Forming Limits for Mild Steel Sheet at High Forming Rates

Abstract

The forming limit diagram for a particular sheet material illustrates the limits of principal strains which it may suffer without failure, in a forming process. The diagram is usually determined experimentally. Displacement rates used in the determination are likely to be low, of the order of 1.0 mm/s. Often in practice, the displacement rates are much higher, of the order of 100 m/s, and a forming limit diagram which has been determined at low rates may not he appropriate. This paper considers the changes in the forming limit ding ram due to higher forming rates. Determinations of the forming limit diagrams for a panel steel, at widely different forming rates, are reported. The forming at a high rate was done hydraulically, the oil pressure being induced dynamically by a falling weight. The displacement rate achieved was about 15 m/s, averaged over the test. Various test-blank configurations were used to obtain a range of minor principal components of strain at failure. The results show the forming limit diagram to be dependent on forming rate with a reduction in formability in stretching, an enhancement in drawing and no change in plane strain, at the high forming rate. The results are consistent with those which have been reported earlier in which an explosive-forming method was used.