Analysis of Strain Nonuniformity Index (SNI) in electrohydraulically formed sheet metal component
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Date
2014
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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.
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Keywords
Electrohydraulic forming, SNI analysis, Thickness strain