Multiscale thermo-mechanical modeling of semi-crystalline polymers : application to additive manufacturing by selective laser sintering

Semi-crystalline polymers exhibit performances that are highly dependent on their micro-structure as induced by the thermo-mechanical processes they are subjected to. In this thesis we developed a multiscale modeling and simulation framework able to predict the thermo-mechanical response of semi-crystalline polymers including crystallization and porosity evolution, with an application to additive manufacturing by selective laser sintering. Firstly, an enhanced phase field model was developed for the numerical simulation of crystallization in semi-crystalline polymers. The model is based on coupling the heat equation with the Allen-Cahn equation, which is derived from the Gibbs-Thomson solid-liquid interface equation. Starting from the nucleation of spherulites, existing phase field models can simulate their evolution in a surrounding liquid and separate the amorphous and crystalline phases. However, the predictions of the morphological characteristics of the spherulites remain qualitative only. Moreover, the predicted spherulite evolution as a function of crystallization temperature is not consistent with experimental results. In our work, existing phase field models were enhanced in order to obtain experimentally consistent results. We used spherulite growth, crystal morphology, and crystallinity degree in spherulite, as measures to determine the model accuracy. The model is numerically implemented using the finite difference method so that 2D and 3D simulation results are presented and compared to experimental data, illustrating the quantitative adequacy of the predictions with experimental evidence. Secondly, full-field micromechanical simulations were conducted on the micro-structures generated by the enhanced phase field model in order to predict the effective mechanical properties. Care is taken to obtain Representative Volume Elements (RVEs) by computing the number of subcells in each spherulite and the number of spherulite nucleations in each RVE. An FFT solver was used to determine the thermo-mechanical properties for a range of crystallinity ratios. Validation against experimental data and comparison with simpler models was done. Thirdly, we developed the constitutive laws enabling the prediction of the thermo-viscoelastic-viscoplastic behavior of semi-crystalline polymers. The crystalline lamellae follow an orthotropic elasto-viscoplastic model based on the activation of slip systems, while the amorphous phase is viscoelastic. Some material parameters were identified directly while others were reverse engineered from experimental measurements. Finally, a mesoscale model was proposed for the density evolution of an initially low density polymer grain powder which becomes compacted after sintering. The approach is based on a generalized self-consistent model, the analogy between linear elasticity and Newtonian fluid and the mass conservation equation. All the models have been implemented in a research version of the Digimat software.(FSA - Sciences de l'ingénieur) -- UCL, 202

Linking a phase field model for polymer crystallization to full-field micromechanical simulations of semi-crystalline polymers

An enhanced phase field model recently proposed by the authors enables to model polymer crystallization and to predict spherulite growth and representative semi-crystalline micro-structures in 2D or 3D. After solidification, the value of the phase field variable in each subcell (2D pixel or 3D voxel) indicates whether the material is amorphous or crystalline. In the present paper, full-field micromechanical simulations are conducted on the microstructures generated by the enhanced phase field model in order to predict the effective mechanical properties. A Fast Fourier Transform (FFT) method with periodic boundary conditions is used. Care is taken to obtain representative volume elements (RVEs) by computing the number of subcells in each spherulite and the number of spherulite nucleations in each RVE. Numerical FFT predictions of elastic properties for a range of crystallinity ratios are validated against experimental data and compared to simpler composite inclusion models. Finite element simulations of thermal shrinkage are also shown. The approach was numerically implemented in a research version of the Digimat software

An enhanced phase field model for the numerical simulation of polymer crystallization

An enhanced phase field model is proposed for the numerical simulation of crystallization in semicrystalline polymers. As with other models, it is based on coupling the heat equation with the Alle–Cahn equation, which is derived from the Gibbs–Thomson solid–liquid interface equation. Starting from spherulite nucleation, existing phase field models can simulate their evolution in a surrounding liquid and separate the amorphous and crystalline phases. However, the predictions of the morphological characteristics of the spherulites remain qualitative only. Moreover, the predicted spherulite evolution as a function of crystallization temperature is not consistent with experimental results. In this work, existing phase field models are enhanced in order to obtain experimentally consistent results. We target these characteristics to make them quantitative: spherulite growth, crystal morphology, and crystalline rate in spherulite. We show the importance of modeling the dependence of the model's parameters with respect to crystallization temperature, because, if assumed constants, the predicted results are not consistent with polymers physics. The model is numerically implemented using the finite difference method in a research version of the Digimat software so that 2D and 3D simulation results are presented and compared to experimental data, illustrating the quantitative adequacy of the predictions with experimental evidence

A ruptured balloon shaft during an angioplasty

Abstract We report a case of balloon shaft rupture during percutaneous coronary intervention. Although the entrapped balloon was not yet deflated when the complication occurred, we successfully retrieved it percutaneously using a trapping technique. This case described a cheap and straightforward technique of device retrieval that helped save our patient

Q fever presenting as myocarditis

We report the case of a 19-year-old healthy adolescent, living in an urban area, admitted because of acute chest pain and extensive anterior ST elevation. Coronary arteries were normal on coronary angiography; troponins were very high, echocardiography revealed a preserved global systolic function but an alteration of the longitudinal strain in the inferolateral wall. Cardiac MRI confirmed the diagnosis of acute myocarditis. As part of the etiological workup, Coxiella burnetii serology showed an acute infection. The diagnosis of Coxiella burnetii myocarditis was retained and the patient was treated with doxycycline and corticosteroid therapy. The myocardial localization of this germ is unusual but can be serious, hence the interest of a Coxiella serology in endemic countries face to any acute myocarditis

147: Infective endocarditis in Tunisia: A changing profile?

BackgroundSince the first description of infective endocarditis (IE), the profile of the disease has evolved continuously with stable incidence.ObjectiveTo update information gained on the epidemiology profile of IE in a Tunisian high-volume tertiary care centerPatients and methodsThis was a descriptive retrospective study of patients treated for IE from January 2001 to December 2010. The patients were divided in two groups according to the date of diagnosis of IE: group 1 (from January 2001 to June 2006) and group 2 (from June 2006 to December 2011). These groups were compared for epidemiologic factors and microbiological differences.ResultsA total of 297 patients admitted during the study period met the modified Duke criteria for IE. The comparison of demographic and microbiological characteristics of IE cases over time between the results of the two groups showed that the annual incidence of IE was stable. The sex incidence rates of IE, in particular, incidence among men was relatively stable across the study period and ranged from 60% in 2001-2006 to 58% in 2006-2010, also there was no significant increase in incidence among women (P=ns). Among incident cases, there was a significant increasing age on presentation, with median age increasing from 39.5 years in 2001-2006 to 51.5 years in 2006-2010 (P=0.05). There were no statistically significant decreases in the proportions of cases affecting the aortic valve (P=0.4) and in IE due to oral streptococci (P= ns) over time. Also there was no significant increase in the incidence of Staphylococcus IE. However, the incidence of IE with negative blood culture has remained stable and continues to be high, nearing 50%.The rate of rheumatic heart disease as an underlying heart disease was stable over time. However, the standardized incidence of IE has increased in patients with prosthetic valves (P= ns). Overall in-hospital mortality rate decreased from ((18.6% in 2001-2006 vs 14.6% in 2006-2010.ConclusionThis study has shown that the annual incidence of IE is stable in Tunisia. However, profound changes in the epidemiological profile of this disease have not been noted in our population

Distal radial approach versus conventional radial approach: A comparative study of feasibility and safety

The distal radial approach (DRA) is suggested to have benefits over the conventional radial approach (CRA) in terms of local complications and comfort of both patient and operator. Therefore, we aimed to compare the feasibility and safety of DRA and CRA in a real life population. We conducted a prospective, observational multicentric trial, including all patients undergoing coronary procedures in September 2019. Patients with impalpable proximal or distal radial pulse were excluded. Thus, the choice of the approach is left to the operator discretion. The primary endpoints were cannulation failure and procedure fail- ure. The secondary endpoints were time of puncture, local complications and radial occlusion assessed by Doppler performed one day after the procedure. We enrolled 177 patients divided into two groups: CRA (n = 95) and DRA (n = 82). Percutaneous intervention was achieved in 37% in CRA group and 34% in DRA group (p = 0.7). Cannulation time was not significantly different between the two sets (p = 0.16). Cannulation failure was significantly higher in DRA group (4.8% vs 2%, p < 0.0008). Successful catheterization was achieved in 98% for the CRA group and in 88% for the DRA group (p = 0.008). Radial artery occlusion, detected by ultrasonography, was found in 3 patients in the CRA group (3.1%) and nobody in the DRA group (p = 0.25). The median diameter of the radial artery diameter was higher in the DRA than the CRA group (2.2 mm vs 2.1 mm; p = 0.007). The distal radial approach is feasible and safe for coronary angiography and interventions, but needs a learning curve

Distal radial approach versus conventional radial approach: a comparative study of feasibility and safety

The distal radial approach (DRA) is suggested to have benefits over the conventional radial approach (CRA) in terms of local complications and comfort of both patient and operator. Therefore, we aimed to compare the feasibility and safety of DRA and CRA in a real life population. We conducted a prospective, observational multicentric trial, including all patients undergoing coronary procedures in September 2019. Patients with impalpable proximal or distal radial pulse were excluded. Thus, the choice of the approach is left to the operator discretion. The primary endpoints were cannulation failure and procedure failure. The secondary endpoints were time of puncture, local complications and radial occlusion assessed by Doppler performed one day after the procedure. We enrolled 177 patients divided into two groups: CRA (n = 95) and DRA (n = 82). Percutaneous intervention was achieved in 37% in CRA group and 34% in DRA group (p = 0.7). Cannulation time was not significantly different between the two sets (p = 0.16). Cannulation failure was significantly higher in DRA group (4.8% vs 2%, p < 0.0008). Successful catheterization was achieved in 98% for the CRA group and in 88% for the DRA group (p = 0.008). Radial artery occlusion, detected by ultrasonography, was found in 3 patients in the CRA group (3.1%) and nobody in the DRA group (p = 0.25). The median diameter of the radial artery diameter was higher in the DRA than the CRA group (2.2 mm vs 2.1 mm; p = 0.007). The distal radial approach is feasible and safe for coronary angiography and interventions, but needs a learning curve

Scorpion Envenomation Among Children: Clinical Manifestations and Outcome (Analysis of 685 Cases)

Our objective was to characterize both epidemiologically and clinically manifestations after severe scorpion envenomation and to define simple factors indicative of poor prognosis in children. We performed a retrospective study over 13 years (1990–2002) in the medical intensive care unit (ICU) of a university hospital (Sfax-Tunisia). The diagnosis of scorpion envenomation was based on a history of scorpion sting. The medical records of 685 children aged less than 16 years who were admitted for a scorpion sting were analyzed. There were 558 patients (81.5%) in the grade III group (with cardiogenic shock and/or pulmonary edema or severe neurological manifestation [coma and/or convulsion]) and 127 patients (18.5%) in the grade II group (with systemic manifestations). In this study, 434 patients (63.4%) had a pulmonary edema, and 80 patients had a cardiogenic shock; neurological manifestations were observed in 580 patients (84.7%), 555 patients (81%) developed systemic inflammatory response syndrome (SIRS), and 552 patients (80.6%) developed multi-organ failure. By the end of the stay in the ICU, evolution was marked by the death in 61 patients (8.9%). A multivariate analysis found the following factors to be correlated with a poor outcome: coma with Glasgow coma score ≤ 8/15 (odds ratio [OR] = 1.3), pulmonary edema (OR = 2.3), and cardiogenic shock (OR = 1.7). In addition, a significant association was found between the development of SIRS and heart failure. Moreover, a temperature > 39°C was associated with the presence of pulmonary edema, with a sensitivity at 20.6%, a specificity at 94.4%, and a positive predictive value at 91.7%. Finally, blood sugar levels above 15 mmol/L were significantly associated with a heart failure. In children admitted for severe scorpion envenomation, coma with Glasgow coma score ≤ 8/15, pulmonary edema, and cardiogenic shock were associated with a poor outcome. The presence of SIRS, a temperature > 39°C, and blood sugar levels above 15 mmol/L were associated with heart failure