Study On The Fracture Parameters Of Concrete Based On The Peak Load

As a typical kind of heterogeneous composite material, the traditional strength theory and linear elastic fracture mechanics theory cannot be used to describe and predict the fracture behavior and failure process of concrete accurately. The double-K fracture model which combines fictitious crack model and elastic equivalent method can be used as a fracture criterion to describe the whole crack propagation process, and has been established as the theoretical basis for DL/T5332-2005《Norm for fracture test of hydraulic concrete》. The fracture mechanics test is an important part in the study of fracture mechanics of concrete. In the general experimental equipment and experimental conditions, a relatively large error may occur in measuring the crack opening displacement, the requirement for determining the peak load is relatively low for the experimental condition. It is of great significance to simplify the process of concrete fracture test, to improve the test accuracy of concrete fracture teat and to rich the calculation method and calculation theory of fracture parameters of concrete by adopting the peak load to study the fracture characteristics of concrete.In this paper, the double-K fracture parameters were determined by the fracture tests of three-point bending beams with different strength grades, different sizes and different initial notch to depth ratios. The relationship among the double-K fracture model, two parameter fracture model and size effect model was analyzed by the peak load method. An improved peak load method, which is based on the double-K fracture model and considers the crack cohesive stress and change characteristics of external load, was proposed to determine the double-K fracture parameters. The boundary effect model and size effect model were adopted to analyze the size effect of the nominal strength of concrete. The comprehensive size effect model which is based on the equivalent elastic fracture mechanics method and a power law was proposed. The specimen size, initial crack length and fracture process zone are taken into account in the comprehensive size effect model. The proposed methods were verified through the experimental data and research data in the existing literature. The research conclusions can be summarized as follows:（1） The influence of the strength grade on the initiation fracture toughness （?） and unstable toughness （?） is significant, （?） and （?） increase with the increase of strength grade of concrete, the influence of the specimen size and initial notch to depth ratio on （?） and （?） is almost negligible.（2） The fracture parameters determined by the peak load method are equivalent to the fracture parameters for infinite structure. The double-K fracture model, two parameter fracture model and size effect model were validated equivalently by the peak load method. The requirement of improved peak load method is relatively low for the experimental condition, measuring the load-crack mouth opening displacement curves is necessary. The calculated results show that the calculated values of improved peak load method agree well with the values obtained by using the conventional analytical method. In the proposed method, both the error may occur in measuring the crack mouth opening displacement during the test and the limit of size or geometry of the specimen can be avoided. The proposed method can ensure the accuracy of the test and simplify the test operation.（3） Both the boundary effect model and size effect model can describe the fracture behavior of concrete well. The critical stress intensity factor K_Ic determined by two methods are very close. However, there are some differences in the values of the tensile strength ft determinated by two models.（4） The comprehensive size effect model can describe and predict the quasi-brittle fracture behavior of concrete material well. In addition,the critical stress intensity factor can be determined only using the peak load P_max of different specimens based on the comprehensive size effect model. The values of critical stress intensity factor determined based on the comprehensive size effect model are very close to those of equivalent elastic fracture models determined by conventional analytical method and numerical analysis method.