A Study On UHPC Crack Simulation And Crack Development Characteristics Based On The Fictitious Crack Model

Ultra-high performance concrete is a new type of cement-based composite material.Its main advantages are high compression resistance,high tensile strength,tensile strain hardening,and excellent durability,which has a wide range of applications in bridge structures.In order to ensure the safety of the bridge structure,it is necessary to predict the crack development in the UHPC structure and explore the crack mechanism.the UHPC bilinear tensile softening curve is established by the inverse analysis method and verified by fortran program.On this basis,the influence of steel fiber content on softening curve parameters and fracture energy was studied,and the influence of steel fiber on cracking load and unstable load was obtained.The study shows:the simulation results are basically the same as the test results,the tensile softening curve obtained by the inverse analysis method in this paper meets the fracture characteristics of UHPC's type I cracks;the tensile softening curve parameters f₁,w₁and G_F increased with the increase of the volume fraction of fiber;the critical crack opening displacement w_c is not affected by the fiber volume fraction.The numerical simulation of the wedge splitting members with different seam height ratio is carried out to study the effect of seam height ratio on the development of a single type?crack;study the influence of seam height ratio on the cracking load and unstable load;a four-point bending beam with multiple initial cracks was simulated to study the type I crack propagation of multiple cracks;the three-point bending beam with three different initial cracks was simulated to study the interaction coefficient of type?cracks.The study shows:for the development of type?cracks:as the seam height ratio increases,the cracking load decreases linearly;when the seam height ratio is larger?>0.4?,the instability load decreases approximately linearly;the seam height ratio is smaller?<0.4?,the seam height ratio has little effect on the unstable load of the structure;for different numbers of initial cracks of the same size,the initial crack at the mid-span will eventually develop into an active crack;when there are two initial cracks with different sizes in the pure bending section,the larger initial cracks will develop into active cracks;when the cracks of impure bends with relatively small size always close eventually,and the location of cracks in the non pure bending section has little effect on the cracking performance and bearing capacity of the structure;the coefficients of interaction at crack B in three-point bending beam span reveal an active role of crack interactions by crack A and crack C in the pre-and post peak regions.After considering the shear force at the crack tip and selecting appropriate shear stress transfer curve,the softening constitutive relationship of UHPC considering shear is determined,and the;study the effect of seam height ratio and initial crack position on P-CMOD and P-CMSD curves;study the effect of seam height ratio and initial crack position on fracture parameters.The study shows:the established I-II composite crack softening constitutive relationship can better reflect the UHPC I-II composite fracture performance;when the ascending section exceeds 60%of the peak load,the type I fracture is more dominant;when the crack height ratio is greater than 0.4,the seam height ratio has no significant effect on the initiation load,but it is the opposite on the instability load;the cracking load decreases linearly with the increase of the distance between the crack and the mid span;the instability load increases first and then decreases as the distance between cracks and mid-span increases.