Numerical investigations on the effect of pit on two-step dimple forming of atomic fuel spacer grid

Abstract

Surface defects, known as pits, due to intervention of impurities are difficult to control during the production of sheet metals for atomic fuel spacer grids. These surface defects can increase the fracture risk during sheet metal forming (SMF) processes and lead to failure due to cracking. In this research, an effect of pits on the formability of zircaloy-4 sheets in two-step stamping process is numerically investigated. Experimentally measured pits with averaged sizes are introduced into finite element (FE) models to investigate the effect pit size and the location on plastic strain distribution around weaker sections. By performing one-step and two-step stamping analyses, strains at the pit locations are compared with an experimental forming limit curve (FLC). Although the two-step stamping model reduces the susceptibility to cracking of the atomic fuel spacer grids over one-step stamping model, this study reveals that the fracture risk is higher, if a pit with relatively small radius and deep depth is located near the edge of the metal sheets. Therefore, it is recommended to identify the pits with critical size around the weaker sections before the SMF processes.