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 ð€€€ N curve) which does not consider the failure due to crack prop- agation. Moreover, the design expressions obtained from S ð€€€ 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 investigation is undertaken in order to characterize the failure behavior under monotonically increasing displacements and fatigue loading of two quasi-brittle materials - graphite and concrete. In the case of graphite, there is sparce experimental data available on basic parameters including the S ð€€€ N curve which is used in the design of components in a nuclear power plant. Hence, in order to create realistic database, experiments are conducted on dumbel specimens subjected to constant amplitude fatigue loading under three di erent stress ratios and the Sð€€€N behavior is established. In addition, tests are conducted on notched graphite beams subjected to monotonically increasing displacements and constant amplitude fatigue loading, to obtain the crack growth curve and estimate the Paris constants.

Concrete being a heterogeneous material having constituents at di erent length scales from nanometers (cement, fly ash, silica fume) to millimeters ( ne and coarse aggre- gates) when subjected to external loading, creates local strain gradients which makes its behavior very complex. The pre-existing flaws act as stress risers causing mi- crocracking at lower scales that grow gradually under repeated loading leading to their coalescence and failure. Strain localization and the development of a large size fracture process zone necessitates the use of nonlinear fracture models whose input parameters have to be obtained from direct tension tests. Hence, in this work, the fracture and fatigue behavior of concrete is studied experimentally under tensile load- ing using the acoustic emission technique. Wedge splitting compact tension specimens are subjected to monotonically increasing crack opening displacements and constant amplitude fatigue loading with three di erent stress levels in a servo-hydraulic testing machine to understand the physics of microcrack formation, their coalescence, macro- crack growth and eventual failure. Important elastic and fracture properties including fracture energy, critical crack mouth opening displacement, critical crack length and size of fracture process zone which are required as input parameters in non-linear fracture mechanics based models are obtained. Most importantly, direct tension test has been carried out on large sized plain concrete specimens of two di erent sizes to get the overal softening load versus crack mouth opening displacement response and to understand the characteristics of the fracture process zone. The results obtained in this work could help in the engineering of new materials with desired properties for design of components subjected to complex fatigue loading. Furthermore, a good understanding of the fracture and damage process has emerged through the comprehensive experimental program and the results would provide useful guidelines for design and failure analysis of concrete structures when subjected to fatigue loading.