| China has a vast territory,with high proportions of mountains and plateaus,and geological disasters frequently occur such as rockfall hazards.The flexible rockfall barrier system was used widely benefiting from its outstanding protective performance,economical cost and convenience for construction.The flexible rockfall barrier system is composed of steel interception net,supporting structure,wire ropes and energydissipating devices.The intercept net is a key component to block falling rocks and consump energy,and the wire-ring net is the most widely used and representative form,directly affectting the performance of the system.If damage occurs in net,the system will completely lose its protection capability.In practice,it is difficult to monitor the disasters in time,which often makes the protection system fail to be maintained in time and suffer the impact of rockfall hazards.In addition,rockfall impacts often occur in series.Therefore repeated impacts may be formed,which easily causes damage accumulation in the net of the protection system,reduces the residual protection capacity,and even causes direct damage to the system.At present,there are few studies on the damage mechanical behavior of interception net under repeated impact loads.Therefore,in the thesis,repeated impact tests of different specifications of ring net under the equal energy impact were carried out,the impact dynamic response of the ring net was tested,and the impact damage features of the net were studied.Accordingly,an impact dynamics numerical model of the net was further established,and the evolution law of damage of the ring net under repeated impacts was revealed.Based on the results of damage evolution analysis,an evaluation index for the remaining bearing capacity of the ring net was established,which provides a quantitative evaluation basis for the remaining bearing capacity and remaining life of the ring net,and can be used to guide the design and maintenance of the actual protection system.The main contents of this study are as follows:(1)The impact mechanical behavior of the flexible rockfall barrier system,the mechanical behavior of the interception net,the dynamic damage,and the evaluation were summarized(Chapter 1);the necessity of studying the damage evolution mechanism of the ring net under repeated impact loads was analyzed,and the research content and technical route were established accordingly.(2)Repeated impact tests of different specifications of ring net were carried out(Chapter 2),a repeated impact test plan for the ring net was developed,and repeated impact tests of netes of three specifications including R5/3/300,R7/3/300,and R9/3/300 were conducted.The test piece resisted 4,3,and 7 shocks respectively under equal energy shock determined by the same calibration rule.Further research and analysis of the dynamic behavior of the specimen under repeated impact loads were conducted,and the deformation characteristics of the specimen,the movement characteristics of the impact block,the damping change of the system and the shrinkage change law of the net ring section were analyzed.(3)Based on the explicit dynamics computational theory and the method of restart analysis,the construction method of the numerical model of the impact damage evolution analysis of the ring net was studied,especially the determination of the element and the equivalent failure strain parameters.The numerical simulation of repeated impacts of three ring netes R5/3/300,R7/3/300 and R9/3/300 was carried out(Chapter 3).Based on the comparison with the test results,the dynamic response characteristics of the ring net were analyzed,including deformation characteristics,impact time history,plastic strain development of the net,energy consumption distribution,and P-Δ effect.(4)The quantitative method of wire-ring net damage was studied,the damage index based on equivalent plastic strain was adopted,and the calculation method of damage variables under this index was further studied(Chapter 4).Based on the development law of equivalent plastic strain of wire-ring net under various calculation conditions,the relationship between the damage variable of the ring net and the number of impacts and the energy of a single impact was revealed.The conclusions of the thesis are as follows: the wire-ring net has the feature of three-stage evolution,i.e.,relaxation slip deformation,tension deformation,and rebound deformation under the impact;compared with the case in static burst test,the damaged parts of the specimens are all at the edge of the impact contact area,indicating that the failure mechanisms of ring net are consistent;with the increase of the number of impacts,the deformation of the wire-ring net gradually decreases,and the damping gradually decreases,showing the stiffness hardening effect,so the accompanying impact force also gradually increases;the plastic strain in the impact contact area of the wire-ring net develops the fastest,showing an“orthogonal cross”pattern that develops outward along the shortest path from the impact center area;the shrinkage change law of the net ring section in the impact contact area is the largest,indicating that the plastic deformation of this part is the largest;the shrinkage change law of the net ring section away from the impact contact area gradually decreases,indicating that the plastic deformation also gradually decreases;under all working conditions,the energy consumption of the ring net under the first impact load reaches about 50%of the input energy,and the energy consumption decreases as the number of impacts increases;based on the equivalent plastic strain,a calculation method for the damage variable of the ring net was established,the correlation between the damage variable of the net,the number of impacts and impact energy was clarified;the damage variable was used to evaluate the residual bearing capacity of the ring net;the method can judge the degree of damage on the ring net according to the rockfall impact energy and the number of impacts obtained by investigation,determine the remaining bearing capacity of the ring net,and judge whether it can continue to bear the load. |
