Experimental Study On Tensile Softening Curve Of Concrete Based On ESPI And DIC Technology

Concrete has the advantages of high bearing capacity,low cost,and convenient construction,but also has the disadvantages of weight and low tensile strength.As the destruction of concrete structures is a brittle failure,it will cause huge economic and property losses once it occurs.In order to prevent the occurrence of brittle failure of the concrete structure and reduce the loss caused by the brittle failure of the concrete,it is crucial to thoroughly understand the basic mechanical properties of the concrete material and the failure mechanism of the material.There are three types of concrete failures:pure opening mode failure（mode I failure）,in-plane shear mode failure（mode II failure）,and antiplane mode failure（mode III failure）.Since mode I crack is the most dangerous in the project,the study of the fracture properties of concrete is mainly based on the research of mode I fracture failure.Due to the presence of the fracture process zone,the concrete material exhibits a nonlinear fracture behavior that is different from other brittle materials.In a series of models used to explain concrete fracture behavior,the Cohesive Crack Model（CCM）is one of the most widely used models.Therefore,as the premise of applying the CCM,the Tension Softening Curves（TSCs）of concrete has become the focus of the research on the fracture properties of concrete.Since the uniaxial tensile test method and the J-integral method of the direct solution cannot provide reliable concrete TSC,the inverse analysis method becomes the only feasible method.As one of the inverse analysis methods,"incremental displacement collocation method"（IDCM）considers the overall response and local response of the specimens at each coordinate point of the TSC,which reduces the ill-posed problems in inverse analysis.Therefore IDCM has become the preferred method for solving concrete TSCs.In order to observe the expansion status of cracks in different stages of the concrete fracture process,and to provide sufficient data for determining the TSC of the concrete,electronic Speckle Pattern Interferometry（ESPI）and digital image correlation（Digital Image Correlation）technology were used during the test to records the surface deformation of the specimens.Through the three-point bending test of specimens with different strengths and different sizes,the load-displacement curves of each concrete specimen during the fracture and failure process were obtained.The comparison of load-displacement curves obtained by ESPI and DIC shows that DIC can be applied to the whole process of concrete three-point bending fracture test with high precision,while ESPI has higher precision in the early stage of the test,and later errors gradually increase.Based on the comparison analysis of the surface deformation measurement results of the same specimen based on ESPI and DIC techniques,the suggestions for reducing the measurement error of ESPI and DIC are given.By analyzing the ESPI and DIC cloud diagrams,the crack extension length and crack shape at different times were obtained,and the crack propagation law during the fracture process of concrete was summarized.The experimental results show that the concrete crack starts at the tip of the prefabricated incision and expands toward the upper bearing.When the coarse aggregate is encountered at the crack tip,the fracture always extends along the path with the least energy consumption,and the complete fracture process zone is presented.Based on the three-point bending test of concrete,the fracture rate of aggregates on the fracture surface of different strength concrete was calculated,and the relationship between the fracture rate of aggregate and the fracture parameters of each concrete was analyzed.From the analysis,the fracture rate of concrete aggregate increases with the increase of concrete strength,and gradually approaches 50%;the concrete fracture parameters CMOD_c,CTOD_c,and w_c are less affected by the concrete aggregate fracture rate;and the concrete fracture energy varies.The increase of the fracture rate of the concrete aggregate increases first and then decreases,and reaches the maximum value when the aggregate fracture rate is about 42.9%.The concrete TSC was constructed using IDCM,and the feasibility of IDCM was verified by the load-displacement curve and fracture energy.Combined with the existing experimental results,the relationship between the parameters of the concrete and the dimensions of the specimen was analyzed.The results show that using IDCM can build a more accurate concrete tensile softening curve;the effect of the test piece size on the tensile softening curve of concrete is small,and the critical crack width wc of concrete decreases with the increase of the strength of the test piece.The concrete test piece size and concrete strength have less influence on CTOD_c,which can be used as the basic material parameters of concrete.Whereas the concrete fracture energy G_F,peak load P_max,CMOD_c andδ_c at peak load are all impacted greater by test piece size and concrete strength,and when the span is greater than 600 mm the CMOD_c remains basically unchanged...