Optical Strain Measurement- Experimental Tool for Validating Sheet Metal Forming Analysis

Automotive sheet metal components involve complex geometry and large surface areas. In addition to complex geometry, thrust for reduction of the new product development cycle demands for virtual simulation before prototyping. However in order to validate the simulation parameters, the numerical model needs to be experimentally verified. Conventional strain measurement techniques like Mylar tape, Traveling microscope are tedious and error prone for sheet metal forming analysis. Recently, optical strain measurement techniques are being used in sheet metal forming industry. Through this, strain measurement is more accurate, less time consuming and repeatable.

This paper discusses a case study in which the analysis results of an automotive sheet metal component are experimentally validated by circular grid analysis using an optical strain measurement method. The circular grids are marked in the sheet metal blanks by screen-printing. Grid circles in the formed component are recorded using a CCD (Charge Coupled Device) camera. A CCD camera consists of an integrated circuit which senses the incoming light due to photoelectric effect and creates electronic images. CCDs are used The major and minor strains are computed using the photogrammetric algorithm. Forming Limit Diagram (FLD) is plotted and percentage reduction in thickness is calculated based on the strains measured. The simulation results are then compared and correlated with the experimental results. The experimental results show good agreement with simulation results.