Prior to the creation of Computer Die Design, Strain Analysis aided in building robustness into the stamping process. To perform strain analysis, technicians first etch a grid pattern of circles onto a metal blank; when the part shape is formed in the stamping press, the circles deform in the direction of metal flow and the magnitude of the deformation is directly related to metal strain. Technicians then measure the deformed circles (ellipsoids) and plot their values on a strain diagram. Next, we establish the location of the forming limit curve on the strain diagram and report strain conditions (magnitude and deformation mode) based on the proximity of these strains to the forming limit curve.
When a part exhibits strains above or near the forming limit curve, we often observe necking, or splits, and recommend permanent adjustments to critical variables in the forming process to reduce the likelihood of failures. Common process adjustments included; increased lubrication, reduced binder force, or a local change in the gap between the binder surfaces (shimming), but each of these suggestions have the potential of impacting part quality in areas, other than just the problematic zone.
To resolve a localized high strain issue, modification to the die geometry may provide a resolution with little impact on the adjacent part quality areas. Beyond the measure of material strain (also reported as elongation), strain analysis also provides knowledge of stress directionality.
Performing manual circle grid reading, the analyst knew the orientation of the major and minor strains. With camera-based strain measurement systems, the system software has the capability of plotting flow (directional) arrows onto a digital part image. In the example below, one could easily assume the major strain direction will be perpendicular to the draw beads and body curvature (part geometry).
Examination of the flow direction arrows of a computer image of surface strains discloses that within small regional areas, substantial variations in material flow are happening. Note significant flow direction changes are occurring in the tail light area, and above / below the termination of the rear window depression. Each of these areas has the potential for producing a surface distortion (low) if we make press or lubricant adjustments. If we chose the wrong solutions, the outcome will resolve the targeted issue but could create a different quality concern.
When performing manual strain analysis, skill and experience are required to choose the correct part areas to focus the analysis; using an ultrasonic thickness measuring tool is time-consuming but helpful in determining the exact circles where strain conditions are potentially problematic. Concerns over unintended consequences shouldn’t diminish appropriate recommendations to increase part robustness, but analysts should thoroughly review all data provided by the computer systems.