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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.
The high cycle fatigue (HCF) behaviour and load sequence effects in HCF of two microalloyed steels are presented. Dislocation structures that resulted from HCF loading were examined. The fatigue ...limits of the two microalloyed (MA) steels (non-failure after 2×10
cycles) correlated well with the cyclic yield strength. The results of two-step loading showed that in both the steels after an initial life fraction of approximately 0.4, the residual life deviated significantly from that predicted by the Palmgren-Miner rule. Crack initiation lives of the MA steels had been determined from the two-step loading experiments. Fatigue functions, which can be used for life prediction under complex loading conditions using the consecutive Wöhler curve approach, were determined from the two-step loading tests. Dislocation cell structures were observed near the fracture surface in the HCF regime. At the fatigue limit, dislocation dipoles and loops were seen.
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.
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