CASA: computer automated strain analyser for the investigation of three-dimensional deformed sheet components
The computer automated strain analyser (CASA) project has delivered a prototype system for strain measurements that fulfils its targets: The feasibility of the adopted measuring strategies and methods has been demonstrated, with steel sheet as an example. Different types are possible, such as uncoated, galvanised, etc. Results can be expressed either as engineering, true strain or equivalent Von Mises or Hill strain. Representation of results is within the CASA-Windows application as a 3-dimensional coloured contour plot or as a FLD-plot. For the latter, strains are plotted for either the full surface or along a user definable section. Output in spreadsheet format is provided. Unattended operation is possible, after fixing the component on the CMM table and instructing the system on the startpoints and endpoints of the zone to be analysed. The size of component is not limited as far as within the measuring volume of the CMM used. The camera is positioned automatically, including automatic change-over from touch probe. Full protection for collision of camera with component within the scanned area is implemented. Overall accuracy of the measuring system is of the order of 1%. Added to this should be the (in)accuracy of the printed grid. The CASA system showed that these grids might show some repetitive patterns resulting in artificial strains, not present on the component. The limits of the system include: The dimensional scanning of the component form takes up most of the processing time. It is anticipated that CMM speeds can be improved in the near future, while maintaining sufficient accuracy. Surfaces with very high reflectance, such as polished stainless steel, might cause some problems with the optics. The camera and the touch probe should be able to physically access the zone to be analysed. Once the form is scanned, the CASA-system automatically checks for camera collision and adjusts its inclination as far as tolerable for valuable strain measurements. However, no system can be set up (yet) to avoid collision of the touch probe for unattended formscanning of a fully unknown part. In case of concave zones, inaccessible either for the touch probe or for the camera, it is suggested to simply reverse the component and to print a grid on both sides of the blank before deep drawing.