Rozprawy doktorskie na temat „Polyethylene Stresses”

Previous studies of high-density polyethylene pipes have been based on the premise that residual stresses have no effects and have almost no role to play in the failure and performance of theses pipes. It is thought that previous studies have underestimated the effects of the residual stresses in pipes at the design stage particularly when assessing pipe-soil interaction and ground loading effects. This project tried to assess the effect of residual stresses in soil-pipe interaction of HDPE (PEIOO) pipes, and opened two main areas of research: (a) to investigate the actual residual stress profile in the pipes as a result of the manufacturing process; and, (b) to take these residual stress profiles, which have been previously ignored, in consideration to assess the performance of the pipe and the surrounding soil. A review of the existing techniques available for the determination of residual stresses was carried out and a literature review was done on how these techniques were used by various authors. On the whole, most of these techniques involved destructive tests that are believed, here, to alter the residual stress profile and magnitude in the pipe wall. The current study has tried to develop an innovative stress-optical technique for the determination of residual stresses with minimal alteration to the residual stress profile and its magnitude. After determining the residual stress profile and magnitudes by the new method, the interaction of the residual stress with internal pressure effects and ground loading effects on pipes is studied. The studies included the numerical modelling of the pipe when the residual stress in the pipe wall incorporated and the pipe subjected to internal pressure loading as well as external loadings from the weight of the surrounding soil. A number of earlier researchers have investigated the soil structure interaction parameters which affect the structural behaviour of buried flexible pipes. However, to date none of the studies have incorporated residual stresses in pipe-soil interaction modelling and the current study has raised awareness of the consequent absence in current design procedure [BS EN 1295-3; 1998].

Magister Scientiae - MSc (Biotechnology)
Water deficit is known to be one of the most detrimental environmental factors to affect crop production and growth in South Africa. This factor has become more apparent with increasing cases of drought in the country.

O presente trabalho foi desenvolvido para avaliar o potencial de utilização de um reator anaeróbico fabricado em PEAD Polietileno de Alta Densidade, produzido pelo processo de rotomoldagem em substituição aos reatores convencionais construídos em concreto e alvenaria, trabalhando em regime de batelada e enterrados no solo. Os estados de tensões e deformações foram avaliados utilizando o programa de Elementos Finitos ABAQUS versão 6.5 e a malha dos nós utilizando o programa MSC PATRAN 2005 formando 7329 nós e 2004 elementos, em uma malha otimizada para as regiões de maior curvatura (pontos concentradores de tensão). O carregamento é formado com uma pressão interna do biogás de 5 kPa acrescido da carga hidrostática de biomassa de 6000 kgf em uma fundação elástica calculada pela razão tensão/recalque a partir do Módulo de Elasticidade equivalente do solo (Esolo). Comparando o estado de tensões avaliado durante o carregamento foi possível constatar que a maior tensão obtida no elemento mais crítico para a utilização mais provável do reator atingiu o valor de 7,46 MPa (não supera 40% do menor valor de resistência à tração e ao cisalhamento do PEAD de 20 MPa) e a maior razão de deformação dR/R foi de 1.0%. O caso mais crítico avaliado foi quando o reator está enterrado, totalmente vazio, em solo com Esolo = 1,55 MPa e o material com EPEAD = 1550 MPa e com uma sobrecarga superficial no terreno de 20kN/m2 gerando uma tensão de 17,80 MPa no elemento 1955 (atingindo 89% do menor valor de resistência à tração e ao cisalhamento do PEAD igual a 20 MPa). Os resultados obtidos comprovam que o reator produzido em PEAD substitui com vantagens os modelos fabricados em concreto ou alvenaria, suportando a pressão interna do biogás e a carga de biomassa.
The present work was developed to evaluate the potential of uses of an anaerobic reactor manufactured in HDPE High Density Polyethylene produced by the rotomolding process in substitution to the conventional reactors built in stonemasonry, working in a batch regime and buried in the soil. The state of tensions and the deformations were assessed using the program of Finite Elements ABAQUS version 6.5 and the mesh of the knots using the program MSC PATRAN 2005 forming 7329 knots and 2004 elements, in an optimized mesh for the areas of larger curvature (tension concentrator points). The loading is formed with an internal pressure of the biogas of 5kPa added of biomass hydrostatic load of 6000 kg in an elastic foundation calculated by the ratio pressure/settling starting from the Module of equivalent Elasticity of the soil (Esolo). Comparing the state of tensions assessed during the loading was possible to verify that the largest tension obtained in the most critical element goes the most probable utilization of the reactor, reached the value of 7, 46 MPa (it doesn't surpass 40% of the smallest resistance value to the traction and to the shearing strain of HDPE of 20 MPa) and the largest ratio of dR/R deformation was of 1.0%. The most critical assessed case was when the reactor is buried in soil with Esolo = 1,55 MPa and material with EPEAD = 1550 MPa, totally empty and with a superficial overload in the land of 20kN/m2 generating a tension of 17,80 MPa in the element 1955 (reaching 89% of the smallest resistance value to the traction and the shearing strain of a 20 MPa HDPE). The obtained results confirmed that the reactor produced in HDPE substitutes with advantages the models manufactured in stonemasonry, supporting the internal biogas pressure and the biomass load.

Thesis (Ph.D.)--Case Western Reserve University, 2009
Title from PDF (viewed on 19 August 2009) Department of Macromolecular Science and Engineering Includes abstract Includes bibliographical references Available online via the OhioLINK ETD Center

Dans ce travail, la caractérisation mécanique de l’interphase entre les zones amorphes et cristallines dans le polyéthylène a été abordée. La caractérisation élastique est effectuée en appliquant deux approches micromécaniques à partir des données de la simulation moléculaire pour la zone interlamellaire. Ces approches micromécaniques sont d’une part le modèle étendu d’inclusion composite, et d’autre part la méthode de double inclusion. Les résultats des deux approches s’accordent parfaitement. Il a été mis en évidence que le tenseur de rigidité de l’interphase n’est pas défini positif, l’interphase est donc mécaniquement instable. La comparaison avec les résultats expérimentaux valide la méthodologie proposée. Pour la caractérisation hyperélastique, l’algorithme hybride proposé consiste à appliquer la loi de comportement d’un milieu continu isotrope, compressible et hyperélastique aux résultats de la simulation de la dynamique moléculaire d’un élément unitaire de polyéthylène. La notion d’optimisation d’un ensemble de fonctions coûts non négatives est l’idée clé de cette partie. Les paramètres hyperélastiques identifiés sont en bon accord avec ceux qui ont été estimés expérimentalement. L’évolution des frontières de l’interphase avec la déformation est le second résultat de cette analyse. La fin du travail est dédiée à la simulation numérique de la grande déformation viscoplastique d’un agrégat de polyéthylène. Le modèle de Gent adopté pour la contrainte de rappel, le tenseur de projection proposé pour l’approche modifiée de Taylor, et l’optimisation multiniveau font parties des contributions apportées
Elastic characterization of the interphase layer in polyethylene is implemented by applying the relationships of two micromechanical approaches, “Extended Composite Inclusion Model” and “Double-Inclusion Method”, to the Monte Carlo molecular simulation data for the interlamellar domain. The results of the two approaches match perfectly. The interphase stiffness lacks the common feature of positive definiteness, which indicates its mechanical instability. Comparison with experimental results endorses the proposed methodology. For the hyperelastic characterization of the interlamellar domain and the interphase layer, the proposed hybrid algorithm consists in applying the constitutive equations of an isotropic, compressible, hyperelastic continuum to the molecular dynamics simulation results of a polyethylene stack. Evolution of the interphase boundaries are introduced as auxiliary variables and the notion of minimizing a set of nonnegative objective functions is employed for parameter identification. The identified hyperelastic parameters for the interlamellar domain arein good agreement with the ones that have been estimated experimentally. Finally, the large, viscoplastic deformation of an aggregate of polyethylene is reexamined. The Gent model adopted for the back stress of the noncrystalline phase, correcting the projection tensor for the modified Taylor approach, and the idea of multilevel optimization are among the contributions made

Este trabalho apresenta resultados de ensaios de fissuramento sob tensão (FST) realizados em geomembranas de polietileno (PE) virgens e degradadas em laboratório. As geomembranas foram degradadas pela exposição à radiação ultravioleta, por envelhecimento térmico em estufa com circulação de ar, e pela compatibilidade química com soda cáustica e com vinhaça. Os resultados destes ensaios demonstram que os processos de degradação a que as geomembranas foram submetidas podem ser considerados catalisadores do fenômeno de FST, pois ocorreram reduções de resistência ao FST da ordem de 50 a 60%, com exceção para a amostra em compatibilidade química com vinhaça, que obteve aumento de 17% na resistência ao FST
This work presents results of stress cracking tests (SC) accomplished in virgin and degraded polyethylene (PE) geomembranes at laboratory. Geomembranes were degraded by exposition to ultraviolet radiation, by thermal aging in oven with circulation of air, and by chemical compatibility with sodium hydroxide and with leachate of alcohol production. The results of these tests demonstrate that the degradation processes of the geomembranes were submitted can be considered catalysts of the phenomenon of SC, because they happened resistance reductions to stress crack resistance of the order from 50 to 60%, with exception for the sample in chemical compatibility with leachate of alcohol production, that had an increase of 17% in the stress crack resistance

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The objectives of this study were: 1) to evaluate the degree of conversion, shrinkage stress, flexural and compressive strength of an experimental resin with a lactic polyacid and castor oil; 2) to evaluate the cytotoxicity of an experimental resin with a lactic polyacid, and castor oil; 3) evaluate, degree of conversion, shrinkage stress, flexural and compressive strength an experimental resin with addition of a high molecular weight polyethylene; 4) to assess the cytotoxicity of an experimental resin with addition of a high molecular weight polyethylene. Initially, it was synthesized a poly (lactic acid) (PLA) via a polycondensation process. After 24 hours, the castor oil (OR) was incorporated by an esterification process with N, N-Dicyclonecylcarbodimide and 99% 4-Dimethylamino-Pyridine in 40 mL of tetrahydrofuran. After this, the blend was characterized by X-ray diffraction (XRPD), Raman spectroscopy, scanning electron microscopy (SEM) and shrinkage stress, where was proven the incorporation of PLA + OR in the experimental composite resin. In the third chapter, PLA + OR were manipulated in two concentrations (1.5% and 3%) in an experimental composite: (2,2 bis [4- (2-hydroxy-3-metacrilatoxypropoxy) phenol] (BisGMA) Triethylene glycol dimethacrylate (TEGDMA), camphorquinone (CQ), N, N-dimethylaminoethyl methacrylate (DEMA-EMA), butylated hydroxytoluene (BHT), silanated barium borosilicate glass, and the tests of degree of conversion, compressive and flexural strength and shrinkage stress were made. The cytotoxicity assay was done; a suspension of 3 x 103 cells/mL of culture medium was prepared. For this, the cells were detached from the bottom of the bottle by the addition of trypsin with EDTA, after incubation in 5% CO2 with 3 mL Eagle's medium supplemented culture. After 24 h incubation period, the culture medium was removed and 200 uL of a solution of culture medium and methylltetrazolium salt (MTT). Subsequently, the analysis of mitochondrial activity was performed in a spectrophotometer (Biotek EL 800, Biotek, Winooski, VT, USA) with a 10 wavelength of 570 nm. The data from each test were analyzed by one-way ANOVA. No significant differences were found (p <0.005) in compressive and degree of conversion tests, and significant differences in shrinkage stress where obtained, up to 69% decrease. and flexion, statistically significant . In the fourth chapter, it was incorporated a high molecular weight polyethylene and polypropiylene, in the experimental resin proposed in chapter two, at two concentrations (1.5% and 3%), and tested by degree of conversion, compressive and flexural strength and shrinkage stress. The data from each test were analyzed by one way ANOVA, finding no significant difference (p> 0.005) in degree of conversion, compressive and flexural strength, and the contraction stress showed a significant reduction of up to 40%. Conclusion: PLA + castor oil and the high molecular weight polyethylene decreases the shrinkage stress and properties like flexural and compressive strength and degree of conversion were not influenced.
Os objetivos deste estudo foram: 1) avaliar o grau de conversão, tensão de contração, resistência à flexão e compressão de uma resina experimental com um poliácido lático e óleo de rícino; 2) avaliar a citotoxicidade de uma resina experimental com um poliácido lático e óleo de rícino; 3) avaliar o grau de conversão, tensão de contração, resistência à flexão e compressão de uma resina experimental com adição de um polietileno de alto peso molecular e 4) avaliar a citotoxicidade de uma resina experimental com adição de um polietileno de alto peso molecular. Previamente realizou-se a síntese de um Poli-ácido lático (PLA) por meio de um processo de policondensação. Após 24 horas, foi incorporado o óleo de rícino (OR) por meio de um processo de esterificação junto a N,N-Diciclonecilcarbodimida e 4-Dimetilamino-Piridina a 99% em 40 mL de tetrahidrofeurano. Em seguida, o conjunto foi caracterizado por difração de espectroscopia Raman, microscopia eletrônica de varredura (MEV) e estresse de contração, onde foi comprovada a incorporação do PLA+OR numa resina composta experimental. No terceiro capítulo foi feita a incorporação do PLA+OR em duas concentrações (1,5% e 3%) numa resina composta experimental: (2,2 bis[4-(2-hidroxi-3-metacrilatoxipropoxi)fenol](BisGMA), Trietilglicoldimetacrilato (TEGDMA), canforoquinona (CQ), N,N– dimetilaminoetil metacrilato (DMAEMA), hidroxitolueno butilado (BHT), vidro de borossilicato de bário silanizado e foram realizados os testes de flexão, compressão, grau de conversão e estresse de contração. Foi feito o teste de citotoxicidade, uma suspensão de 3 x 103 células/mL de meio de cultura foi preparada. Para isso, as células foram descoladas do fundo da garrafa por meio da adição de tripsina com EDTA, após incubação em estufa com 5% de CO2 e 3 mL de meio de cultura Eagle suplementado. Após o período de incubação de 24 h, o meio de cultura foi removido e 200 μL de uma solução de meio de cultura e sal metiltetrazólio (MTT).Posteriormente, a análise da atividade mitocondrial foi realizada em espectrofotômetro (Biotek EL 800, Biotek, Winooski, VT, EUA) com comprimento de onda de 570 nm. Os dados obtidos de cada teste foram analisados com ANOVA um fator. Não encontrou-se diferença significativa (p> 0.005) nos testes de compressão e grau de conversão; já para o teste de estresse de contração, houve diminuição estatisticamente significativa de até 69% e no teste de resistência à flexão . No quarto capítulo foi feita a incorporação de um polipropileno (PP) e o polietileno de ultra-alto peso molecular (UTEC) a uma resina experimental com os mesmos materiais manipulados no capítulo dois, em duas concentrações (1,5% e 3%), e realizados os testes de compressão, flexão, grau de conversão e estresse de contração. Os dados de cada teste foram analisados com ANOVA um fator. Não houve diferença significativa (p> 0.005) nos testes de compressão, flexão e grau de conversão; já para o teste de estresse de contração houve diminuição estatisticamente significativa de até 40%. Conclusão: OPLA+óleo de rícino e o polietileno de ultra alto peso molecular em 1,5%, diminuíram o estresse de contração e propriedades como resistência à flexão, compressão e grau de conversão não foram influenciadas.

This thesis is dealing with the issue of residual stress present in the wall of a polymer pipe and the influence on its lifetime. Experimental data obtained by the ring slitting method are evaluated and the tangential residual stress in the wall of polypropylene pipe is determined. The evaluation is carried out using a new methodology based on the curved beam theory. The method is verified using numerical simulation. 3D numerical model is used to verify the behavior of the pipe, when there are both tangential and axial residual stresses present in the pipe wall, because the presence of axial stress causes a rise in the magnitude of tangential residual stress. A correction of the tangential stress values corresponding to the pipe length is then proposed. It is shown, that the distribution of the tangential residual stress does not depend significantly on the dimensions or ma-terial of the extruded pipe and a general equation is proposed to describe the distribution. This general distribution is then involved in the calculations of the pipe lifetime that are carried out using a method based on the linear elastic fracture mechanics. A significantly lower lifetimes are obtained when taking the residual stress into account.

In the present work, experimental and numerical investigations into the load-displacement responses of a human lumbar Truncated Vertebral Unit (TVU) under quasi-static and impact loading conditions have been carried out for aiding in the design of orthopaedic implants and countermeasures for vehicle occupant and pedestrian safety. TVU samples obtained from the lumbar spinal column of an adult human male cadaver were initially subjected to quasi-static compressive tests. Impact tests were then conducted on a similar TVU sample in a drop-weight testing device instrumented with a piezoelectric load cell and a high-speed data acquisition system. An explicit nonlinear finite element model of the TVU was developed for predicting the experimental quasi-static and impact dynamic responses. Using the validated modelling approach mentioned, insights have been generated on adjoining vertebral stresses due to disc arthroplasty, and single and multi-level disc fusions as well as posterior fusions with and without posterior instrumentation. The numerical study is further extended to another crucial orthopaedic domain i.e. the assessment of the performance of variants of TKR (Total Knee Replacement) implants under ISO-specified dynamic gait cycle. In the latter investigation, a detailed and realistic finite element model of a representative human knee complex was developed by capturing relevant tissues such as femoral and tibial bones, medial and lateral collateral ligaments, and the components of a typical TKR implant including femoral component, tibial tray and UHMWPE (Ultra High Molecular Weight Polyethylene) insert. Substantive contribution has been made in the current research work towards assessment of vehicle occupant and pedestrian safety by applying the previously mentioned advanced finite element modelling approaches for representing complex vehicle structures, anthropomorphic test devices (commonly called as “dummies”), and pedestrian leg-forms. To this end, keeping in mind computational efficiency and need for optimization, a truncated finite element modelling approach capable of predicting the occupant response for a passenger car subject to a full-frontal US-NCAP test has been developed. Using the modelling tools mentioned and a nonlinear explicit LS-DYNA solver, it has been shown that meeting pedestrian safety standards need not be an isolated exercise of designing the front bumper of a vehicle only but can be combined with meeting NCAP occupant safety requirements leading to weight reduction of the front structure of a vehicle with gages of parts such as front rails in addition to bumper parts being included as design variables. For the first time, with the help of a comparative study carried out with a Hybrid 3 dummy and detailed biomechanical models of human lower extremity, the susceptibility of knees with TKR implants to periprosthetic injuries during frontal collisions has been demonstrated pointing out to a need for higher knee-protection countermeasures in vehicles.

The increasing use of polyethylene in diverse applications motivates the need for understanding how its molecular properties relate to the overall behaviour of the material. Although microstructure and mechanical properties of polymers have been the subject of several studies, the irreversible microstructural rearrangements occurring at large deformations are not completely understood. The purpose of this thesis is to describe how the concepts of Continuum Damage Mechanics can be applied to modelling of polyethylene materials under different loading conditions. The first part of the thesis consists of the theoretical formulation and numerical implementation of a three-dimensional micromechanical model for crystalline polyethylene. Based on the theory of shear slip on crystallographic planes, the proposed model is expressed in the framework of viscoplasticity coupled with degradation at large deformations. Earlier models aid in the interpretation of the mechanical behaviour of crystalline polyethylene under different loading conditions; however, they cannot predict the microstructural damage caused by deformation. The model, originally due to Parks and Ahzi (1990), was further developed in the light of the concept of Continuum Damage Mechanics to consider the original microstructure, the particular irreversible rearrangements, and the deformation mechanisms. Damage mechanics has been a matter of intensive research by many authors, yet it has not been introduced to the micromodelling of semicrystalline polymeric materials such as polyethylene. Regarding the material representation, the microstructure is simplified as an aggregate of randomly oriented and perfectly bonded crystals. To simulate large deformations, the new constitutive model attempts to take into account existence of intracrystalline microcracks. The second part of the work presents the theoretical formulation and numerical implementation of a three-dimensional constitutive model for the mechanical behaviour of semicrystalline polyethylene. The model proposed herein attempts to describe the deformation and degradation process in semicrystalline polyethylene following the approach of damage mechanics. Structural degradation, an important phenomenon at large deformations, has not received sufficient attention in the literature. The modifications to the constitutive equations consist essentially of introducing the concept of Continuum Damage Mechanics to describe the rupture of the intermolecular (van der Waals) bonds that hold crystals as coherent structures. In order to model the mechanical behaviour, the material morphology is simplified as a collection of inclusions comprising the crystalline and amorphous phases with their characteristic average volume fractions. In the spatial arrangement, each inclusion consists of crystalline material lying in a thin lamella attached to an amorphous layer. To consider microstructural damage, two different approaches are analyzed. The first approach assumes damage occurs only in the crystalline phase, i.e., degradation of the amorphous phase is ignored. The second approach considers the effect of damage on the mechanical behaviour of both the amorphous and crystalline phases. To illustrate the proposed constitutive formulations, the models were used to predict the responses of crystalline and semicrystalline polyethylene under uniaxial tension and simple shear. The numerical simulations were compared with experimental data previously obtained by Bartczak et al. (1994), G‘Sell and Jonas (1981), G‘Sell et al. (1983), Hillmansen et al. (2000), and Li et al. (2001). Our model’s predictions show a consistently good agreement with the experimental results and a significant improvement with respect to the ones obtained by Parks and Ahzi (1990), Schoenfeld et al. (1995), Yang and Chen (2001), Lee et al. (1993b), Lee et al. (1993a), and Nikolov et al. (2006). The newly proposed formulations demonstrate that these types of constitutive models based on Continuum Damage Mechanics are appropriate for predicting large deformations and failure in polyethylene materials.

Environmental stress cracking (ESC) is one of the main, and probably the most common, failure mechanisms involved in polymer fractures. This type of failure is critically important as it occurs suddenly, without any visible pre-fracture deformation. Such failure can be catastrophic and costly in cases where structural integrity is required. In polyethylene (PE), ESC occurs through a slow crack growth mechanism. Cracks initiate from stress-concentrated imperfections, propagate through the bulk of PE, and ultimately result in a brittle fracture. In order to predict the environmental stress cracking resistance (ESCR) of PE, it is necessary to fully understand the molecular structure of the resin. In this thesis, attempts were made to find relationships between molecular structure characteristics and material responses, mainly inter-lamellar entanglements and strain hardening behaviour of PE resins, through mechanical and rheological experiments. Inter-lamellar entanglements are believed to be the main factor controlling slow crack growth of PE. Extent of entanglements and entanglement efficiency were investigated by monitoring the strain hardening behaviour of PE resins in the solid state through a uniaxial tensile test, and in the melt state, through extensional rheometry. ESCR is usually assessed by unreliable and time consuming testing methods such as the notch constant load test (NCLT) on notched PE specimens in the presence of an aggressive fluid and elevated temperatures. In this thesis, a practical, yet reliable, tensile test was developed for the evaluation and prediction of ESCR. The developed test offers a more reliable and consistent ESCR picture without the drawbacks of the subjective notching process and presence of aggressive fluids. Through this test, a factor called ???corrected hardening stiffness (cHS)??? was developed, which can easily be used for a relative ranking of ESCR of different PE resins. Studies were next extended to the melt state via shear and extensional rheometry. Through studies in the shear mode, a molecular weight-normalized average characteristic relaxation time (??N) was found to be efficient in predicting the extent of chain entanglements in resins. This provided a potential melt indicator for a relative measure of ESCR, for linear low density polyethylene (LLDPE), with different short chain branching levels. Extensional studies were conducted to evaluate the strain hardening behaviour in the melt state. An inverse correlation was obtained between ESCR and the melt strain hardening coefficient (MSHC), found from Sentmanat Extensional Rheometry (SER). This indicated an inverse relationship between ESCR and chain extensibility in the melt. In addition, a new factor called ???melt hardening stiffness (mHS)??? was developed from the slope of a stress-strain line, obtained from SER. This factor, analogous to cHS, can be used for a practical and reliable ranking of ESCR of PEs. ESCR is usually associated with classical crystalline phase property indicators, such as crystallinity and lamella thickness. In this thesis, the effect of processing and post processing temperature on the extent of inter-lamellar entanglements were investigated, evaluated, and correlated to ESCR. Also, analysis of the lamella surface area (LSA) was pursued since LSA reflects changes in phase interconnectivity more precisely. The focus of this part of the study was on the effect of temperature on LSA to identify the optimum processing and post-processing conditions which yield a higher LSA. It was reasonable to presume that PE with larger lamella lateral surface areas will have more inter-lamellar entanglements, hence higher ESCR. Finally, a well-controlled ultraviolet (UV) photoinitiated reactive extrusion (REX) process was developed for selective formation of long chain branches in the PE structure. This was conducted to impose restrictions against stretching of the polymer chain, which consequently enhanced ESCR.

Environmental stress cracking (ESC) can result in catastrophic failure of polyethylene (PE) structures without any visible warning. The use of PE in more demanding applications, such as trenchless piping, can accelerate ESC failure of the material. Besides public safety issues, the replacement and remediation of these failed polyethylene structures also cost both in money and labour. This thesis is part of a collaborative project between the disciplines of chemical and civil engineering to study environmental stress cracking resistance (ESCR) of polyethylene. By combining structural mechanics and (micro)molecular science, new insights into the ESCR behaviour of polyethylene could be achieved. The test commonly used for determining ESCR of polyethylene can be time consuming and rather imprecise. In our study a new testing method has been developed which compares ESCR of resins based on the more direct measure of “hardening stiffness” rather than strain-hardening modulus. Our new method is much simpler than those proposed previously because it is conducted under ambient conditions and does not require specialized equipment for true stress-strain measurements. Comparisons between the conventional ESCR test method and the strain hardening test show that strain hardening can be used to rank ESCR of polyethylene in a reliable fashion. The strain hardening test developed in this thesis has the potential to replace the standard ESCR test that has been in use in industry for the past twenty five years. Most ESCR research has so far focused on bridging-tie-molecules as the main source of inter-lamellar connections. We take a fresh approach and demonstrate in this thesis that physical chain entanglements also contribute to the formation of inter-lamellar linkages. Chain entanglements in the melt state are known to be preserved in the polymer upon solidification, therefore, rheological determination of the molecular weight between entanglements (Me) is used as a measure of chain entanglements for PE. A lower Me value means a higher number of entanglements in the system. The inversely proportional relationship between Me and ESCR indicates that low network mobility due to increasing number of chain entanglements increases ESCR of PE. With the understanding that strain hardening is related to ESCR of polyethylene, the relationship between chain entanglements and tensile strain hardening has also been investigated. By combining experimental observations and parallel micromechanical modeling results, the presence of physical chain entanglements in the amorphous phase was demonstrated to be the factor controlling the strain hardening behaviour of polyethylene. Studies of the effect of inter-lamellar linkages on ESCR of polyethylene have traditionally focused on changes in the amorphous phase. In this thesis, percentage crystallinity and lamella thickness of polyethylene resins were studied to determine their effects on ESCR. The study of the effect of the crystalline phase on ESCR was extended to investigate the lateral surface characteristics of the lamella. An increase in ESCR was observed with increases in lateral lamella area of resins. It was postulated that a larger lateral lamella area results in a higher probability of formation of inter-lamellar linkages. This increase in phase interconnectivity directly results in an increasing ESCR for the resins. Finally, in order to facilitate practical applications of polyethylene (especially in pipes), attempts were made to develop a predictive tool for the quantitative estimation of the long-term ESCR of polyethylene based on the short-term notched constant load test (NCLT). Although previous work on slow crack growth models showed little sensitivity to crack activation energy, the ESC model pursued herein was found to be exponentially dependent on this parameter. Further refinement of the ESC model is needed but the modeling investigation proved fruitful in highlighting several other relationships amongst chemical, physical and mechanical properties of PE resins, such as, that between ESC crack activation energy and the α-relaxation energy of polyethylene.

碩士
中原大學
醫學工程研究所
86
Total hip replacement (THR) surgery has been performed for over thirty years for patients with severe hip joint problems. Although THR surgery has provided satisfactory outcomes, there are still failures, such as stem loosening, osteolysis, and bone resorption, which occur occasionally. It has raised great attention by orthopaedic surgeons and biomedical engineers on how to improve implant designs and surgical techniques in order to avoid THR failures. It is widely believed that osteolysis is mainly caused by the wear debris of the ultra high molecular weight polyethelyne (UHMWPE) insert in the acetabular cup. Therefore, how to reduce the contact stress in the UHMWPE insert so as to reduce wear debris has become an important issue. In this research, the stress distributions on the UHMWPE insert of the acetabular cup at different inclination angles and anteversion angles were investigated. Different variations for the inclination and anteversion angles were investigated with twenty five different finite element models. Finite element analyses were performed by using a commercial finite element package - MARC K6.2. The linear and volumetric wear rates of the UHMWPE insert for two different sizes of femoral heads were also evaluated. Results showed that as the inclination angle and anteversion angle increased, the peak stress on the UHMWPE insert increased substantially and vice versa. This result can provide a guideline for surgeons when selecting implantation angles for the acetabular cup. In the wear rate prediction, for a femoral head with 26 mm diameter, the linear wear rate was 0.34 mm per year while the volumetric wear rate was 154.1 mm3 per year. For a femoral head with 32 mm diameter, the linear wear rate was 0.29 mm per year while the volumetric wear rate was 210.0 mm3 per year. These results all fall within the range observed from earlier clinical follow-up studies. Further parametric studies on the design of acetabular components can be performed based on this wear prediction model.

After Total Knee Arthroplasty, contact stresses at the surface and stresses at the implant-cement-bone interface are directly related to the joint contact forces. These stresses are a major factor in wear and fatigue, aseptic loosening, stress shielding and osteoporosis. Implant contact stresses influence the wear and fatigue damage of the Ultra High Molecular Weight Polyethylene (UHMWPE) articulating surface, decreasing the longevity of the implant. The contact stresses are influenced by the kinematics, the bearing congruency of the articulating surfaces and insert thickness. Thus, various studies have focused on the prediction and optimization of kinematics at the joint interface, contact areas, and stresses in different knee implant designs. As a result, the successful total knee replacement designs depend on joint kinematics and the contact stresses. The objective of this study was to perform contact stress analysis on a newly designed surface guided knee implant, in order to evaluate the design with respect to the potential of polyethylene wear. In order to test the performance of this design, Finite Element Modeling (FEM) was used as a good medium to analyze the design’s specifications, and to evaluate the results of the stress analysis of the design. For validation and also comparison with previous studies, results of this study were compared with those of related work with similar loading and constraints. Based on the gathered data from FE analysis of the design, it can be concluded that the new surface guided knee implant shows lower peak contact pressure than other previously evaluated implants.
October 2015

No
The effect of matrix polymer and filler content on the rheological behavior of hydroxyapatite-filled injection molding grade high-density polyethylene (HDPE) has been studied. Studies of the flow curves revealed that the matrix and the composite exhibit three distinct regions in the flow curve, namely, a pseudoplastic region at low to moderate shear rates, a plateau and a second pseudoplastic region at high shear rates. The shear stress corresponding to the plateau (Tc) is dependent on both the filler concentration and the melt temperature. Addition of HA in the HDPE matrix increases the value of Tc and decreases compressibility of the melt. An increase in temperature also raises the value of Tc. From the nature of flow curves it is concluded that the matrix polymer largely decides the rheology of the composite.

Indiana University-Purdue University Indianapolis (IUPUI)
Large protein molecules are abundant in biological cells but are very difficult to study in physiological conditions due to molecular disorder. For large proteins, most structural information is obtained in crystalline states which can be achieved in certain conditions at very low temperature. X-ray and neutron crystallography methods can then be used for determination of crystalline structures at atomic level. However, in solution at room or physiological temperatures such highly resolved descriptions cannot be obtained except in very few cases. Scattering methods that can be used to study this type of structures at room temperature include small-angle x-ray and neutron scattering. These methods are used here to study two distinct proteins that are both classified as glycoproteins, which are a large class of proteins with diverse biological functions. In this study, two specific plasma glycoproteins were used: Fibrinogen (340 kDa) and Alpha 1-Antitrypsin or A1AT (52 kDa). These proteins have been chosen based on the fact that they have a propensity to form very large molecular aggregates due to their tendency to polymerize. One goal of this project is to show that for such complex structures, a combination of scattering methods that include SAXS, SANS, and DLS can address important structural and interaction questions despite the fact that atomic resolution cannot be obtained as in crystallography. A1AT protein has been shown to have protective roles of lung cells against emphysema, while fibrinogen is a major factor in the blood clotting process. A systematic approach to study these proteins interactions with lipid membranes and other proteins, using contrast-matching small-angle neutron scattering (SANS), small angle x-ray scattering (SAXS) and dynamic light scattering (DLS), is presented here. A series of structural reference points for each protein in solution were determined by performing measurements under osmotic stress controlled by the addition of polyethylene glycol-1,500 MW (PEG 1500) in the samples. Osmotic pressure changes the free energy of the molecular mixture and has consequences on the structure and the interaction of molecular aggregates. In particular, the measured radius of gyration (Rg) for A1AT shows a sharp structural transition when the concentration of PEG 1500 is between 33 wt% and 36 wt%. Similarly, a significant structural change was observed for fibrinogen when the concentration of PEG 1500 was above 40 wt%. This analysis is applied to a study of A1AT interacting with lipid membranes and to a study of fibrinogen polymerization in the presence of the enzyme thrombin, which catalyzes the formation of blood clots. The experimental approach presented here and the applications to specific questions show that an appropriate combination of scattering methods can produce useful information on the behavior and the interactions of large protein systems in physiological conditions despite the lower resolution compared to crystallography.

Polymer pipes, typically high density polyethylene (HDPE), can be pulled-into-place, avoiding traditional cut-and-cover construction, using pipe bursting and horizontal directional drilling (HDD) pipe installation techniques. Of particular interest, are the ground displacements, induced by cavity expansion, associated with these techniques and the strains that develop in existing pipes in response to these displacements. Further, the axial stress-strain response of the new HDPE pipe during and after the cyclic pulling force history required to pull the pipe into place is of interest. Surface displacements and strains in an adjacent polyvinyl chloride (PVC) pipe induced by static pipe bursting were measured during the replacement of a new unreinforced concrete pipe. For the pipe bursting geometry tested, the maximum vertical surface displacement measured at the ground surface was 6 mm, while the distribution of vertical surface displacements extended no more than 2 m on either side of the centreline. The maximum longitudinal strain measured in the PVC pipe was less than 0.1% and its vertical diameter decreased by only 0.5%, suggesting that pipe bursting did not jeopardize the long-term performance of the water pipe tested. In addition, results from identical stress relaxation and creep tests performed on whole pipe samples and coupons trimmed from a pipe wall were compared, and these demonstrated that the coupons exhibited higher modulus than the pipe samples. Therefore, isolated pipe samples, as opposed to coupons, were tested to quantify the stress-strain response of HDPE pipe during the simulated installation, strain recovery, and axial restraint stages of HDD. Axial strains were found to progressively accumulate when an HDPE pipe sample was subjected to the cyclic stress history used to simulate an HDD installation. It was shown that existing linear and nonlinear viscoelastic models can serve as predictive design tools for estimating the cyclic strain history of HDPE pipe during installation. For the specific conditions examined, the tensile axial stresses redeveloped in the pipe samples, once restrained, were not large enough to lead to long-term stress conditions conducive to slow crack growth even when the short-term performance limits were exceeded by a factor of 1.5.
Thesis (Ph.D, Civil Engineering) -- Queen's University, 2009-04-01 18:19:24.434

Indiana University-Purdue University Indianapolis (IUPUI)
Reduction of the polymerization shrinkage stress inherent of dimethacrylate-based resin composites has been a work in progress since the phenomenon was first described by Dr. Rafael L. Bowen in 1967. Contemporary efforts to modify the composites or the curing protocols for polymerization have proven a challenging task with controversial results. Influenced by existing mathematical models relating exposure, curing time and depth of cure of resin composites, a novel method for the reduction of polymerization shrinkage stress is proposed. By polymerizing through a single aperture mask, a dental light curing unit is transformed from a planar light source to a point light source, and a fully cured, three-dimensional “bullet” shaped curing front is predicted for the cured resin below. So long as the edges of the bullet do not touch the cavity walls or floor, the shrinkage stress of the bullet is not transferred. Follow-up with an unmasked curing unit then fully polymerizes the restoration. By reducing the volume of uncured composite in contact with the cavity walls and floor, shrinkage stress of the restoration is also reduced. The objective of the present study was to demonstrate this curing phenomenon with a model resin composite using masks with aperture diameters of 0.5, 0.4, and 0.25 mm and curing times of 10, 20, 30, and 40 seconds. The resulting curing front was evaluated quantitatively and qualitatively. From this, mathematical models of the curing front were derived. Selected combinations of aperture mask and curing time were then investigated to evaluate the influence of this phenomenon on the degree of conversion, Knoop hardness, and polymerization shrinkage stress of the same model resin composite. Group differences were analyzed using a one-way ANOVA at 5% significance.