Vitor Anes

A New Criterion for Evaluating Multiaxial Fatigue Damage under Multiaxial Random Loading Conditions

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.

AZ31 Magnesium Alloy Multiaxial LCF Behavior: Theory, Simulation and Experiments

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.

On the assessment of multiaxial fatigue damage under variable amplitude loading

In this work, the performance of the SSF criterion is evaluated under variable amplitude loading conditions. The main objective was to inspect the validity of the hypothesis in which the SSF damage map remains valid for any high strength steel. In order to achieve that, fatigue life correlation of the 1050QT steel and 304L stainless steel was analyzed under multiaxial loading conditions. The loading block considered in the study comprises 360 proportional loading cycles with different stress amplitude ratios and stress levels. Despite being made of proportional branches, this loading block is a non-proportional loading due to its principal directions variation. This feature allows the evaluation of combined loading effects under variable amplitude loading conditions, which makes this loading block suitable to mimic the loading effects usually found in the field. Results show very good agreements, which reinforces the aforementioned hypothesis

Evaluation of the AZ31 cyclic elastic-plastic behaviour under multiaxial loading conditions

Components and structures are designed based in their material’s mechanical properties such as Youngs modulus or yield stress among others. Often those properties are obtained under monotonic mechanical tests but rarely under cyclic ones. It is assumed that those properties are maintained during the material fatigue life. However, under cyclic loadings, materials tend to change their mechanical properties, which can improve their strength (material hardening) or degrade their mechanical capabilities (material softening) or even a mix of both. This type of material behaviour is the so-called cyclic plasticity that is dependent of several factors such as the load type, load level, and microstructure. This subject is of most importance in design of structures and components against fatigue failures in particular in the case of magnesium alloys. Magnesium alloys due to their hexagonal compact microstructure have only 3 slip planes plus 1 twining plane which results in a peculiar mechanical behaviour under cyclic loading conditions especially under multiaxial loadings. Therefore, it is necessary to have a cyclic elastic-plastic model that allows estimating the material mechanical properties for a certain stress level and loading type. In this paper it is discussed several aspects of the magnesium alloys cyclic properties under uniaxial and multiaxial loading conditions at several stress levels taking into account experimental data. A series of fatigue tests under strain control were performed in hour glass specimens test made of a magnesium alloy, AZ31BF. The strain/stress relation for uniaxial loadings, axial and shear was experimentally obtained and compared with the estimations obtained from the theoretical elastic-plastic models found in the state-of-the-art. Results show that the AZ31BF magnesium alloy has a peculiar mechanical behaviour, which is quite different from the steel one. Moreover, the state of the art cyclic models do not capture in full this peculiar behaviour, especially the cyclic magnesium alloys anisotropy. Further, an analysis is performed to identify the shortcomings inherent to the actual cyclic models in the capture of the magnesium alloys cyclic behaviour. Several conclusions are drawn.