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).

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[pt] Medindo a rigidez de corpos de prova em testes de propagação de trinca por fadiga, Elber identificou que a ponta da trinca só está totalmente aberta durante uma parte do ciclo de carregamento, e nomeou o carregamento onde a trinca abre totalmente de carga de abertura Pab. Baseado nisso, Elber assumiu que o dano à frente da trinca era induzido apenas pela fração do carregamento acima da Pab, propondo que a força motriz na propagação de trincas por fadiga é a gama do fator de intensidade de tensões efetivo (delta)Kef. Para verificar esta hipótese, neste trabalho foram examinados diferentes cenários em testes de propagação de trinca por fadiga. Primeiramente, em corpos DC(T) de aço AISI 1020 e em corpos DC(T) e C(T) de alumínio 6351-T6 foi propagada uma trinca com (delta)K e Kmax quase constantes, medindo Pab em campo próximo e em campo distante, com extensômetros e com um sistema 3D de correlação digital de imagens (DIC). Depois, usando novos corpos DC(T) de aço e de alumínio, foi propagada uma trinca com (delta)K e Kmax quase constantes, antes e depois de um evento de sobrecarga, medindo a Pab ao longo do teste, em campo próximo e campo distante. Cabe salientar que nesses testes as espessuras dos corpos de prova foram projetadas para que a trinca propagasse em tensão plana e em deformação plana. Finalmente, em testes de propagação de trinca com (delta)K e Kmax quase constantes, em corpos DC(T) de aço e de alumínio foi medido o campo de deformação à frente da ponta da trinca com um sistema de microscópio estéreo DIC, para analisar o comportamento dentro da zona plástica, medindo também a Pab com os métodos mencionados anteriormente. Dos resultados dos testes três comportamentos foram particularmente relevantes. Nos primeiros testes de propagação com deltaK e Kmax quase constantes, a razão Kab/Kmax diminuía enquanto a trinca propagava com uma taxa constante. Nos testes de sobrecarga o valor mínimo de (delta)Kef estava defasado em relação ao valor mínimo de taxa de propagação (da/dN). Já nos testes onde foi medido o campo deformação com o sistema de microscópio estéreo DIC, a deformação 0.1mm à frente da ponta da trinca mostrou que existia dano para cargas abaixo da Pab. Portanto, estes resultados não podem ser explicados pela hipótese de Elber, e contestam o (delta)Kef como a força motriz na propagação de trincas por fadiga. / [en] Measuring the stiffness of a fatigue-cracked plate during its loading cycle, Elber discovered that this crack only completely opened after reaching the crack opening load Pop. Based on this, Elber assumed that the damage ahead of the crack is induced by the loading part above the Pop. In this way, he proposed that the range of the effective stress intensity factor (delta)Keff is the driving force in fatigue crack growth. In order to verify this hypothesis, this study investigated different scenarios in fatigue crack growth tests. For this, DC(T) specimens of AISI 1020 steel, and DC(T) and C(T) specimens of 6351-T6 aluminum were tested. The fatigue crack growth tests were performed under quasi-constant K and Kmax conditions. The Pop measurements in the near and far field were obtained from strain gage readings and 3D Digital Image Correlation (DIC) analysis. In addition, simple variable amplitude tests were performed in steel and aluminum DC(T) specimens. A single tensile overload was introduced in the fatigue crack growth experiments under quasi-constant (delta)K and Kmax conditions, also measuring the Pop throughout the test in the near and the far field. It is important to note that the thickness of the specimens was designed to perform a crack propagation in plane stress and plane strain conditions. Moreover, near tip strain measurements in steel and aluminum DC(T) specimens were obtained with a stereo microscope DIC system, to analyze the behavior within the plastic zone, also measuring the Pop with the methods previously mentioned. From the experimental results, three behaviors were particularly relevant. In the first fatigue crack growth tests with quasi-constant {(delta)K, Kmax} conditions, the ratio Kop/Kmax decreased as the crack propagates under a constant rate. In the fatigue crack growth tests with overload application, the minimum value of (delta)Keff was lagged in relation to the minimum value of the propagation rate (da/dN). Finally, in the strain measurements performed with the stereo microscope DIC system, the de-formation at 0.1mm ahead of the crack tip showed the existence of damage in load values below Pop. Therefore, these results cannot be explained by Elber s hypothesis, and question the (delta)Keff as the driving force in fatigue crack growth.

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.

Fracture mechanics is an effective approach for characterizing material resistance to interfacial failure and for making interface reliability predictions. Because interfacial bond integrity is a major concern for performance and reliability, the need to evaluate the fracture and delamination resistance of an interface under different environmental conditions is very important. This study investigates the effects of temperature, solution chemistry and environmental preconditioning, in several solutions on the durability of silicon/epoxy and glass/epoxy systems. A series of experiments was conducted using wedge test specimens to investigate the adhesion performance of the systems subjected to a range of environmental conditions. Both silicon and glass systems were relatively insensitive to temperature over a range of 22-60°C, but strongly accelerated by temperatures above 60°C, depending on the environmental chemistry and nature of the adhesive system used. Silicon/commercial epoxy specimens were subjected to preconditioning in deionized (DI) water and more aggressive solution mixtures prior to wedge insertion to study the effect of prior environmental exposure time on the system. The wedge test data from preconditioned specimens were compared with standard wedge test results and the system was insensitive to preconditioning in DI water but was affected significantly by preconditioning in aggressive environments. Plots describing - G (crack velocity versus applied strain energy release rate) characteristics for a particular set of environmental conditions are presented and a comparison is made for different environmental conditions to quantify the subcritical debonding behavior of systems studied. A kinetic model to characterize subcritical debonding of adhesives for microelectronic applications is also proposed based on molecular interactions between epoxy and a silane coupling agent at the interface and linear elastic fracture mechanics, which could help predict long-term deterioration of interfacial adhesion. / Master of Science

The main objective of the project was to establish a fracture-mechanics energy-based approach for the design of structural adhesive joints under cyclic loading. This required understanding how an adhesive system behaved near its fatigue threshold, and how the key factors affected this behavior in a fresh undegraded joint. The investigated factors were mode ratio (phase angle), substrate material, surface treatment and surface roughness (both affecting the bond strength), bondline thickness and load ratio. It was first required to understand how the adhesive system behaved under quasi-static loading by examining a fracture mechanics-based design approach for adhesive systems with different substrate materials and geometries. Experiments were initially performed to characterize the strength of aluminum and steel adhesive systems based on the fracture envelope, critical strain energy release rate as a function of the mode ratio. Ultimate failure loads of aluminum and steel adhesive joints, having different overlap end conditions and different geometries were then experimentally measured. These values were compared with the failure loads extracted from the fracture envelope. Considering the toughening behavior of the adhesive in the fracture mechanics analyses, a very good agreement (average of 6%) was achieved between the predictions and experiments for all types of overlap end conditions and geometries. Different fatigue threshold testing approaches, which are commonly used in the literature or suggested by the ASTM standard, were evaluated for the cracked and intact fillet joints. Based on the experimental and analytical studies, the most appropriate technique for fatigue testing and characterization of adhesive systems was suggested. Comparing the mixed-mode near-threshold behavior of different adhesive systems with the fracture behavior and fatigue mode-I and mixed-mode high crack growth rates showed the high sensitivity of the mixed-mode near-threshold fatigue to the subtle changes in the interfacial bond strength. In order to make a baseline for the design of adhesive joints under cyclic loading, similar to the previous fracture tests and following the energy-based approach, fatigue behavior was characterized as a function of the loading mode ratio for aluminum and steel adhesive joints. The effect of substrate material, surface treatment, bondline thickness, surface roughness and fatigue testing load ratio on the near-threshold fatigue behavior of adhesives joints was evaluated experimentally. The experimental observations were then explained using finite element modeling. To generalize the conclusions, the majority of experiments and studies covered a broad range of crack growth rates, as low as fatigue threshold and as high as 10-2 mm/cycle. Having understood the significant testing and design parameters, an adhesive system can be designed based on a safe cyclic load that produces an insignificant (for automotive industry) or reasonably low but known crack growth rate (for aerospace industry).

Die Arbeit ruht auf zwei Säulen: Die eine besteht in der Aufbereitung, Erprobung und konsequenten Anwendung von Methoden der Skaleninvarianzanalyse, die andere in einem breiten Fundus an experimentellen Daten für aushärtbare Aluminiumknetlegierungen in der Form dünner Bleche, die unter gleichartigen, streng kontrollierten Bedingungen gewonnen worden sind. Als methodische Weiterentwicklungen sind die Fundierung des Umgangs mit der algebraischen Korrelation zwischen Vorfaktor und Exponent einer beliebigen Potenzgleichung, die Übertragung des Ansatzes der finiten Skaleninvarianz auf die Ermüdungsrißausbreitung sowie die Kombination der Idee eines geschwindigkeitsbestimmenden Schrittes mit der Dimensionsanalyse der umgebungsabhängigen Ermüdungsrißausbreitung bis hin zur Kartierung der geschwindigkeitsbestimmenden Schritte zu nennen. Auf experimenteller Seite wurde eine Datensammlung mit gemessenen Streubändern für die Ermüdungsrißausbreitung und das Verfestigungsverhalten von 39 Orientierungen bzw. Auslagerungszuständen von Aluminiumlegierungen aufgebaut. Diese Sammlung wird durch ausgewählte Messungen der Ermüdungsrißausbreitung im schwellenwertnahen Bereich, Restfestigkeitsversuche, Rißschließmessungen, Rauheitsmessungen an Bruchflächen, frequenzabhängige Messungen zum Umgebungseinfluß sowie Untersuchungen an drei Stählen und einer Magnesiumlegierung sinnvoll ergänzt. Auf der Basis der Meßdaten und der Analysemethoden wurde der Werkstoffeinfluß auf die Ermüdungsrißausbreitung in dünnen Blechen aus Aluminiumknetlegierungen bei Belastung mit konstanter Amplitude im Gültigkeitsbereich der linear-elastischen Bruchmechanik untersucht. Dabei wurden folgende Größen als wesentliche Einflußfaktoren identifiziert: - für die Gruppenzugehörigkeit: der Kohärenz- und Ordnungsgrad der festigkeitsbestimmenden Ausscheidungen und die resultierende Gleitverteilung, - für den gemeinsamen Vorfaktor der Legierungen der Gruppe 1: die elastischen Eigenschaften und das Spannungsverhältnis (Translation der Paris-Geraden), - für die Exponenten der Legierungen der Gruppe 1: 0,2%-Dehngrenze, athermischer Verfestigungsparameter, Probendicke und Kc-Wert als dimensionsloses Potenzprodukt (Rotation der Paris-Geraden), - für die Legierungen der Gruppe 2: das Ausmaß der Rißablenkung und eine bleibende Mode-II-Komponente der Rißöffnungsverschiebung, - für den Umgebungseinfluß der Legierung 6013 T6: Frequenz und Schwingbreite des Spannungsintensitätsfaktors. Die Diskussion umfaßt den wertenden Vergleich der experimentellen Ergebnisse mit Befunden und Modellen aus der Literatur, Erklärungsansätze für die Ursachen der Einflußnahme der wesentlichen Parameter sowie einen Modellansatz für die Legierungen der Gruppe 1 auf der Basis einer Mischungsregel. Dabei hatte sich erwiesen, daß keines der aus der Literatur bekannten Modelle alle Befunde richtig wiedergibt. Einige der ausgearbeiteten Erklärungsansätze bedürfen der zukünftigen Vertiefung. / The work is based upon two essentials: the first one is the preparation and application of techniques of scale invariance analysis, the second one consists in a database of experimental results for heat-treatable thin-sheet wrought aluminium alloys obtained under uniform conditions. Progress with respect to methodology was achieved regarding, first, the algebraic correlation between sets of coefficients and exponents of any power law, second, the transfer of the concept of finite scale invariance to the phenomenon of fatigue crack growth (FCG), and third, the combination of the ideas of a rate-controlling step and dimensional analysis of environmental-assisted FCG including the mapping of rate-controlling steps. In the experimental part, a database containing both measured scatterbands of FCG and strengthening characteristics for several orientations and aging conditions of aluminium alloys amounting to a total of 39 different material conditions was established. This database was supplemented with results of selected measurements of near-threshold FCG rates, residual strength, crack closure, roughness of fatigue cracks, and frequency-dependent environmental-assisted FCG as well as investigations of three plain-carbon steels and a magnesium alloy. Based on these prerequisites, the influence of the material on the FCG behaviour of thin-sheet wrought aluminium alloys under constant-amplitude loading was investigated within the limits of validity of linear-elastic fracture mechanics. The following influence factors were identified to be essential: The assignment of alloys to one out of two groups is mainly determined by the degrees of coherency and order of the strength-controlling precipitates and the resulting type of slip distribution. The normalized-Paris-law coefficient for the first group is mainly dependent on the modulus of elasticity and the stress ratio. The Paris-law exponents for the first group are dominated by a dimensionless power monomial of the 0.2% proof stress, the athermal strengthening coefficient, sheet thickeness and the critical stress intensity factor. The retardation of the FCG rates of alloys of the second group relative to the first group is mainly determined by the amount of crack deflection and by a residual mode-II component of crack opening displacement. Finally, the environment-assisted FCG for aluminium alloy 6013 T6 reveals a coupled dependence on loading frequency and cyclic stress intensity factor. The discussion covers the evaluation of the results in relation to observations and models from the literature, the explanation of the modes of operation of the major influence factors and a model based on a mixing rule for the alloys of the first group. It turned out that there is not any model that reflects all of the observations simultaneously. Some of the ideas presented require to be worked out in more detail.

[pt] A resistência a fadiga dos elementos estruturais pode ser limitada pela presença de trincas curtas, as quais podem passar despercebidas em inspeções não-destrutivas tradicionais. Para modelar o comportamento dessas trincas pode-se utilizar o tamanho característico das trincas curtas ao, proposto por El Haddad, Topper e Smith (ETS), que permite ajustar adequadamente os dados experimentais do diagrama de Kitagawa-Takahashi. Partindo do modelo ETS, neste trabalho se apresenta o modelo do Gradiente de Tensão (GT), desenvolvido para prever o efeito das trincas curtas que nascem em pontas de entalhes. Este modelo reconhece que trincas podem ser facilmente geradas por fadiga em entalhes afiados, que introduzem efeitos de concentração de tensão elevados nas suas pontas. Mas devido ao alto gradiente de tensão que atua em torno das pontas desses entalhes, as trincas curtas que deles emanam também podem parar de crescer por fadiga após terem se propagado por uma pequena distância, tornando-se assim trincas não-propagantes que podem ser toleradas em serviço. Corpos de prova tipo C(T) modificados foram projetados especificamente para estudar a geração e o crescimento inicial dessas trincas em ensaios de fadiga, e usados para verificar experimentalmente as previsões do modelo proposto. / [en] The fatigue strength of structural elements may be limited by the presence of short cracks, which can not be detected with traditional nondestructive inspections. The behavior of these cracks can be modeled using the characteristic size of short cracks a0, proposed by El Haddad, Topper and Smith (ETS), which allows one to properly adjust experimental data from the Kitagawa-Takahashi diagram. Based on the ETS model, this work presents the Stress Gradient (SG) model, developed to predict the effect of short cracks that grow from the notch tips. This model considers that cracks are easily generated due to fatigue in sharp notches, which introduce high stress concentration effects at their tips. Because of the high stress gradient acting at these notch tips, the short cracks can also stop to grow by fatigue after having propagated through a small distance, thereby becoming non-propagating cracks that can be tolerated in service. C(T) modified specimens were specifically designed to study the generation and initial growth of these cracks in fatigue tests, and used to experimentally verify the predictions of the proposed model.