A new yield criteria including the effect of lode angle and stress triaxiality

AbstractAccording to several experiments reported in the literature, the elastoplastic behaviour of metals depends not only on the first stress invariant (triaxiality) for the ductile damage and on the second stress invariant (equivalent von Mises stress) for the yield, but also on the third stress invariant (normalized Lode angle X) which may affect at the same time the yielding and the ductile failure.In this paper a new yield model is presented, where the yield surface depends on the Lode Angle and, eventually, also on the triaxiality ratio.The proposed model is identified by a calibration parameter expressing the degree of nonlinearity of the yield with respect to the Lode angle, and a calibration function expressing the maximum variability of the hardening stress at the two extremities of the Lode angle range, corresponding to the uniaxial and to the pure shear stress states.The proposed model has been tested against several experimental data from the literature on the Titanium alloy Ti6Al4V, including mixed tension-torsion loading which allowed to control the evolution of X and to confine its values into different narrow ranges for better investigating the Lode angle effects on the yield response

INTERACTION OF STRAIN RATE AND NECKING ON THE STRESS-STRAIN RESPONSE OF UNIAXIAL TENSION TESTS BY HOPKINSON BAR

Abstract The effect of the necking combined to that of the strain rate is analysed in dynamic split Hopkinson bar (SHTB) tests, by both experiments and finite elements. Experiments from the literature by Noble et al. are considered here together with other tests ran at the University of Catania. Two different characterization procedures are used for modeling the materials, leading to strain and strain rate-dependent flow stress according to the Johnson-Cook model for the Remco Iron by Noble et al., and to an MLR-based calibration for the FeN steel implemented by fortran subroutines, respectively. After satisfactory validation of the finite elements results and of the dynamic hardening models via comparison to the experimental stress-strain, a detailed investigation on the way the necking perturbation of the stress interacts with the strain rate is carried out, expecially investigating how the ratio of the flow stress/true stress evolves with the strain and the strain rate. Special modifications are introduced to the subroutine modeling the strain rate-promoted dynamic amplification of the stress; the related response from finite elements confirms the outcomes of previous papers, unveiling a new feature of the dynamic stress in SHTB tests and providing new information about the suitability and the accuracy of the modern procedures for the dynamic stress-strain characterization

Closed-loop control of an open cavity flow using reduced-order models

International audienceThe control of separated fluid flow by reduced-order models is studied using the two-dimensional incompressible flow over an open square cavity at Reynolds numbers where instabilities are present. Actuation and measurement locations are taken on the upstream and downstream edge of the cavity. A bi-orthogonal projection is introduced to arrive at reduced-order models for the compensated problem. Global modes, proper orthogonal decomposition (POD) modes and balanced modes are used as expansion bases for the model reduction. The open-loop behaviour of the full and the reduced systems is analysed by comparing the respective transfer functions. This analysis shows that global modes are inadequate to sufficiently represent the inputoutput behaviour whereas POD and balanced modes are capable of properly approximating the exact transfer function. Balanced modes are far more efficient in this process, but POD modes show superior robustness. The performance of the closed-loop system corroborates this finding: while reduced-order models based on POD are able to render the compensated system stable, balanced modes accomplish the same with far fewer degrees of freedom. © 2009 Cambridge University Press

static and dynamic response of titanium alloy produced by electron beam melting

Abstract The suitability of Titanium alloys for many specialized applications requiring excellent performances at both static and dynamic strain rates, benefits of modern manufacturing technologies like the additive manufacturing, oriented toward the obtainment of complicated component shapes. The EBM methodology for the production of Ti6Al4V components is based on the localized melting of alloy powders by way of guided electron beams scanning the powder volume by successive planar trajectories; for this reason, the whole production process may confer a certain degree of anisotropy to the components. The material behavior of the EBM alloy may be orientation-dependent in terms of stress-strain elastoplastic response as well as in terms of damage sensitivity and ductile fracture under given triaxiality histories. The static and dynamic behavior of a sintered Ti6Al4V alloy is investigated here by way of quasistatic tension-torsion tests and dynamic tensile Hopkinson bar (SHTB) tests. The outcome of the latter experiments, compared to similar tests results from the literature concerning Ti alloy obtained by classical metallurgical techniques, gives some indications about how the technological process may affect the final performance of the material and the component

Stage III Kienböck's Disease Treated with Hyperbaric Oxygen: the Role of an Unusual Approach to a Rare Condition

Kienböck's disease is a rare condition characterised by avascular necrosis of the lunate bone. Its natural history and aetiopathogenesis have not yet been clarified, nor are its triggering factors identified. We present a case of a 17-year-old male gymnast, without relevant medical/family history, with stage IIIA Kienböck's disease diagnosed in 2016. Initially, submitted to conservative treatment that proved to be insufficient. Consequently, surgical treatment was proposed, but refused. The patient instead underwent experimental treatment with hyperbaric oxygen (120 sessions, 100% oxygen at 2.5 atm, for 70 min periods, once daily, five times per week). In April 2018, a favourable clinical and radiological evolution was observed, with an improvement in the patterns of pain, motion and strength and an almost complete involution of the process of aseptic necrosis of the semilunar. To the best of our knowledge, this is the first report of Kienböck's disease treated with hyperbaric oxygen.info:eu-repo/semantics/publishedVersio

Blood pressure and cardiac autonomic nervous system in obese type 2 diabetic patients: Effect of metformin administration

Background: Hyperinsulinemia/insulin resistance and elevated plasma free fatty acids (FFA) levels are involved in the hypertension and cardiac sympathetic overactivity. Metformin improves insulin action and lower plasma FFA concentrations. We investigate the possible effect of metformin on arterial blood pressure (BP) and cardiac sympathetic nervous system. Methods: One hundred twenty overweight type 2 diabetic patients were treated by placebo (n = 60) + diet or metformin (850 mg twice daily) (n = 60) + diet for 4 months, to evaluate the effect of metformin treatment on the cardiac autonomic nervous system. Insulin resistance was measured by the Homeostasis Model Assessment (HOMA) index. Heart rate variability (HRV) assessed cardiac sympathovagal balance. Results: Metformin treatment, but not placebo treatment, was associated with a decrease in fasting plasma glucose (P < .05), insulin (P < .05), triglyceride (P < .05), and FFA (P < .03) concentrations and HOMA index (P < .03). Metformin treatment was also associated with a significant improvement in cardiac sympathovagal balance but not in mean arterial BP. Furthermore, in a multivariate analysis, delta change in sympathovagal balance index (LF/HF ratio) were associated with delta change in plasma FFA concentrations and HOMA index independently of gender and delta change in plasma triglyceride and HbA1c concentrations. Conclusions: Our study demonstrated that metformin treatment might be useful for improving cardiac sympathovagal balance in obese type 2 diabetic patients

Evapotranspiration from horizontal subsurface flow constructed wetlands planted with different perennial plant species

This paper presents the results of an experiment carried out in Southern Italy (Sicily) on the estimation evapotranspiration (ET) in pilot constructed wetlands planted with different species (Chrysopogon zizanioides, Myscanthus x giganteus, Arundo donax, Phragmites australis, and Cyperus papyrus). In the two monitored growing seasons, reference ET0 was calculated with the Penman-Monteith formula, while actual ET and crop coefficients were measured through a water balance and the FAO56 approach, respectively. The highest average seasonal ET value was observed in Phragmites australis (17.31mmd-1) followed by Arundo donax (11.23mmday-1) Chrysopogon zizanioides (8.56 mm day-1), Cyperus papyrus (7.86 mm day-1), and Myscanthus x giganteus (7.35 mm day-1). For all plants, crop coefficient values showed different patterns in relation to growth stages and were strongly correlated with phenological parameters. Myscanthus x giganteus and Arundo donax showed a water use efficiency values significantly higher than those observed for the other tested species. Results of this study may contribute to select appropriate plant species for constructed wetlands located in semi-arid regions, especially when the use of reclaimed water and/or the use of aboveground biomass are planned

On the performance of a pilot hybrid constructed wetland for stormwater recovery in Mediterranean climate

Abstract The overall efficiency of a pilot-scale hybrid constructed wetland (H-CW), located on a retail store's parking area in Eastern Sicily, for alternative treatment of stormwater runoff and of sequential batch reactor (SBR) effluent was evaluated. Experimental activities were focused on system performances, including wastewater (WW) quality and hydraulic monitoring. System design, macrophyte growth and seasonal factors influenced the pilot plant performance. Very high removal efficiency for microbial indicators were reported within the subsurface horizontal flow unit (HF), playing a strategic role for Clostridium perfringens. The algal growth occurred in the free water surface (FWS) unit and inhibited removal efficiencies of total suspended solids (TSS), biochemical oxygen demand (BOD5) and chemical oxygen demand (COD), impairing water quality. The whole H-CW showed good efficiency in trace metals removal, especially for Pb, Zn, and Cu. Preliminary results suggested the reliability of the H-CW technology in decentralised water treatment facilities for enhancing water recovery and reuse