Influence of the hydrostatic and the deviatoric components of stress on ductility of metals

Failure of ductile metals occurring as microvoids growth is known to strongly depend on the stress triaxiality parameter which is a conventional measure of the first stress invariant normalized with respect to the equivalent von Mises stress, which in turn is a conventional measure of the second stress invariant. The triaxiality parameter is usually assumed to not influence the stress-strain behavior of metals according to the von Mises plasticity. The effects of the Lode angle and of the stress triaxiality are investigated here with regard to ductile failure and to the stress-strain behavior. Experimental results and numerical predictions are compared for different metals and different specimen geometries, by investigating about various failure criteria and a procedure for the post-necking stress-strain characterization. Tensile specimens and notched plates are loaded up to failure, then finite elements simulations are verified by comparison with experimental data. The verified numerical data are then used to calculate local variables within the volume of failing specimens; the knowledge of these local variables enables to predict global and local failure conditions according to the selected failure models

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

From CO2 to Cell: Energetic Expense of Creating Biomass Using the Calvin-Benson-Bassham and Reductive Citric Acid Cycles Based on Genomic Data

Abstract The ubiquity of the Calvin-Benson-Bassham cycle (CBB) amongst autotrophic organisms suggests that it provides an advantage over a wide range of environmental conditions. However, in some habitats, such as hydrothermal vents, the reductive citric acid cycle (rCAC) is an equally predominant carbon fixation pathway. It has been suggested that the CBB cycle poses a disadvantage under certain circumstances due to being more energetically demanding compared to other carbon fixation pathways. The purpose of this study was to compare the relative metabolic cost of cell biosynthesis by an autotrophic cell using either the CBB cycle or the rCAC. For both pathways, the energy, in ATP, required to synthesize the macromolecules (DNA, RNA, protein, and cell envelope) for one gram of biomass was calculated, beginning with CO2. Two sulfur-oxidizing chemolithoautotrophic proteobacteria, Thiomicrospira crunogena XCL-2, and Sulfurimonas autotrophica were used to model the CBB cycle and rCAC, respectively while Escherichia coli was used to model both pathways because it has had its cell composition extremely well-characterized. Since these organisms have had their genomes sequenced, it was possible to reconstruct the biochemical pathways necessary for intermediate and macromolecule synthesis. Prior estimates, based solely on the ATP cost of pyruvate biosynthesis, suggested that the cellular energetic expense for biosynthesis from the CBB cycle was more than that from the rCAC. The results of this study support this conclusion; however the difference in expense between the two pathways may not be as extreme as suggested by pyruvate synthesis. Other factors, such as oxygen sensitivity, may act in concert with energetic expense in contributing to the selective advantages between different autotrophic carbon fixation pathways

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

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

Bosentan and macitentan prevent the endothelial-to-mesenchymal transition (EndoMT) in systemic sclerosis: in vitro study.

Background: Systemic sclerosis (SSc) is characterized by early vascular abnormalities and subsequent fibroblast activation to myofibroblasts, leading to fibrosis. Recently, endothelial-to-mesenchymal transition (EndoMT), a complex biological process in which endothelial cells lose their specific markers and acquire a mesenchymal or myofibroblastic phenotype, has been reported in SSc. In the present study, we evaluated the ability of endothelin-1 (ET-1) dual receptor antagonists bosentan (BOS) and macitentan (MAC) to antagonize EndoMT in vitro. Methods: Ten women with limited SSc were enrolled. They underwent double skin biopsy (affected and nonaffected skin). Fibroblasts and microvascular endothelial cells (MVECs) were isolated from biopsies. We performed mono- or coculture of MVECs (isolated from nonaffected skin) with fibroblasts (isolated from affected skin and stimulated with ET-1 and transforming growth factor beta [TGF-\u3b2]). In cocultures, the MVEC layer was left undisturbed or was preincubated with BOS or MAC. After 48 h of coculture, MVECs were analyzed for their tube formation ability and for messenger RNA and protein expression of different vascular (CD31, vascular endothelial growth factor-A [VEGF-A], VEGF-A165b) and profibrotic (alpha-smooth muscle actin [\u3b1-SMA], collagen type I [Col I], TGF-\u3b2) molecules. Results: After 48 h, MVECs showed a reduced tube formation ability when cocultured with SSc fibroblasts. CD31 and VEGF-A resulted in downregulation, while VEGF-A165b, the antiangiogenic isoform, resulted in upregulation. At the same time, mesenchymal markers \u3b1-SMA, Col I, and TGF-\u3b2 resulted in overexpression in MVECs. Tube formation ability was restored when MVECs were preincubated with BOS or MAC, also reducing the expression of mesenchymal markers and restoring CD31 expression and the imbalance between VEGF-A and VEGF-A165b. Conclusions: With this innovative EndoMT in vitro model realized by coculturing nonaffected MVECs with affected SSc fibroblasts, we show that the presence of a myofibroblast phenotype in the fibroblast layer, coupled with an ET-1-TGF-\u3b2 synergic effect, is responsible for EndoMT. BOS and MAC seem able to antagonize this phenomenon in vitro, confirming previous evidence of endothelium-derived fibrosis in SSc and possible pharmacological interferenc

Fatigue assessment by energy approach during tensile tests on AISI 304 steel

Estimation of the fatigue limit for steel ductile materials using non-destructive methods is a topic of great interest to researchers today. In recent years, the method adopted has implemented infrared sensors to detect the surface temperature and correlate it with the fatigue limit. In previous paper, a new energy approach was proposed to investigate the fatigue limit during tensile test. The numerical procedure proposed by Chrysochoos is adopted to clean infrared images and applied to analyse the surface heat sources during tensile test. AISI 304 specimens with rectangular cross-sections are tested. Moreover fatigue tests at increasing loads were carried out on steel by a stepwise succession, applied to the same specimen, for applying the thermographic method. The predictions of the fatigue limit, obtained by the analysis of the energy evolution during the static tests, were compared with the predictions obtained applying the thermographic method during fatigue tests

A Monitoring Framework with Integrated Sensing Technologies for Enhanced Food Safety and Traceability

A novel and low-cost framework for food traceability, composed by commercial and proprietary sensing devices, for the remote monitoring of air, water, soil parameters and herbicide contamination during the farming process, has been developed and verified in real crop environments. It offers an integrated approach to food traceability with embedded systems supervision, approaching the problem to testify the quality of the food product. Moreover, it fills the gap of missing low-cost systems for monitoring cropping environments and pesticides contamination, satisfying the wide interest of regulatory agencies and final customers for a sustainable farming. The novelty of the proposed monitoring framework lies in the realization and the adoption of a fully automated prototype for in situ glyphosate detection. This device consists of a custom-made and automated fluidic system which, leveraging on the Molecularly Imprinted Polymer (MIP) sensing technology, permits to detect unwanted glyphosate contamination. The custom electronic mainboard, called ElectroSense, exhibits both the potentiostatic read-out of the sensor and the fluidic control to accomplish continuous unattended measurements. The complementary monitored parameters from commercial sensing devices are: temperature, relative humidity, atmospheric pressure, volumetric water content, electrical conductivity of the soil, pH of the irrigation water, total Volatile Organic Compounds (VOCs) and equivalent CO (Formula presented.). The framework has been validated during the olive farming activity in an Italian company, proving its efficacy for food traceability. Finally, the system has been adopted in a different crop field where pesticides treatments are practiced. This has been done in order to prove its capability to perform first level detection of pesticide treatments. Good correlation results between chemical sensors signals and pesticides treatments are highlighted