Design Protocols and Analytical Strategies that Incorporate Structural Reliability Models

Ceramic matrix composites (CMC) and intermetallic materials (e.g., single crystal nickel aluminide) are high performance materials that exhibit attractive mechanical, thermal, and chemical properties. These materials are critically important in advancing certain performance aspects of gas turbine engines. From an aerospace engineers perspective the new generation of ceramic composites and intermetallics offers a significant potential for raising the thrust/weight ratio and reducing NO(sub x) emissions of gas turbine engines. These aspects have increased interest in utilizing these materials in the hot sections of turbine engines. However, as these materials evolve and their performance characteristics improve a persistent need exists for state-of-the-art analytical methods that predict the response of components fabricated from CMC and intermetallic material systems. This need provided the motivation for the technology developed under this research effort. Continuous ceramic fiber composites exhibit an increase in work of fracture, which allows for 'graceful' rather than catastrophic failure. When loaded in the fiber direction these composites retain substantial strength capacity beyond the initiation of transverse matrix cracking despite the fact that neither of its constituents would exhibit such behavior if tested alone. As additional load is applied beyond first matrix cracking, the matrix tends to break in a series of cracks bridged by the ceramic fibers. Any additional load is born increasingly by the fibers until the ultimate strength of the composite is reached. Thus modeling efforts supported under this research effort have focused on predicting this sort of behavior. For single crystal intermetallics the issues that motivated the technology development involved questions relating to material behavior and component design. Thus the research effort supported by this grant had to determine the statistical nature and source of fracture in a high strength, NiAl single crystal turbine blade material; map a simplistic future strength envelope of the material; develop a statistically based reliability computer algorithm; verify the reliability model and computer algorithm-, and model stator vanes for rig tests. Thus establishing design protocols that enable the engineer to analyze and predict the mechanical behavior of ceramic composites and intermetallics would mitigate the prototype (trial and error) approach currently used by the engineering community. The primary objective of the research effort supported by this short term grant is the continued creation of enabling technologies for the macro-analysis of components fabricated from ceramic composites and intermetallic material systems. The creation of enabling technologies aids in shortening the product development cycle of components fabricated from the new high technology materials

Design Protocols and Analytical Strategies that Incorporate Structural Reliability Models

Ceramic matrix composites (CMC) and intermetallic materials (e.g., single crystal nickel aluminide) are high performance materials that exhibit attractive mechanical, thermal and chemical properties. These materials are critically important in advancing certain performance aspects of gas turbine engines. From an aerospace engineer's perspective the new generation of ceramic composites and intermetallics offers a significant potential for raising the thrust/weight ratio and reducing NO(x) emissions of gas turbine engines. These aspects have increased interest in utilizing these materials in the hot sections of turbine engines. However, as these materials evolve and their performance characteristics improve a persistent need exists for state-of-the-art analytical methods that predict the response of components fabricated from CMC and intermetallic material systems. This need provided the motivation for the technology developed under this research effort. Continuous ceramic fiber composites exhibit an increase in work of fracture, which allows for "graceful" rather than catastrophic failure. When loaded in the fiber direction, these composites retain substantial strength capacity beyond the initiation of transverse matrix cracking despite the fact that neither of its constituents would exhibit such behavior if tested alone. As additional load is applied beyond first matrix cracking, the matrix tends to break in a series of cracks bridged by the ceramic fibers. Any additional load is born increasingly by the fibers until the ultimate strength of the composite is reached. Thus modeling efforts supported under this research effort have focused on predicting this sort of behavior. For single crystal intermetallics the issues that motivated the technology development involved questions relating to material behavior and component design. Thus the research effort supported by this grant had to determine the statistical nature and source of fracture in a high strength, NiAl single crystal turbine blade material; map a simplistic failure strength envelope of the material; develop a statistically based reliability computer algorithm, verify the reliability model and computer algorithm, and model stator vanes for rig tests. Thus establishing design protocols that enable the engineer to analyze and predict the mechanical behavior of ceramic composites and intermetallics would mitigate the prototype (trial and error) approach currently used by the engineering community. The primary objective of the research effort supported by this short term grant is the continued creation of enabling technologies for the macroanalysis of components fabricated from ceramic composites and intermetallic material systems. The creation of enabling technologies aids in shortening the product development cycle of components fabricated from the new high technology materials

Vibration fatigue reliability analysis of aircraft landing gear based on fuzzy theory under random vibration

The failure of aircraft landing gear (ALG) is major caused by vibration fatigue. And its main failure mode is fatigue fracture. Currently the reliability of ALG is usually calculated by the stress strength interference (SSI) model, which is based on the binary state assumption. While in reality, the strength is degraded with time and the boundary of the failure and success is blur, so the binary state assumption is deviated from the fact. To overcome this problem, this paper uses the membership function (MF) to represent fuzzy safe state which caused by the strength degradation under the failure mode of vibration fatigue. Moreover, a fuzzy reliability model (FRM) of ALG is proposed based on fuzzy failure domain (FFD). Finally, the feasibility of method is tested through a simulation example. By comparing the simulation results (SRs) of the FRM with SRs of the static SSI model and the dynamic SSI model, the rationality of the method is verified. The FRM can calculate the reliability without the gradual degradation processes, thus it is used more widely

Computational Modeling of Fracture Failure in Mineralized and Prosthetic Biomaterials

Natural mineralized tissues, e.g., teeth and bone, have the capacity to tolerate the daily physiological loading. However, due to their high mineralized composition, they have been recognized as a class of relatively brittle biomaterials. The inherent brittle nature and fairly high susceptibility to mechanical failure present a more critical problem in biomedical research field. To replace such diseased or damaged mineralized tissues, prosthetic materials are largely applied in the areas of dental and osteo clinical treatments. Ceramic materials provide numerous favourable characteristics, including biocompatibility and chemical resistance. In addition to the dental industry, applications of osteofixation/osteosynthiesis devices are considered fundamental to stabilize various treatments of bone defects for promoting osteointegration and reconstruction. However, clinical observations and specialized literature have revealed that dental restorative materials and prosthetic fixation devices are often subject to high stress, leading to fracture either by catastrophic overloading or cyclic fatigue failure. The aim of this thesis is to develop a computational modelling framework on the basis of the extended finite element method (XFEM) to investigate the fracture behaviors of mineralised and synthetic biomaterials in various medical applications. The XFEM modelling results are validated by being compared with the in-vitro experiments and/or clinical observations. Through the research in this thesis studies, XFEM has been demonstrated to be a powerful tool to analyse fracture behaviors in the bio-structures subjected to not only static loadings but also cyclic loadings. The outcomes generated in this thesis help gain some insightful understanding failure of the native or prosthetic structures, which is anticipated to provide some clinical guidelines for the design optimisation of patient-specific prosthetic devices to ensure their reliability and longevity

Currently used systems of dental posts for endodontic treatment

An advanced stage of a tooth decay promotes an extremely damaged tooth that needs endodontic treatment to be restored. When satisfactory coronal tooth structure remains, an artificial crown can be placed without a post. On the other hand, the treatment of seriously damaged teeth often require an endodontic post. The main reason for using post is to enable rebuilding of the tooth structure prior to crown restoration. Dentists believe that endodontic posts provide a stable and solid restoration of the tooth, as well as providing strengthening of the tooth root, which constitutes the solid basis for a dental crown restoration. However, some authors reported that the strength of the tooth is directly related to the remaining dentin, and because of that, an endodontic treatment can present a higher risk of biomechanical failure. A number of different materials have been used for the manufacturing of dental posts. The fundamental posts requirements include high tensile strength, high fatigue resistance to occlusal and shear loading and a good distribution of the forces affecting the tooth root. The purpose of this article is to review the current literature and identify the various characteristics of a dental post, as well as some principles on the endodontic treatment for tooth decay.info:eu-repo/semantics/publishedVersio

A critical review and finite element study of structural relaxation in veneered 3Y-TZP dental structures

Yttria stabilized Tetragonal Zirconia Polycrystal (3Y-TZP) materials veneered with porcelain are preferred for clinical practice due to their bio-functionality, biocompatibility, and affordability. However, these materials are often subjected to premature clinical failures due to combination of subsurface flaws and tensile stresses within porcelains. This work focuses on performing a critical literature review and evaluating veneer characteristics, i.e. structural relaxation, to form a suitable explanation for the development of subsurface stresses. User material subroutines, UEXPAN and UTRS are developed, validated and integrated in ABAQUS finite element solver. With the use of validated models, it is shown that faster cooling rates and high veneer thickness results in higher subsurface tensile stresses due to lack of structural relaxation. Slow cooling rates and lower veneer thickness result in desired compressive stresses in the subsurface. This work shows that structural relaxation in veneer can be used for tailoring the desired stress for the extending lifespan of veneered 3Y-TZP dental restorations

Endodontic access in all-ceramic dental restorative crown materials

A diagnosis of irreversible pulpitis can occur after a crown has been permanently cemented. This necessitates the need for endodontic treatment, often with the crown in situ. Increasing trends indicate that all-ceramic crowns are provided as the preferred restorative option to metal-ceramic crowns. This is because modern all-ceramic restorations can now provide excellent aesthetic solutions combined with high mechanical strength properties, compared with earlier, weaker ceramic materials. It is a considerable operative challenge for the dental practitioner to prepare an endodontic access cavity in vivo, due to the high mechanical properties of modern dental ceramic materials. The inherent nature of ceramic materials is that they are brittle, sensitive to damage and moisture, with failure occurring in an unpredictable manner. The difficulties in performing endodontic treatment in all-ceramic crowns and subsequently repairing the access cavity is relatively unexplored in the dental literature, more research is needed to inform clinical practice in this area. A systematic review of the literature aimed to identify influential treatment factors of endodontically accessed and repaired all-ceramic crowns and report the evidence of damage around the endodontic access cavity as a result of preparing the cavity in an all-ceramic crown. Eight studies were selected to address the aims. The inadequate volume of literature was highlighted with, the earliest relevant publication identified in 1962 and since the last electronic search (2016) only 26 additional references were identified in the subject area. Potentially noteworthy strength controlling factors were identified to be related to the crown material, its baseline strength, the grit size of the diamond bur used to create the access cavity, the ratio of access cavity to crown dimension, the cement used to lute the crown and the presence of radial cracks after access cavity preparation. The effect of two variables, namely, cavity dimension and modulus of elasticity of the resin composite repair material on the equibiaxial flexural strength of lithium disilicate glass-ceramic (IPS e.max® Press, Ivoclar Vivadent) material was investigated. Disc specimens with representative access cavities were used as a model system to examine these variables. Within the study limitations, the results indicate that cavity size and not the repair material, influence the equibiaxial flexural strength. The shear bond strength of the resin composite material used to repair the access cavity in a lithium disilicate glass-ceramic was determined to be comparable to those values as found in the literature (see Appendices). Model mandibular first molar crowns were fabricated from lithium disilicate glass-ceramic to examine the impact of cavity size on failure load. The failure load for the intact crowns and crowns with a rhomboidal (based on the presence of three-canals) or rectangular (based on the presence of four-canals) endodontic access cavity, with and without a resin composite repair were measured and analysed statistically. Within the limitations, the results show that a rectangular access cavity significantly reduces the failure load which was then restored to the original values upon repair with resin composite. The preparation of a rhomboidal access cavity did not reduce the failure load compared with the intact crown. The novel use of Finite Element Analysis (FEA) was successfully demonstrated in this subject area. Solid geometric models of lithium disilicate glass-ceramics (LDGC) crowns with three endodontic access options repaired with a resin composite (Tetric EvoCeram®, Ivoclar Vivadent) were modelled. The models were subsequently subjected to clinically relevant loads and a stress analysis was performed using FEA. This work showed that high curvature access cavity designs produced the highest stress scenario and therefore should be avoided. In an attempt to compliment the in vitro study computer models of LDGC discs were modelled using Finite Element (FE). The models were successfully validated, similar variables were modelled and concentric ring loading conditions were applied as per the in vitro study. It was determined that the size and not the stiffness of the repair material was more critical to the strength of LDGC discs. In conclusion, this study has addressed some of the aspects of problems encountered when endodontic access cavities are prepared in all-ceramic dental crowns, however it is an area where substantial literature is lacking and therefore further research is warranted

Applied Fracture Mechanics

The book "Applied Fracture Mechanics" presents a collection of articles on application of fracture mechanics methods to materials science, medicine, and engineering. In thirteen chapters, a wide range of topics is discussed, including strength of biological tissues, safety of nuclear reactor components, fatigue effects in pipelines, environmental effects on fracture among others. In addition, the book presents mathematical and computational methods underlying the fracture mechanics applications, and also developments in statistical modeling of fatigue. The work presented in this book will be useful, effective, and beneficial to mechanical engineers, civil engineers, and material scientists from industry, research, and education

The influence of different surface and heat treatments on the biaxial flexural strength of veneering ceramics for zirconia and strength reliability and mode of fracture of veneering ceramics

Tese de doutoramento, Medicina Dentária (Reabilitação Oral), Universidade de Lisboa, Faculdade de Medicina Dentária, 2011New processing techniques have facilitated the use of Zirconia core materials in all-ceramic dental prostheses. Zirconia has many potential advantages compared to existing core materials; however its performance when layered with veneering ceramics has not been clearly evaluated. Moreover the veneering ceramics used with Zirconia may be ground, polished or glazed during laboratory procedures and/or clinical adjustments. These treatments may affect their strength by introducing microscopic flaws and defects. The purposes of this study were to investigate the effects of surface treatments on the mean biaxial flexural strength of three feldspathic veneering ceramics used to layer Zirconia cores: NobelRondoTM Zirconia veneer ceramic (Nobel BiocareTM AB, Sweden), LavaTM Ceram veneer ceramic (3MTM, ESPETM, Germany), and Vita® VM®9 veneer ceramic (Vita®, Zahnfabrick, Germany) and compare the mean biaxial flexural strength, its reliability, and mode of fracture of bilayered Zirconia discs veneered with the three feldspathic veneering ceramics. For the first part of the study one hundred and eighty monolithic disc specimens (12.7 mm x 2.2mm), sixty for each feldspathic veneering ceramic were prepared according to the manufacturer’s instruction and divided into eighteen groups, 6 groups for each feldspathic veneering ceramic with 10 specimens for each group. The six groups for each feldspathic veneering ceramic were untreated, grounded, grounded and polished, grounded and glazed, grounded polished and glazed and polished and glazed. Mean biaxial flexural strength and Weibull modulus were appraised. Statistical significance Summary 236 among groups of population was analyzed using one-way and two-way ANOVA and Fisher's PLSD comparison tests. For the second part of the study sixty bilayered disc specimens (12.7 mm x 2.2 mm), twenty for each feldspathic veneering ceramic were prepared using sixty Zirconia core discs (12.7 mm x 1.1 mm) layered with the three feldspathic veneering ceramics according to the manufacturer’s instruction and divided into 6 groups of 10 specimens for each material. Mean biaxial flexural strength and Weibull modulus were appraised, and a scanning electron microscope was used to describe surface features. Statistical significance among groups of population was analyzed using two-way ANOVA, Fisher's PLSD and Student’s t-test comparison tests. For the first part of the study and when the veneering ceramics were analyzed together the data provided strong evidence that there was a significant difference in biaxial flexural strength between the grinding groups and all other groups. When the feldspathic veneering ceramics were analyzed individually data revealed more heterogeneity between the mean biaxial strength of different groups. However, grinding decrease the strength of veneering ceramics and in some cases significantly. For this reason, grinding should always be avoided if any other procedure is to be done, as this will either create or change the developed crack dimensions or increase the volume loss. Conversely polishing and glazing improved the strength of all materials and in same cases significantly. These procedures are recommended to counteract the detrimental effects of grinding which was related to the ability of the procedures to improve the condition of the ceramic’s surface and free it from various defects and flaws. The Weibull modulus values for the veneering ceramics tested varied with different treatments. They showed higher values for polished, glazed and untreated groups, and lower values for ground groups. For the bilayered specimens when the veneering ceramics were Summary 237 analyzed together, specimens with the core material on the bottom surface were statistically stronger and more reliable than those with the veneering ceramics on the bottom surface. When analyzed individually only in the NobelRondoTM Zirconia veneer ceramic there was no significant difference when the core material or the veneering ceramic was on the bottom surface. Two different modes of fracture were observed in the bilayered specimens according to which material was on the bottom surface. The material that underwent tensile stress dictated the strength, reliability, and fracture mode of the specimens. The design of the restorations and the actual distribution of the tensile stresses must be taken into account, otherwise the significant contribution of stronger and tougher core materials to the performance of all-ceramic Zirconia-based restorations may be offset by the weaker veneering ceramics.Com o objectivo de ultrapassar as limitações dos materiais em cerâmica pura tradicionais, diversas companhias introduziram em medicina dentária reabilitadora um material de elevada resistência composto por cerâmica de Zircónia. A sua aplicação em prostodontia está a emergir devido fundamentalmente às suas excelentes propriedades mecânicas, biológicas e estéticas e ao desenvolvimento de novas tecnologias, como a tecnologia CAD/CAM, que permitem a confecção de coroas unitárias e próteses parciais fixas de uma forma standardizada e eficiente. Apesar das enormes e aparentes vantagens da Zircónia, comparativamente aos materiais cerâmicos tradicionais utilizados como infraestrutura de restaurações protéticas, o seu desempenho clínico quando estratificada com cerâmicas feldspáticas de revestimento não tem sido até agora avaliada com evidência. Do ponto de vista da selecção de material, a substituição de materiais cerâmicos tradicionais ou mesmo alumina por Zircónia com maior resistência deveria melhorar a performance clínica das coroas tendo como referência a origem da fractura. No entanto, a resistência das cerâmicas feldspáticas, e consequentemente de uma restauração em cerâmica pura com núcleo de Zircónia, está dependente do grau de polimento final da restauração e dos diferentes procedimentos de fabricação no laboratório e ou ajustes clínicos que possibilitem uma correcta adaptação e ou oclusão. Os procedimentos de processamento e ou ajustes clínicos são passíveis de provocar pequenos defeitos microcópicos e ou fissuras sub-críticas, que poderão ser Resumo 240 acompanhados por uma alteração e consequente redução de resistência à fractura. A presença destas fissuras pode como consequência de carga clínica e ou presença de humidade crescer para uma situação crítica limite levando a falha catastrófica. O efeito dos procedimentos de processamento de materiais cerâmicos tem sido estudado por numerosos investigadores. No entanto, existe ainda controvérsia no que respeita ao melhor método para produzir a superfície mais polida e resistente. No sentido de avaliar todas estas suposições os objectivos deste estudo foram avaliar a resistência à fractura medida através da resistência à flexão biaxial de cerâmicas feldspáticas de revestimento de Zircónia quando submetidas a tratamentos de superfície nomeadamente, desgaste, polimento e tratamento térmico; e avaliar a resistência, a fiabilidade e o modo de fractura de restaurações em cerâmica pura com infra-estrutura de Zircónia estratificadas com diferentes cerâmicas de revestimento. Para alcançar estes objectivos, foram efectuadas avaliações quantitativas da resistência à flexão biaxial, avaliações qualitativas da ultramorfologia e modo de fractura da interface cerâmica de revestimento – infraestrutura de Zircónia. A estratégia seguida levou à formulação das seguintes hipóteses experimentais: H1.0: Não existem diferenças significativas na resistência à fractura, medida em termos de resistência à flexão biaxial, entre os diferentes tratamentos de superfície no conjunto das cerâmicas feldspáticas de revestimento. H1.1: Existem diferenças significativas na resistência à fractura, medida em termos de resistência à flexão biaxial, entre os diferentes tratamentos de superfície no conjunto das cerâmicas feldspáticas de revestimento. Resumo 241 H2.0: Não existem diferenças significativas na resistência à fractura, medida em termos de resistência à flexão biaxial, entre os diferentes tratamentos de superfície em cada uma das cerâmicas feldspáticas de revestimento. H2.1: Existem diferenças significativas na resistência à fractura, medida em termos de resistência à flexão biaxial, entre os diferentes tratamentos de superfície em cada uma das cerâmicas feldspáticas de revestimento. H3.0: Não existem diferenças significativas na resistência à fractura, medida em termos de resistência à flexão biaxial, no conjunto das cerâmicas feldspáticas de revestimento/infra-estruturas de Zircónia independentemente do material submetido a stress tensional. H3.1: Existem diferenças significativas na resistência à fractura, medida em termos de resistência à flexão biaxial, no conjunto das cerâmicas feldspáticas de revestimento/infra-estruturas de Zircónia independentemente do material submetido a stress tensional. H4.0: Não existem diferenças significativas na resistência à fractura, medida em termos de resistência à flexão biaxial, em cada uma das cerâmicas feldspáticas de revestimento/infra-estruturas de Zircónia independentemente do material submetido a stress tensional. H4.1: Existem diferenças significativas na resistência à fractura, medida em termos de resistência à flexão biaxial, em cada uma das cerâmicas feldspáticas de revestimento/infra-estruturas de Zircónia independentemente do material submetido a stress tensional. H5.0: Não existem diferenças significativas no modo de fractura das cerâmicas feldspáticas de revestimento/infra-estruturas de Zircónia Resumo 242 independentemente do material submetido a stress tensional. H5.1: Existem diferenças significativas no modo de fractura das cerâmicas feldspáticas de revestimento/infra-estruturas de Zircónia independentemente do material submetido a stress tensional. Na primeira parte da investigação uma amostra de conveniência de cento e oitenta (180) espécimes em forma de disco (12.7 mm x 2.2 mm) foram preparados e usados neste estudo. Os discos foram fabricados com cerâmica feldspática utilizada para estratificar infra-estruturas de Zircónia 3YTZP de três (3) marcas comerciais: NobelRondoTM Zirconia veneer ceramic (Nobel BiocareTM AB, Suécia), LavaTM Ceram veneer ceramic (3MTM, ESPETM, Alemanha), e Vita® VM®9 veneer ceramic (Vita®, Zahnfabrick, Alemanha). Sessenta (60) espécimes monolíticos de cada cerâmica foram fabricadas de acordo com as instruções ISO/DIS 6872: 1995 (three-point and biaxial flexural strength) usando um molde de aço inoxidável (ISO/DIS 6872: 1995). O líquido de mistura e o pó cerâmico foram combinados nas proporções recomendadas pelo fabricante. A mistura resultante de cerâmica feldspática foi vibrada e compactada no molde e posteriormente sinterizada em forno específico para cerâmica (Programat P500, Ivoclar Vivadent AG, Liechtenstein). Após a primeira sinterização mais cerâmica foi adicionada por forma a compensar a contracção resultante da primeira sinterização. Os discos de cerâmica produzidos foram examinados com um estereomicroscópio (Nikon SMZ-U, Tokyo, Japan) com uma ampliação X75 para avaliar a presença de pequenas fissuras ou poros. Espécimes que demonstrassem defeitos visíveis foram substituídos. A superficie de todas os espécimes foi posteriormente polida com discos de carbureto de silício (grão P220, P500, P1200 - Ultra-Prep, Buehler Ltd., Lake Bluff, IL, EUA) numa lixadora mecânica (Ecomet® 3, Buehler Ltd., Lake Buff, IL, EUA) de acordo Resumo 243 com ISO 6344-1: 1998 (ISO/DIS 6344-1: 1998). Este procedimento foi efectuado até serem obtidos espécimes com 2.2(± 0.1) mm por 12.7(± 0.1) mm de espessura e diâmetro respectivamente. Um transportador especial de aço inoxidável foi utilizado para assegurar a uniformização da espessura e paralelismo das superficies durante o corte e polimento. As dimensões dos espécimes foram avaliadas através da utilização de um medidor digital (Digimatic Caliper Series 500, Mitutoyo America Corporation, Aurora, IL, EUA) por forma a garantir espessura e diâmetros exactos. Finalmente, todas os espécimes foram limpas com água destilada num banho de ultra-sons (Eurosonic® 4D, Euronda, Vicenza, Italia) durante 15 minutos e posteriormente colocados no forno específico para cerâmica onde foram auto-glazeados. Após o glaze, a espessura final e o diâmetro foram novamente avaliados através da utilização do mesmo medidor digital até ao centésimo de milimetro. Os cento e oitenta (180) espécimes foram aleatoriamente distribuidos por dezoito grupos, seis grupos para cada cerâmica, cada grupo composto por dez espécimes. Os seis grupos experimentais de cada cerâmica foram fabricados como anteriormente descrito e submetidos aos seguintes tratamentos de superfície: 1) preparação segundo as instruções do fabricante (grupo de controlo) (C), 2) com desgaste/corte da superfície com instrumento de diamante (G), 3) com desgaste/corte da superfície com instrumento de diamante seguido de glaze (GG), 4) com desgaste/corte da superfície com instrumento de diamante seguido de polimento (GP), 5) com desgaste/corte da superfície com instrumento de diamante seguido de polimento e glaze (GPG), 6) Com polimento da superfície seguido de glaze (PG). A resistência máxima à fractura foi medida através do método “pistonon- three-ball” utilizando uma máquina de testes mecânicos universal Instron (Modelo TT-BM Instron Corp., Canton, MA), e de acordo com o standard ISO/DIS 6872 para cerâmicas dentárias (ISO/DIS 6872: 1995). A resistência Resumo 244 máxima à fractura (N) foi registada e conjugada com a seguinte formula (ASTM F 394-78, 1996), por forma a obter e calcular a resistência à flexão biaxial para cada espécime: S = - 0.2387 P(X – Y)/d2 (ISO/DIS 6872: 1995). Conjuntamente foi calculado o Weibull modulus para a resistência à flexão biaxial obtida. A análise estatística dos resultados foi efectuada através da utilização do método one-way and two-way ANOVA com ajustamento de Fisher's PLSD post-hoc para comparações múltiplas (p=0.05), para avaliar as diferenças de resistência à fractura entre grupos. Na segunda parte da investigação uma amostra de coveniência de sessenta (60) espécimes em forma de disco (12.7 mm x 2.2 mm) foram preparados e usados neste estudo. Os discos foram fabricados utilizando sessenta discos de Zircónia 3Y-TZP produzidos por CAD/CAM (Nobel BiocareTM AB, Suécia) (12.7 mm x 1.1 mm) que foram revestidos com cerâmica feldspática utilizada para estratificar infra-estruturas de Zircónia 3YTZP de três (3) marcas comerciais: NobelRondoTM Zirconia veneer ceramic (Nobel BiocareTM AB, Suécia), LavaTM Ceram veneer ceramic (3MTM, ESPETM, Alemanha), e Vita® VM®9 veneer ceramic (Vita®, Zahnfabrick, Alemanha). A preparação dos sessenta (60) espécimes, vinte (20) de cada cerâmica foi efectuada de acordo com as instruções ISO/DIS 6872: 1995 (three-point and biaxial flexural strength) (ISO/DIS 6872: 1995) usando as recomendações específicas de cada fabricante e de acordo com o procedimento efectado para os espécimes monolíticos de cada cerâmica. Os sessenta (60) espécimes foram aleatoriamente distribuidos por seis grupos, dois grupos para cada cerâmica, cada grupo composto por dez espécimes. A resistênicia máxima à fractura foi medida através do método pistonon- three-ball utilizando uma máquina de testes mecânicos universal Instron Resumo 245 (Modelo TT-BM Instron Corp., Canton, MA), e de acordo com o standard ISO/DIS 6872 para cerâmicas dentárias (ISO/DIS 6872: 1995). Em trinta (30) espécimes, dez por cerâmica, a força foi aplicada na cerâmica felsdpática de revestimento. Nos restante trinta (30), dez por cerâmica, a força foi aplicada na infra-estrutura de Zircónia. O objectivo de inverter a posição dos espécimes foi perceber a influência que a cerâmica de revestimento teria na origem interna ou externa da fractura do material. A máxima resistência à fractura (N) foi registada e conjugada com a seguinte formula (ASTM F 394- 78, 1996), por forma a obter e calcular a resistência à flexão biaxial para cada especime: S = - 0.2387 P(X – Y)/d2 (ISO/DIS 6872: 1995). Conjuntamente foi calculado o Weibull modulus para a resistência à flexão biaxial obtida. A análise estatística dos resultados foi efectuada através da utilização do método two-way ANOVA com ajustamento de Fisher's PLSD post-hoc e Student’s t-test para comparações múltiplas (p=0.05), para avaliar as diferenças de resistência à fractura entre grupos. Após os testes de fractura todos os espécimes foram analisados com um estereomicrocópio (Nikon SMZ-U, Tokyo, Japan) com uma ampliação de X75 no sentido de caracterizar a origem e modo de fractura. A caracterização morfológica dos diferentes tipos de fractura registados na interface Zircónia/cerâmica feldspática de revestimento foi efectuada através da utilização de microcopia electrónica de varrimento (SEM) (Amray 1820, Bedford, MA, USA). Seis espécimes representativos, dois de cada grupo, foram seleccionados e fotografias de diferentes amplitudes foram efectudas. Os resultados para a primeira parte do estudo demonstraram que quando as cerâmicas feldspáticas de revestimento são analizadas em conjunto, os grupos de desgaste (G) apresentarm diferenças significativas em relação a todos os outros grupos. Foram encontradas também diferenças significativas entre os grupos de controlo (C) e os grupos de polimento/glaze (PG). Nenhuma outra diferença significativa de resistência à flexão biaxial, Resumo 246 entre os restantes grupos de tratamento de superfície foi encontrada. Quando as cerâmicas feldspáticas de revestimento foram analizadas individualmente, os resultados encontrados da resistência à flexão biaxial entre os diferentes grupos de tratamento de superfície, foram mais heterogéneos. No entanto, em todos as cerâmicas o tratamento de superfície de degaste/corte (G) provocou uma diminuição de resistência à flexão biaxial, e em alguns casos de forma estatisticamente significativa. Estes resultados sugerem que o desgaste da superfície destas cerâmicas deve ser sempre evitado se nenhum outro tratamento de superfície for efectuado posteriormente, uma vez que o desgaste ou corte com instrumento de diamante poderá criar ou alterar as dimensões de fissuras ou poros préexistententes diminuindo a resistência do material. Pelo contrário, os resultados da resistência à flexão biaxial para os tratamentos de superficie de polimento e/ou glaze (GG, GP, GPG, PG) melhoraram a resistência à flexão biaxial dos materiais cerâmicos, e em alguns casos de forma estatisticamente significativa. Os resultados sugerem que estes tratamentos de superfície limitam os efeitos do desgaste/corte, devido à sua capacidade para melhorar as condições da superfície da cerâmica, através da eliminação ou diminuição de defeitos, fissura e/ou poros. Os resultados sugerem que as diferenças de distribuição da resistência à flexão biaxial entre os diferentes grupos de tratamento de superfície estão mais dependentes da rugosidade de superfície de cada cerâmica e consequentemente do tratamento de superfície efectuado, do que com a sua estrutura; excepto quando a estrutura interna do material possa provocar uma concentração de stress superior aquela originada pela rugosidade de superfície e/ou a presença de poros ou fissuras. Os resultados encontrados demonstraram também, que os valores obtidos para as cerâmicas feldspáticas de revestimento para Zircónia (grupo de controlo), se encontram dentro dos valores que as cerâmicas feldspáticas Resumo 247 de revestimento nos sistemas metálicos apresentam. Os resultados encontrados para o Weibull modulus das três cerâmicas testadas são semelhantes aos valores obtidos para outras cerâmicas feldspáticas de revestimento. Valores mais elevados foram encontrados para os grupos de polimento, glaze e de controlo, demonstrando maior homogeneidade de valores obtidos, do que para os grupos de desgaste. Os resultados para a segunda parte do estudo demonstraram que, quando os valores da resistência à flexão biaxial das cerâmicas feldspáticas de revestimento/infra-estrutura Zircónia são analizadas em conjunto, os espécimes que foram testados com a infra-estrutura de Zircónia na superfície inferior, apresentaram valores estatisticamente superiores aqueles que apresentaram as cerâmicas feldspáticas de revestimento sob tensão. Quando analizadas individualmente apenas a NobelRondoTM Zirconia veneer ceramic não demonstrou diferenças significativas independentemente do material que era colocado sob tensão. Estes resultados demonstram, que o contributo de infra-estruturas mais resistentes no desempenho clínico de restaurações em cerâmica pura, poderá não ser significativo se o desenho da restauração não tiver em atenção a distribuição do stress tensional sobre ela exercido. Os resultados encontrados para o Weibull modulus das três cerâmicas testadas são semelhantes aos valores obtidos para outras cerâmicas feldspáticas de revestimento quando estratificadas sobre infra-estruturas diferentes da Zircónia. Valores mais elevados foram encontrados para os grupos que apresentavam a Zircónia sob tensão, evidenciando maior homogeneidade de valores obtidos. Dois modos de fractura diferentes foram predominantemente encontrados nos espécimes, dependendo do material que era colocado em tensão. Quando a Zircónia era colocada em tensão, um cone Hertziano estava presente na superfície da cerâmica feldspática em practicamente todos os espécimes. A presença deste cone, era acompanhado por traços de Resumo 248 fractura que se propagavam lateralmente causando eventual delaminação parcial da cerâmica feldspática de revestimento sem fractura da infraestrutura de Zircónia. Pelo contrário, quando as cerâmicas de revestimento eram colocadas sob tensão, a fractura

An Investigation of Reliability Models for Ceramic Matrix Composites and their Implementation into Finite Element Codes

The development of modeling approaches for the failure analysis of ceramic-based material systems used in high temperature environments was the primary objective of this research effort. These materials have the potential to support many key engineering technologies related to the design of aeropropulsion systems. Monolithic ceramics exhibit a number of useful properties such as retention of strength at high temperatures, chemical inertness, and low density. However, the use of monolithic ceramics has been limited by their inherent brittleness and a large variation in strength. This behavior has motivated material scientists to reinforce the monolithic material with a ceramic fiber. The addition of a second ceramic phase with an optimized interface increases toughness and marginally increases strength. The primary purpose of the fiber is to arrest crack growth, not to increase strength. The material systems of interest in this research effort were laminated ceramic matrix composites, as well as two- and three- dimensional fabric reinforced ceramic composites. These emerging composite systems can compete with metals in many demanding applications. However, the ongoing metamorphosis of ceramic composite material systems, and the lack of standardized design data has in the past tended to minimize research efforts related to structural analysis. Many structural components fabricated from ceramic matrix composites (CMC) have been designed by "trial and error." The justification for this approach lies in the fact that during the initial developmental phases for a material system fabrication issues are paramount. Emphasis is placed on demonstrating feasibility rather than fully understanding the processes controlling mechanical behavior. This is understandable during periods of rapid improvements in material properties for any composite system. But to avoid the ad hoc approach, the analytical methods developed under this effort can be used to develop rational structural design protocols