Experimental Investigation On Initial Cracking Criterion And Numerical Simulation Of Crack Propagation For Ⅰ-Ⅱ Mixed Mode In Concrete

The mechanical behaviors of concrete are closely associated with crack propagation and the research on crack propagation theory has become a foundational subject for concrete. However,owing to the heterogeneities and non-linear behaviors of concrete,different from the ideal brittle materials,the fracture process of concrete is complex and shows three typical stages:crack initiation,crack stable propagation and unstable propagation.In addition,due to the asymmetry of the structure or load and the anisotropic behaviors of the materials,the crack in structure is usually under combined stress condition,which means the stress field at the crack tip is not absolutely classified as modeⅠ(opening),or modeⅡ(sliding),or modeⅢ(tearing),but a combined effect of both modeⅠand modeⅡ,or even that of the three.This kind of crack is defined as mixed mode crack.In engineering,theⅠ-Ⅱmixed mode crack occurs more frequently.Therefore,the whole process of crack initiation,stable propagation and unstable propagation of the mixed mode fracture,as well as its corresponding fracture parameters,has been widely concerned.Then the work is done in this dissertation using experimental and numerical simulation methods as follows:1,Experimental investigation on initial cracking criterionWith the total number of 75 specimens for concrete fracture,13m~3 in volume,and 900×200×200mm,1300×300×200mm,1700×400×200mm,2100×500×200mm, 2500×600×200mm respectively in sizes,the fracture tests of three-point bending beams and four-point shearing beams were carried out.Based on the initial load measured in the test, the corresponding K_Ⅰand K_Ⅱcan be calculated using finite element method.Besides,a more rational load distribution under four-point shearing condition was obtained and the initial cracking criterion for the specimens with five different sizes is given.Finally,a crack initial cracking criterion forⅠ-Ⅱmixed mode crack is proposed and its size effect is discussed.2,Study on crack propagation process ofⅠ-Ⅱmixed mode crack in concrete by means ofphotoelastic coatingsFor the research of the concrete fracture theory,especially for the observation of the crack propagation in specimens,the photoelastic coatings have many advantages,such as lossless,live,intuitive,precise,easier to reserve,etc.19 four-point shearing specimens with five different sizes were utilized to systematically investigate the crack initiation,stable propagation and unstable failure of concrete using photoelastic coatings.The complete crack propagation process of concrete appeared in the form of interference fringes on the photoelastic coatings and was recorded using camera.On this basis,the propagation lengths in different loading stages were obtained.The whole process of stable propagation and unstable development of the main crack was recorded by photographic pictures,and the strain distribution around the crack tip was also obtained based on the colored strain sequence.Test results show that both the initial load and the critical crack propagation length are increased with the increasing of theΠmode portion for those specimens with the same size.While for those specimens with the same crack position but different sizes,the initial load is increased with the increasing of the size.3,Numerical simulation of crack propagation forⅠ-Ⅱmixed mode in concreteA crack propagation criterion is proposed.Once the difference between the stress intensity factor caused by the applied load and the one caused by the cohesive force satisfies the initial fracture equation,the crack begins to propagate.Based on the method,only if the uniaxial tensile strength,the elastic modulus,poisson's ratio and the initial fracture toughness of concrete are obtained,the whole process ofⅠ-Ⅱmixed mode fracture propagation,the complete P-CMSD curve,the initial load,the maximum load,the critical crack propagation length and the crack trajectory in concrete can be given respectively using the proposed method.The same method can be applied into numerical simulation ofⅠmode fracture,and hence the complete P-CMOD curve,the initial load,the maximum load,the critical crack propagation length and the KR-curve can be also obtained.Upon the comparison with the experimental results,the calculated ones show good agreement for these fracture parameters. Therefore,the proposed method in this dissertation is feasible.