Browsing by Author "Santos Silva, Maria"

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    Experimental methods for creation of artificial imperfections via creep and fatigue in 13CrMo4-5 steel
    (2022-08-22) Svartbäck, Niklas; Santos Silva, Maria; Insinööritieteiden korkeakoulu; Vilaça, Pedro
    This work contributes with the development of experimental conditions and methods for creating artificial imperfections in low-alloy 13CrMo4-5 steel. This steel is a heat-resistant special structural steel. The steel plate studied had a thickness of 20 mm. The work is implemented on the scope of a fundamental research project to evaluate early imperfections in special materials for energy applications, such as power plants. The methods applied to create these imperfections are fatigue and creep tests. In particular, the installation of the creep testing conditions was one main development of this work, as it was not previously available at Aalto laboratories. Additionally, several adapters and clamps were designed and created dedicated for the existing equipment and ensure fit with the dimensions of the specimens to be extracted from the tested steel. In parallel, other tests were applied to further characterize the mechanical metallurgical features of the 13CrMo4-5 steel. These other tests are hardness test, and tensile tests for the designed specimen. Some of the conducted tensile tests are featured with digital image correlation analysis, where localized strain is evaluated during tensile tests. A new dedicated hourglass-shape specimen was designed and implemented, with geometrical features to be compatible with the stable operation requirements for creep, fatigue, and tensile testing. This will enable future combination of testing actions with same specimen, namely creep after fatigue, and fatigue after creep. This aim at the creation of complex, located in multiple positions and more realistic imperfections. The cross section of the specimens will also enable the extraction of samples for thermal desorption spectroscopy, which is a major strategic testing method on the scope of the main research project. The creep life of the specimen was found to be approximately 250 hours, at a temperature of 662°C. The test was run at constant load, corresponding to an initial stress of 50 MPa. A large amount of corrosion was obtained in the sample, during testing, and this may have affected the results. The results in fatigue tests shows that the specimen ruptured after approximately 500 000 cycles at a stress of 4 % above its initial yield strength, which is an indication of the evolution of the strengthening mechanisms, with energy loading, inherent to this special material composition. Specimens tested below the yield strength presented infinite life and were stopped after 1 300 000 cycles when no fracture occurred. Tensile tests and hardness tests for the base material showed slightly higher values in strength and hardness when comparing the material to various sources in literature.
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    Review of conventional and advanced non-destructive testing techniques for detection and characterization of small-scale defects
    (Elsevier Ltd, 2023-09) Santos Silva, Maria; Malitckii, Evgenii; Santos, Telmo G.; Vilaça, Pedro; Department of Mechanical Engineering; Materials to Products; Universidade Nova de Lisboa
    Inspection reliability of small-scale defects, targeting dimensions below 100 µm, is crucial for structural safety of critical components in high-value applications. Early defects are often possible to repair, contributing for the circular economy and sustainability by allowing extended life and reuse of components. During in-service operation, the small-scale defects are typically originated from creep, fatigue, thermal cycles, and environmental damage, or any combination of these multiphysical loading conditions. What are thresholds in Non-Destructive Testing (NDT) techniques to detect and reliably characterise small-scale defects? What is the state of the art of NDT-based solutions, in terms of small-scale defects located at surface, and interior of materials? Examples of small-scale defects in engineering materials are established, and a holistic review is composed on the detectability in terms of sensitivity and resolution. Distinguishable high detection accuracy and resolution is provided by computed tomography paired with computer laminography, scanning thermal microscopy paired with Raman spectroscopy, and NDT techniques paired with machine learning and advanced post-processing signal algorithms. Other promising techniques are time-of-flight diffraction, thermoreflectance thermal imaging, advanced eddy currents probes, like the IOnic probe, micro magnetic bridge probe used in magnetic flux leakage, driven-bacterial cells, Quantum dots and hydrogen-as-a-probe.