Development of innovative methods for residual stress analysis in advanced materials

Residual stress assessment is a key factor in engineering design owing to its impact on engineering properties of materials and structural components. Improper management of residual stresses can result in distortion, reduced fatigue life, degraded corrosion resistance, brittle fracture, and even failure during component fabrication, especially with technologies such as welding and additive manufacturing. Therefore, in the context of sustained industrial development for producing advanced components for cutting-edge applications, there is a need to advance residual stress measurement methods to make assessments faster and more reliable. In this thesis, two residual stress measurement techniques have been adopted and improved: the contour method and hole drilling. The first part of the thesis focuses on the contour method. A critical aspect of this technique is the time required to accurately measure the deformed surfaces after cutting the part. Therefore, in this thesis we adopted a full-field optical technique to expedite the surface acquisition step of the contour method. However, due to this approach, it was necessary to modify the subsequent processing of the point clouds of the deformed cut surfaces. Data located at the perimeter of the point clouds had to be excluded. Nevertheless, when compared to the data obtained by coordinate measuring machines, which is the standard profile acquisition technique in the contour method, the initial data points are closer to the perimeter, with a distance of only 0.2 mm. Using the optical technique, we obtained richer data that allowed us to measure the residual stress field in specimens only 2 mm thick, which is at the limits of the contour method. The contour method is attractive because of its insensitivity to microstructural changes, which makes it particularly suitable for the study of joints between different materials. In this thesis, the contour method was used to characterize residual stress fields in various types of advanced multi-material joints fabricated by laser welding and metal additive manufacturing. The second part of the thesis is dedicated to the hole drilling technique. A tool based on a probabilistic machine learning framework has been developed to reduce instability problems notoriously associated with this technique. The proposed approach also enables quantification of the uncertainty of the final measurement output related to the fitting procedure of the strains measured using strain gauge rosettes. The developed methodology was finally applied to measure the residual stresses in advanced laser shock-peened aluminum AA 7050-T7451 specimens.