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•The fatigue experiments were performed on notched three-point bending tests.•In total, four concrete types have been studied C 50/60, HPC, HSC, and AAC.•The novel methodology for ...transition from the S-N field to Paris’ law is prosed.•The proposed methodology was validated by back-calculation of the S-N curve.•The recommendation of the number of specimens is presented and discussed.
Fatigue behaviour of concrete materials is often investigated on un-notched specimens under the compressive or bending loads. In this experimental study, a notched three-point bending (TPB) specimen is used in high-cycle fatigue experiments to obtain the Wohler’s curve. Based on this approach, a novel, yet relatively simple transition from the traditional Wohler’s curve to the Paris’ law curve is proposed. Such a methodology allows one to obtain the Paris’ law material constants, which are used to determine the fatigue failure of the structure or a component. The constants of the experimentally determined material, measured in four different concrete mixtures, have been verified by recalculating the number of cycles until the fatigue failure Nf by the integration of the Paris’ law equation. The back-calculated number of cycles and the approximation of the S-N curve allowed for a comparison with the experimental data. Furthermore, the initial notch tip was extended in this approximation by the value of the critical distance. Such an extension allowed us to cover a wide range of the experimental data and provided a better prediction of fatigue life. The proposed method was verified on all four studied materials and showed satisfactory results.
A framework for characterising the static and fatigue crack resistance of a fully supported beam on an elastic foundation is presented. The crack resistance is given as a function of the resisting ...cohesive stress and represented mathematically with a set of simple linear regression equations that can be easily integrated within a mechanistic-empirical design framework. The static crack resistance is derived using a weight function integral and reciprocal energy formulation that may be extended to more complex structural systems containing two-dimensional crack growth. Finally, a modified Paris law that utilises the crack resistance curves is used to develop a modified S-N diagram that can capture the fatigue response under various support conditions and different material characteristic lengths and structural sizes.
The fatigue and crack growth behavior of Inconel 718 and AL6XN alloys joined with an ERNiFeCr-2 filler by the gas tungsten arc welding process was evaluated. A comparison between Wöhler curves for ...the monolithic IN718 and AL6XN alloys and the IN718-AL6XN dissimilar welded joint were obtained. Although Vickers microhardness profiles revealed the presence of a soft region in the heat affected zone of the IN718 base material, the fatigue crack nucleated at the fusion zone near to the AL6XN and propagated across the weld bead. The welded joint and the AL6XN base material presented a similar high cycle fatigue behavior despite the presence of an unmixed zone at the AL6XN/weld metal interface. The results of the fatigue crack growth test indicated that the weld metal has the highest growing rate owing to the presence of brittle secondary phases such as Laves and NbC. The fatigue crack growth rates obtained from experimental data were compared with measurements of the distance between striations on the fracture surfaces, both were very close.
This paper investigates the influence of molecular properties (molecular weight, polydispersity (PDI) and molecular weight distribution (MWD)) on the mechanical fatigue of model systems of anionic ...synthesized polystyrene (PS, low PDI), their blends (bimodal MWD) and commercial PS (broad PDI) under strain-controlled oscillatory shear (torsion). A power-law relation between lifetime Nf and strain amplitude γ0 (Wöhler curve) was found, with the exponent independent, but the pre-factor highly dependent on molecular characteristics. Additionally, nonlinear changes of the stress response, due to ongoing fatigue, were quantified via Fourier Transform (FT), to analyze besides the storage modulus G′, the intensities of the second (I2) and third (I3) harmonics over the fundamental one (I1). The behavior of G′ and I3/1 was found to depend on molecular properties, with I3/1 being more sensitive to failure onset than G′, while an increase of I2/1 above the noise level could be correlated with the occurrence of macroscopic cracks.
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•Fatigue resistance is highly function of polymer average molecular weight.•Number average is better than weight average to represent the fatigue resistance.•There is a transition around Mn = 180 kg/mol for polystyrene.•Fourier transform is a powerful tool to analyze on-line test under oscillation.
An investigation was performed where 6061-T6 extrusions were exposed to a 3.5% NaCl solution at pH 2 for 2 days and 24 days to create distinct surface flaws. The effect of these flaws on the rotating ...beam fatigue life was then investigated and analyzed by using Wöhler curves, Weibull statistics and scanning electron microscopy (SEM). It was determined that corrosion damage reduced the fatigue life significantly and specimens corroded for both 2-days and 24-days exhibited similar fatigue lives. Statistical analyses showed that fatigue life of all three datasets followed the 3-parameter Weibull distribution and the difference between the fatigue lives of two corroded datasets was statistically insignificant. Analysis of fracture surfaces showed that sizes of pits that led to fatigue crack initiation were very different in the two corroded datasets. Implications of the similarity in fatigue lives despite disparity in surface condition are discussed in detail in the paper.
Abstract
Betonstrukturen wie Verkehrsflächen oder Industrieböden sind zyklischen Beanspruchungen aus Verkehr oder Maschinenvibrationen ausgesetzt. Zur Abschätzung der Ermüdungslebensdauer dienen ...Wöhlerlinien, die in der Regel experimentell an Referenzprobekörpern bei einachsiger Beanspruchung konstanter Frequenz, Amplitude und Oberspannung validiert werden. Unter realen Betriebsbedingungen treten Einwirkungen jedoch häufig mehrachsig und mit variabler Lasthöhe auf, sodass die im Zeitverlauf progressiv zunehmende Materialschädigung rechnerisch zu akkumulieren ist. In der Praxis hat sich dazu die lineare Akkumulationsregel nach Palmgren‐Miner etabliert, obwohl diese bekannte Effekte aus der Belastungsreihenfolge vernachlässigt. Im Beitrag wird zunächst das Wöhlerkonzept durch Einführung einer Vergleichsspannung auf mehrachsige Spannungszustände erweitert und anschließend ein modifizierter Schädigungsakkumulationsansatz entwickelt, welcher die Auswirkungen von Be‐ und Entlastungen mehrstufiger Lastprozesse auf verbleibende Bruchlastspielzahlen berücksichtigt. Der Ansatz wird anhand von Literaturdaten verifiziert und in Form eines Bemessungshilfsmittels zur praktischen Nutzung aufbereitet. Die Anwendung ist am Beispiel demonstriert.
Abstract
Fatigue of concrete under multiaxial and multistage loads
Concrete structures like traffic areas or industrial floors are subjected to cyclic loads induced by traffic or machine vibrations. To assess fatigue lifes, Wöhler‐curves serve, which are usuallay validated by experiments. During testing the reference specimens are exposed to uniaxial and constant amplitude fatigue although in practice cyclic loads often occur multiaxial and with alternating frequency and magnitude. As a consequence the material's progressively increasing degradation has to be mathematically captured for design purposes until failure. In this regard, the linear accumulation hypothesis of Palmgren‐Miner is well established although it neglects distinct sequence effects. In the paper, Wöhler's concept is generalized to multiaxial stress states by introducing an equivalent stress. Additionally, a modified damage accumulation procedure is derived to gather the impact of increasing or decreasing loads on residual numbers of cycles to failure in case of variable amplitude fatigue. The approach is verified to experimental data taken from the literature and elaborated to a design chart. Its practical application is demonstrated exemplarily.
For over two decades, vascular stents have been widely used to treat clogged vessels, serving as a scaffold to enlarge the narrowed lumen and recover the arterial flow area. High-purity ...oligocrystalline austenitic steel is usually applied for the production of stents. Despite the popularity and benefit of stenting, it still may cause serious clinical adverse issues, such as in-stent restenosis and stent fracture. Therefore, the study of the mechanical properties of stents and in particular the prediction of their life cycles are in the focus of materials research. In our contribution, within the finite element method, a two-scale model of crack initiation in the microstructure of stents is elaborated. The approach is developed on the basis of the physically based Tanaka–Mura model (TMM), considering the evolution of shear bands during the crack initiation phase. The model allows for the analysis of the microstructure with respect to the life cycles of real materials. The effects of different loading conditions, grain orientation, and thickness of the specimen on Wöhler curves were analysed. It was found that the microstructural features of oligocrystals are very sensitive to different loading conditions with respect to their fatigue behaviour and play a major role in fatigue crack initiation. Different grain-orientation distributions result in qualitative and quantitative differences in stress distribution and in the number of cycles for crack initiation. It was found that presence of a neutral zone in the cut-out of the microstructure under three-point-bending loading conditions changes the qualitative and quantitative patterns of stress distribution and affects the number of cycles for crack initiation. It was found that under both tensile and bending loading conditions, thicker specimens require more cycles for crack initiation. The Wöhler curves for crack initiation in oligocrystalline microstructures of stents could be compared with the ones in the experiment, taking into account that for high cyclic fatigue (HCF), typically, more than 70% of the cycles refer to crack initiation. The developed numerical tools could be used for the material design of stents.
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