Study On Time-Dependent Fracture Crack Propagation Rate Of Concrete

Under long-term loads,the crack of concrete and quasi-brittle materials will expand with passage of time due to time-dependent effects,which affects the structural safety.Given the significant difference between quasi-static fracture behavior without temporal correlation which has maturely researched and the time-dependent fracture behavior of concrete with time correlation,it is necessary to study the law of crack propagation of concrete under long-term loading.In this paper,propagation rate and critical threshold value of concrete time-dependent fracture cracks are analyzed and studied,and the main content is as follows:1.The relationship curve CMOR（t）-t between the crack mouth opening rate and time for time-dependent cracks of the standard three-point bending beam test specimen exhibits three-stage characteristics.In the first stage,the crack mouth opening rate gradually decreases;in the second stage,the crack mouth opening rate is relatively stable;and in the third stage,the crack mouth opening rate increases rapidly until the test piece is damaged.2.The crack propagation rate under long-term loads is calculated using the method for calculating fatigue crack propagation rate.Then,the relationship between the crack propagation rate da/dt（t）and the stress intensity factor K（t）is studied with the Paris formula which is widely used in the study of metal fatigue.The results show:the relationship between the crack propagation rate da/dt（t）and the stress intensity factor K（t）in the second stage can be well expressed with the Paris formula;the crack propagation rate is increased with the increase of loads,but exhibits non-linear relationship;the loads have a greater impact on the value of parameter C in the Paris formula and a relatively small impact on the value of parameter m.The analysis of the influencing factors of time-dependent fracture life with Paris formula shows that there is a negative correlation between the crack propagation life and the constant loads.3.The stress intensity factor K（t）of crack propagation at the critical points in each stage is calculated and analyzed.The results show:the dispersion of stress intensity factor K_Ⅰ-Ⅱat the critical point between stage I and stage II and that of stress intensity factor K_Ⅱ-Ⅲat the critical point between stage II and stage III are small;after the crack propagation reaches the next stage,there is a stable threshold value of the stress intensity factor;since the intensity factor at the critical point between stage II and stage III is similar to the mean value of unstable fracture toughness K_Ic^unin the monotonically loaded static fracture test,the stability of crack under long-term loads can be evaluated with the fracture toughness obtained from the monotonically loaded static fracture test.