Fracture mechanics of bi-material interfaces for composite pipe repair

PhD ThesisComposite repairs evolved as popular choice for rehabilitation of oil and gas pipelines from corrosive material loss. However, corrosion can develop into local through-wall defects. In this case, fluid pressure acts directly on repairs, forms blisters and applies stresses on repair-pipe interface bonds. A practical model is needed to evaluate and design composite repairs against interface failure. This study investigated fracture mechanics aspects of failure through crack propagation along the repair-pipe interface. Thick fibre-reinforced plates were examined as repairs for circular ‘sharp-edged’ through-holes in stiff metal substrates. Blister formation and propagation onset were analysed. Energy release rates were investigated as measure of interface failure. Two types of blister tests were conducted; using fluid pressure and shaft-loading with different punch heads. A novel method of determining energy release rates in pressure blister tests was developed. Digital image correlation was used to track blister volumes, which are directly related to energy release rates. Existing and newly derived analytical solutions for each test method were compared with measurements and simulations using the virtual crack closure technique. Energy release rates were found to be influenced differently by repair, defect and shaft geometries. Contrary, critical loads could be plotted as function of defect size to repair thickness ratio. To reduce geometry dependence, ‘volumetric’ energy release rates were introduced by adjusting for defect and repair geometries. Similar to load curves, these are functions of defect size to repair thickness ratios, potentially simplifying fracture criteria. Shaft-loading could not be recommended as general fluid pressure replacement, because of differences between the test methods. Design criteria against debonding by ISO/TS 24817 [1] and ASME PCC-2/4 [2] based on energy release rates were reviewed against results presented. An improvement to the formulation in the standards was suggested. An alternative process for qualification and dimensioning was proposed based on empirical formulations for volumetric energy release rates and critical loads as design criteria

Determination of Mechanical Properties of Materials by Small Punch and Other Miniature Testing Techniques: Proceedings of the 5th International Small Sample Test Techniques Conference SSTT2018

Topics include Small Scale Testing for Tensile and Fracture Behaviour of Steel, Alternative Miniaturised Test Methods, Impression Creep, Miniaturised Tensile Testing, Small Scale Testing of Advanced Materials, Small Scale Creep Testing, Small Scale Testing Methodologies and Standardisation

Experimental Characterization and Synthesis of Nanotwinned Ni-Mo-W Alloys

Microelectromechanical systems (MEMS) have transformed consumer and industrial products through the integration of mechanical and electrical components within a single package. MEMS are ubiquitous in society, found predominantly in consumer electronics and automotive industries, providing interconnectivity across a wide variety of devices and everyday objects. To date, the materials selection for the structural element of many MEMS devices has been limited to a relatively small subset of materials, with silicon being the dominant choice. Employing MEMS sensors and switches in extreme environments will need advanced materials with a synergistic balance of properties, e.g. high strength, density, electrical conductivity, dimensional stability, and microscale manufacturability, but MEMS materials with this suite of properties are not readily available. Metallic systems are especially attractive for these applications due to their high density, strength and electrical conductivity. For this reason, metal MEMS materials are the motivation and focus for this dissertation. The synthesis of nanotwinned nickel-molybdenum-tungsten (Ni-Mo-W) alloys resulted in thin films with a very favorable suite of properties. Combinatorial techniques were employed to deposit a compositional spread of Ni85MoxW15-x, alloys and to investigate their physical and mechanical properties as a function of alloy chemistry. The addition of Mo and W was shown to significantly decrease the coefficient of thermal expansion (CTE) and provide a route for tailoring the CTE and its temperature dependence with compositional control. The measured CTE values for Ni-Mo-W matched that of commercial glass substrates currently employed in MEMS devices, broadening the spectrum of materials with the requisite dimensional stability for use in layered structures. Microscale mechanical testing was used to measure the in-plane tensile properties; a linear-elastic response with fracture strengths ranging from 2-3 GPa was uncovered. The ultrahigh tensile strengths are attributed to the presence of highly-aligned nanotwins and their effectiveness as obstacles to dislocation motion. In situ micropillar experiments demonstrated compressive strengths of 3-4 GPa and extremely localized plasticity, both of which are strongly orientation dependent. The nanoscale twins underpinning this mechanical behavior do not impede motion of electrons, and nanotwinned Ni-Mo-W thin films were found to posses the electrical conductivity of bulk Ni alloys. Taken as a whole, this study highlights the balance of physical, thermal and mechanical properties for Ni-Mo-W, driven by nanoscale twin formation. Deposition of Ni-Mo-W films displayed a wide processing window for the formation of the requisite nanotwinned microstructure and attendant properties (CTE, strength, ductility and electrical resistivity). Microcantilever beams were designed and fabricated using traditional integrated circuit processing to translate thin film properties into prototype MEMS device structures. Laser interferometry was used to certify the dimensional stability of the cantilever beams as-fabricated and after thermal exposure at elevated temperatures associated with wafer bonding. Micromachined cantilever beams showed excellent dimensional stability with beam deflection profiles on the order of tens of nanometers, elucidating a path beyond outstanding material properties to actual device structures for next generation metal MEMS devices

Modelling alpha case formation and embrittlement for Ti-6Al-4V produced by additive manufacturing and subjected to thermomechanical post-processing

Comunicación presentada en: 5th Iberian Conference on Structural Integrity que corresponde con el 38 Congreso del Grupo Español de Fractura GEF2022 celebrado en Coimbra, Portugal del 30 de marzo al 1 de abril de 2022La Fabricación Aditiva permite la producción optimizada de geometrías complejas. Las propiedades de tracción entallada de la aleación Ti-6Al-4V se estudian numéricamente en este trabajo considerando el método de Selective Laser Melting (SLM) y diferentes condiciones de post-proceso. La formación de una capa frágil enriquecida con oxígeno o “alpha case” también se reproduce aquí para atmósferas no inertes aprovechando esquemas numéricos para el agrietamiento asistido por hidrógeno. La reducción local de la energía de fractura en función de la concentración de oxígeno se implementa en un modelo de Phase Field para la nucleación y propagación de grietas. Se modelizan diferentes probetas de tracción entallada en Comsol Multiphysics. El análisis es secuencial: la entrada y difusión de oxígeno se simulan para diferentes temperaturas y tiempos de permanencia, reproduciendo procesos típicos de SLM y HIP y diferentes presiones parciales de oxígeno. En un segundo paso, se simula el ensayo mecánico de tracción y se resuelve el problema de Phase Field considerando una reducción lineal de la tenacidad a fractura en función de la concentración de oxígeno. Se evalúan los efectos del valor de la longitud característica y del comportamiento plástico. Los resultados muestran que el marco numérico actual, después de una necesaria calibración de parámetros, es capaz de predecir la influencia del post-proceso termomecánico en la fractura de componentes entallados.Additive Manufacturing enables cost-effective production of complex geometries. Notched tensile properties of Ti-6Al4V alloy are here numerically studied considering the Selective Laser Melting (SLM) method and different postprocessing conditions. The formation of an oxygen-enriched brittle layer or an “alpha case” in Ti-6Al-4V is also reproduced for non-inert atmospheres. Exploiting other Finite Element frameworks for environmentally assisted cracking, e.g. hydrogen embrittlement models, the local reduction of fracture energy as a function of oxygen concentration is implemented in a Phase Field model for crack nucleation and propagation. Different notched tensile specimens are modelled in Comsol Multiphysics. The analysis is sequential: oxygen uptake and diffusion are simulated for different temperatures and dwell times, reproducing typical SLM and HIP processes and different partial pressures of oxygen. In a second step, mechanical tensile testing is simulated, and the damage Phase Field scheme is solved considering a linear reduction of fracture toughness as a function of oxygen concentration. The effects of the characteristic length value and of plastic behaviour are evaluated. Results show that the present framework, after parameter calibration, is able to predict the influence of thermomechanical post-processing on notch fracture.The authors gratefully acknowledge financial support from the Junta of Castile and Leon through grant BU002-P20, co-financed by FEDER funds

Selected Papers from Experimental Stress Analysis 2020

This Special Issue consists of selected papers from the Experimental Stress Analysis 2020 conference. Experimental Stress Analysis 2020 was organized with the support of the Czech Society for Mechanics, Expert Group of Experimental Mechanics, and was, for this particular year, held online in 19–22 October 2020. The objectives of the conference included identification of current situation, sharing professional experience and knowledge, discussing new theoretical and practical findings, and the establishment and strengthening of relationships between universities, companies, and scientists from the field of experimental mechanics in mechanical and civil engineering. The topics of the conference were focused on experimental research on materials and structures subjected to mechanical, thermal–mechanical, and dynamic loading, including damage, fatigue, and fracture analyses. The selected papers deal with top-level contemporary phenomena, such as modern durable materials, numerical modeling and simulations, and innovative non-destructive materials’ testing

Conjunto de ecuaciones para el cálculo del factor de intensidad de tensiones en fisuras internas de barras cilíndricas sometidas a tracción

Comunicación presentada en: 5th Iberian Conference on Structural Integrity que corresponde con el 38 Congreso del Grupo Español de Fractura GEF2022 celebrado en Coimbra, Portugal del 30 de marzo al 1 de abril de 2022La propagación de fisuras por fatiga iniciadas a partir de defectos internos es uno de los principales mecanismos de fallo en piezas sometidas a alto número de ciclos (HCF) y muy alto número de ciclos (VHCF). La propagación de estos defectos internos en barras sometidas a esfuerzos de tracción tiende a adquirir una forma circular, independientemente de la forma irregular que presente el defecto iniciador, estableciendo un patrón conocido como fish-eye. Para el estudio de la propagación de este tipo de defectos se presenta en este trabajo un conjunto de ecuaciones que permiten obtener el factor de intensidad de tensiones (FIT) en función de tres parámetros adimensionales (tamaño, posición y relación de aspecto de la fisura). Se presenta también la metodología necesaria para su aplicación al estudio de la propagación de un defecto inicial. Los caminos de propagación calculados son comparados con los obtenidos en ensayos experimentales obteniendo una muy buena predicciónFatigue crack propagation initiated from internal defects is one of the main mechanisms observed in components created using additive manufacturing techniques, as well as one of the most common mechanisms in high cycle fatigue (HCF). The propagation of these internal defects in round bars subjected to tensile loads tends to a circular shape, regardless of the irregular shape of the initiating defect, establishing a pattern known as fish-eye. In order to study the fatigue crack propagation of this type of defects, a series of closed-form equations are presented which allow the stress intensity factor (SIF) to be obtained in this geometry as a function of three dimensionless parameters (size, position and aspect ratio). The methodology necessary for its application to the study of fatigue crack propagation is also presented. The propagation paths calculated are compared with those obtained in experimental tests, obtaining a very good prediction.Los autores desean agradecer la financiación recibida a través del proyecto de la JCyL referencia BU-002-P20, cofinanciada con fondos FEDER

Advanced Testing of Soft Polymer Materials

Manufacturers of soft polymer products, as well as suppliers and processors of polymers, raw materials, and compounds or blends are compelled to use predictive and advanced laboratory testing in their search for high-performance soft polymer materials for future applications. The collection of 12 publications contained in this edition therefore presents different methods used to solve problems in the characterization of various phenomena in soft polymer materials, asks relevant questions and offers appropriate solutions