Strains measured on plane and curved surfaces by means of the shearographic method ‐ Part 2

Abstract

The shearographic interferometry is employed as a nondestructive full field, optical testing method without contact. Fringes of constant strain (so called isotase, tasis (Greek)=strain) can be observed in real time on the surface of the investigated machine parts and are represented by the shearograms. Using shearography two states of deformation are recorded by doubly exposing a Holotest film in an ordinary camera or stored by an image processing system. In the telelens of the camera a shearing element inside or outside the focus is integrated. Rigid body motions of the specimen are not recorded. Local deformation irregularities caused by a defect under or on the surface of the specimen create strain concentrations; the homogeneous surrounding is poorly superimposed by an interference pattern, e.g., in the case of the internal pressure differential for testing tubes1. The shearogram shows dark and bright fringes which are the functions of the displacement derivative. The holographic interferometry measures the out of plane deformations directly. A term of the out of plane strain can be determined by the shearographic method2 as well as the in plane strain fringes under certain conditions which are described in the following. For this purpose the fringe patterns generated by the basic types of loading are presented and compared with the photoelastic patterns showing obvious analogies. The magnitude of the shearographic strain components is nearly the same when measuring by means of strain gauges or determining theoretically as is shown for one case.