Necking and radial cracking around perforations in thin sheets at normal incidence

The Formation of multiple necks and cracks around perforations in ductile materials is investigated. Expressions are obtained for the number of plane-strain radial necks formed by conical penetrators (with or without starter holes in the flat target), and also by round-ended projectiles into flat targets having starter holes or, in plain targets with no starter holes, after fracture of the initial circumferential neck has detached a circular cap of material ("discing"). Likewise, expressions are derived for the number of radial cracks which form subsequently in both cases and lead to "petalling". The number of cracks is smaller than the number of necks. Experiments seem to agree with the analyses, although accurate counting of necks in thin sheets is illusory. (C) 1998 Elsevier Science Ltd. All rights reserved.