Macro-meso Scale Fracture Numerical Simulation Research On Cementitious Composites

The ordinary concrete and steel fiber reinforced cementitious composites(SFRC)are typical cementitious composites,the meso-heterogeneity of their internal structure has significant influence on the macroscopic mechanical properties.The influence of internal components(i.e.aggregates,matrix,ITZ and fibers)on the macro-scale mechanical properties are difficult to be analyzed clearly only depend on macro-scale mechanical test or the homogeneous numerical model.The aligned steel fibre reinforced cementitious composite(ASFRC)is a typical anisotropy material.At present,the research on failure characteristics of ASFRC and the reinforced mechanism of aligned fibers for toughness are not sufficient and clear.Therefore,the failure mechanism of cementitious composites was studied based on the meso-scale numerical model in this paper,the contents of this research are as follows:(1)The meso-scale finite element model for the concrete was established due to the region of crack propagation was considered as meso-heterogeneity materials.The whole process of meso-fracture was simulated based on the extended finite element method(XFEM).After verified the reliability of the meso-model,the influences of aggregates'distribution and volume content on the crack propagation were studied.The results showed that the descending branch of the P-CMOD curve is affected by the aggregates'distribution,while the peak load and the crack propagation trajectory are less influenced.The cracking trajectory also affected by the aggregate volume content in the local region,while the overall direction of which is almost unaffected.(2)The random number was generated using the mixed congruence algorithm,and which was used to simulate the position of fiber.Furthermore,the 3D numerical model for the steel fiber reinforced cementitious composites was established based on the criteria proposed in this paper for determining the intersecting of fibers.Fiber contents within the different sections of model were counted,and the statistical result was compared with test result.Considering bond-slip between fiber and matrix,the whole process of tensile fracture for ASFRC with different fiber content was simulated based on the XFEM.The result shows that,the statistical result of model and the complete curve of numerical simulation match well with test result,and the failure pattern of ASFRC accompanied multiple cracking behavior under uniaxial tension for high fiber content.(3)The complete fracture process for the ASFRC three-point bending beam was simulated using cohesive crack model,and the reliability of model was verified by the comparison of numerical and existing test results.The influence of different cohesive-laws of matrix on the whole fracture process for ASFRC was investigated,and the complete failure processes of specimens with different size were analyzed.Besides,the numerical model incorporated aligned fibers with different orientation were simulated.The result shows that in this study,simulated results of finite element model for ASFRC beam matched well with test results;the types of cohesive-law have little influence on the complete load-pan deflection curve;with the increase of specimen's size,the nominal strength of ASFRC has a obvious size effect;since the angle between fiber orientation and principal tensile stress over the 60~o,the reinforcement of steel fibers for peak load of bending failure is not significant.(4)The rate-dependent behavior was added to the cohesive-law of matrix and equivalent constitutive law of fibers,and the subroutine VUMAT was establish in this paper based on that.The complete fracture processes for the SFRC and ASFRC three-point bending beam under impact loading were simulated using this VUMAT respectively.The influence of different impact velocities on the impact resistance of the SFRC and ASFRC three-point bending beam was compared and analyzed.The result shows that,the impact resistance of cementitious composites can be singnifcantly reinforced due to the dissipative energy of aligned fibers is significantly higher than that of dispersed fibers.The maximum value of dissipative energy for aligned fibers increases with drop-weight impact velocity,while for the dispersed fibers,which decreases slightly.The dissipative energy curve of steel fibers for SFRC and ASFRC specimens decreases abruptly after peak value,and the descending branch of curve is close to the result of low impact velocity.