Study On Fracture Characteristics Of U-notched Concrete Beams

The tensile strength and fracture toughness of concrete are important material parameters.How to obtain these two important parameters through effective testing has always been the key research content of fracture analysis of quasi-brittle materials such as concrete.Due to the heterogeneous nature of the concrete material and the moderate difficulty of the sample,it is difficult to obtain the tensile strength of the concrete by direct stretching.The fracture toughness obtained by the conventional concrete three-point bending test has a size effect,etc.,and needs to seek new tests.The method is to obtain these two material parameters that are independent of the geometry or size of the sample.The study of concrete fracture mechanics is usually similar to the case of straight cracks or sharp V-shaped cuts made under metal materials.However,in actual production,it is difficult to make specimens with sharp cracks by means of steel sheet insertion,etc.,and the pores always contain certain Curvature.At the same time,the actual situation of non-sharp cracks may sometimes occur in civil engineering.Therefore,the influence of concrete fracture problems including non-sharp defects such as U-shaped openings on the accuracy of concrete fracture test and its own strength characteristics are studied.necessary.The critical distance theory is the local fracture failure criterion of the microscopic properties of the material and the macroscopic fracture characteristics of the material.This paper attempts to obtain the tensile strength and fracture toughness of the material by the critical distance theory and the fracture analysis of the concrete beam with U-shaped orifice.It provides a new idea and research method for the fracture mechanics analysis of concrete.In order to study the influence of U-notched depth,root radius of grooving and maximum particle size on the fracture characteristics of concrete quasi-brittle materials,four U-notched depths,four grooving root radii and three kinds are designed.A total of 66 concrete beam specimens with a maximum particle size of 400mm?100mm?100mm.The beam is loaded by three-point bending method,the cracking load value of the U-notched concrete beam is measured by the resistance strain gauge method,and the length of the fracture process zone and the opening displacement of the notch are obtained by digital image method(NMOD The critical distance strength factor and the failure stress of the beam are studied by the critical distance theory?TCD?,and compared with the results of the energy density criterion?SED?and the finite fracture mechanics?FFM?.The main conclusions of this paper are as follows:?1?The strain gauge method is used to measure the cracking load of concrete specimens during three-point bending loading.The test results show that the cracking load becomes smaller with the increase of the grooving depth under the same condition,and becomes larger as the kerf root radius increases,and the maximum aggregate particle size is the same when the grooving depth and the root radius are the same.WhenD_max?28?20mm,the cracking load is the largest;?2?The failure load of the U-notched concrete beam during the three-point bending loading process.Under the same condition,the failure load becomes smaller as the grooving depth increases,and becomes larger as the kerf root radius increases.When the grooving depth and the root radius are the same,the maximum aggregate particle sizeD_max?28?20mm is the maximum cracking load;?3?The length of the fracture process zone and the open displacement at the bottom end of the groove were measured by digital image method during the three-point bending loading of the U-notched concrete specimen.The test results show that under the same conditions,the length of the fracture process zone decreases with the increase of the grooving depth,and increases with the increase of the kerf root radius.When the other factors are the same,the maximum particle size of the aggregate isD_max?28?20mm.The length of the fracture process zone is the shortest.The NMOD value is positively correlated with the load.Under the same load,the NMOD value becomes larger as the grooving depth,root radius and maximum aggregate size increase;?4?Using ANSYS structural analysis software to model the stress concentration factor of U-shaped grooved concrete beam.The calculation results show that the change of the stress concentration coefficient with the increase of the depth of the groove increases first and then decreases and becomes smaller as the radius of the root of the groove increases;?5?Using the"two-parameter test method"derived from the critical distance midline method to measure the fracture toughness and tensile strength of concrete and comparing with the calculation results of the"double K fracture criterion"it is found that the two-parameter test is measured.The fracture toughness results are close to the instability fracture toughness in the double K fracture criterion;?6?Calculating and analyzing the generalized strength factor and failure stress of U-notched concrete beam by the line method in critical distance theory and comparing with the energy density criterion and the analysis results of finite fracture mechanics the critical distance theory is found.The analysis results of the other two fracture theories are similar indicating that the critical distance theory can be used to analyze the fracture problem of U-notched concrete beams.