Research On Mechanical Properties Of Concrete Fracture Process Zone Based On Digital Image Correlation Method

Nonlinear fracture characteristic in concrete especially size-dependent fracture toughness have always been a hot and significant issue for both researchers and engineers in civil engineering.The most important feature of concrete fracture is fracture process zone(FPZ)at the crack tip,which resulted in the nonlinear process of fracture and the size effect of concrete.Many concrete nonlinear fracture models have been proposed.However,due to limit of experiment method,the characteristic of fracture process zone in concrete is not very clear.Based on experiment and theoretical analysis,crack propagation process of concrete is research in this thesis,along with mechanical characteristics of the fracture process area.A more convenient fracture model of concrete for engineering application is established.The main content of this paper includes the following aspects:(1)The process of propagation and failure of concrete and mortar notched three-point bending beam is measured by using digital image correlation method(DIC).Failure experiments of three-point bending concrete specimen with different sizes and different initial crack-depth ratio were undertaken.The crack propagation process is observed by DIC method and the whole field displacement was obtained.(2)Based on virtual crack model and bilinear softening curve hypothesis,mathematical expression of opening displacement along concrete fracture process zone is derived,by solving Fredholm? integral equations.Taking the cracks at the center of the infinite plate and three-point bending specimen as an example respectively,the displacement on the fracture process zone and cohesive force distribution is obtained.The influence of specimen size,softening curve parameters and initial seam height ratio on fracture characteristics is also investigated.(3)Assuming linear displacement distribution along fracture process zone of concrete,a new cohesive crack model was proposed and solved.By comparing the experimental data and model calculation,validity of the model is verified.And further analysis on the development of the fracture process zone length,the initial ratio,specimen size and the influence of softening is also undertaken.(4)Using multiple linear regression method,the method of using experimental load-opening displacement curve(P-CMOD)solving parameters of softening curve is proposed.The GUI program was written using MATLAB software for engineering application convenience.