Study Of Dynamic Compressive And Tensile Behavior Of Concrete Materials With Large-diameter Hopkinson Bar

The large-diameter Hopkinson bar technique is an important type of apparatus for studying dynamic mechanical properties of concrete and other heterogeneous materials,but the basic assumptions of traditional split Hopkinson bar(SHB)may not always hold with the increase of diameter,which has brought new challenges to experimental technique,data processing and the analysis of experimental results.The main work and results of this study include the following six aspects:1.Research results and progress of large-diameter Hopkinson bar technique at home and abroad were carefully investigated.Firstly,the brief development history of largediameter Hopkinson bar technique was reviewed.Then the wave dispersion effect,stress uniformity in specimen,none-parallel end-faces of specimen and the transverse inertia effect problems due to large diameter were summarized,as well as the corresponding solutions.Lastly,the research direction and hotspots of large diameter SHB experimental technique were discussed.2.In order to study the influence of rise time and duration of incident wave on stress uniformity of specimen in dynamic compressive experiments with large-diameter Hopkinson bar apparatus,concrete was considered as a two-phase composite material which composed of coarse aggregates and cement matrix in this paper,the cylindrical threedimensional concrete aggregate model was established and applied to SHPB simulation experiment.By means of applying five kinds of trapezoidal loads,stress-strain curves of the concrete specimen at three different strain rate were obtained.Afterwards,the comparative analysis between simulation results and experimental results were carried out.The results showed that this model can be effectively applied in the SHPB simulation experiment of concrete and reflect the dynamic mechanical behaviors.The corresponding macroscopic failure mode was differ from the traditional homogeneous material model,which also provided convenient to mesoscopic numerical simulation research of many other heterogeneous materials.At last,the concrete aggregate models of different aggregates content and aggregate size were established,the effects of aggregates volume fraction and dimension on dynamic strength and stress uniformity of concrete specimen were studied.3.A split Hopkinson pressure bar device in two-bar form,including bar bundle form and single cylindrical bar form,was designed in response to demand from the dynamic mechanical experiments for brittle materials such as concrete,rock,etc.The stress waveform generated through projectile impacting two different kinds of incident bars has been studied based on one dimension stress wave theory and numerical simulation method.At last,we offered corresponding solutions to improve the stress waveform and provide support to the processing and development of this experimental device.The results showed that this designed device has expanded the application scope of conventional SHPB device,and the wave dispersion effect exists in large diameter bar could be evidently reduced when we employed the bar bundle form,which is more suitable for the dynamic mechanical experiments of concrete-like heterogeneous materials.4.Dynamic tensile tests of concrete materials at high strain rates are primarily splitting experiments and spalling experiments at present,however,certain limitations exist in these methods to indirectly study dynamic tensile mechanical behaviour of concrete materials.Thus it is necessary to perform direct dynamic tensile study of concrete materials with a large-diameter split Hopkinson tensile bar(SHTB).Concrete was considered as a two-phase composite material which composed of coarse aggregates and cement matrix in this paper,then cylindrical and dumbbell-shaped three-dimensional mesoscale concrete models were established and applied to SHTB simulation research.Three contact modes between concrete specimen and bar members were discussed based on existing experiments and our numerical simulation analysis.The results showed that established finite element models can effectively simulate SHTB tests and mesoscale concrete models also reflect proper tensile behaviour at high strain rates,which also provides convenience to mesoscopic numerical simulation research of many other heterogeneous materials.At last,a proper contact mode was chosen to perform mesomechanics analysis of concrete materials subjected to four types of tensile impact wave,then five mesoscale concrete models with different aggregates content were established and corresponding numerical simulation analysis were conducted.5.Mechanical analysis of a new type of SHTB apparatus was performed in numerical simulation method,then corresponding incident tensile stress wave was studied and optimize improvement measures for partial components were also proposed.The partly improved SHTB apparatus reconciled the demands of glued connect mode,hooked connect mode and so on.At last,concrete was considered as a two-phase composite material which composed of coarse aggregates and cement matrix,the annulus three-dimensional concrete aggregate model was established and applied to SHTB simulation experiment.The comparison between numerical simulation results and experimental results verified the effectiveness of partly improved SHTB apparatus,which also provided research directions for dynamic tensile responses of mesoscopic concrete model.6.Practical experiments were performed for commissioning of large-diameter SHPB and SHTB apparatus,which belong to dynamic mechanics laboratory of civil engineering and mechanics college of Xiangtan University.The experimental data acquisition system,testing fundamentals and commissioning process were introduced in detail.With regard to large-diameter SHPB apparatus,the measured waveforms which employed pulse shaper technique were given,the striker velocity and incident wave amplitude at different launch pressure were also recorded to provide references for establishment of experimental schemes.With regard to large-diameter SHTB apparatus,the tensile waveforms at different launch pressure were given.Then changes of incident tensile waveform at relatively high launch pressure were analyzed.After that,the influence factor of tensile wave transmission was studied by conducting a series of glued tests,which made preparations for dynamic mechanical property tests of materials.