Combinations Of Load Parameters Research Articles

Representative volume elements for plasticity and creep measured from high-resolution microscale strain fields

Homogenization techniques have been widely used in upscaling microscale numerical results to predict the macroscopic response of materials. Often homogenization is based on the concept of a representative volume element (RVE) and thus, measuring the RVE size and understanding what factors influence it are key elements in successful material modeling. Here, the size of the RVE for an austenitic stainless steel alloy is experimentally determined under, both separately and combined, plasticity and creep loading conditions with varying parameters (namely stress, temperature, and creep hold time). We use a high-resolution optical digital image correlation (DIC) methodology capable of discerning residual strain inhomogeneities at the microstructural level. Furthermore, by combining the strain results from DIC with surface microstructural information from electron backscatter diffraction (EBSD), the localized strains near grain boundaries can be isolated allowing for quantitative observations related to the deformation mechanisms responsible for strain accumulation. Finally, by comparing the results for RVE size and localized normal to shear strain ratios for different combinations of loading parameters, the relationship between grain-boundary sliding and the resulting heterogeneity of the strain field is explored. Cases where grain-boundary sliding was the dominant deformation mechanism (i.e., at elevated temperature) had considerably smaller RVE sizes (from 4 to 6 times the average grain size) when compared to samples where sliding was not as prevalent (around 10 times the average grain size).

Assessment of fatigue damage in steam turbine shafting due to torsional vibrations

We present a technique and results of fatigue damage evaluation of steam turbine shafting material under conditions of torque vibrations occurring at turbogenerator abnormal modes of operation (short circuit, random paralleling start, etc.). Combinations of loading parameters, which provide the shafting limiting conditions, are identified.

A Theoretical Study of Mechanical Stability of Arteries

This study proposes a mathematical model for studying stability of arteries subjected to a longitudinal extension and a periodic pressure. An artery was considered as a straight composite beam comprised of an external thick-walled tube and a fluid core. The dynamic criterion for stability was used, based on analyzing the small transverse vibrations superposed on the finite deformation of the vessel under static load. In contrast to the case of a static pressurization, in which buckling is only possible if the load produces a critical axial compressive force, a loss of stability of arteries under periodic pressure occurs under many combinations of load parameters. Instability occurs as a parametric resonance characterized by an exponential increase in the amplitude of transverse vibrations over several bands of pressure frequencies. The effects of load parameters were analyzed on the basis of the results for a dynamic and static stability of a rabbit thoracic aorta. Under normal physiological loads the artery is in a stable configuration. Static instability occurs under high distending pressures and low longitudinal stretch ratios. When the artery is subjected to periodic pressure, an independent increase in the mean pressure, amplitude of the periodic pressure, or frequency, most often, but not always, increases the risk of stability loss. In contrary, an increase in longitudinal stretch ratio most likely, but not certain, stabilizes the vessel. It was shown that adaptive geometrical remodeling due to an increase in mean pressure and flow does not affect artery stability.

Influences of generalized loading parameters on FLD predictions for aluminum tube hydroforming

Abstract The present study intends to investigate the prediction of forming limit diagrams (FLDs) for tube hydroforming from the perspective of selecting various combinations of loading parameters based on plastic instability. From this perspective, certain related research efforts on plastic instability of thin-walled tubes subjected to internal fluid pressure and axial force are reviewed and six combinations of loading parameters have been identified. There are nine combinations of loading parameters, including six found in the literature, being employed to determine forming limit curves (FLCs) for tube hydroforming in the present study. The predicted FLCs are compared with experimental data obtained in a well-controlled laboratory condition. Comparisons indicate that the combination of the internal pressure p and the resultant axial force Fz + πr2p provides the best predicted right side of FLC among the nine combinations and a reasonably well-predicted left-hand side of FLC. In addition, comparison also demonstrated that the combination of p and the applied axial stress Fz/2πrt can also give a reasonable predicted left-hand side of FLC.