Experimental Study On Mechanical Properties And Failure Characteristics Of Cracked Cement-based Brittle Materials

With the application of repeated loads and the impact of the external environment on existing engineering structures,defects on the surface and the inside of the structure that cause greater damage to the structure(such as holes,cracks,etc.)appear.The existence of these defects seriously affects the stability of the structure.Cracks are more common defects in concrete structures.Existing cracks further expand under the load(static load and dynamic load),eventually leading to the instability and failure of the structure.There are large differences in the number of cracks and stress angles in the structure,and the failure modes are different.Common external loads include static and dynamic loads(such as earthquakes).Therefore,the dynamic and static mechanics of crack-containing cement-based materials is studied.The characteristics,the interaction mechanism between cracks and their failure modes are of great significance for taking effective crack arrest measures and ensuring the safety and stability of the structure.In this paper,a variable rate uniaxial compression test is performed on cracked cement-based brittle materials to comprehensively analyze the compressive strength,strain field evolution law,energy evolution law,and fractal characteristics of cracked cement-based brittle materials under different strain rates.To improve its static and dynamic mechanical properties,the main conclusions are as follows:(1)A variable rate uniaxial compression test was performed on a specimen containing a prefabricated crack through a laboratory test.The results show that its static characteristics are: as the crack inclination angle increases,the compressive strength continues to increase;when the number of cracks and inclination angle are the same As the strain rate increases,the compressive strength increases.(2)The CCD camera was used to monitor the crack evolution law,the static characteristics of precast cracks were obtained.As the crack inclination angle increased,the crack inclination angle generated during the specimen failure process gradually paralleled the axial force direction,and the main The crack gradually transferred from the crack generated at the prefabricated crack tip to the far-field crack generated at the edge of the specimen.With the same number of cracks and inclination,as the strain rate increases,the far-field cracks generated by the pre-cracked specimens gradually increase.(3)The digital strain correlation method(DIC)was used to analyze the strainfield of the specimen with prefabricated cracks,and the evolution of the strain field of the specimen under uniaxial compression was obtained.The results show that the failure of the specimen is mainly controlled by radial strain under static loading With the increase of the inclination of the crack,the effect of the prefabricated crack tip on the radial strain field of the sample gradually decreases,and the radial strain cloud of the sample gradually approaches the radial strain cloud of the complete sample.With the same inclination and different strain rates,as the strain rate increases,the influence of radial strain on the specimen gradually increases,and the influence of shear strain on the specimen gradually decreases.(4)The law of energy evolution under static and dynamic mechanics of prefabricated crack samples is mainly that under static mechanics,as the crack inclination angle increases,the total energy,pre-peak accumulation energy and post-peak dissipation energy show an upward trend with the overall crack inclination angle;As the strain rate increases,the total energy,accumulated energy,and dissipated energy show an upward trend as a whole.The main reason is that with the increase of the strain rate,when the specimen fails,the far-field cracks gradually increase,and the specimen produces The main crack is gradually parallel to the axial force direction.The crack propagation path is longer and the twists and turns are accompanied by the development of secondary cracks.The surface energy required for crack growth is greatly increased.(5)Using PFC numerical simulation software combined with laboratory tests,it is found that the inclination angle,number and length of the prefabricated cracks are the main factors that cause the mechanical properties of the sample to deteriorate.In actual engineering,a comprehensive analysis of the geometry of the cracks is required Take appropriate crack arrest measures.