Application of an image processing technique in strain measurement in sheet metal forming

Abstract

In sheet metal forming, strains or strain increments are evaluated usually from the shape change of grids marked previously on the surface of the workpiece, the present understanding of formability in a forming process being based largely on the knowledge gained from the strain measurement. The theory of square-grid analysis is a great help in gaining knowledge of sheet metal forming. In the application of this theory, nodal points on a sheet metal surface etched with square-grids have to be measured. Conventional grid measurements are performed manually, which is either time-consuming or of low accuracy. It is known that the image processing technique is very powerful in digitizing the image of an object. A newly developed image processing device which consists of a video camera, a monitor and a personal computer, has therefore been introduced into strain measurement. The computer is equipped with an AD card and with data processing software. With such a device, the measurement of a gridded surface can be performed conveniently, with accuracy and efficiency. The combination of the image processing technique and the theory of square-grid strain analysis permits rapid measurement and strain analysis over the surface of a workpiece marked with a large number of nodal points.