[pt] Os Ensaios Não-Destrutivos são técnicas utilizadas na inspeção de materiais e equipamentos sem dano a integridade física, podendo ser executados nas etapas de pós-fabricação, montagem, operação e manutenção. São utilizados pela indústria como ferramenta de controle e garantia da qualidade de materiais, produtos e processos. Esta pesquisa teve como objetivo identificar trincas e estimar o seu comprimento em corpos de prova de aço DIN 42CrMo4, adotando diferentes técnicas de END. Os corpos de prova foram de um eixo virabrequim de um motor Diesel, utilizado como unidades geradora em usinas termoelétricas, que falhou por fadiga de baixo ciclo. Os corpos de prova foram projetados de acordo com as normas ASTM E1820, ASTM E1290 e ISO 15653, bem como adotando sugestões de inspetores de ultrassom quanto a uma geometria que otimizasse a eficiência das medições. Subsequente ao trincamento dos corpos de prova em fadiga, os mesmos foram inspecionados com o emprego das técnicas de Radiografia Digital (RD), Ultrassom Phased Array (UT-PA), métodos Pulso-Eco (PE) e Time os Flight Diffraction (TfOD), e Tomografia Computadorizada (TC). Em sequência, os corpos de prova foram fraturados com o uso de uma carga em flexão e nitrogênio liquido, o que permitiu a medição real dos comprimentos das trincas. As técnicas de UTPA, RD e TC indicaram tamanhos de trincas próximos entre si. Nos métodos de PE e ToFD, de UT-PA, se observou que a indicação das trincas sofre influência da distancia entre o cabeçote do equipamento e a posição da trinca. Em trincas menores, essa influência é mais acentuada, levando a uma medição errônea do comprimento da trinca. A técnica de RD apresentou um bom resultado da medida lateral da trinca, porem foi necessário um processamento digital da imagem para obtenção de melhores resultados, explicado devido a sensibilidade do ensaio à geometria do corpo de prova e da densidade do material utilizado. Já a TC apresentou imagens melhores e medidas de trinca similares se comparada com a RD, o que pode influenciar em outros tipos/dimensões de defeitos. Em compensação, sofre mais influencia do volume de material presente no entorno do defeito, o que impossibilitou a detecção de defeitos em algumas amostras. / [en] Non-Destructive Testing (NDT) are techniques applied to materials and equipment inspection without integrity degradation and can be performed in the post-manufacture, assembly, operation and maintenance stages. Those techniques are used by the industry to control a guarantee the quality of materials, products and processes. The aim of this research was to identify cracks and estimate their length on DIN 42CrMo4 steel specimens adopting different NDT techniques. The material to the test specimen was taken from a crankshaft of a Diesel engine, utilized as generating units in thermoelectric power plant, which has failed due to low cycle fatigue. The test specimen were designed according to ASTM E1820, ASTM E1290 and ISO 15653 standards, as well as suggestions made by NDT inspectors for a geometry that optimized the efficiency of the measurement. Subsequent to the fatigue induced crack on the specimens, they were inspected using, Digital Radiography (RD), Phased Array Ultrasound (UT-PA), Pulse-Echo (PE) and Time of Flight Diffraction (ToFD) methods, and Digital Tomography (TC). The specimens were then fractured with a bending load and liquid nitrogen, which allowed the measurement of the real crack length. UT-PA, PE and ToFD, and RD techniques showed a closer estimate crack size among then. It was observed that both UT-PA methods, PE and ToFD, presented an influence of the distance between the equipment head and the crack position, on the crack measurement. This was more pronounced in smaller cracks, leading to an erroneous measurement of the crack length. The RD technique presented a good a good result of lateral crack length, but a good digital image processing was necessary to obtain these results, explained by the sensitivity of the test to the geometry to the geometry of the specimen and to the material density. TC presented better imaging and similar lateral crack lengths when compared to RD, which could influence the detection of other types/dimensions of defects. In contrast, it is more influenced by the volume of material around the object, which made it impossible to detect defects in some samples.

Dans les installations industrielles, des inspections régulières sont planifiées pour évaluer l’état de santé interne des composants. Si des fissures sont révélées, il est souhaitable de savoir si l’exploitation de la structure peut se poursuivre ou si un régime de fonctionnement dégradé pourrait être envisagé. En se basant sur la mécanique élastique linéaire de la rupture, les travaux présentés traitent donc dans le cas de composants fissurés de la dispersion de durée de vie résiduelle relative aux incertitudes sur les paramètres du modèle de prévision. La longueur de fissure initiale, les propriétés du matériau ainsi que les paramètres d’entrée de la loi de Paris ont été considérés comme des variables aléatoires, dont les distributions ont été déterminées expérimentalement puis ajustées par des lois statistiques adéquates. Des contrôles ultrasonores par mesure du temps de vol de l’onde diffractée – Time Of Flight Diffraction (TOFD) en anglais – et des mesures de champs obtenues par corrélation d’images numériques ont été utilisés pour quantifier la propagation d’une fissure dans une éprouvette à défaut soumise à des sollicitations cycliques uniaxiales. Les données expérimentales recueillies ont été utilisées pour initialiser les calculs et valider les résultats numériques. Les distributions de taille de fissure obtenue après un nombre donné de cycles de sollicitation et de nombre de cycles de sollicitation conduisant à une taille définie de fissure ont été obtenues par une méthode de Monte-Carlo appliquée au modèle de prévision. L’ajustement de ces distributions par de lois log-normales a fourni des outils analytiques d’estimation probabiliste de propagation de fissure. Cela a notamment permis la réalisation de cartographies de risques et l’évaluation de l’évolution de la fiabilité du composant étudié. Enfin, les effets d’une actualisation de la connaissance de la longueur de fissure au cours de la vie de ce composant en termes d’incertitude de prévision et d’extension de durée de vie résiduelle prévisionnelle ont été étudiés. En particulier, afin de limiter le coût des campagnes de contrôle non destructifs dans le cas industriel, une stratégie d’optimisation de l’actualisation de cette connaissance basée sur l’étude de fiabilité a été proposée. / In industrial plants, regular inspections are planned to assess the internal state of installations. If some cracks are revealed, it is desirable to know whether the structure can still be used or if a degraded mode of operation should be considered. Starting from a linear elastic fracture mechanics model, the work presented studied the scatter of the remaining life of such cracked parts due to the uncertainties on the parameters of the prediction model. Initial crack size, material properties and input parameters of Paris’ law have been considered as random variables and their distributions have been experimentally identified and fitted with convenient statistical laws. Time Of Flight Diffraction (TOFD) and field measurement technique based on Digital Image Correlation (DIC) were used to monitor the crack propagation initiated from a notch introduced in specimen submitted to uniaxial cyclic loading. Experimental crack length results were used to initiate computations and as a mean to validate numerical results. Both the crack lengths distributions resulting from the application of loading cycles and the distribution of the number of cycles leading to a given crack length were obtained from a Monte-Carlo method applied to the prediction model. The fit of these distributions with log-normal laws provided analytical tools to assess probabilistic crack propagation. It allowed for risk mapping and for the evaluation of the studied component’s reliability evolution. Last, the effects of an actualisation of crack length knowledge along the life of this component in terms of assessment uncertainty and predicted residual life extension has been studied. Especially, to limit the cost of non-destructive techniques inspection in industrial cases, a reliability-based strategy has been proposed for the optimisation of the crack knowledge actualisation.