Fatigue Behaviour of Steel Reinforcement Bars at Very High Number of Cycles

Rocha, Marina (Rocha Pinto Portela Nunes, Marina)
Advisors: Brühwiler, Eugen; Nussbaumer, Alain

A large portion of reinforced concrete (RC) bridges in the western world were built in the second half of the last century. RC bridge deck slabs are nowadays often subjected to increased traffic loading and volume than originally designed for and thus, steel reinforcement bars (rebars) are more susceptible to fatigue damage. Fatigue life of rebars can be largely affected by the crack initiation phase characterised by the growth of short cracks. The approaches available for fatigue damage evaluation of rebars fail to predict the crack initiation phase. Microstructural barriers control the short crack behaviour which can be significantly different from the stable long crack growth described by Paris' law. The stochastic nature of the fatigue life comes mainly from the scatter of these short cracks. Research on this domain is attractive since it can help to understand more accurately the fatigue behaviour of rebars. A better understanding can result in more accurate fatigue damage evaluation of RC elements. The aim of this thesis is to predict the scatter and fatigue behaviour of hot rolled (HR), cold worked (CW) as well as quenched and self-tempered (QST) rebars incorporating the crack initiation phase. The research commences with an experimental investigation on the fatigue strength of QST rebars under high and very high cycle fatigue (HCF-VHCF) at constant amplitude (R= 0.1). A non-destructive inspection technique was applied for surface crack detection based on the frequency change monitored during the tests. Surface and near surface macro residual stresses on QST rebars were determined by X-ray diffraction and Cut Compliance techniques. Surface imperfections and roughness were identified with Scanning Electron Microscopy mainly near the ribs. A parametric study of the rebar geometry, using 3D Finite Element Models, allowed to determine the influence of rib inclination and rebar diameter on the stress concentration factors. A short crack growth model was developed to study the scatter resulting from the interaction between short crack and microstructural barriers. The model includes dispersion of grain orientation ratio, grain size variation and different phases (ferrite-pearlite and martensite). This model was then model to include the surface roughness effects and long crack propagation. The stress concentration factor was considered as a constant parameter. The model predicted the fatigue behaviour of HR-CW and QST rebars.

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