Prediction of Transient and Steady-State Flexural Fatigue Crack Propagation in Concrete Using a Cyclic R-Curve

Abstract

Fatigue damage in plain portland cement concrete is a nonlinear process that exhibits two distinct crack stages: a deceleration stage (transient crack growth) and an acceleration stage (steady-state crack growth). In the past, either the transient crack growth stage was completely neglected or was described using a power law that used crack extension as its argument. This works well for constant amplitude loading and for one geometry. However, to extend the fatigue prediction to variable amplitude loading, and also account for size effect, a fatigue crack resistance curve should be obtained so that the bridging stresses behind the crack tip can be calculated. In this paper, a new method has been developed to determine the fatigue crack resistance curve from crack rate and stress intensity data obtained from three point bending single edge notched specimens. The results of this study suggest that the fatigue crack resistance curve has a similar shape to the quasi-static crack resistance curve. Primarily, the critical crack extensions are equivalent, and the maximum fatigue crack resistance value is approximately 40–45% of the quasi-static fracture toughness at the critical crack extension. In addition, the results suggest that the KIC in fatigue is larger than the KIC in quasi-static loading. The reason for this is that the quasi-static resistance continues to rise after peak load, which will tend to increase the fracture toughness in fatigue. Finally, it is shown that by using a fatigue crack resistance curve, a unique set of Paris parameters C and n can be defined for both the transient and steady-state crack stages, which can then be used to predict the fatigue life without having to separate the two crack stages.