UBC Theses and Dissertations
Compact ESPI device for isotropic measurements of residual stress Melamed, Samuel
Residual stresses are self-equilibrating stresses that exist within bodies without any external loads acting on them. Monitoring residual stresses is vital to engineer safe and reliable structures. A reliable means of measuring residual stress is by drilling a small hole in the body and observing the resulting redistribution of the stress using an electronic speckle pattern interferometer (ESPI). Current ESPI systems are not suitable to measure stress on large or immovable structures because they depend on highly coherent laser sources that are bulky, delicate and not suitable for rugged field conditions. Additionally, ESPI is limited to a single direction of measurement. If the measurement axis is misaligned with the principal stress direction accuracy is reduced. This presents a practical challenge when measuring unknown residual stress states in the field. A compact ESPI device providing an isotropic residual stress measurement is presented here. The compactness of the device is possible by a novel optical arrangement that uses a diffraction grating and a miniature laser diode. This optical arrangement can be geometrically tuned to provide high quality measurements even with low coherence laser diodes. Furthermore, the device is constructed with two orthogonal measurement axes to reduce the influence of instrument alignment on measurement accuracy and to improve the overall precision by doubling the quantity of data. Experimentation with a calibrated bend specimen showed that the device has an accuracy ranging 9-12MPa and precision of 13MPa. This integration of ESPI and hole drilling modules into a compact, stand-alone unit is a significant milestone for in-field residual stress measurements.
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