Shear Stability, Instability and Localization of Materials Exhibiting Thermal Softening, Strain Rate Sensitivity and Strain Hardening

1989 ◽  
Vol 111 (4) ◽  
pp. 250-253 ◽  
Author(s):  
N. Charalambakis
Keyword(s):  
Test Problem ◽  
Shear Banding ◽  
Homogeneous Solution ◽  
Rate Sensitivity ◽  
Thermal Softening ◽  
Simple Shearing ◽  
Shear Forces ◽  
Localization Of Plastic Deformation ◽  
Dependent Flow

We consider the test-problem of simple shearing of a thermoviscoplastic solid subject to steady or time-dependent boundary velocities or shear forces. Previously derived stability and nonlocalization criteria are presented. The influence of boundary conditions on the time-asymptotic “solution,” the role of nonuniformities and the localization of plastic deformation are discussed. Finally, a perturbation analysis of homogeneous solution under steady boundary velocities or stresses of a material with a gradient-dependent flow stress is presented and “shear-banding” criteria are derived.

A Large Deformation Plasticity Model With Rate Sensitivity and Thermal Softening

1984 ◽  
Vol 106 (4) ◽  
pp. 388-392
Author(s):  
D. W. Nicholson ◽  
K. C. Kiddy
Keyword(s):  
Large Deformation ◽  
Shear Banding ◽  
Small Deformation ◽  
Rate Sensitivity ◽  
Thermal Softening ◽  
Dynamic Loads ◽  
Extended Model ◽  
Adiabatic Shear Banding ◽  
Deformation Plasticity ◽  
Applied Stresses

In this paper, a previously published small deformation constitutive model with rate sensitive plasticity and thermal softening is extended to large deformation. The extended model appears suitable for describing a deleterious thermoplastic process manifested by adiabatic shear banding in materials such as titanium under severe dynamic loads. The nature of the instability admitted by the model is described. Also, calculations are reported on the rapid extension of a titanium strip. For applied stresses several times the yield stress, a deleterious temperature is attained in times of the order of 10−2 s.

scholarly journals Clinical Assessment of Endothelial Function in Convalescent COVID-19 Patients Undergoing Multidisciplinary Pulmonary Rehabilitation

Biomedicines ◽  
2021 ◽  
Vol 9 (6) ◽  
pp. 614
Author(s):  
Pasquale Ambrosino ◽  
Antonio Molino ◽  
Ilenia Calcaterra ◽  
Roberto Formisano ◽  
Silvia Stufano ◽  
...  
Keyword(s):  
Endothelial Function ◽  
Pulmonary Rehabilitation ◽  
Pulmonary Function Tests ◽  
Care Facility ◽  
Forced Expiratory Volume ◽  
Vascular Events ◽  
Inclusion Study ◽  
History Of ◽  
Dependent Flow

Background: Growing evidence points to a key role of endothelial dysfunction in the pathogenesis of COVID-19. In this study, we evaluated changes in endothelium-dependent flow-mediated dilation (FMD) in a cohort of convalescent COVID-19 patients undergoing pulmonary rehabilitation (PR). Methods: After swab test negativization, convalescent COVID-19 patients referring to a post-acute care facility for PR were consecutively screened for inclusion. Study procedures were performed at the time of hospitalization and discharge. Results: We enrolled 82 convalescent COVID-19 patients (85.4% males, mean age 60.4 years). After PR, a significant improvement in most pulmonary function tests and exercise capacity was documented. FMD changed from 2.48% ± 2.01 to 4.24% ± 2.81 (p < 0.001), corresponding to a 70.9% increase. Significantly higher changes in FMD were found in patients without a history of vascular events as compared to those with (+2.04% ± 2.30 vs. +0.61% ± 1.83, p = 0.013). Values of forced expiratory volume in 1 s (FEV1%), forced vital capacity (FVC%) and diffusion capacity for carbon monoxide (DLCO%) significantly and directly correlated with FMD both at baseline and after PR. Patients with normal FEV1% (≥80% predicted) during the overall study period or those normalizing FEV1% after PR showed a more significant FMD change as compared to patients with persistently impaired FEV1% (<80% predicted) (p for trend = 0.029). This finding was confirmed in a multivariate analysis. Conclusions: Clinically evaluated endothelial function improves after PR in convalescent COVID-19 patients. A direct and persistent association between the severity of pulmonary and vascular disease can be hypothesized. Endothelial function testing may be useful in the follow-up of convalescent COVID-19 patients.

scholarly journals Comodulation of NO-Dependent Vasodilation by Erythroid Band 3 and Hemoglobin: A GP.Mur Athlete Study

2021 ◽  
Vol 8 ◽  
Author(s):  
Kate Hsu ◽  
Yen-Yu Liu ◽  
Wei-Chin Tseng ◽  
Kuang-Tse Huang ◽  
Chia-Yuan Liu ◽  
...  
Keyword(s):  
Blood Pressure ◽  
Arterial Occlusion ◽  
Band 3 ◽  
Flow Mediated Dilation ◽  
Red Cell Membrane ◽  
Anion Exchanger 1 ◽  
Before And After ◽  
Dependent Flow ◽  
Plasma Nitrite

GP.Mur, a red blood cell (RBC) hybrid protein encoded by glycophorin B-A-B, increases expression of erythroid band 3 (Anion Exchanger-1, SLC4A1). GP.Mur is extremely rare but has a prevalence of 1–10% in regions of Southeast Asia. We unexpectedly found slightly higher blood pressure (BP) among healthy Taiwanese adults with GP.Mur. Since band 3 has been suggested to interact with hemoglobin (Hb) to modulate nitric oxide (NO)-dependent hypoxic vasodilation during the respiratory cycle, we hypothesized that GP.Mur red cells could exert differentiable effects on vascular tone. Here we recruited GP.Mur-positive and GP.Mur-negative elite male college athletes, as well as age-matched, GP.Mur-negative non-athletes, for NO-dependent flow-mediated dilation (FMD) and NO-independent dilation (NID). The subjects were also tested for plasma nitrite and nitrate before and after arterial occlusion in FMD. GP.Mur+ and non-GP.Mur athletes exhibited similar heart rates and blood pressure, but GP.Mur+ athletes showed significantly lower FMD (4.8 ± 2.4%) than non-GP.Mur athletes (6.5 ± 2.1%). NO-independent vasodilation was not affected by GP.Mur. As Hb controls intravascular NO bioavailability, we examined the effect of Hb on limiting FMD and found it to be significantly stronger in GP.Mur+ subjects. Biochemically, plasma nitrite levels were directly proportional to individual band 3 expression on the red cell membrane. The increase of plasma nitrite triggered by arterial occlusion also showed small dependency on band 3 levels in non-GP.Mur subjects. By the GP.Mur comparative study, we unveiled comodulation of NO-dependent vasodilation by band 3 and Hb, and verified the long-pending role of erythroid band 3 in this process.

The Role of Rate Effects and of Thermomechanical Coupling in Shear Localization

1997 ◽  
Vol 119 (4) ◽  
pp. 322-331 ◽  
Author(s):  
Y. Estrin ◽  
A. Molinari ◽  
S. Mercier
Keyword(s):  
Numerical Simulations ◽  
Strain Localization ◽  
Mechanical Energy ◽  
Shear Banding ◽  
Rate Sensitivity ◽  
Thermomechanical Coupling ◽  
Face Centered Cubic ◽  
Adiabatic Shear Banding ◽  
Thin Walled Tube ◽  
Face Centered

A dislocation density related constitutive model that accounts for the strain-rate sensitivity of the flow stress and, notable, of the strain-hardening coefficient was applied to describe adiabatic shear banding in face centered cubic metals. The parameter values of a prototype material, for which numerical simulations were carried out, are close to those of copper. The effect of the material parameters, especially of those reflecting the two rate sensitivities, on the occurrence of strain localization in a thin-walled tube under torsion containing a geometrical defect was investigated systematically. The results obtained provide some guidance with respect to design of materials with high resistance to strain localization and high mechanical energy absorption. Another outcome of this study is the recognition that for the problem in question linear stability analysis cannot provide a reliable criterion even for the onset of strain localization, and that numerical simulations have to be invoked.

THE ROLE OF THE GHOST FIELDS IN THE RENORMALIZED t–J LAGRANGIAN MODEL

2009 ◽  
Vol 23 (04) ◽  
pp. 493-519
Author(s):  
O. S. ZANDRON
Keyword(s):  
Thermal Softening ◽  
Lagrangian Model ◽  
Lagrangian Formalism ◽  
Spin Polaron ◽  
Renormalization Procedure ◽  
Ghost Field

The present work treats the role of ghost fields in the renormalization procedure of the Lagrangian perturbative formalism of the t–J model. We show that by introducing proper ghost field variables, the propagators and vertices can be renormalized to each order. In particular, the renormalized ferromagnetic magnon propagator coming from our previous Lagrangian formalism is studied in detail, and it is shown how the thermal softening of the magnon frequency is predicted by the model. The antiferromagnetic case is also analyzed, and the results are confronted with the previous one obtained by means of the spin-polaron theories.

Role of salt sources in density-dependent flow

2009 ◽  
Vol 45 (5) ◽  
Author(s):  
Juan J. Hidalgo ◽  
Jesús Carrera ◽  
Agustín Medina
Keyword(s):  
Density Dependent ◽  
Dependent Flow ◽  
Density Dependent Flow

Photochemistry of triphenylamine (TPA) in homogeneous solution and the role of transient N-phenyl-4a,4b-dihydrocarbazole. A steady-state and time-resolved investigation.

2021 ◽  
Author(s):  
Stefano Protti ◽  
Mariella Mella ◽  
Sergio Mauricio Bonesi
Keyword(s):  
Steady State ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Homogeneous Solution ◽  
Laser Flash ◽  
Time Resolved ◽  
Homogeneous Media

Abstract. The photoreactivity of triphenylamine in homogeneous media has been investigated by means of laser flash photolysis spectroscopy and preparative experiments. The goal of this study consists in the evaluation...

Cardiovascular and respiratory responses to slow ramp carotid sinus pressures in the dog

1990 ◽  
Vol 68 (4) ◽  
pp. 1465-1474 ◽  
Author(s):  
R. Jung ◽  
P. G. Katona
Keyword(s):  
Carotid Sinus ◽  
Rate Sensitivity ◽  
Cardiovascular Responses ◽  
Pressure Change ◽  
Arterial Pco2 ◽  
Ganglionic Blockade ◽  
Central Processing ◽  
Carotid Baroreceptor ◽  
Cardiovascular Variables

The respiratory and mean arterial pressure (MAP) responses to slow ramp pressure stimulation of carotid baroreceptors were compared in pentobarbital-anesthetized vagotomized dogs breathing 100% O2. Carotid sinus pressure (CSP) was raised from 50 (control) to 220 mmHg and then returned to control as linear ramps (+/- 1 mmHg/s) in isolated sinuses. MAP, heart rate (HR), ventilation (VE), frequency (f), and tidal volume (VT) were expressed as percent of control. The maximum difference between responses to positive and negative ramps at a given CSP (MAX) and the average difference (AVG) served as indicators of the hysteresis for each response. In 27 dogs MAP changed monotonically with varying CSP with insignificant (P = 0.27, MAX) or barely significant (P = 0.03, AVG) hysteresis, monotonic function being one that is continuously nondecreasing or continuously nonincreasing. Similar responses were obtained for HR. VE decreased as CSP increased, but the change was not monotonic. During negative ramp, VE increased back to control with an overshoot. Hysteresis for VE was pronounced (P less than 0.0001, both measures). The VE response was primarily determined by f; VT increased with CSP. To eliminate secondary respiratory effects due to alterations in MAP, in seven dogs similar experiments were performed after ganglionic blockade with hexamethonium. Hysteresis in VE and f persisted. To assess the role of changing arterial PCO2 (PaCO2) on VE, the CSP was held constant (after a ramp rise) at 140, 150, or 180 mmHg before reducing it at -1 mmHg/s to 50 mmHg; however, a significant hysteresis in VE was still observed. Further experiments, to eliminate secondary reflexes due to altered PaCO2, were performed in seven dogs after ganglionic blockade and paralysis with Flaxedil, with phrenic nerve activity as an indicator of (“neural”) respiration. The hysteresis in VE and f were no longer significant. In summary, the results indicate that 1) slow ramp carotid baroreceptor stimulation elicits both VE and cardiovascular responses, the VE response showing a dramatically higher hysteresis than the cardiovascular responses; 2) the ventilatory hysteresis is partially explained by the secondary changes in PaCO2 and perhaps by cardiovascular variables; and 3) the central processing of the baroventilatory reflex appears to be rate sensitive at a slower rate of pressure change than that which causes rate sensitivity in the baropressure reflex.

A survey of instrumented indentation studies on metallic glasses

2004 ◽  
Vol 19 (1) ◽  
pp. 46-57 ◽  
Author(s):  
C.A. Schuh ◽  
T.G. Nieh
Keyword(s):  
Metallic Glasses ◽  
Structural Changes ◽  
Shear Banding ◽  
Depth Sensing ◽  
Future Directions ◽  
Fundamental Physics ◽  
New Approach ◽  
The Past ◽  
Rate Dependent

The development of instrumented nanoindentation equipment has occurred concurrently with the discovery of many new families of bulk metallic glass during the past decade. While indentation testing has long been used to assess the mechanical properties of metallic glasses, depth-sensing capabilities offer a new approach to study the fundamental physics behind glass deformation. This article is a succinct review of the research to date on the indentation of metallic glasses. In addition to standard hardness measurements, the onset of plasticity in metallic glasses is reviewed as well as the role of shear banding in indentation, structural changes beneath the indenter, and rate-dependent effects measured by nanoindentation. The article concludes with perspectives about the future directions for nanocontact studies on metallic glasses.

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