Very high cycle fatigue tests with quenched and self-tempered reinforcement steel bars

Investigations on the fatigue strength of steel reinforcement bars (rebars) mainly involves fatigue tests with hot rolled (HR) and cold worked (CW) steels. However, in the last few decades, HR and CW rebars were replaced by quenched and selftempered (QST) rebars with hardened surface layer. There still remains a lack of research on fatigue strength of QST rebars especially in the very high cycle domain i.e., number of stress cycles surpassing 5 million. This study is part of a further detailed investigation on the fatigue behaviour of HR, CW and QST rebars in the very high cycle domain. It aims to investigate the fatigue performance of QST rebars axially tested at number of stress cycles in the range of 106–108. A preliminary study of the gripping method is followed by fatigue test results including nondestructive inspection of the rebar surface and fractographic analyses. The rebar surface is examined with liquid penetrant to reveal fatigue crack location and size in specific frequency interval monitored during the tests. Fractured surface analyses are performed by scanning electron microscopy to detect the location from where fatigue cracks initiate. Cross sectional area reduction resulting from fatigue crack propagation is also determined. Fractographic investigations are compared with the fractured surfaces of HR, CW and QST rebars from the literature.

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

Marina Rocha Pinto Portela Nunes

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.

EPFL2014

Geometrical and material characterisation of quenched and self-tempered steel reinforcement bars

Eugen Brühwiler, Alain Nussbaumer, Marina Rocha Pinto Portela Nunes

Quenched and self-tempered (QST) steel reinforcement bars (rebars) are manufactured by Thermex or Tempcore processes. Characterization studies of QST rebars are mostly limited to reveal the hardened outer layer and some improved mechanical properties compared to hot-rolled (HR) and cold-worked (CW) rebars. However, investigations on residual stresses and imperfections originating from the manufacturing process as well as stress concentrations arising from the ribbed profile are rarely found in the literature. Surface residual stress may be beneficial or detrimental to the fatigue performance of rebars although they have been studied only on the subsurface of QST rebars. Surface imperfections are zones of stress concentration from where fatigue cracks may initiate. Imperfections are usually identified in the fractured cross section resultant from fatigue tests of rebars. Stress concentrations can also arise from the rib geometry. Although the rib geometric parameters affect the stress concentration factor Kt on rebars, stress concentration analysis are restricted to two-dimensional (2D) finite-element models (FEMs). The study presented herein is part of a further detailed investigation on the fatigue behavior of HR-CW and QST rebars in the very high cycle domain. In the present work, characterization analyses of QST rebars include (1) experimental investigation of surface and subsurface residual stresses on QST rebars by cut compliance and X-ray diffraction techniques (residual stresses from both techniques are discussed); (2) identification of surface imperfections by scanning electron microscopy (SEM) analysis; and (3) three-dimensional (3D) finite-element analysis of stress concentrations on the ribbed profile. The influence of the rib geometry such as radius, width, height, and inclination as well as the rebar diameter on Kt values are analyzed. The critical zones are determined along the ribs. (C) 2016 American Society of Civil Engineers.

Asce-Amer Soc Civil Engineers2015

Prediction of fatigue life of reinforced concrete bridges using Fracture Mechanics

Eugen Brühwiler, Marina Rocha Pinto Portela Nunes

With the occurrence of higher and more frequent axle loads, bridges are more solicited by fatigue loading. Bridge elements like deck slabs are subjected to a high number of stress cycles at relatively small stress magnitudes. The application of Fracture Mechanics as a useful tool for the analysis of fatigue crack growth in steel elements was demonstrated by Paris et al. in the early 1960s. With respect to reinforced concrete, the fatigue strength of the steel reinforcement is determinant. The fatigue behaviour of the steel reinforcement is similar to that of structural steel. The fatigue relevant parameters are the stress ranges, the number of fatigue cycles and stress concentrations. This paper presents a study to predict the fatigue life of steel reinforcement based on the Paris law. The method will be validated by a case study.