A Study of the Strain Histories And Drawing-In in a Pressed Steel Shell

Summary

The control of metal flow in sheet metal forming is generally aimed at avoiding strain concentrations leading to fracture and those compressive strains which lead to puckering. Flow can be controlled by different means, such as draw beads, shape and size of blank, holes, shape of die, and so on. In practice, the details of the flow are usually unknown to the engineer; he sees only the final result, and that only in terms of the presence or absence of puckers.

For sheet metal to be deformed into a particular shape, it must have formability, but the determination of this quality is not an easy task. A particular metal may form excellently for one pressing, and fail when used for another of a different shape. The pressing considered in this investigation had proved difficult to produce, due to fracture at a critical section and the presence of excessive puckering. The history of the strains occurring in the critical section was obtained from measurements of deformed grids on progressively formed shells and the data recorded in triangular co-ordinates, thus enabling strain histories and the mode of deformation of particular elements to be observed.

Measurement of drawing-in were made from tracings of the periphery of the formed pressings and plotted against the punch penetration, so that the rate and amount of drawing-in during forming could be shown. The influence of drawing-in is discussed. Finally deformation rotation of the elements was observed from the deformed grid, and its influence on the results discussed.