The mechanical engineering department at the University of Idaho is conducting a project with the purpose of developing a complete system for investigating creep-, creep-fatigue- and fatigue properties of metallic materials at elevated temperatures up to 650 ˚C with Compact Tension (CT) specimens. Considerable efforts have been made to study and understand these phenomena, although numerous problems still exist. It is important to explore more extensively the complicated phenomena of creep, fatigue and of creep-fatigue interactions. The Direct Current Potential Drop (DCPD) method is a common method used to investigate, for example, the initiation of cracks, crack growth rates and to monitor crack growth. The technique utilizes the fact that the electrical resistance of a CT specimen changes with crack growth. By applying a constant current over the specimen and measuring the resulting voltage over the crack, the crack length can be related to the voltage, and the difference in crack length with difference in voltage. Standards from the American Society for Testing of Materials (ASTM) were used as guidance when designing the DCPD system and CT specimen. The development and implementation processes were divided into an analytical and an experimental stage. The final product consisted of a high temperature extensometer, to measure crack mouth opening displacement (CMOD), and a DCPD system, to measure crack growth, controlled by separate control units. The DCPD system consisted of a DC supply and a nano voltmeter along with Constantan wire and NiCr60 wire respectively, that were mechanically fastened. The DCPD system delivered overall satisfying results and was able to generate sufficient data to produce a crack growth curve, da/dN vs. ΔK. Although, by taking advantage of resistance welding equipment to attach the DCPD wires, along with implementing one shared control unit for the DCPD system and the extensometer, more accurate and accessible measurements and correlations could be extracted. / Mechanical engineering avdelningen på University of Idaho genomför just nu ett utvecklingsprojekt med syftet att utveckla ett komplett system för undersökning av krypnings, krypnings-utmattnings- samt utmattnings- egenskaper av metalliska material vid höga temperaturer upp till 650 ˚C med hjälp av kompakta spänningsprovstavar (CT specimens). Betydande ansträngningar har gjorts för att undersöka och förstå dessa fenomen, men flera problem kvarstår. Det är viktigt att djupare undersöka kopplingen mellan krypnings- och utmattningsegenskaper. Direct Current Potential Drop (DCPD) metoden är en vanlig metod vilken används för att undersöka, exempelvis, sprickinitiering, spricktillväxthastigheter och spricktillväxt. Tekniken utnyttjar faktumet att den elektriska resistansen i en provstav ändras med spricktillväxt. Genom att föra en konstant ström genom provstaven och sedan mäta den resulterande spänningen över sprickan, kan spricklängden relateras till uppmätt spänning. På samma sätt kan spricktillväxt relateras till spänningsförändringar. Standarder från American Society for Testing of Materials (ASTM) användes för att designa ett DCPD system samt en CT provstav. Utvecklings- och implementeringsprocessen var uppdelad i en analytisk och en experimentell del. Den slutgiltiga produkten bestod av en extensometer, för mätning av spricköppning vid höga temperaturer, och ett DCPD system, för mätning av spricktillväxt vid höga temperaturer, vilka kontrollerades av separata kontrollenheter. DCPD systemet bestod av en strömkälla och en nanovoltmeter tillsammans med Constantan kablar respektive NiCr60 kablar, vilka fastsättes mekaniskt. DCPD systemet levererade generellt sett tillfredställande resultat och hade kapacitet att generera tillräckligt precisa data för att producera en spricktillväxtkurva, da/dN vs. ΔK. Däremot, genom att utnyttja en resistanssvets, för att fastsätta DCPD-kablarna, tillsammans med en gemensam kontrollenhet för extensometern och DCPD systemet, kan det tänkas att bättre och mer tillgängliga resultat kunde åstadkommas.

Main extrinsic and intrinsic modifiers factors of crack growth rate in AA7050-T7451 were assessed in order to provide tools for aeronautical structures designers. These tools cover most necessary information to project aircraft\'s structures using the studied alloy, under damage tolerance philosophy. The experimental methodology consisted of use CT specimens, on TL and LT rolling direction to test its behavior under different conditions of stress ratio, force waveform, and the environment. The stress ratio values were 0.1 and 0.5, the force waveform used were sine and trapezoidal or Dwell under normal air laboratory conditions and salt fog 3.5%NaCl weight in order to simulate the marine environment. In Dwell tests, results were checked with the electrical potential drop technique (DCPD) in addition to the crack opening displacement (COD) method. Using the Walker coefficients, calculated on the present research, could be projected accurately the crack propagation behavior on Paris region and do fatigue life predictions using da/dN and S-N diagrams for different stress ratio values. The corrosion environment increases both crack growth rate and ΔKth due to oxides formation on the crack path that generates a crack closure effect. Dwell carrying makes decrease the crack growth rate by decreasing the slope of the Paris line on log (da/dN) versus log (ΔK) curve, instead of shifting down the line as occurs on titanium alloys. Rolling direction change from LT to TL increase the FCG rate in both threshold and Paris region, where the rate change use to be small. / Os principais fatores modificadores extrínsecos e intrínsecos da taxa de propagação de trincas na liga AA7050-T7451 foram avaliados para fornecer subsídios para projetistas de estruturas aeronáuticas, com base na filosofía de tolerância ao dano. A metodologia experimental consistiu em ensaiar corpos de prova do tipo compact tension (CT) da liga nas direções de laminação TL e LT, para verificar seu comportamento sob diferentes razões de tensões, forma de onda e condição ambiente. Os valores de razão de tensão estudados foram 0,1 e 0,5, as formas de onda foram senoidal e trapezoidal ou de Dwell, em condições normais de laboratório, ao ar, e névoa salina 3,5% NaCl, em massa, para simular um ambiente marinho. No caso dos ensaios Dwell, os resultados foram conferidos pelo método de queda de potencial eléctrico (QPE), além do método de flexibilidade elástica. Usando os coeficientes de Walker calculados a partir dos resultados obtidos, pôde-se projetar com precisão o comportamento da propagação de trinca na região de Paris e prever a vida em fadiga usando os diagramas da/dN e S-N para diferentes valores da razão de tensões. O ambiente corrosivo aumenta tanto a taxa de propagação de trinca, quanto o valor de ΔKth por causa da formação de óxidos na trajetória da trinca, que geram um efeito de fechamento sobre a mesma. Quanto à forma de onda, verificou-se que o carregamento Dwell diminui a taxa de propagação de trinca, diminuindo a inclinação das curvas log (da/dN) versus log (ΔK) na região de Paris, ao invés de deslocá-la paralelamente como ocorre com ligas de titânio. A mudança da direção de laminação de LT para TL aumenta a taxa de propagação de trinca por fadiga (PTF) tanto na região de threshold, quanto na região de Paris, onde a mudança de taxa é pequena.

Main extrinsic and intrinsic modifiers factors of crack growth rate in AA7050-T7451 were assessed in order to provide tools for aeronautical structures designers. These tools cover most necessary information to project aircraft\'s structures using the studied alloy, under damage tolerance philosophy. The experimental methodology consisted of use CT specimens, on TL and LT rolling direction to test its behavior under different conditions of stress ratio, force waveform, and the environment. The stress ratio values were 0.1 and 0.5, the force waveform used were sine and trapezoidal or Dwell under normal air laboratory conditions and salt fog 3.5%NaCl weight in order to simulate the marine environment. In Dwell tests, results were checked with the electrical potential drop technique (DCPD) in addition to the crack opening displacement (COD) method. Using the Walker coefficients, calculated on the present research, could be projected accurately the crack propagation behavior on Paris region and do fatigue life predictions using da/dN and S-N diagrams for different stress ratio values. The corrosion environment increases both crack growth rate and ΔKth due to oxides formation on the crack path that generates a crack closure effect. Dwell carrying makes decrease the crack growth rate by decreasing the slope of the Paris line on log (da/dN) versus log (ΔK) curve, instead of shifting down the line as occurs on titanium alloys. Rolling direction change from LT to TL increase the FCG rate in both threshold and Paris region, where the rate change use to be small. / Os principais fatores modificadores extrínsecos e intrínsecos da taxa de propagação de trincas na liga AA7050-T7451 foram avaliados para fornecer subsídios para projetistas de estruturas aeronáuticas, com base na filosofía de tolerância ao dano. A metodologia experimental consistiu em ensaiar corpos de prova do tipo compact tension (CT) da liga nas direções de laminação TL e LT, para verificar seu comportamento sob diferentes razões de tensões, forma de onda e condição ambiente. Os valores de razão de tensão estudados foram 0,1 e 0,5, as formas de onda foram senoidal e trapezoidal ou de Dwell, em condições normais de laboratório, ao ar, e névoa salina 3,5% NaCl, em massa, para simular um ambiente marinho. No caso dos ensaios Dwell, os resultados foram conferidos pelo método de queda de potencial eléctrico (QPE), além do método de flexibilidade elástica. Usando os coeficientes de Walker calculados a partir dos resultados obtidos, pôde-se projetar com precisão o comportamento da propagação de trinca na região de Paris e prever a vida em fadiga usando os diagramas da/dN e S-N para diferentes valores da razão de tensões. O ambiente corrosivo aumenta tanto a taxa de propagação de trinca, quanto o valor de ΔKth por causa da formação de óxidos na trajetória da trinca, que geram um efeito de fechamento sobre a mesma. Quanto à forma de onda, verificou-se que o carregamento Dwell diminui a taxa de propagação de trinca, diminuindo a inclinação das curvas log (da/dN) versus log (ΔK) na região de Paris, ao invés de deslocá-la paralelamente como ocorre com ligas de titânio. A mudança da direção de laminação de LT para TL aumenta a taxa de propagação de trinca por fadiga (PTF) tanto na região de threshold, quanto na região de Paris, onde a mudança de taxa é pequena.

The main goal of this thesis is to get an insight into a field of non-destructive testing using potential drop techniques that have nowadays become the standard not only in the fatigue and loading tests but also in the industry. These methods can serve to non-destructively and continuously measure material specimens, thickness, corrosion losses, deformations, spectroscopy and detection and analysis of crack geometry. They can help to identify materials and measure material changes over time. The second part of this thesis deals with designing the aperture for detection of cracks in steam and product piping using potential drop technique modified by Ing. Ladislav Korec, CSc. Last part deals with extensive testing, experimenting and evaluation of the aperture.