Curvature measurement of hydroformed axisymmetrical shells using shearography

Abstract

It is commonly known that the membrane stresses in a hydroformed shell are directly related to the local principal curvatures. Accurate measurement of the local shape of thin axisymmetrical shells is therefore of fundamental importance during the hydroforming process. Previous works involving local curvature measurement in hydroformed shells utilized such device as the extensometer and photogrid method. In recent years, the more conventional methods, such as the strain gage, bevel protractor and travelling microscope techniques are used. However, these techniques always require direct contact with the specimen surface and, in certain instances, could result in minor damage of the object surface. In a previous paper by the authors, a non-contact optical technique called shearography is developed for the measurement of slope in simple objects. In this paper, the technique, which employs a laser and an image-shearing camera, is further used to measure the slope of the meridional profile in hydroformed shells. The local curvatures of the shells are derived from the slopes obtained from the shearographic fringe patterns produced with a moving light source. The results obtained are compared with those derived using the travelling microscope technique.