In this research, the fracture processes in concrete subjected to monotonic and fatigue loadings are characterized and the differences in failure mechanisms are studied using acoustic emission (AE) and digital image correlation (DIC) techniques. Experiments are performed on notched plain concrete beams under three-point bending. The micro and macro structural activities are classified based on acoustic energy levels to differentiate between the formation of a fracture process zone (FPZ) under monotonic and fatigue loadings. It is observed that a FPZ within its conventional definition is formed under monotonic loading. On the contrary, the AE results under fatigue loading indicate isolated and dispersed micro cracks up to 95% of fatigue life. A damage index based on AE energy is proposed for concrete subjected to fatigue loading which could be used in health monitoring of structures.
Frequency of loading plays a signicant role in influencing the fatigue response of concrete. It is widely accepted in the literature that concrete fatigue life increases with increase in loading frequency. This behaviour is counter-intuitive, as the specimen is expected to fail in less number of cycles when subjected to higher loading frequency. This paper aims at understanding the fracture and failure mechanisms responsible for this counter intuitive behaviour through an experimental investigation. The flexural fatigue experiments are performed on concrete beam specimens subjected to three different loading frequencies: 0.5 Hz, 2 Hz and 4 Hz with the aid of acoustic emission technique. The mechanical and acoustic emission results reveal that the increased fatigue life for specimens subjected to higher loading frequencies is attributed to the widely distributed and randomly oriented micro cracks that can blunt the effect of high stresses by increasing the energy required to cause failure. The concrete specimens subjected to lower loading frequencies tend to exhibit brittle behaviour and consequently fail at lower number of fatigue load cycles. 1
Abstract. Damage quantification of concrete subjected to variable amplitude load is not a simple task. In this study, a procedure is proposed to convert variable amplitude load acting on concrete structures to an equivalent constant amplitude load. The procedure described in this work is developed using an existing fatigue crack propagation model and an energy dissipation model which are both based on the energy approach. Further, the damage incurred by concrete is quantified using damage index, which is obtained by normalizing the cumulative energy dissipated with the total cumulative energy dissipated at the critical crack length of the concrete specimen.
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