| Many mechanical and exploratory studies have been performed on the concrete materials in the past decades.However,how scientifically understand the dynamic behavior of concrete under complex static-dynamic coupling loading conditions is still a hard challenge.Because of the complexity of the concrete materials,the differences of experimental specimens and the limitations of testing equipment.Based on previous works,this study is performed by applying the consistent concrete specimens and a novel Split Hopkinson Pressure Bar（SHPB）testing system which can achieve the true triaxial static-dynamic coupling loading on the specimen.The dynamic mechanical behaviors of concrete under static-dynamic coupling loading conditions are studied based on the literature,experimental testing and mechanism analysis as the pre-stress conditions are considered.The main contents and results of this study can be written as follows:（1）The dynamic tensile behaviors of concrete and cement disc specimens are studied at different strain rates.The results show that both concrete and cement have strong strain rate effects,the dynamic tensile strength（4.9 MPa～20.5 MPa）increases with the increase of strain rate(10-5 s-1～18 s-1).The nonlinear functional relation between the dynamic tensile strength and strain rate effect of concrete materials shows that the strain rate effects of concrete specimens are more sensitive than that of cement.Three typical failure patterns that central cracking,central cracking with multiple crushed cracks and crushed fracture zone with shear damage can be concluded according to their fracture modes of dynamic behaviors of Brazilian disc experiments.The third typical failure mode is that the crack initiates at the contact interface between the specimen and the elastic bars rather than the center of disc specimen,which usually occurs at high strain rate(20 s-1).Therefore,it is difficult to ensure the realization of the stress balance of the Brazilian disc specimen and the crack initiation can be checked by the digital electronic technique（DIC）and the testing by multiple strain gauges.（2）Based on the uniaxial dynamic compression experiments at different strain rates,the dynamic peak stress,dynamic peak strain and failure mechanism of concrete under different static-dynamic coupling loading conditions were studied.The influences of pre-stress conditions on their dynamic behaviors were analyzed.The results show that the concrete is sensitive to the strain rate and the pre-stress conditions.The dynamic peak stress（73.6 MPa～97.5 MPa）of concrete specimens increases with the increase of strain rate(60 s-1～180 s-1),while the dynamic peak strain presents a decreasing tendency(4.46×10-3～2.07×10-3)under the without pre-stress conditions.At the same pre-stress（20 MPa/30 MPa）,the dynamic peak stress also increases with the increase of strain rate.At the same strain rate(90 s-1/140 s-1/180 s-1),the dynamic peak stress decreases with the increase of the pre-stress and the normal peak stress（total peak stress）presents increasing tendency.With the increase of strain rate(90 s-1～180 s-1),the nominal peak stress increasing tendency to decrease.（3）The dynamic axial peak stress/strain,the strain rate effect considering confining stress and the typical failure characteristics of the concrete specimens were investigated based on the dynamic experiments of concrete specimens at the strain rate of 50 s-1～250s-1 under the confining and without confining condition.The results proved that the dynamic peak stress（41 MPa～65 MPa）and strain of the specimens significantly increases with the increase of strain rate(50 s-1～250 s-1)under the uniaxial dynamic loading.The dynamic stress-strain curves of specimen present obvious strain-softening stage as the dynamic stress unloading stage.The dynamic peak axial stress（193 MPa～319 MPa）,strain(7.48×10-3～2.57×10-2)and dynamic peak confining stress（43.9 MPa～74 MPa）increase with the increase of strain rate(75 s-1～225 s-1),and the dynamic stress-strain curves are no obvious strain-softening stage under the confining condition,while the stage can be found in the without confining state.The concrete specimens usually broken into small pieces or powers under the uniaxial dynamic loading at high strain rate.While the specimens present several fractured cracks and shear failure under dynamic loading at confining state.The dynamic increase factors（DIF）prove that the strain rate effect of concrete specimen without confining pressure is more obvious than that under the confining condition.（4）The dynamic compression behaviors of concrete cube specimens were conducted under different static-dynamic coupling loading and strain rate conditions(60s-1～180 s-1)by a novel SHPB experimental system,which can realize the true triaxial static-dynamic coupling loading on the specimen.The results reveal that the dynamic peak stress in the direction of the principal stress increases of concrete specimen,while the dynamic peak stress in the direction of intermediate principal stress decreases with the increase of the pre-stress（15 MPa,20 MPa and 25 MPa）of the direction of intermediate principal stress at similar strain rate under the biaxial static-dynamic coupling loading conditions.The specimens present a layered failure parallel to the plane,which is formed by the principal stress and intermediate principal stress.The breakage of concrete specimens increases and the particle size decreases with the increase of strain rate.The dynamic response peak stress（strain）of the direction of intermediate and minimum principal stress is not uniform under dynamic loading of the hydrostatic pre-stress（10 MPa）,which is the results of anisotropy/heterogeneity of concrete specimens.The difference in dynamic response（stress）seems to decrease with the increase of the hydrostatic pre-stress.The dynamic response peak stress/strain of the specimens increase with the increase of the hydrostatic pre-stress（0+MPa,10 MPa and 20 MPa）at the same strain rate.The dynamic response peak stress of specimen increases with the increase of strain rate(50 s-1～120 s-1)at the same hydrostatic pre-stress.The dynamic peak stress of specimens show an increasing tendency with the increase of strain rate(45 s-1～110 s-1)under true triaxial static-dynamic coupling loading conditions,while the dynamic elastic modulus seems not affected by the strain rate condition.Then the typical compression shear failure patterns are found in the concrete specimens under dynamic loading.（5）Combined with the experimental researches and previous works,the dynamic behaviors of concrete are analyzed at different static-dynamic coupling loading conditions,the mechanism of influence of pre-stress on the dynamic performance of specimens and the physical mechanism of strain rate effects are summarized.Two main research contents in the dynamic mechanical behaviors of concrete under complex pre-stress conditions.First content is the pre-stress state of concrete specimen:it is the uniaxial,biaxial or triaxial state,which is key problem that the state of shear stress inside concrete specimen under these stress conditions.Second content is the properties of dynamic loading applying on specimen.Cracks of different sizes inside specimen synchronously propagate due to the dynamic loading,which the size is determined by the strain rate condition.Therefore,the dynamic stress/deformation behaviors of concrete specimens are closely related to the strain rates condition.The thermal vibration of specimen is strengthened by the dynamic loading,then a large number of cracks with different sizes are evoked due to the fracture of atomic bonds which the result of the strengthened thermal vibration.These microcracks simultaneously expand under the dynamic loading,which greatly increases the energy required for crack growth.Therefore,the strength of concrete specimen is increased which embodied in the strain rate effects.（6）The variation of nominal peak stress of concrete specimens under different static-dynamic coupling loading conditions was studied,and the results show that the cohesion（c0:31.89 MPa～42.26 MPa）of the specimen significantly changed with the confining,and the internal friction angle（φ:26.38°～29.16°）seems not sensitive to the confining conditions.Based on the microscopic perspective and the theory of dynamic fracture mechanics,a dynamic failure criterion of concrete is constructed under multiaxial stress conditions.The experimental nominal peak stress and the theoretical value by the failure criterion were compared and analysed,the results show that the dynamic mechanical behavior of concrete can be well reflected by the dynamic failure criterion. |
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