Experimental studies on the fracture and fatigue failure processes under direct tension in quasi brittle materials Graphite and Concrete

Abstract

Abstract Fatigue phenomenon is a gradual, permanent, micro structural change that takes place in a material due to the application of repeated loading. In the case of quasi-brittle materials like graphite and concrete, due to inhomogeneous nature and presence of pre-existing structural defects, the crack initiation phase is generally not considered in the estimation of fatigue life unlike comparatively more homogeneous materials like steel. The fatigue behavior of these quasi-brittle materials is not well explored and the complexities involved in conducting experiments in addition to costs and time have added to the lack of understanding of the internal mechanisms that lead to failure. In design of structural components, the fatigue problems are handled based on the safe life approach (S and#1048576; N curve) which does not consider the failure due to crack prop- agation. Moreover, the design expressions obtained from S and#1048576; N curve concept does not consider the fundamental material parameters, due to which the data meant for one design case is not applicable to other design cases with di erent applied loading, material and boundary conditions. With the emergence of the concepts and princi- ples of fracture mechanics, the design of structures subjected to fluctuating stresses has changed the direction and mechanistic methods have been developed by di erent researchers to predict the crack growth behavior along with the life prediction. Using this methodology, fatigue failure is avoided by keeping lower safety margins and us- ing damage tolerant mechanism. The theory of damage tolerant design predicts the fatigue crack propagation (FCP) as well as the number of load cycles required for an existing crack or a flaw at a critical location to reach critical crack length leading to failure of the structure. Furthermore, with improved instrumentation in terms of servo-hydraulics and acoustic emission sensing, the mechanisms of crack nucleation, initiation and growth could be well understood. In this work, an experimental...