Application of ESPI-method for strain analysis in thin wall cylinders

The Centre for Industrial and Engineering Optics, DIT, Ireland, have recently developed and applied new optical techniques for the measurement of mechanical strain, one of which was based on electronic speckle pattern interferometry (ESPI). The accuracy of this optical technique when used on flat surfaces is well established. In this research the technique is tested on curved surfaces, and results are compared with the theoretical hoop strain as predicted by the bi-axial strain equation modified for the thin cylinder, and by those obtained by electrical resistance strain gauge (ERSG) undoubtedly the principal method of measuring mechanical strain. For testing procedures, a special unit was designed and produced for holding and loading the thin cylinder. Also the basic equation for hoop strain by ESPI for flat surfaces was modified for applications on curved surfaces. Thin cylinder hoop strain obtained by ESPI and calculated by a modified equation show remarkably good correlation to the predicted theoretical value as well as to the results obtained by ERSG. Since the ESPI theory was originally developed for use on flat surfaces, there are reasonable grounds for further investigating this relationship especially where curved surfaces are involved.