Crack Growth In Jointed Rock-Like Material Plate Under Dynamic Loading:Experimental Observation And Mechamism

Major civil and military infrastructures are?sually located in mountaineous areas featuring complex geological structures.When subjected to dynamic loads due to blasting,large numbers of pre-existing flaws or jointswill grow in the hosting rock mass,which has serious adverseimpact on the safety of rock engineering projects.Thereforefor the sake of the stability evaluation of rock mass engineering constructions,it is of great significance to study the crack initiation and propagation mechanism of fractured rock structure under dynamic loads.In this dissertation,cement mortar is used to fabricate plate specimens containingflaws(joints),which are in the orientation from 0°to 90°with respect to the loading direction.The split Hopkinson bar(SHPB)is used as the dynamic loading system.The failure characteristics of specimens under different loading rates are analyzed in combination with ultra-high speed digital image correlation(DIC)technology.The crack initiation and propagation criterion of the simulated jointed rock mass under dynamic loading conditions are revealed.The main research contents are as follows:(1)Experimental study on dynamic failure of rock-like material plates with a single open joint:The failure mode of singlejoint specimens is mainly X-type tensile-shear damage.The dynamic nominal strength increases with the loading rate,revealing the socalled rate dependency phenomenon.The crack angle also influences the strength of the specimens.At the same time,the dynamic facture process of the prefabricated crack is recorded,and the correlation between the dynamic crack initiation toughness and the loading rateis analyzed.(2)Dynamic fracture of filled single-joint rock-like material plates:Compared with open crack specimens,the randomness of the failure mode of resin filled specimens is augmented,showing non-standard X-type damage.The strength also shows an overall enhancementas compared with open crack specimens.The overall enhancement of the strength with the increasing loading rate is the same for both types of specimens.The difference of peak stress variation at the crack tip is obvious:the crack initiation peak stress of the open-type jointed specimen is smaller than that of the filled jointed specimen,and the strength is significantly affected by the mixing degree of K_II/K_I,while the resin filled joint is obviously compressed,resulting in a negative value of K_I.The variation of K_IIis also obviously different at the failurebetween the two types of jointed specimens.(3)Dynamic fractureof open-type parallel jointed rock plates:As the joint angle changes gradually from 0°to 90°,the strength increases gradually first and then decreases before it increases again.The double jointed specimensfail overall in the X-type.The strain concentration was firstly found at the specimen center,and then the rock bridge was fractured and destroyed.There are two main failure types of the rock bridge,one is the Z-type pattern connected by the diagonal of the two joint tips,and the other is a closed circle.In addition,higher loading ratesmay lead to the change of position of the Z-type failure pattern.It is also found that under the same loading condition,the joint angle has a significant influence on the stress field around the crack tip.(4)Dynamic fracture of filled parallel jointed rock plates:The strength of filled double jointed specimens are also strongly rate dependent,and shows a W-type change with the increasing joint angle.The strength of the filled double jointedspecimens is greater than that of the open ones,and presentmore randomness in the failure patterns.With the increase of loading rate,the peak stress distribution of K_IIis mainly parabolic-like at the crack tip,and it is approcimately inversely proportional to that of K_I.(5)Analysis of dynamic failureof jointed rock-like plates under deep buried condition:The finite element method was used to simulate the failure process of specimens under laboratory conditions.And the numerical modeling was proved to be reliable by comparing the failure modes,displacement field distribution,shear stress field and the law of intensity variationwiththe experimental results.Then the validated model is used to analyze the damage characteristics of specimens under different confining pressures and different axial compression conditions.(6)Fracture pattern classification based on DIC technology data:employing the ultra-high speed camera and DIC diagnostic system method,thedesplacemnt profile of the digital makers is used to accurately obtain the local coordinate system for thequantitative analysis of the fracture evolution.There are four fracture modes observed,and the coupling relationship between the fracture mode and the fracture angle is determined by the analysis.For the first time,the dissertation quantitatively analyzed the failure modes,displacement field and stress field evolution of crack tips in jointed plate specimen under dynamic loads.Specimens with different joint configurations(sheet joints,filling joints,shear joints)and angles were considered.The DIC technique helped accurately obtain the local coordinate system and distinguish the fracture mode.The research results can provide theoretical support for the prevention and control of geological disasters in rock engineering infrastructures,therefoer posessing considerable economic values.