Modeling the Non-linear Viscoelastic Response of High Temperature Polyimides

A constitutive model is developed to predict the viscoelastic response of polyimide resins that are used in high temperature applications. This model is based on a thermodynamic framework that uses the notion that the `natural configuration' of a body evolves as the body undergoes a process and the evolution is determined by maximizing the rate of entropy production in general and the rate of dissipation within purely mechanical considerations. We constitutively prescribe forms for the specific Helmholtz potential and the rate of dissipation (which is the product of density, temperature and the rate of entropy production), and the model is derived by maximizing the rate of dissipation with the constraint of incompressibility, and the reduced energy dissipation equation is also regarded as a constraint in that it is required to be met in every process that the body undergoes. The efficacy of the model is ascertained by comparing the predictions of the model with the experimental data for PMR-15 and HFPE-II-52 polyimide resins.Comment: 16 pages, 4 figures, submitted to Mechanics of Material

On Maxwell fluid with relaxation time and viscosity depending on the pressure

We study a variant of the well known Maxwell model for viscoelastic fluids, namely we consider the Maxwell fluid with viscosity and relaxation time depending on the pressure. Such a model is relevant for example in modelling behaviour of some polymers and geomaterials. Although it is experimentally known that the material moduli of some viscoelastic fluids can depend on the pressure, most of the studies concerning the motion of viscoelastic fluids do not take such effects into account despite their possible practical significance in technological applications. Using a generalized Maxwell model with pressure dependent material moduli we solve a simple boundary value problem and we demonstrate interesting non-classical features exhibited by the model.Comment: 16 pages, 14 figures, submitted to International Journal of Non-Linear Mechanic

On Modeling the Response of Synovial Fluid: Unsteady Flow of a Shear-Thinning, Chemically-Reacting Fluid Mixture

We study the flow of a shear-thinning, chemically-reacting fluid that could be used to model the flow of the synovial fluid. The actual geometry where the flow of the synovial fluid takes place is very complicated, and therefore the governing equations are not amenable to simple mathematical analysis. In order to understand the response of the model, we choose to study the flow in a simple geometry. While the flow domain is not a geometry relevant to the flow of the synovial fluid in the human body it yet provides a flow which can be used to assess the efficacy of different models that have been proposed to describe synovial fluids. We study the flow in the annular region between two cylinders, one of which is undergoing unsteady oscillations about their common axis, in order to understand the quintessential behavioral characteristics of the synovial fluid. We use the three models suggested by Hron et al. [ J. Hron, J. M\'{a}lek, P. Pust\v{e}jovsk\'{a}, K. R. Rajagopal, On concentration dependent shear-thinning behavior in modeling of synovial fluid flow, Adv. in Tribol. (In Press).] to study the problem, by appealing to a semi-inverse method. The assumed structure for the velocity field automatically satisfies the constraint of incompressibility, and the balance of linear momentum is solved together with a convection-diffusion equation. The results are compared to those associated with the Newtonian model. We also study the case in which an external pressure gradient is applied along the axis of the cylindrical annulus.Comment: 25 pages, 11 figures, accepted in Computers & Applications with Mathematic

Diffusion of a fluid through a viscoelastic solid

This paper is concerned with the diffusion of a fluid through a viscoelastic solid undergoing large deformations. Using ideas from the classical theory of mixtures and a thermodynamic framework based on the notion of maximization of the rate of entropy production, the constitutive relations for a mixture of a viscoelastic solid and a fluid (specifically Newtonian fluid) are derived. By prescribing forms for the specific Helmholtz potential and the rate of dissipation, we derive the relations for the partial stress in the solid, the partial stress in the fluid, the interaction force between the solid and the fluid, and the evolution equation of the natural configuration of the solid. We also use the assumption that the volume of the mixture is equal to the sum of the volumes of the two constituents in their natural state as a constraint. Results from the developed model are shown to be in good agreement with the experimental data for the diffusion of various solvents through high temperature polyimides that are used in the aircraft industry. The swelling of a viscoelastic solid under the application of an external force is also studied.Comment: 26 pages, 7 figures, submitted to International Journal of Solids and Structure

Low-cost Adsorbents for the Removal of Mercury (11) from Aqueous Solution-A Comparative Study

The establishments of the Ministry of Defence, specifically ordnance factories and public sector undertakings (like Bharat Electronics Ltd), carry out operations like electroplating, metal1surface finishing, solid-state wafer processing, and initiatory manufacturing (lead azide, mercury fulminate), which generate waste water contaminated with hazardous heavy metals. Mercuryand its compounds are known to be highly toxic, both for the living organisms and theenvironment. To protect public health, a regulatory discharge standard of mercury, as low as 0.01 mgll, has been imposed and is expected to be even stricter in the future. A promising method for effective mercury discharge control is to employ suitable adsorbents for the removal of mercury from the contaminated aqueous stream.This paper describes the effectiveness of low cost and locally available, untreated and chemically-treated adsorbents for the removal of mercury from the aqueous solution. Their effectiveness has been compared with that of chemically-treated granular activated carbon. Treated sawdust and untreated weathered coal were found to be the most suitable low-cost adsorbents in addition to treated granular activated carbon for the removal of mercury from aqueous solution. Under the optimised conditions, ie, adsorbent dose 10 gll, pH 6, contact time 48 h, and initial concentration of mercury 3 mgll, the removal of mercury was found to be 99.8 per cent, 99.8 per cent, and 99.7per cent, using treated granular activated carbon, treated sawhust, and untreated weathered coal, respectively.The adsorption parameters were determined using both Langmuir and Freundlich isotherm models. Surface complexation and ion exchange were the major removal mechanisms involved.The adsorption isotherm studies clearly indicated that the Langmuir model is in good agreement, with the experimental data on the adsorptive behaviour of mercury on treated granular activated carbon, whereas, the experimental data on adsorptive behaviour of mercury on weathered coal and treated sawdust follow both Langmuir and Freundlich isotherm models. The paper presents the results of the experimental studies as well as the model parameters

A model for the degradation of polyimides due to oxidation

Polyimides, due to their superior mechanical behavior at high temperatures, are used in a variety of applications that include aerospace, automobile and electronic packaging industries, as matrices for composites, as adhesives etc. In this paper, we extend our previous model in [S. Karra, K. R. Rajagopal, Modeling the non-linear viscoelastic response of high temperature polyimides, Mechanics of Materials, In press, doi:10.1016/j.mechmat.2010.09.006], to include oxidative degradation of these high temperature polyimides. Appropriate forms for the Helmholtz potential and the rate of dissipation are chosen to describe the degradation. The results for a specific boundary value problem, using our model compares well with the experimental creep data for PMR-15 resin that is aged in air.Comment: 13 pages, 2 figures, submitted to Mechanics of Time-dependent Material

Degradation and healing in a generalized neo-Hookean solid due to infusion of a fluid

The mechanical response and load bearing capacity of high performance polymer composites changes due to diffusion of a fluid, temperature, oxidation or the extent of the deformation. Hence, there is a need to study the response of bodies under such degradation mechanisms. In this paper, we study the effect of degradation and healing due to the diffusion of a fluid on the response of a solid which prior to the diffusion can be described by the generalized neo-Hookean model. We show that a generalized neo-Hookean solid - which behaves like an elastic body (i.e., it does not produce entropy) within a purely mechanical context - creeps and stress relaxes when infused with a fluid and behaves like a body whose material properties are time dependent. We specifically investigate the torsion of a generalized neo-Hookean circular cylindrical annulus infused with a fluid. The equations of equilibrium for a generalized neo-Hookean solid are solved together with the convection-diffusion equation for the fluid concentration. Different boundary conditions for the fluid concentration are also considered. We also solve the problem for the case when the diffusivity of the fluid depends on the deformation of the generalized neo-Hookean solid.Comment: 24 pages, 10 figures, submitted to Mechanics of Time-dependent Material

Socioeconomic Mediators of Racial and Ethnic Disparities in Congenital Heart Disease Outcomes: A Population-Based Study in California.

Background Racial/ethnic and socioeconomic disparities exist in outcomes for children with congenital heart disease. We sought to determine the influence of race/ethnicity and mediating socioeconomic factors on 1-year outcomes for live-born infants with hypoplastic left heart syndrome and dextro-Transposition of the great arteries. Methods and Results The authors performed a population-based cohort study using the California Office of Statewide Health Planning and Development database. Live-born infants without chromosomal anomalies were included. The outcome was a composite measure of mortality or unexpected hospital readmissions within the first year of life defined as >3 (hypoplastic left heart syndrome) or >1 readmissions (dextro-Transposition of the great arteries). Hispanic ethnicity was compared with non-Hispanic white ethnicity. Mediation analyses determined the percent contribution to outcome for each mediator on the pathway between race/ethnicity and outcome. A total of 1796 patients comprised the cohort (n=964 [hypoplastic left heart syndrome], n=832 [dextro-Transposition of the great arteries]) and 1315 were included in the analysis (n=477 non-Hispanic white, n=838 Hispanic). Hispanic ethnicity was associated with a poor outcome (crude odds ratio, 1.72; 95% confidence interval [CI], 1.37-2.17). Higher maternal education (crude odds ratio 0.5; 95% CI , 0.38-0.65) and private insurance (crude odds ratio, 0.65; 95% CI , 0.45-0.71) were protective. In the mediation analysis, maternal education and insurance status explained 33.2% (95% CI , 7-66.4) and 27.6% (95% CI , 6.5-63.1) of the relationship between race/ethnicity and poor outcome, while infant characteristics played a minimal role. Conclusions Socioeconomic factors explain a significant portion of the association between Hispanic ethnicity and poor outcome in neonates with critical congenital heart disease. These findings identify vulnerable populations that would benefit from resources to lessen health disparities

Characterizing peritoneal dialysis catheter use in pediatric patients after cardiac surgery

ObjectiveChildren who undergo cardiac surgery are at high risk for renal insufficiency and abdominal compartment syndrome. Peritoneal dialysis catheter (PDC) implantation is used in this population for abdominal decompression and access for dialysis. However, there is no consensus regarding PDC use, and the practice varies widely. This study was undertaken to assess associated factors, outcomes, and variability in the use of PDC in patients who have undergone cardiac surgery.MethodsThe cohort was obtained from the Kids' Inpatient Database, years 2006 and 2009. Patients who underwent cardiac surgery were included and the subset that underwent PDC implantation during the same hospitalization was identified. Univariable and multivariable analyses assessed factors associated with PDC and survival.ResultsA cohort of 28,259 patients underwent cardiac surgery, of whom 558 (2%) had PDCs placed. In the PDC group, 39.1% (n = 218) had acute renal failure whereas 3.5% or patients (n = 974) in the non-PDC group had acute renal failure. Among patients receiving PDC, mortality was 20.3% (n = 113; vs 3.4% overall mortality, n = 955). Excluding patients with acute renal failure, mortality remained 12% (n = 41) for the PDC group. Factors associated significantly with PDC placement in the overall cohort were younger age, greater surgical complexity, nonelective admission, hospital region, use of cardiopulmonary bypass, and acute renal failure.ConclusionsPatients receiving PDC after cardiac surgery had 20% mortality, which remained 12% after excluding patients with acute renal failure. Given the variability in PDC use and poor outcomes, further research is needed to assess the possible benefit of earlier intervention for peritoneal access in this high-risk cohort

Mild postoperative acute kidney injury and outcomes after surgery for congenital heart disease

ObjectiveThe effect of mild acute kidney injury (AKI) on outcomes after heart surgery in children is unclear. We sought to characterize the epidemiology of mild AKI associated with surgery for congenital heart disease (CHS-AKI) in children.MethodsWe conducted a single-center, retrospective cohort study of 693 patients (aged 6 days-18 years) who underwent heart surgery in 2009. The prevalence of AKI within 72 hours of surgery was determined using the 3-stage Acute Kidney Injury Network criteria. Factors associated with both hospital length of stay and AKI were used in a proportional hazards model to test the association of stage 1 AKI with hospital length of stay.ResultsThe median age of the patients was 11.5 months (interquartile range, 3-54 months). Eighteen percent of the cohort had single ventricle heart disease and 54% underwent RACHS-1 category 3 or higher surgery. The prevalence of stages 1, 2, and 3 AKI in this cohort was 11% (n = 77), 3% (n = 19), and 1% (n = 8), respectively. Factors independently associated with AKI were prematurity, single ventricle physiology, peak postoperative lactic acid concentration, cardiopulmonary bypass time, and a history of heart surgery. Stage 2 or greater CHS-AKI was associated with hospital length of stay (adjusted hazard ratio [AHR], 0.53; 95% confidence interval [CI], 0.33-0.87; P = .01), but stage 1 was not (AHR, 0.85; 95% CI, 0.66-1.10; P = .22).ConclusionsAKI occurs after surgery for congenital heart disease but may be less common than previously reported. Although moderate to severe CHS-AKI is independently associated with prolonged recovery after heart surgery, mild disease does not appear to be