Mechanics of thin ultra-light stainless steel sandwich sheet material Part II. Resistance to delamination

This study concerns three variants of a novel type of thin sandwich sheet. Details of the core structures, and also the results of an investigation into elastic properties, were presented in the first part of this pair of papers. A study was also made of the tensile properties of single fibres of the type present in the core of these sheets. In this second paper, an investigation is presented of the resistance offered by these materials to delamination of the two faceplates. In one variant of the material, in which the fibres lie approximately normal to the plane of the sheet, delamination occurs predominantly by frictional pull-out of fibres from their sockets in the adhesive. The mode I fracture energy has been measured at about 340 J m(-2). This value is consistent with predictions from a model based on shear-lag theory, with a fibre-adhesive interfacial shear strength of about 5 MPa. It is noted that there should be scope for improving the fracture energy somewhat by raising the strength of the fibre-adhesive bond. For the other two variants studied, in which the fibres are softer (as a result of heat treatment during sintering) and are inclined close to the plane of the sheet, the measured fracture energy is appreciably lower at about 30 J m(-2). In this case, delamination occurs by fracture of the fibres near the mid-plane. Application of a simple model for prediction of the fracture energy in this case leads to the conclusion that some of the fracture was probably of sintered necks between fibres, rather than the fibres themselves, and that this process required considerably less energy. (C) 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.