Advanced Materials Research Vols. 891-892

Abstract: Generally, mechanical components or structures are subjected to random and a three-dimensional stress state; there are very few field loading paths which can be experimentally fully simulated in laboratory. Loading path parameters such as load sequence, stress level or proportionality/non-proportionality presences are unknown variables with unknown levels under random loading conditions which are impossible to modulate in laboratory because the load spectra is unknown. The load spectrum depends on numerous factors such as environmental, mechanical or user behavior. At design stages the fatigue life estimation is based on typical loading paths or typical loading spectra, however that assumption may be very different from the usage regime. From here it can be concluded that the random multiaxial fatigue issue is of utmost importance to monitoring the in-field damage accumulation. This work presents a proposal to estimate the accumulated damage resulted from multiaxial random loadings based on the SSF equivalent stress and SSF virtual cycle counting concept.

Abstract: In this work it is studied the low-cycle fatigue behavior of the magnesium alloy AZ31-B at several total strains under uniaxial and multiaxial cyclic loading conditions. Cyclic tests were carried out in a biaxial servo hydraulic machine under strain control at room temperature. Test specimens were machined in an hourglass shape from extruded rods. The total strain amplitudes started at 0,2% and ended at 1.4% regarding the von Mises equivalent strain. The particular mechanical behavior inherent to this type of materials, hexagonal closed pack microstructures, leads to conclude that it is necessary to have a numeric elastoplastic model based in experimental tests. In this paper is presented a numerical model based on stress-strain experimental data. The objective is to modulate several physical mechanisms inherent to the magnesium elastoplastic behavior. In order to validate the achieved model the numeric estimations were correlated with the experimental data and with the Jiang & Sehitoglu plasticity model. Results show that the implemented model modulation is in agreement with the experimental data. Some differences between the Jiang & Sehitoglu and the implemented model regarding the magnesium hysteresis loop modulation are pointed out.

Abstract: This article describes a flexible modeling framework which leads to the construction of a probabilistic, multiaxial Kitagawa-Takahashi diagram. This framework has been developed following experimental observations that clearly indicate that two independent fatigue damage mechanisms can be activated, at the same time, in metallic materials. Specifically, one damage mechanism is associated with crack initiation and the other with crack arrest. It is postulated that these damage mechanisms are more appropriately modeled using two different fatigue criteria or, more specifically, two completely different approaches to fatigue (i.e. a classical multiaxial fatigue criterion and a LEFM type criterion). Hence, the proposed modeling framework provides the possibility of combining any two suitable criteria, in a probabilistic framework based on the weakest link hypothesis and results in the continuous description of the Kitagawa diagram for any multiaxial stress state. It is shown that under certain conditions this approach is equivalent to the classical El Haddad approach to the short crack problem encountered in LEFM. However, the proposed framework is easily extended to multiaxial loading conditions. This modeling framework is demonstrated in detail via its application to multiaxial fatigue data for data taken from the literature.

Abstract: In the present paper, two fundamental concepts are discussed for formulating fatigue strength diagrams of notched metals. The first is a hypothesis of cyclic plastic adaptation that reflects mechanical behavior/property inherent in a persistent slip band. From this hypothesis, an equivalent cyclic stress ratio , which is the corresponding parameter between the cyclic stress condition of a notched and un-notched specimen, is derived. is extended to multi-axial cyclic stress conditions by use of the potential stress defined by Mises', Kawamoto's criterion, etc. The fatigue strength of a notch specimen is diagramed as a relation between and the notch root stress range . As a result, the fatigue strength is characterized into the two types of and , which can be replaced by the fatigue strength in the surface layer of the un-notch specimen and by the threshold stress of the crack specimen with the same crack depth as the notch depth, respectively. The second idea is how to express parametrically notch size factors based on a notch behavior/property map. For the type of , the notch size factor is expressed by a power function of a square root of a product of a notch root radius and a notch depth , and for the type of as a power function of the notch depth . These factors can be also applied to multi-axial cyclic stress conditions.

Abstract: This study discusses fatigue properties of low carbon steel under multiaxial non-proportional loading and an evaluation of failure life. Multiaxial fatigue tests under non-proportional loading with various stress amplitudes were carried out using a hollow cylinder specimen in low and high cycle regions at room temperature. In the test, three types of strain/stress path were employed. They are a push-pull, a reversed torsion and a combined push-pull and reversed torsion loadings in which stress amplitudes used were constant and random. This study evaluates an effect of non-proportional loading on fatigue life in the high cycle fatigue region to discuss the applicability of Δε_NP proposed by Itoh et al. on life evaluations in the high cycle region and under random loading.

Abstract: This study presents definitions of principal stress/strain range and mean stress/strain introduced by utilizing Itoh-Sakane criterion for multiaxial loading including non-proportional loading, and shows the method of calculating the non-proportional factor which expresses the severity of non-proportional loading under the multiaxial 3D loading. This paper also shows a method of visually presenting the stress/strain, the non-proportionality of loading and the damage evaluation.

Abstract: With modern laser beam sources welding processes can be developed, that allow the joining of otherwise barely realisable material and geometrical constellations such as dissimilar welded, thick-walled shaft-hub joints for powertrain systems. Current design recommendations do not offer solutions to account for the cyclic strength under torsional loading for welded structures. In order to bridge the gap between cost and time consuming prototype testing and laboratory tests of basic homogeneous material samples, a test system combining axial and torsional loading was used. For this purpose application oriented test parts are designed to mimic the weld seam geometry, stiffness and heat dissipation conditions of the real structural part at its best. The dissimilar joints were realised for two material combinations: cast iron GJS-600-3 with case hardened steel 16MnCr5 and 42CrMo4 with 16MnCr5. The latter combination showed only a slightly higher cyclic strength compared to the cast iron/steel combination. A systematic optimization of the laser beam welding process leads to a fatigue behaviour under multi-axial loading conditions, where the cast iron/case hardened steel combination still met the strength specification required.

Abstract: An experimental study was conducted to evaluate notch effects on fatigue behavior of a neat polymer (PP impact co-polymer) and a composite made of 30 wt% short glass fibers in polybutylene terephthalate (PBT). A plate-type specimen geometry with a central circular hole was used. The experiments were conducted at room temperature in uniaxial tension-tension (R = 0.1) and tension-compression (R = -1) loading conditions. Some analytical methods including Neubers rule and the method of critical distances were used in addition to FEA to predict fatigue life of notched specimens. Neubers rule commonly used for metallic materials proved to be an accurate method for predicting the notched fatigue life of the thermoplastics considered.

Abstract: A high-temperature ultrasonic fatigue testing system was developed to evaluate the gigacycle fatigue properties of single-crystal superalloys used in aircraft engine turbine blades. In this development, a commercial ultrasonic fatigue testing machine was considerably modified to achieve high-temperature fatigue testing. The developed system took account of temperature dependency of Youngs modulus, and also had a function to evaluate the Youngs modulus. In order to protect the testing system from the heat of a specimen, straight and round rods were inserted between the testing system and the specimen. Other modifications achieved accurate control of temperature, edge displacement and resonance frequency, which were necessary for accurate control of stress amplitude. The testing system was first applied to a heat-resistant steel at 650 °C to check its accuracy, and next to SC superalloy samples at 1000 °C. In the conventional fatigue tests on the heat-resistant steel, the results were coincident in a frequency range from 1 Hz to 800 Hz, suggesting that comparable results would be obtained in ultrasonic fatigue testing at 20 kHz. In case of the SC superalloy samples, conventional fatigue tests were conducted at only 10 Hz, so the frequency effects were not clarified. In both cases, ultrasonic fatigue testing showed good agreement with conventional fatigue testing. The accuracy of the developed system is therefore high, even at 1000 °C. In these results, the SC superalloys showed no fatigue limit, indicating gigacycle fatigue tests to be necessary.

Abstract: This paper is devoted to the initiation of fatigue crack in Armco iron from low cycle fatigue to gigacycle fatigue. It is shown that the basic mechanisms of initiation are very similar from a physical point of view: PSB and Grain boundary cracking. But the mechanical aspect is specific in LCF and in GCF.