3D Analyses of crack propagation in torsion

The initiation and propagation of fatigue cracks during cyclic torsion loading was studied in a castAl alloy (A357) with a relatively large (~500 ?m) grain size. 2D observations, revealed that multi site crackinitiation occur on the {111} slip planes exhibiting the highest slip activity (iso-strain Taylor analysis),preferentially on planes nearly perpendicular to the sample axis. Within the first grain, the cracks have apronounced crystallographic propagation mode (mode II crack growth) and strongly interact with the grainboundaries. In situ 3D monitoring of torsion fatigue tests using synchrotron X-ray tomography reveal thatpropagation towards the interior of the samples occurs first along the sample periphery in mode II leading torelatively shallow cracks which penetrate towards the sample center only after a large number of cycles isreached .The values of mode I, II and III stress intensity factors have been calculated from finite elementsimulation at the tip of the shallow mode II crack. Those values are used to analyse the bulk propagation of theobserved cracks

Effect of water availability pattern on yield of pearl millet in semi-arid tropical environments

Throughout much of the semi-arid tropics, fluctuations in grain yield can largely be attributed to differences in timing and intensity of drought stress. Since seasonal rainfall in these environments is often poorly related to grain yield, the aim of this paper was to establish a relationship between water availability and grain yield for pearl millet (Pennisetum glaucum (L.) R. Br.), grown across 24 semi-arid tropical environments in India. We used a simple soil water budget to calculate a water satisfaction index (WSI) throughout the season. The cumulative WSI at maturity explained 76% of the variance in grain yield. This was three times as much as explained by actual rainfall, because WSI accounted for differences in water losses and pan evaporation. A classification of environments into four groups of water availability patterns explained 75% of the environmental sum of squares for grain yield. For a subset of 13 environments, environmental differences in grain number could also be explained by water availability patterns, whereas differences in grain mass were related to both water availability and temperature. Our results indicate that cumulative WSI, which is an integrated measure of plant-available water, can provide an adequate estimation of the environmental potential for yield in environments where grain yield is mainly limited by variable availability of water

Multisite model prediction of texture induced anisotropy in brass

Twinning is commonly observed above a certain level of deformation at room temperature in face centred cubic (fcc) metals with low stacking fault energy (SFE). Twins carry a part of the plastic deformation and they act as barriers to dislocation motion, leading to anisotropic hardening already at the crystal level. The influence of deformation twinning on the development of texture and the related anisotropy and hardening is investigated in this study by means of experimental observations as well as an advanced crystal plasticity model. The latter "multisite" model takes account of short range interaction among adjacent grains. Anisotropy is assessed based on mechanical tests in uniaxial tension and torsion. When simulation of torsion rely on a simplified one-dimensional finite element modeling, the crystal plasticity model yields qualitative predictions of the strengthening, the texture development and the Swift effect. More accurate predictions would require three-dimensional simulation of the torsion test. © 2010 Springer-Verlag France

Experimental and numerical investigation on fatigue damage in micro-lattice materials by Digital Volume Correlation and μCT-based finite element models

This work addresses the fatigue behaviour of two strut-based topologies of micro-lattice materials, focusing in particular on the damage evolution throughout the fatigue life. An experimental campaign is performed on micro-lattice specimens tested in fatigue with different stress ratio (R = 0, R = -1 and R = 10). A methodological approach for metal foams is adopted to analyse the experimental tests and to characterize the fatigue damage evolution. To validate this approach the Digital Volume Correlation technique is applied on two in-situ experimental fatigue tests conducted inside a Micro-Computed Tomography scanner. An analysis based on a finite element model of the as-manufactured geometry and the application of a suitable multiaxial fatigue criterion to evaluate the local stress field shows good results in predicting the fatigue failure location

Experimental and numerical investigation on fatigue damage in micro-lattice materials by Digital Volume Correlation and μ CT-based finite element models

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