Fatigue performance test of concrete and study on damage assessment model

Fatigue performance tests on concrete specimens were conducted using a three-point equal amplitude loading method. As the number of fatigue loadings increased, microcracks inside the concrete extended and expanded, forming through-cracks that gradually developed into macroscopic, large cracks that rapidly propagated across the entire cross-section. Under different stress levels, the specimens exhibited a relatively stable ultimate residual strain value when fatigue failure occurred, while the maximum strain varied significantly. The development of bending fatigue deformation followed a three-stage pattern. Microscopic testing methods such as SEM electron microscope scanning and mercury intrusion porosimetry (MIP) were employed to analyze the whole process of deterioration and evolution of microcracks and pore structures in fatigue-damaged concrete. Fatigue loading damage resulted in a significant reduction in harmless and less harmful pores, or micropores, inside the concrete, while the proportion of harmful and more harmful pores increased substantially. Macroscopically, this was manifested as further intensification of fatigue performance deterioration. The entire fatigue development pattern is a process of continuous damage accumulation, culminating in instantaneous brittle fatigue fracture at failure. Based on the fundamental theory of continuum damage mechanics, the damage degree D was determined by quantifying the cumulative damage of concrete using residual strain. A fatigue damage evolution equation was derived from theoretical deductions, and a calculation model for the fatigue damage degree of concrete was proposed.