Papers by Keyword: Short Fatigue Cracks

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Authors: A.J. McEvily
Abstract: The behavior of short fatigue cracks is a matter of importance not only because much of the fatigue lifetime is spent in propagating these cracks, but also because the boundary between propagation and non-propagation separates the safe from the potentially unsafe fatigue regimes. The method of analysis is based upon the following equation:
Authors: Ulrich Krupp, Alexander Giertler, Stephanie Siegfanz, Wilhelm Michels
Abstract: The correlation between the microstructure, the mechanical properties and the fatigue life of the common aluminum cast alloy Al-7Si-0.3Mg (A356) was investigated. By variation the solution heat treatment temperatures and times the precipitation strengthening effect in the dendritic aluminum solid solution phase and the spheroidization of the eutectic silicon were modified. The results of fully reversed fatigues tests revealed an increase in the fatigue life of specimens that were heat treated at higher temperatures. This observation was supported by analyzing the fatigue crack propagation behavior using the direct current potential drop technique (DCPD). With (i) increasing heat treatment temperature, i.e., increasing dendritic α-Al strength and (ii) roundness of the eutectic silicon particles the resistance to technical fatigue crack initiation, expressed by the threshold value of the stress intensity range Kth, was shifted to higher values.
Authors: Ulrich Krupp, Alexander Giertler, Stefanie Siegfanz, Wilhelm Michels
Abstract: Specimens of the common cast alloy Al-7Si-0.3Mg (A356) were solution heat treated at different temperatures and times to modify (i) the precipitation strengthening effect in the aluminum solid solution phase and (ii) to spheroidize the eutectic silicon. Just 15 minutes of solution heat treatment at a temperature of 540°C are sufficient to reach the desired effect. Cyclic loading experiments revealed an increase in fatigue life as compared to specimens heat treated at lower temperatures. In particular in the HCF regime, fatigue cracks that were originally initiated at pores follow crystallographic slip bands under shear control (mode II) as it was proven by automated EBSD (electron back scatter diffraction) measurements. The smoothly polished surface of the fatigue specimens was observed continuously by a long-distance microscope and discontinuously by SEM (scanning electron microscopy) to identify the fatigue crack propagation mechanisms. It was shown that da/dN decreases strongly when the crack tip interacts with the eutectic areas. Obviously, the blocking effect of the eutectic silicon particles makes the crack leaving the straight slip-band path.
Authors: Ulrich Krupp, Helge Knobbe, Olaf Düber, Hans Jürgen Christ, Philipp Köster, Boris Künkler, Claus Peter Fritzen
Abstract: In the present paper examples for propagating and non-propagating conditions of slip bands and short fatigue cracks in a ferritic-austenitic duplex steel are given, which were quantified by means of SEM in combination with automated EBSD. To classify the results within the scope of predicting the service life under HCF- and VHCF-loading conditions a numerical model based on the boundary-element method has been developed, where crack propagation is described by means of partially irreversible dislocation glide on crystallographic slip planes in a polycrystalline model microstructure (Voronoi cells). This concept is capable to account for the strong scattering in fatigue life for very small strain amplitudes and to contribute to the concept of tailored microstructures for improved cyclic-loading behaviour.
Authors: A.J. McEvily
Abstract: Many of the recent advances in the understanding of the fatigue crack growth process have resulted from an improved realization of the importance of fatigue crack closure in the crack growth process. Two basic crack closure processes have been identified. One of which is known as plasticity-induced fatigue crack closure (PIFCC), and the other is roughness-induced fatigue crack closure (RIFCC). Both forms occur in all alloys, but PIFCC is a surface-related process which is dominant in aluminum alloys such as 2024-T3, whereas RIFCC is dominant in most steels and titanium alloys. A proposed basic equation governing fatigue crack growth is (1) where where Kmax is the maximum stress intensity factor in a loading cycle and Kop is the stress intensity factor at the crack opening level. is the range of the stress intensity factor at the threshold level which is taken to correspond to a crack growth rate of 10-11 m/cycle. The material constant A has units of (MPa)-2, and therefore Eq. 1 is dimensionally correct. Eq.1 has been successfully used in the analysis of both long and short cracks, but in the latter case modification is needed to account for elastic-plastic behavior, the development of crack closure, and the Kitagawa effect which shows that the fatigue strength rather than the threshold level is the controlling factor determining the rate of fatigue crack growth in the very short fatigue crack growth range. Eq. 1 is used to show that The non-propagating cracks observed by Frost and Dugdale resulted from crack closure. The behavior of cracks as short as 10 microns in length can be predicted. Fatigue notch sensitivity is related to crack closure. Very high cycle fatigue (VHCF) behavior is also associated with fatigue crack closure.
Authors: Ulrich Krupp, Marcus Söker, Tina Waurischk, Alexander Giertler, Benjamin Dönges, Hans Jürgen Christ
Abstract: As being used for structural applications, where a high corrosion resistance is required, the fatigue behavior of duplex stainless steels (DSS) is governed by the partition of cyclic plasticity to the two phases, ferrite and austenite, respectively. Under very high cycle fatigue (VHCF) loading conditions, the heterogeneous distribution of crystallographic misorientations between neighboring grains and phases yields to a pronounced scatter in fatigue life, ranging from 1 million to 1 billion cycles for nearly the same stress amplitude. In addition, the relevant damage mechanisms depend strongly on the atmosphere. Stress corrosion cracking in NaCl-containing atmosphere causes a pronounced decrease in the VHCF life. By means of ultrasonic fatigue testing at 20kHz in combination with high resolution scanning electron microscopy, electron back-scattered diffraction (EBSD), focused ion beam milling (FIB) and synchrotron tomography, the microstructure heterogeneities were quantified and correlated with local fatigue damage. It has been shown that the fatigue process is rather complex, involving redistribution of residual stresses and three-dimensional barrier effects of the various interfaces. The application of a 2D/3D finite element model allows a qualitative prediction of the fatigue-damage process in DSS that is controlled by stochastic local microstructure arrangements.
Authors: Pavel Hutař, Stanislav Seitl, Zdeněk Knésl
Abstract: A two-parameter constraint-based fracture mechanics approach is used to explain the effect of the constraint on the apparently anomalous behavior of short fatigue cracks. The different levels of stress constraint are quantified by the T-stress, and microstructurally as well as mechanically short cracks are discussed. Short cracks generally behave more sensitively to the constraint than the long ones. It is shown that in most cases, the existence of short cracks goes hand in hand with an intrinsic loss of the constraint, which contributes to a decrease of their fatigue threshold values and accelerates their growth. In this paper, the above effect is quantified and conclusions concerning the applicability of the fracture mechanics parameters and approaches to the estimation of the residual fatigue life of structures are discussed.
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