Analysis of Strain Nonuniformity Index (SNI) in electrohydraulically formed sheet metal component

Abstract

In sheet metal parts formed by Electrohydraulic (free forming) process, a spherically shaped reflector is predicted to result in a highly non-uniform thickness strain distribution. As expected, the thickness strain (and hence thinning) is maximum at the top of the formed dome. The probability of failure is therefore highest at this location. A finite element model is formulated in ABAQUS explicit to generate the strain distribution in sheet metal. The model considers a water filled chamber to transmit the pressure pulse. The effect of the electrical discharge between the electrodes is modeled as a pressure wave of a certain magnitude originating from the location of the electrodes. The constitutive relation for the sheet is described by the Johnson-Cook model and non-deformable/ stiff tools are used. The extent of non-uniformity in thickness strain distribution has been expressed in terms of Strain Non-uniformity Index (SNI). It is the difference between the peak thickness strain (PTS) and the average thickness strain (ATS). For successful forming, the value of SNI should be as low as possible. This is enabled by an alternate design of the reflector which is compared with the spherical shape. The pros and cons of using either shape of reflector are examined in the paper.