ARGUS employs an advanced form of optical photogrammetry to measure and visualize the quality and degree of the stamping process on sheet metal. With high precision it automates conventional circle grid analysis. For the measurement, the sheet is marked with a regular grid of dots, using chemical etching, laser marking or other marking techniques. The sheet is stamped or formed (hydroforming, magnetic forming, etc.). For the measurement, one or two scale bars and some coded markers are placed directly on the part or close around it. A digital CCD camera is then used, with real time viewing of the actual view on the PC, to take shots from different viewing directions.

After the acquisition of the images, ARGUS will define in each image the exact center point of all marked dots, to within a hundredth of a pixel. Then, using photogrammetry techniques, the images are virtually assembled to represent the object, as a 3D model. From this virtual assembly, the center of each marked dot on the object is defined in true 3D coordinates. These calculated 3D points define the exact form of the stamped sheet part. From the local distortion of the regular grid pattern, which was applied on the flat sheet metal, the local strain introduced by the stamping process is calculated in many thousand local data points. These strain values define the degree of the forming, which are typically viewed as major strain (principle strain 1) and minor strain (principle strain 2), and especially thickness reduction. These values are graphically displayed and validated in relation to the Forming Limit Curve (FLC) of the selected material or in relation to the maximal allowed reduction of the sheet thickness by the stamping process. The accuracy of the strain values is better than +/- 0.5%, thus also hardening investigations on parts with a low forming degree (hood, roof) can be done.

For the ARGUS measurement of a simple stamped sheet metal part, only few images are taken and a typical measurement to validate the stamping process is finished in a few minutes.

Fig. 1: Sheet metal part with etched dots after stamping

In the following example, a sheet metal part with 25 x 25 cm (10 inch) size and 8 cm depth was captured with seven images. The ARGUS calculated are the approximately 2,000 etched points on the object surface. The complete measurement, with the setup of the part, the image acquisition, the calculation of the centers of the dots in the images and then the calculation of the actual 3D coordinates of these dots on the stamped part (3D model), and then the calculation of the pattern distortion (local strain) and graphical display was completed in less than ten minutes.

Fig. 2: Color plot of the major strain distribution	Fig. 3: Forming limit diagram with measurement data and forming limit curve	Fig. 4: Major strain in the section which is indicated in Fig. 2

To measure the stamping quality of complex and large parts, additional views are needed to cover the complete surface and all of the detail areas. For these applications, ARGUS accepts and integrates many images into a virtual assembly (3D model) and is therefore able to measure the complete range of formed shapes used in sheet metal stamping. Even cylindrical parts from hydroforming or other processes are ideal applications for the technology.

The following example shows the stamping evaluation of a complete B pillar with a starting thickness of 0.9 mm, with a 1.1 meter length and 0.35 meter width, which was provided by Salzgitter AG. The applied dot pattern was etched with a dot diameter of 1.5 mm and a dot spacing of 3 mm. For the complete measurement typically 100 images are taken. From these images typically 30,000 measuring points were defined on the part and an evaluation time of three hours is needed.

Fig. 5 : B--pillar	Fig. 6 : Color plot of the major strain distribution of the B-pillar
Fig. 7 :Detailed View of the B--pillar	Fig. 8 :Detailed View of the B--pillar: Major Strain Distribution
Fig. 9 : Section as given in Fig. 8 with the corresponding strain values

Trilion/GOM measurement systems are used daily in many steel and aluminum producing companies, in automotive enterprises and their suppliers. The systems deliver exact and relevant data of the quality and degree of the stamping process. The ARGUS system is used on the shop floor and in research applications, for simple parts as well as for very complex and large stamping parts, for part and die verification, and for documented proof of part quality.

The ARGUS measurements were made in cooperation with Salzgitter AG.