Size Effect Of Fatigue Fracture Of Mode ? Crack In Concrete

The mass concrete structure represented by the dam has many internal micro cracks.The micro crack will increase and interconnect under fatigue loads such as external seismic load or cyclic water pressure which can cause macro cracks format,propagate and affect the normal use of the dam seriously.In engineering practice,the fatigue crack propagation law of mass concrete structures is often analyzed by using the fatigue fracture test results of small-sized concrete specimens in the laboratory,but the analysis results are not accurate due to the size effect of fracture parameters.Therefore,it has been the focus of engineering research to establish a method to obtain actual fatigue crack propagation law of mass concrete structures by analyzing the fatigue fracture test results of small-sized concrete specimens.At present,the research of the size effect of static load fracture of concrete has developed extensively and achieved ideal results at home and abroad,but the existing research results on the size effect of fatigue fracture of concrete are difficult to be applied to engineering practice and have great limitations.Therefore,the fatigue fracture test of three-point bending beams with similar size of specimen depth is carried out in this paper,and a new method is proposed to establish the fatigue fracture propagation law of mass structures in engineering practice through the fatigue fracture test results of small-sized specimens which are made in the laboratory.The main work and conclusions are as follows:?1?Non-standard three-point bending beams were used for static load fracture tests and constant amplitude fatigue load fracture tests with stress levels of 0.80,0.85 and 0.90respectively in this paper.The surface crack propagation length is measured by clamp extensometers which are distributed equally along the ligament,and then the?_s-N curve of fatigue fracture test,the value of stress intensity factor of fatigue fracture failure and the static fracture toughness are obtained by calculation.The study shows that the properties of K_IC of the static load fracture test and the value of stress intensity factor of fatigue fracture failure K_Iff are equal when the specimen has the same size.Therefore,the K_If of the fatigue fracture test can be substituted by the K_IC of the static load fracture test when the size of the specimen is same.And the K_{I C} and K_If also has size effect,its value increases with the increase of the specimen depth.?2?Three scatter distribution zones which are approximately parallel are obtained by substituting the fatigue fracture test results into the Paris law because of the size effect of the fatigue fracture intensity factor.Due to the fact that the scatter distribution zones cannot overlap into a single distribution zone,the fatigue crack propagation rules of specimens of different sizes cannot be described by the uniform Paris formula.Therefore,the Paris formula is not suitable for calculating the fatigue crack propagation rate to represent the fatigue crack propagation rule of concrete specimens of different heights.?3?The Forman formula is adopted to analyze the data of the fatigue fracture test because of the phenomenon that K_If increases with increasing specimen depth.After substituting the fatigue fracture test data of part?1?into the Forman formula,it shows that the Forman formula of different stress level and specimen depth is same,which makes the formula can be used to analyze the fatigue crack propagation law of mass concrete structures in engineering practice through the fatigue fracture test of small-sized specimens in the laboratory.The standard normal distribution statistics method is used for error analysis and it shows that the prediction result has a 95%assurance rate.?4?It shows that the fatigue crack propagation law is independent of the stress level after substituting the fatigue fracture test data of part?1?into the Paris law and the Forman formula respectively,which verifies that the fatigue crack propagation law of the same sized three-point bending beams under different stress levels can be analyzed by the same Forman formula.The research results above show that the Forman formula can be used as size effect formula to analyze the size effect of fatigue fracture of mode I crack in concrete,and it provides a new research idea for predicting the fatigue crack propagation of mass concrete structures in engineering practice by means of fatigue fracture test results of small-sized specimens which are made in the laboratory.