Different Approaches for Predicting Air Jet Spun Yarn Strength

V současné době tryskové dopřádání dosáhlo po půlstoletí svého vývoje průmyslového uznání a zaujalo své místo na trhu. Cílem této práce je přispět k poznání procesu tvorby příze, zmapovat vliv vybraných technologických parametrů tryskového dopřádacího stroje na vlastnosti příze, zejména její pevnost a především poskytnout širší náhled na problematiku predikce pevnosti tryskové příze. V práci je provedena trojrozměrná numerická simulace průtokového pole vzduchu uvnitř spřádací trysky tryskového dopřádacího stroje Rieter Air-jet. Byla analyzována distribuce rychlosti a tlaku vzduchu s cílem popsat princip tvorby příze. Analýza složek rychlosti a statického tlaku vzduchu ukázala, jak jsou uvnitř trysky tvořeny vzduchové víry, stejně jako způsob, jakým se příze formuje.Byla provedena numerická simulace spolu s experimentální verifikací, která zkoumala vliv tlaku kroutícího vzduchu na pevnost příze. Výsledky simulace přinesly dobrou shodu s experimentem. Výsledky ukázaly, že zvyšující se tlak v trysce vedl zpočátku ke zlepšení pevnosti příze, ale při vysokém tlaku vzduchu se pevnost zhoršila.Ve stěžejní části práci jsou prezentovány a popsány různé možnosti přístupů k predikce pevnosti příze. Jedním z nich je statistické modelování založené na experimentálních měřeních. V rámci tohoto přístupu byl sledován vliv délkové hmotnosti příze, odtahové rychlosti a nastaveného spřádního tlaku vzduchu. Pro analýzu kombinovaného vlivu těchto parametrů pomocí responzních povrchů byl použit vícenásobný regresní model. Na základě různých kombinací mezi sledovanými technologickými veličinami byly získány optimální parametry nastavení pro testovaný materiál.Jako druhý z možných přístupů k predikci pevnosti tryskové příze je navržen matematický model. Pomocí tohoto modelu lze predikovat pevnost 100% viskozové a 100% tencelové tryskové příze na krátkých upínacích délkách. Model je založen na výpočtu pevnosti jádra příze, jakožto paralelního svazku vláken, výpočtu pevnosti obalové vrstvy vláken jakožto svazku vláken ovinutého ve šroubovici kolem jádra příze. V modelu je rovněž zohledněn interakční účinek mezi vlákny v obalu a vlákny v jádru příze. Jako vstupní parametry modelu pro výpočet teoretické pevnosti příze na krátkých upínacích délkách jsou použity parametry vláken i strukturální parametry příze. Výsledky ukázaly, že přesnost navrhovaného modelu je uspokojivá pro soubor experimentálních přízí. Jako další z možných přístupů k predikci pevnosti příze (na krátkých úsečkách) je prezentován statistický model. Pomocí modelu je zkoumán vliv upínací délky příze v trhacím přístroji na pevnost a variační koeficient pevnosti tryskové, prstencové a rotorové příze. Model vychází z Peirceova modelu a předpokládá tříparametrové Weibullovo rozdělení hodnot pevnosti příze. Mezi experimentálními a predikovanými hodnotami byla zaznamenána přiměřená shoda. Model úspěšně zachytil změny pevnosti příze a její variační koeficient při různých upínacích délkách. Výsledky potvrdily, že při větších upínacích délkách pevnost příze klesá a její variační koeficient se rovněž snižuje.Air jet spinning process has reached an industrial acceptance stage having developed through half a century. This study aims to contribute to the knowledge of air-jet yarn formation process by investigating the influence of selected technological parameters of the Rieter air-jet spinning machine on yarn properties, especially its strength. Furthermore, to shed light on the problem of the prediction of yarn strength. A three-dimensional numerical simulation of the airflow field inside Rieter air jet spinning nozzle has been presented. The velocity and pressure distribution were analyzed to describe the principle of yarn formation. The analysis of velocity components and static pressure revealed how the air vortices are created inside the nozzle as well as how the yarn is spun.A numerical simulation along with experimental verification were performed to investigate the influence of nozzle pressure on air jet yarn tenacity and the results were in good agreement. The results show that increasing nozzle pressure resulted initially in improving yarn tenacity, but at high-pressure, tenacity deteriorates.Different approaches have been used to predict the tenacity of air jet yarn. One of these approaches is a statistical model, where the effect of yarn linear density, delivery speed and nozzle pressure on yarn strength were investigated and a multiple regression model was used to study the combined effect of these parameters and response surfaces were obtained. Based on the different combinations of processing variables, optimal running conditions for tested materials were obtained. As a second possible approach to predict yarn strength, a mathematical model that predicts the strength of Viscose and Tencel air jet spun yarn at short gauge length has been presented which is based on an earlier model. The model is based on calculating the core fiber strength as a parallel bundle of fibers. Also, calculating the wrapper fiber strength as a bundle of fibers in the form of helical path and considering the interaction effect between the wrapper and core fibers. Fiber parameters in addition to yarn structural parameters were used to obtain the theoretical yarn tenacity at short gauge length. Results showed that the accuracy of the proposed model is satisfactory for the tested yarns set.As an alternative approach to predict air jet yarn strength, a statistical model has been presented. By using this model, the influence of the tensile tester gauge length on the ring, rotor, and air jet spun yarn tenacity and its coefficient of variation has been investigated. The model correlates yarn tenacity and coefficient of variation of yarn tenacity to gauge length. The model is based on Peirce model and assuming the 3-parameter Weibull distribution of yarn strength values. A reasonable agreement has been shown between the experimental and the predicted values. The model successfully captured the change in yarn strength and its coefficient of variation at different gauge lengths. Results confirmed that at longer gauge lengths, yarn strength decreases and its coefficient of variation decreases as well

Burnout among surgeons before and during the SARS-CoV-2 pandemic: an international survey

Background: SARS-CoV-2 pandemic has had many significant impacts within the surgical realm, and surgeons have been obligated to reconsider almost every aspect of daily clinical practice. Methods: This is a cross-sectional study reported in compliance with the CHERRIES guidelines and conducted through an online platform from June 14th to July 15th, 2020. The primary outcome was the burden of burnout during the pandemic indicated by the validated Shirom-Melamed Burnout Measure. Results: Nine hundred fifty-four surgeons completed the survey. The median length of practice was 10 years; 78.2% included were male with a median age of 37 years old, 39.5% were consultants, 68.9% were general surgeons, and 55.7% were affiliated with an academic institution. Overall, there was a significant increase in the mean burnout score during the pandemic; longer years of practice and older age were significantly associated with less burnout. There were significant reductions in the median number of outpatient visits, operated cases, on-call hours, emergency visits, and research work, so, 48.2% of respondents felt that the training resources were insufficient. The majority (81.3%) of respondents reported that their hospitals were included in the management of COVID-19, 66.5% felt their roles had been minimized; 41% were asked to assist in non-surgical medical practices, and 37.6% of respondents were included in COVID-19 management. Conclusions: There was a significant burnout among trainees. Almost all aspects of clinical and research activities were affected with a significant reduction in the volume of research, outpatient clinic visits, surgical procedures, on-call hours, and emergency cases hindering the training. Trial registration: The study was registered on clicaltrials.gov "NCT04433286" on 16/06/2020

Numerical simulation of the yarn formation process in Rieter air jet spinning

This paper presents a three-dimensional numerical simulation of the airflow field inside Rieter air jet spinning nozzle. The velocity and pressure distribution were analyzed to describe the principle of yarn formation. The analysis of velocity components and static pressure revealed how the air vortices are created inside the nozzle. A theoretical study along with experimental verification was performed to investigate the influence of nozzle pressure on air jet yarn tenacity and the results were in good agreement. The results show that increasing nozzle pressure resulted initially in improving yarn tenacity but at high pressure tenacity deteriorates. © 2016 The Textile Institut

Theoretical Porosity of Elastic Single Jersey Knitted Fabric Based on 3D Geometrical Model of Stitch Overlapping

To improve the dimensional stability of weft knitted fabrics, spandex yarns are incorporated with the base yarn by the plaiting technique. Adding spandex leads to stitch overlapping, increases the fabric thickness, and affects the pore’s size and distribution. This in turn affects the geometrical and physical properties of weft knitted fabrics such as air permeability. In this paper, the theoretical 3D modeling of stitch overlap, maximum set, and open structures is presented by using AutoCAD software. Fabric thickness was divided into several sections, and the theoretical porosity at each section was analyzed and calculated. Furthermore, single jersey knitted fabrics SJKF with and without spandex were produced to obtain overlapping (by using spandex weight percent, SWP 8%), and open structures. The calculated and theoretical overall porosity were calculated and compared. The study showed the influence of stitch overlapping on fabric porosity. Moreover, the results show that the proposed model captured to some extent the change in fabric porosity as its structure changes