| The dynamic strength of concrete is an aged but hot problem,which has been attracting sustaining and extensive attention worldwide.Awareness of the problem will directly affect the dynamic safety design of concrete engineering structures.The test results are discrete getting from the lots dynamic test due to the difference of equipment,experimental methods,specimen types and loading rate,and the results lead to that,the mechansim of concrete dynamic mechanical properties is difficult to form a clear consensus.At mesoscale,concrete may be regarded as a three-phase composite consisting of mortar matrix,coarse aggregate,interfacial transition zone and initial imperfection.The macroscopic mechanical behaviour of the concrete depends on the properties of its components.It has been noted that the physical explanation of concrete macroscopic mechanical behaviour could be given by adopting a multi-scale analysis approach.In view of this,the dynamic characteristics of concrete materials are studied through the theoretical analysis,the discrete element numerical test,the finite element numerical simulation and the physical test verification in this paper base on the mesoscale physics of concrete,and gradually refine and deepen the understanding of strain rate effect of dynamic strength for concrete materials.Research work of this paper can be summarized as the following aspects:(1)The causes of the strain rate effect of dynamic strength for concrete materials are screened,The causes of concrete strain rate effect are of dynamic strength attributed to two aspects,the structural properties of materials and dynamic loading characteristics,and putted forward the root of dynamic strength enhancement as results of the combined effect of materials inhomogeneity and inertia force.(2)Starting with the difference of static failure mode and dynamic failure mode of conceret materials,and prepsese that the energy release rate dependent principle is followed by crack opening when concrete materials are damaged.The strain energy in the material releases slowly and the cracks propagate in the weak interface of material at static failure,which the least energy consumption principle is followed at static crack opening.While the strain energy in the material releases instantly at dynamic fracture,the cracks propagate along the path which energy release is the fastest,which the energy release rate dependent principle is followed.The analytical expression of static and dynamic strength for concrete-like brittle materials is established based on the principle,the connotation of concrete analytical expression and the crack development path are analytical studied at different strength ratio of motar and aggregate.(3)Research on the influence of gradation and aggregate content for the physical and mechanical properties of concrete,in order to provide references for reveal the influence mechanism of aggregate content and gradation on its mechanical behavior.he virtual experiment which had aggregate destruction function on concrete under uniaxial compression based on granular flow platform was realized,the concrete virtual experiment of different aggregate(50%,60%,70%,80%)and different gradation(one-gradation,two-gradation,three-gradation,four-gradation)was designed,through analyzing the failure mode,the crack propagation process,the stress-strain curve and the breaking energy evolution law of material.The results show that the structure stability of concrete had significant effect on the strength,and the least energy consumption principle which the static cracks propagate in the weak interface of material is verified.(4)Based on 3-dimensional random aggregate model,this paper deals with the static and dynamic failure limit load and failure morphology of three dividing-point bending concrete beam under different loading rates,takes the effects of each component material strength upon dynamic enhancement factor into consideration and also discusses the functions of inertia force played in the different strain rate stages.The mechanism of strain rate effect of dynamic strength is studied from the inhomogeneity of concrete material and the inertia force has different effect on different strain rates range,the energy release rate dependent principle of concrete dynamic failure is verified through numerical experiments.(5)Based on the CT number of each resolution cell,the numerical reconstruction model which can reflect the true mesostructure of concrete is established(including aggregate,mortar,interface and initial defects).Dynamic uniaxial compression and dynamic uniaxial tension tests were carried out on the numerical models,the influence of the initial defects on the dynamic mechanical properties of concrete was revealed.Dynamic loading tests were carried out on the non-defective concrete model and homogeneous mortar model,compare the results of three samples,the influence mechanism of meso-structure on the dynamic mechanical properties of concrete was revealed,the relationship between inhomogeneity and inertial effect is further discussed,the rationality of the theory proposed in this paper is explained from the perspective of mesoscopic mechanics.(6)The least energy consumption principle of static failure and the energy release rate dependent principle of dynamic failure proposed in this paper are verified by means of the specially designed quasi-static loading test and different loading rates impact test for nonuniform brittle materials three-point bending beam,and the results are basically in accordance with the theoretical assumption. |
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