No description available.

No description available.

No description available.

Objetivos: determinar a resistência à flexão em três pontos (f), módulo de Weibull (m), coeficiente de susceptibilidade ao crescimento subcrítico de trinca (n) e tenacidade à fratura (KIC) de três cerâmicas usadas para confecção de infraestrutura (IE) de próteses parciais fixas (PPFs) (YZ- zircônia tetragonal parcialmente estabilizada por óxido de ítrio; IZ- cerâmica a base de alumina infiltrada por vidro e reforçada com zircônia; AL alumina policristalina) e duas porcelanas (VM7 e VM9); avaliar o efeito da configuração (uma, duas ou três camadas) nos valores de f e modo de fratura dos corpos-de-prova (CP); avaliar a influência do material de IE, do tamanho dos conectores e da ciclagem mecânica (CM) na carga de fratura (CF) e distribuição de tensões de PPFs; relacionar o comportamento mecânico dos materiais cerâmicos testados na configuração de barra e de PPF. Material e Método: Foram produzidos três tipos de CP em forma de barra (2mm x 4mm x 16mm): monolítico, duas camadas e três camadas. As IE das PPFs foram confeccionadas utilizando o sistema CAD-CAM e recobertas com porcelana. Os ensaios de f foram realizados em saliva artificial a 37ºC. Os valores de m e n foram determinados pela análise de Weibull e ensaio de fadiga dinâmica, respectivamente. As PPFs foram carregadas no centro do pôntico até a fratura. Oito PPFs de cada grupo foram submetidas a CM com freqüência de 2 Hz e carga de 140 N durante 106 ciclos e, posteriormente, ensaiadas até a fratura. A distribuição de tensões nas PPFs foi avaliada com análise de elementos finitos (AEF). Os princípios da fractografia foram utilizados para determinar o padrão de fratura e os valores de KIC. Os dados de f e CF foram analisados estatisticamente com Kruskal-Wallis e Tukey (95%). Resultados: A YZ obteve o maior valor de f (860 MPa) seguida dos materiais IZ (411 MPa) e AL (474 MPa) que não apresentaram diferença estatística. Os menores valores de f foram encontrados para as porcelanas (65 MPa). Os valores de m foram semelhantes para os materiais, com exceção dos grupos IZ e VM7, que apresentaram diferença significante. Os maiores valores de n foram encontrados para as cerâmicas YZ (76) e AL (72), seguidos pela IZ (54) e pelas porcelanas (40). A YZ apresentou o maior valor de KIC. O material submetido à tensão de tração durante o ensaio determinou o valor de f das estruturas. As PPFs de YZ com conector de 16mm2 suportaram os maiores valores de CF. Houve influência significativa do tamanho de conector para o material YZ. A CM não influenciou os valores de CF das PPFs. O padrão de distribuição de tensões foi semelhante entre as PPFs. Foi observada uma boa relação entre os valores de tensão de fratura dos materiais de IE em forma de barra e de PPF. Foram encontradas diferenças no modo de falha dos CP em forma de barra e de PPF. Conclusão: a YZ apresentou o melhor comportamento mecânico tanto na configuração de barra como de PPF. / Objectives: to determine the flexural strength (f), Weibull modulus (m), slow crack growth coefficient (n) and fracture toughness (KIC) of three ceramics used as framework materials for fixed partial dentures (FPDs) (YZ- yttria partially stabilized zirconia tetragonal polycrystals; IZ- alumina-based zirconia-reinforced glass infiltrated ceramic; AL alumina polycrystals) and two veneering porcelains (VM7 and VM9); to evaluate the effect of the specimen design (one, two or three layers) in the f and fracture mode; to evaluate the influence of the framework material, connector size and mechanical cycling (MC) in the fracture load (FL) and stress distribution of FPDs; to relate the mechanical behavior of the ceramic materials tested using bar-shaped specimens and FPDs. Materials and Methods: Bar-shaped specimens (2mm x 4mm x 16mm) were produced in three different designs: monolithic, bilayer and trilayer. FPD frameworks were built using CAD-CAM system and veneered with porcelain. Specimens were tested for three point bending in 37ºC artificial saliva. Weibull analysis and dynamic fatigue testing were used to determine m and n values, respectively. FPDs were tested with a load applied in the middle of the pontic. Eight FPDs of each group were subjected to MC using a frequency of 2Hz and load of 140N for 106 cycles and were subsequently loaded to failure. Stress distribution for FPDs was evaluated using finite element analysis (FEA). Fractography principles were used to determine the fracture mode and KIC values. f and FL data were analyzed using Kruskal-Wallis and Tukey (95%). Results: YZ had the highest f value (860 MPa) followed by IZ (411 MPa) and AL (474 MPa). Lower f values were observed for the porcelains (65 MPa). Except for IZ and VM7, m values were similar among the ceramic materials. Higher n values were found for YZ (76) and AL (72), followed by IZ (54) and the veneering materials (40). YZ presented the highest KIC value. The f values were influenced by the material subjected to tensile stress during testing. YZ FPDs with 16mm2 connector showed higher FL values. There was significant influence of the connector size on the FL values for YZ material. MC had no influence in the FL values for the FPDs. The stress distribution was similar for all FPDs. Considering the framework material, there was a good agreement between the fracture strength values obtained for bar-shaped specimens and FPDs. Different fracture modes were observed for bar-shaped specimens and FPDs. Conclusion: YZ presented the best mechanical performance in both bar-shaped and FPD specimen configuration.

Objetivos: determinar a resistência à flexão em três pontos (f), módulo de Weibull (m), coeficiente de susceptibilidade ao crescimento subcrítico de trinca (n) e tenacidade à fratura (KIC) de três cerâmicas usadas para confecção de infraestrutura (IE) de próteses parciais fixas (PPFs) (YZ- zircônia tetragonal parcialmente estabilizada por óxido de ítrio; IZ- cerâmica a base de alumina infiltrada por vidro e reforçada com zircônia; AL alumina policristalina) e duas porcelanas (VM7 e VM9); avaliar o efeito da configuração (uma, duas ou três camadas) nos valores de f e modo de fratura dos corpos-de-prova (CP); avaliar a influência do material de IE, do tamanho dos conectores e da ciclagem mecânica (CM) na carga de fratura (CF) e distribuição de tensões de PPFs; relacionar o comportamento mecânico dos materiais cerâmicos testados na configuração de barra e de PPF. Material e Método: Foram produzidos três tipos de CP em forma de barra (2mm x 4mm x 16mm): monolítico, duas camadas e três camadas. As IE das PPFs foram confeccionadas utilizando o sistema CAD-CAM e recobertas com porcelana. Os ensaios de f foram realizados em saliva artificial a 37ºC. Os valores de m e n foram determinados pela análise de Weibull e ensaio de fadiga dinâmica, respectivamente. As PPFs foram carregadas no centro do pôntico até a fratura. Oito PPFs de cada grupo foram submetidas a CM com freqüência de 2 Hz e carga de 140 N durante 106 ciclos e, posteriormente, ensaiadas até a fratura. A distribuição de tensões nas PPFs foi avaliada com análise de elementos finitos (AEF). Os princípios da fractografia foram utilizados para determinar o padrão de fratura e os valores de KIC. Os dados de f e CF foram analisados estatisticamente com Kruskal-Wallis e Tukey (95%). Resultados: A YZ obteve o maior valor de f (860 MPa) seguida dos materiais IZ (411 MPa) e AL (474 MPa) que não apresentaram diferença estatística. Os menores valores de f foram encontrados para as porcelanas (65 MPa). Os valores de m foram semelhantes para os materiais, com exceção dos grupos IZ e VM7, que apresentaram diferença significante. Os maiores valores de n foram encontrados para as cerâmicas YZ (76) e AL (72), seguidos pela IZ (54) e pelas porcelanas (40). A YZ apresentou o maior valor de KIC. O material submetido à tensão de tração durante o ensaio determinou o valor de f das estruturas. As PPFs de YZ com conector de 16mm2 suportaram os maiores valores de CF. Houve influência significativa do tamanho de conector para o material YZ. A CM não influenciou os valores de CF das PPFs. O padrão de distribuição de tensões foi semelhante entre as PPFs. Foi observada uma boa relação entre os valores de tensão de fratura dos materiais de IE em forma de barra e de PPF. Foram encontradas diferenças no modo de falha dos CP em forma de barra e de PPF. Conclusão: a YZ apresentou o melhor comportamento mecânico tanto na configuração de barra como de PPF. / Objectives: to determine the flexural strength (f), Weibull modulus (m), slow crack growth coefficient (n) and fracture toughness (KIC) of three ceramics used as framework materials for fixed partial dentures (FPDs) (YZ- yttria partially stabilized zirconia tetragonal polycrystals; IZ- alumina-based zirconia-reinforced glass infiltrated ceramic; AL alumina polycrystals) and two veneering porcelains (VM7 and VM9); to evaluate the effect of the specimen design (one, two or three layers) in the f and fracture mode; to evaluate the influence of the framework material, connector size and mechanical cycling (MC) in the fracture load (FL) and stress distribution of FPDs; to relate the mechanical behavior of the ceramic materials tested using bar-shaped specimens and FPDs. Materials and Methods: Bar-shaped specimens (2mm x 4mm x 16mm) were produced in three different designs: monolithic, bilayer and trilayer. FPD frameworks were built using CAD-CAM system and veneered with porcelain. Specimens were tested for three point bending in 37ºC artificial saliva. Weibull analysis and dynamic fatigue testing were used to determine m and n values, respectively. FPDs were tested with a load applied in the middle of the pontic. Eight FPDs of each group were subjected to MC using a frequency of 2Hz and load of 140N for 106 cycles and were subsequently loaded to failure. Stress distribution for FPDs was evaluated using finite element analysis (FEA). Fractography principles were used to determine the fracture mode and KIC values. f and FL data were analyzed using Kruskal-Wallis and Tukey (95%). Results: YZ had the highest f value (860 MPa) followed by IZ (411 MPa) and AL (474 MPa). Lower f values were observed for the porcelains (65 MPa). Except for IZ and VM7, m values were similar among the ceramic materials. Higher n values were found for YZ (76) and AL (72), followed by IZ (54) and the veneering materials (40). YZ presented the highest KIC value. The f values were influenced by the material subjected to tensile stress during testing. YZ FPDs with 16mm2 connector showed higher FL values. There was significant influence of the connector size on the FL values for YZ material. MC had no influence in the FL values for the FPDs. The stress distribution was similar for all FPDs. Considering the framework material, there was a good agreement between the fracture strength values obtained for bar-shaped specimens and FPDs. Different fracture modes were observed for bar-shaped specimens and FPDs. Conclusion: YZ presented the best mechanical performance in both bar-shaped and FPD specimen configuration.

O objetivo deste trabalho foi realizar a caracterização microestrutural e das propriedades mecânicas de diferentes cerâmicas odontológicas para entender sua influência nos parâmetros de crescimento de trinca subcrítico (SCG), n (coeficiente de susceptibilidade ao SCG) e ?f0 (parâmetro escalar), determinados por diferentes métodos. Cinco cerâmicas foram avaliadas: porcelanas VM7 (Vita) e D (d.Sign, Ivoclar), vitrocerâmicas E1 (IPS Empress, Ivoclar) e E2 (IPS Empress 2, Ivoclar) e compósito IC (In-Ceram Alumina, Vita). Espécimes em forma de disco (12 mm x 1 mm) foram confeccionados segundo as recomendações dos fabricantes. A caracterização microestrutural dos materiais foi realizada. A tenacidade à fratura (KIc) foi determinada pelo método da fratura por indentação (IF) e indentation strength. Os parâmetros de SCG foram determinados por fadiga dinâmica (com diferentes taxas de tensão constante), fadiga estática (com diferentes níveis de tensão constante) e fadiga por IF. Os resultados mostraram que todas as cerâmicas constituíram-se de fase vítrea e cristalina, com exceção da VM7, composta somente por fase amorfa. D e E1 apresentaram partículas de leucita, em frações volumétricas iguais a 0,16 e 0,29, sendo que E1 apresentou distribuição homogênea destes cristais, enquanto que D apresentou distribuição heterogênea, formando aglomerados. E2 apresentou cristais alongados de dissilicato de lítio, em fração volumétrica de 0,58, que apresentaram tendência de alinhamento com seu maior eixo alinhado perpendicularmente à direção de compactação, que variou angularmente a partir do ponto de injeção. O compósito IC apresentou-se constituído de cristais de alumina parcialmente sinterizados (65% em volume), infiltrados por vidro de baixa fusão. Independentemente do método, IC apresentou maior tenacidade (2,81 MPa.m1/2), seguido por E2 (1,81 MPa.m1/2) e E1 (0,96 MPa.m1/2) e, por último pelas porcelanas D (0,84 MPa.m1/2) e VM7 (0,67 MPa.m1/2). A deflexão de trincas foi o principal mecanismo de tenacificação dos materiais com partículas cristalinas. Para o ensaio de fadiga dinâmica, o valor de n dependeu do material estudado, ficando entre 17,2 e 31,1. Com relação ao parâmetro ?f0, foram encontrados valores entre 48 e 384 MPa. Para materiais a base de silicato de alumínio e potássio (porcelanas VM7 e D e vitrocerâmica E1), a composição química das fases vítreas dos materiais estudados parece estar relacionada com as diferenças entre os valores de n. O parâmetro ?f0 apresentou correlação com KIc e com a microestrutura, tendendo a aumentar com o aumento da fração volumétrica de cristais. No ensaio de fadiga estática, realizado apenas com D, foram obtidos valores de n e ?f0 respectivamente iguais a 31,4 e 47 MPa, sendo considerados similares aos obtidos por fadiga dinâmica. Ao se compararem os valores de n determinados nos ensaios dinâmico e por IF, nota-se que os materiais com maior volume de fase vítrea (VM7, D e E1) apresentaram valores de n próximos para os dois métodos, ao passo que materiais com maior fração de cristais (E2 e IC), apresentaram maiores discrepâncias entre os valores de n. O ensaio de fadiga por IF também permitiu a determinação dos limites de fadiga (KI0), encontrando-se valores de entre 0,48 e 2,89 MPa.m1/2. / The objective of this study was to determine the mechanical properties and the microstructure of different dental ceramics in order to understand their influence on the slow crack growth (SCG) parameters, n (crack growth exponent) and ?f0, (scaling parameter), determined by different methods. The five ceramics tested were: porcelains VM7 (Vita) and D (d.Sign, Ivoclar), glass-ceramics E1 (IPS Empress, Ivoclar) e E2 (IPS Empress 2, Ivoclar) and composite IC (In-Ceram Alumina, Vita). Disc specimens (12 mm x 1 mm) were prepared according to manufacturers\' instructions. The microstructure of the materials was carried out. The fracture toughness (KIc) was determined by means of the indentation fracture technique (IF) and indentation strength. The slow crack growth parameters were determined by dynamic fatigue test (constant stress rate), static fatigue test (constant stress) and the indentation fracture method. The results showed that all ceramic materials were composed by glassy matrix and crystalline phases, except for VM7 (vitreous porcelain). D and E1 presented leucite particles, in volume fractions of 0.16 and 0.29. For E1, the leucite crystals were homogeneously distributed in the glassy matrix, while in D, leucite formed agglomerates. E2 presented lithium dissilicate crystals (58% in volume) that presented an alignment tendency, with their major axis oriented perpendicularly to the pressing direction, which varied angularly from the injection point. IC presented alumina crystals (65% in volume) partially sintered, infiltrated by a lanthanum glass. Regardless of the method, the fracture toughness values were higher for IC (2.81 MPa.m1/2), followed by E2 (1.81 MPa.m1/2) and E1 (0.96 MPa.m1/2), and were lower for the porcelains D (0.84 MPa.m1/2) and VM7 (0.67 MPa.m1/2). Crack deflection was the main toughening mechanism observed for the ceramics containing crystalline phases. Regarding the dynamic fatigue test, the n values depended on the material, ranging from 17,2 to 31,1. With respect to ?f0, the values obtained for this parameter ranged from 48 e 384 MPa. For the ceramics based on potassium and aluminum silicate (porcelains VM7 and D and glass-ceramic E1), the chemical composition of the glass matrix seems to be related to the differences observed in the n values. The ?f0 parameter presented a positive correlation with KIc and volume fraction of crystalline particles. For the static fatigue test, used only for porcelain D, the n and ?f0 values were, respectively, 31,4 e 47 MPa, considered similar to the ones obtained by the dynamic method. When comparing the crack growth exponents determined by the dynamic and indentation fracture tests, it can be noted that n values for the ceramics with high volume fraction of glassy phase (VM7, D and E1) were similar for both methods, but for ceramics with higher crystalline content (E2 and IC), large discrepancies were observed. The static fatigue limit (KI0) was also determined for the five materials, ranging from 0.48 e 2.89 MPa.m1/2.

Les céramiques sont sensibles à la fissuration lente qui résulte de l'effet conjoint entre un chargement mécanique et l'environnement (taux d'humidité et température). A partir d'études atomistiques disponibles dans la littérature, un modèle cohésif représentant localement la rupture assistée par l'environnement est proposé dans le cadre d'une formulation thermiquement activée. Nous montrons que cette description est capable de rendre compte de la fissuration lente en fatigue statique de monocristaux de céramiques, ainsi que la fissuration lente intergranulaire de polycristaux. Nous soulignons qu'une représentation de la fissuration lente avec la vitesse de propagation V en fonction du taux de restitutions d'énergie G rend compte des caractéristiques intrinsèques de la cinétique de rupture et est préférable à une présentation V-K. Le modèle cohésif permettant d'incorporer une longueur caractéristique dans la description, des effets de taille de grains sont explorés. La prise en compte des contraintes initiales d'origine thermique liées à l'élaboration est nécessaire pour prédire de manière réaliste l'accroissement du seuil de chargement en-dessous duquel aucune propagation n'a lieu ainsi que la résistance à la fissuration lente avec la taille de grains augmentant. La vitesse fissuration lente et le seuil de chargement K0 sont sensibles à l'environnement et notamment à la température et à la concentration d'eau. En augmentant la concentration d'eau et/ou la température, le seuil K0 diminue et la vitesse de fissuration lente augmente. Pour rendre compte de l'influence du taux d'humidité sur la fissuration lente, il est nécessaire de considérer une énergie d'activation ainsi qu'un seuil d'amorçage du mécanisme de réaction-rupture diminuant avec la concentration locale en eau. L'effet de la température est prédit de manière réaliste avec le modèle cohésif proposé et en tenant compte des contraintes initiales thermiques. Nous avons comparé les réponses en fissuration lente de l'alumine et de la zircone et montré qu'intrinsèquement et en l'absence de transformation de phase, la zircone résiste mieux à la fissuration lente que l'alumine. A partir de ces résultats, nous avons abordé l'étude de la fissuration lente de céramiques élaborées par projection plasma. Un endommagement initial de la microstructure à l'échelle des splats est observé sans qu'il n'influence la fissuration lente intra-splats en termes V-G. / Ceramic materials are prone to slow crack growth (SCG)due to the combined effect of the mechanical loading and the environment (moisture and temperature).Based on atomistic studies available in the literature,a thermally activated cohesive model is proposed to represent the reaction-rupture mechanism underlying slow crack growth. The description is shown able to capture SCG under static fatigue on ceramic single crystals as well as intergranular SCG in polycrystals.We emphasize that the representation of SCG with the crack velocity versus the energy release rate G accounts for the intrinsic characteristics of SCG, which is preferable than a usual plot with V-K curves.The cohesive model incorporates a characteristic length scale, so that size effects can be investigated. SCG is grain size dependent with the decrease of the crack velocity at a given load level and improvement of the load threshold with the grain size. To capture this observation, account for the initial thermal stresses related to the processing is mandatory. SCG is also dependent on the concentration of water with an increase of the crack velocity and a decrease of the load threshold with the relative humidity increasing. To predict this effect, the cohesive description needs to account for activation energy and a threshold to trigger the reaction-rupture that depends on the concentration of water. The influence of the temperature on SCG shows an increase in the crack velocity and a decrease of the load threshold for SCG due to the reduction in the initial thermal stresses. The SCG behavior of the alumina and zirconia is compared. Zirconia exhibits a better resistance to SCG compared to that of alumina, in the absence of any phase transformation due to lower kinetics of its reaction-rupture. Based on these results, SCG is investigated in plasma sprayed ceramic. An initial damage at the scale of the splats is observed without effect on load threshold G0 for SCG in V-G plots.

O objetivo deste trabalho foi realizar a caracterização microestrutural e das propriedades mecânicas de diferentes cerâmicas odontológicas para entender sua influência nos parâmetros de crescimento de trinca subcrítico (SCG), n (coeficiente de susceptibilidade ao SCG) e ?f0 (parâmetro escalar), determinados por diferentes métodos. Cinco cerâmicas foram avaliadas: porcelanas VM7 (Vita) e D (d.Sign, Ivoclar), vitrocerâmicas E1 (IPS Empress, Ivoclar) e E2 (IPS Empress 2, Ivoclar) e compósito IC (In-Ceram Alumina, Vita). Espécimes em forma de disco (12 mm x 1 mm) foram confeccionados segundo as recomendações dos fabricantes. A caracterização microestrutural dos materiais foi realizada. A tenacidade à fratura (KIc) foi determinada pelo método da fratura por indentação (IF) e indentation strength. Os parâmetros de SCG foram determinados por fadiga dinâmica (com diferentes taxas de tensão constante), fadiga estática (com diferentes níveis de tensão constante) e fadiga por IF. Os resultados mostraram que todas as cerâmicas constituíram-se de fase vítrea e cristalina, com exceção da VM7, composta somente por fase amorfa. D e E1 apresentaram partículas de leucita, em frações volumétricas iguais a 0,16 e 0,29, sendo que E1 apresentou distribuição homogênea destes cristais, enquanto que D apresentou distribuição heterogênea, formando aglomerados. E2 apresentou cristais alongados de dissilicato de lítio, em fração volumétrica de 0,58, que apresentaram tendência de alinhamento com seu maior eixo alinhado perpendicularmente à direção de compactação, que variou angularmente a partir do ponto de injeção. O compósito IC apresentou-se constituído de cristais de alumina parcialmente sinterizados (65% em volume), infiltrados por vidro de baixa fusão. Independentemente do método, IC apresentou maior tenacidade (2,81 MPa.m1/2), seguido por E2 (1,81 MPa.m1/2) e E1 (0,96 MPa.m1/2) e, por último pelas porcelanas D (0,84 MPa.m1/2) e VM7 (0,67 MPa.m1/2). A deflexão de trincas foi o principal mecanismo de tenacificação dos materiais com partículas cristalinas. Para o ensaio de fadiga dinâmica, o valor de n dependeu do material estudado, ficando entre 17,2 e 31,1. Com relação ao parâmetro ?f0, foram encontrados valores entre 48 e 384 MPa. Para materiais a base de silicato de alumínio e potássio (porcelanas VM7 e D e vitrocerâmica E1), a composição química das fases vítreas dos materiais estudados parece estar relacionada com as diferenças entre os valores de n. O parâmetro ?f0 apresentou correlação com KIc e com a microestrutura, tendendo a aumentar com o aumento da fração volumétrica de cristais. No ensaio de fadiga estática, realizado apenas com D, foram obtidos valores de n e ?f0 respectivamente iguais a 31,4 e 47 MPa, sendo considerados similares aos obtidos por fadiga dinâmica. Ao se compararem os valores de n determinados nos ensaios dinâmico e por IF, nota-se que os materiais com maior volume de fase vítrea (VM7, D e E1) apresentaram valores de n próximos para os dois métodos, ao passo que materiais com maior fração de cristais (E2 e IC), apresentaram maiores discrepâncias entre os valores de n. O ensaio de fadiga por IF também permitiu a determinação dos limites de fadiga (KI0), encontrando-se valores de entre 0,48 e 2,89 MPa.m1/2. / The objective of this study was to determine the mechanical properties and the microstructure of different dental ceramics in order to understand their influence on the slow crack growth (SCG) parameters, n (crack growth exponent) and ?f0, (scaling parameter), determined by different methods. The five ceramics tested were: porcelains VM7 (Vita) and D (d.Sign, Ivoclar), glass-ceramics E1 (IPS Empress, Ivoclar) e E2 (IPS Empress 2, Ivoclar) and composite IC (In-Ceram Alumina, Vita). Disc specimens (12 mm x 1 mm) were prepared according to manufacturers\' instructions. The microstructure of the materials was carried out. The fracture toughness (KIc) was determined by means of the indentation fracture technique (IF) and indentation strength. The slow crack growth parameters were determined by dynamic fatigue test (constant stress rate), static fatigue test (constant stress) and the indentation fracture method. The results showed that all ceramic materials were composed by glassy matrix and crystalline phases, except for VM7 (vitreous porcelain). D and E1 presented leucite particles, in volume fractions of 0.16 and 0.29. For E1, the leucite crystals were homogeneously distributed in the glassy matrix, while in D, leucite formed agglomerates. E2 presented lithium dissilicate crystals (58% in volume) that presented an alignment tendency, with their major axis oriented perpendicularly to the pressing direction, which varied angularly from the injection point. IC presented alumina crystals (65% in volume) partially sintered, infiltrated by a lanthanum glass. Regardless of the method, the fracture toughness values were higher for IC (2.81 MPa.m1/2), followed by E2 (1.81 MPa.m1/2) and E1 (0.96 MPa.m1/2), and were lower for the porcelains D (0.84 MPa.m1/2) and VM7 (0.67 MPa.m1/2). Crack deflection was the main toughening mechanism observed for the ceramics containing crystalline phases. Regarding the dynamic fatigue test, the n values depended on the material, ranging from 17,2 to 31,1. With respect to ?f0, the values obtained for this parameter ranged from 48 e 384 MPa. For the ceramics based on potassium and aluminum silicate (porcelains VM7 and D and glass-ceramic E1), the chemical composition of the glass matrix seems to be related to the differences observed in the n values. The ?f0 parameter presented a positive correlation with KIc and volume fraction of crystalline particles. For the static fatigue test, used only for porcelain D, the n and ?f0 values were, respectively, 31,4 e 47 MPa, considered similar to the ones obtained by the dynamic method. When comparing the crack growth exponents determined by the dynamic and indentation fracture tests, it can be noted that n values for the ceramics with high volume fraction of glassy phase (VM7, D and E1) were similar for both methods, but for ceramics with higher crystalline content (E2 and IC), large discrepancies were observed. The static fatigue limit (KI0) was also determined for the five materials, ranging from 0.48 e 2.89 MPa.m1/2.

The thesis deals with the study of the fracture behaviour of the pipe grades of high density polyethylene (HDPE). Detailed study of current state of knowledge was performed in the first part. The focus was put on those types of polymers used for pipeline applications, the limit states of the plastic pipes, and the key parameters affecting the fracture behaviour of plastic pipes’ materials. The methods used for testing the plastic pipes and native pipeline materials were summarized. In the second part of this work, the relationship between the processing conditions, internal structure and fracture behaviour of the native types of HDPE pipe grades was evaluated. Than, the correlation between short-term alternative tests and the standardized accelerated full notch creep test (FNCT) was studied. An experimental basis for numerical model for evaluation of the fracture behaviour of multilayer pipes was established. Two types of non-homogeneous specimens for evaluating the resistance of multilayer pipes against slow crack growth (SCG) were suggested. This evaluation was based on the parameters of linear elastic fracture mechanics (LEFM) and elasto-plastic fracture mechanics (EPFM).

Environmental stress cracking (ESC) is a significant problem that has plagued the plastics industry since its discovery nearly 70 years ago. The accelerated brittle failure brought about when a stressed polymer comes in contact with an aggressive environment can happen suddenly with destructive results. Many classes of polymers are susceptible to this type of slow crack growth; however special emphasis has typically been placed on polyolefins due to their wide range of working environments, market dominance and their seemingly chemical resistance. Much research has been focused on formulating environmentally resistant materials, while the evaluation techniques for gauging environmental stress cracking resistance (ESCR) seem to have been left behind. This research focuses on developing a reliable testing technique for evaluating the ESCR of polyethylene resins. Passive acoustic monitoring was adapted to an industrially accepted ESCR test in an attempt to hear polymer damage before it was visually apparent. It was discovered that the low energy released during the early stages of damage and excessive background noise masked passive signals, making this method of evaluation impractical. Alternatively, active ultrasonic monitoring through velocity and attenuation measurements was investigated to see if probing techniques could be used to detect structural damage. Active ultrasonic monitoring of static and tensile stressed samples were able to differentiate plasticization after ageing, however no indication of ESCR properties could be inferred. A novel forced based monitoring system was developed in response to the acoustic testing techniques. Force monitoring was able to provide useful information regarding the failure cycle of ESC and the acquired profiles could describe a failure onset time. Several ageing environments were also tested with force monitoring and a traditional ESCR test to reveal the stress cracking ability of biodiesel, an important finding. / Thesis / Master of Applied Science (MASc) / Accelerated failure of stressed plastics can occur upon exposure to fluids through a phenomenon known as environmental stress cracking (ESC). The following research outlines the development of a novel testing technique to gauge a material’s environmental stress cracking resistance (ESCR). Adaption of passive acoustics to an existing stress cracking test was unable to provide any indication of ESCR, however the use of active ultrasonics was able to show sample plasticization. A novel forced based measuring technique was found to uniquely map the failure progression of a sample undergoing ESC, providing valuable information for understanding the phenomenon. Additional testing was also completed on various environmental fluids to reveal biodiesel’s ability to provoke ESC, an important observation.