Current formability measurements derived from fundamental and stimulative test data often do not correlate with press performance results. The formability value obtained is usually influenced more by test procedure than by the quality of the material itself. Amounts of stretch and draw in the stamping do not always match those of the formability test.
Strain distributions and maximum strain values are measured from a grid type pattern composed of small diameter (0.25 0.05 in.) circles, which indicate directly the principal (maximum) strain direction and magnitude. The grids are imprinted on the blanks by a rapid and accurate electrochemical marking system.
The grid creates a visual display of the high strain areas, which can provide a clue for eliminating failures.
A failure analysis can be conducted which will indicate the proximity of a stamping to failure. The empirical failure criterion is the largest allowable (critical) strain in the surface of the sheet. The level of this critical strain increases with increasing surface strain. Failure is anticipated for strain conditions above this critical level.
The critical strain level is presently limited to annealed and lightly skin passed steel, copper, brass, and aluminum subjected to tensile-tensile surface strains. Studies are being conducted on the effect of cold work and on tensile compressed strain states.
A report of an instrument panel mounting plate illustrates one example where analysis of the circular grid provided a solution the breakage problem.
Strain distribution analysis may be used to detect critical stampings, reduce production breakage, monitor die modification, and evaluate material specifications.