Ž . Materials and Design 21 2000 217236 Springback and fracture in v-die air bending of thick stainless steel sheets Nader Asnafi  Granges Technology, SE-612 81 Finspang, Sweden ¨ ˚ Received 2 June 1999; accepted 5 July 1999 Abstract In this investigation, the attention is focused on the springback and fracture of thick stainless steel sheets. Nine different stainless grades and various thicknesses are tested. The thinnest sheet is 7.9 mm, whilst the thickest sheet is 31.3 mm. A consistent analytical model is constructed for prediction of the springback, the inner sheet radius prior to and after unloading, and the smallest die width. The springback calculated by this analytical model is in all cases smaller than that found experimentally. The correspondence between theory and practice is, however, very good, although the shift in the position of the neutral axis and thinning are neglected in the theoretical analysis. Fracture did not occur in any of the conducted bending operations. It is commonly assumed that fracture in v-die air bending is related to the reduction in the cross-section area at fracture, Z, in tensile testing. Z was greater than 70% for the majority of the studied materials. It is shown that particularly the Ž . mode of fracture fracture through shear bands or by necking should be studied in future investigations.  2000 Elsevier Science Ltd. All rights reserved. Keywords: Springback; Fracture; Stainless steel; V-die bending; Sheet metal forming; Modelling 1. Introduction Plastic bending has been addressed in many scientific papers. In contrast to many other published theoretical investigations into plastic bending, this paper does not  comprise an account of the works conducted by Hill 1 ,   Proska 2 , Shaffer and Ungar 3 , Lubahn and Sachs    4 , Crafoord 5 , Verguts and Sowerby 6 , Dadras and   Majlessi 7 , and Queener and de Angelis 8 , etc. Those interested in a literature survey of the analytical models constructed for plastic bending are referred to  Asnafi 9 . The aim of this study was to construct and test a consistent mathematical model, which can be used by  Tel.: 46-122-170-00; fax: 46-122-124-87. Ž . E-mail address: nader.asnafi@techno.graenges.se N. Asnafi . Technical Customer Ser  ice or Application Support at material suppliers for quick estimations of the: Ž .  smallest bending radius fracture ;  springback; Ž .  least die width fracture ; and  maximum required bending force. In the majority of the published papers studied by the author, the parameters above are treated sepa- rately or it is, for instance, assumed that the bending radius is equal to the punch radius. Therefore, the term consistent is italicised above. The inner sheet radius at the end of air bending Ž . before unloading , R could be classified as one of the i Ž . most significant parameters in v-die bending Fig. 1 . This radius determines the strain level in the bending region, and thereby, whether there is a risk of fracture. 0261-306900$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. Ž . PII: S 0 2 6 1 - 3 0 6 9 99 00051-5