cracks of unequal length in infinite plate(Model Ⅱ),collinear holes and cracks of equal length in finite plate(Model Ⅲ),collinear holes and cracks of equal length in semi-finite plate(Model Ⅳ),and non-collinear holes with cracks of equal length in infinite plate(Model Ⅴ).In view of the T-stress and specimen geometry,a modified maximum tangential stress(MTS)criterion considering T-stress is established,taking into account the model geometry,i.e.crack length,distance between two crack tips,distance of the circular hole from the model boundary,model height,and crack transverse and vertical offset distances.The effect of T-stress on crack initiation and propagation behavior was investigated by changing factors such as the offset distance of the crack;the results of numerical calculations found that when the height of Model Ⅲ is small,the crack tip generates positive T-stress,and based on the modified maximum tangential stress criterion considering T-stress,the crack in Model Ⅲ may generate "hook" phenomenon,the simulation of the crack path of Model Ⅲ resulted in the The simulation of the crack path in Model Ⅲ results in the phenomenon of "hooking",which indicates that the T stress is the main reason for the deflection of the crack under quasi-static loading and finally the phenomenon of "hooking".(2)In order to study the influence of T-stress on the dynamic crack initiation behavior,a three-dimensional model of Hopkinson(SHPB)impact and static notch was established.Firstly the validity of the numerical method was verified,and then the stress intensity factor at the notch end and the variation law of T-stress with time were studied considering the change of impact velocity and notch length.Finally,the dynamic fracture toughness was predicted using the critical stress criterion,and the mechanism of the influence of T-stress on the predicted dynamic fracture toughness was analyzed;the calculation results showed that for different impact velocities,the dynamic stress intensity factor K at the notch end showed a tendency to increase first and then smoothly,and the dynamic K at the notch end increased as the impact velocity increased;for different notch lengths,the dynamic K at the notch end showed an overall increasing trend for different notch lengths;a relatively large negative T-stress was generated at the notch end at the initial stage of the type Ⅰ tension stress field generation,which increased rapidly with time to a relatively small positive value and then leveled off;the larger the absolute value of the negative T-stress,the greater the predicted dynamic fracture toughness.(3)To address the shortcomings of the research content(2)in which the notch is static,the changes of stress intensity factor and T-stress during crack proprgation stage are unknown,and the dynamic fracture toughness is predicted to be obtained,in order to verify the validity of the conclusions of the numerical simulation,Hopkinson impact experiments based on digital image correlation(DIC)technology are conducted,and the static displacement field equation of the crack tip considering T-stress and displacement,the interaction between the stress intensity factor and T-stress at the crack initiation stage was studied,and the dynamic crack tip displacement field equation and displacement extrapolation method considering T-stress and crack velocity were used to calculate the stress intensity factor and T-stress during the crack proprgation stage,and the experimental results show that the larger the absolute value of the negative T-stress,the larger the stress intensity factor at the crack initiation moment,which is consistent with the conclusions obtained in content(2).The stress intensity factor in the crack propagation process showed a trend of oscillating decay and then decreasing,and the value of stress intensity factor was 0.94 MPa*(?) when the crack was stopped;the T-stress in the crack propagation process showed a trend of oscillating increase and then increasing,and the value of T-stress was-5.42 MPa when the crack was arrest.(4)To address the deficiency that the crack propagation behavior in the research content(1)which the crack propagation velocity was not considered,model Ⅲ is selected as the research object,and the crack propagation velocity is introduced into the modified MTS criterion to establish the dynamic MTS criterion considering the T-stress and crack propagation velocity,and by changing the influencing factors such as plane state,Poisson’s ratio,crack length and model height,the dynamic fracture criterion is considereded.The results of numerical calculations show that the crack deflection angle gradually increases with the increase of crack propagation velocity,the plane state,the increase of Poisson’s ratio,the increase of crack length and the decrease of model height,that is,the larger the crack propagation velocity is,the larger the crack deflection angle is.Compared with the crack propagation path simulated by the static MTS criterion,the deflection angle calculated by the dynamic MTS criterion is slightly larger than that calculated by the static MTS criterion,and the distance between the crack deflection position and the crack tip is smaller than that of the static MTS criterion.The distance between the crack deflection position and the crack tip in the static MTS criterion is smaller than that in the static MTS criterion.That is,when the dynamic MTS criterion is used to simulate the crack propagation path,the crack deflects earlier.