Shear and mode II fracture of PUR foams Liviu Marsavina a, ⁎, Dan M. Constantinescu a,b , Emanoil Linul a , Tudor Voiconi a , Dragos A. Apostol a,b a Politehnica University of Timisoara, Blvd. M. Viteazu, No. 1, Timisoara, Romania b Politehnica University of Bucharest, Splaiul Independentei, No. 313, Bucharest, Romania article info abstract Article history: Received 18 December 2014 Received in revised form 16 March 2015 Accepted 5 May 2015 Available online 6 June 2015 Polyurethane (PUR) foam materials are widely used as cores in sandwich composites, for pack- ing and cushioning. They are made of interconnected networks of solid struts and cell walls in- corporating voids with entrapped gas. The main characteristics of foams are lightweight, high porosity, high crushability, and good energy absorption capacity. Fracture toughness in mixed mode loading is of particular interest because foam cracking weakens the structure’s capacity for carrying loads. Present paper assesses the shear elastic (shear modulus) and mechanical (shear strength) properties of polyurethane foams. Also, three different types of specimens were used to deter- mine mode I and mode II fracture toughness. The shear modulus, shear strength and fracture toughness increases with increasing foam density. Also the effect of loading direction and load- ing speed is investigated. The authors propose a micromechanical model to estimate fracture toughness based on the tensile strength of the solid material and the topology of the cellular structure. © 2015 Elsevier Ltd. All rights reserved. Keywords: Polyurethane foams Fracture criteria Fracture toughness Mixed mode loading 1. Introduction Sandwich structures are widely used due to their ability to provide high stiffness coupled with light weight. Because of this, sandwich panels are often used in applications where weight saving is critical: in aviation applications in recent years, for isola- tions, helicopter rotor blades, tail and wing components. The core of such structures is often made of plastic foams Polyurethane (PUR), Polyvinylchloride (PVC), Polymethylmethacrylate (PMMA) or metallic foams. Failure of sandwich structures occurs due to: – the yielding or fracture of the faces [1,2]; – the compressed face may “wrinkle” by local buckling of the skin into the core, or it may “dimple” by a local buckling of the compression face [3]; – the core can fail, usually in shear, Fig. 1a, but tensile, compression or local crushing can also occur [4]; – the bond between the face and the core can fail; and since resin adhesives are usually brittle, debonding is by brittle fracture [5,6]; – the sandwich component can fail by indentation of the faces and core at the loading point [1]. Engineering Failure Analysis 58 (2015) 465–476 ⁎ Corresponding author. Tel.: +40 256403577; fax: +40 256403523. URL: msvina@mec.upt.ro (L. Marsavina). http://dx.doi.org/10.1016/j.engfailanal.2015.05.021 1350-6307/© 2015 Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Engineering Failure Analysis journal homepage: www.elsevier.com/locate/efa