Polymer Testing 23 (2004) 51–57 www.elsevier.com/locate/polytest Test Method Characterization of the shear/compression failure envelope of Rohacell foam D. Benderly a,* , S. Putter b a Carmel Olefins R&D, P.O. Box 1468, Haifa 31014, Israel b Rafael, Structures and Solid Mechanics Group, Haifa 31021, Israel Received 24 February 2003; accepted 16 April 2003 Abstract Four-point flexural testing of sandwich structures is often used for the determination of the shear strength of the core material. The accuracy of this test was evaluated using finite-element modeling. The model results show that the measured average shear stress may be inaccurate due to shear stress variations within the sample and the presence of supermiposed compressive stresses. A modified experimental test method was developed to ensure a more homogeneous shear stress field. Using this test method, controlled combinations of shear and compressive stress can be generated within the polymeric foam, allowing evaluation of the shear/compression failure envelope. Results for a Rohacell polymeric foam show that, at the temperatures evaluated, the compression/shear failure envel- ope is elliptical. On a normalized plot of the failure envelope, the data for all temperatures coincide on a circular plot. Use of this failure envelope approach can reduce the amount of testing required to acquire design data.  2003 Elsevier Ltd. All rights reserved. Keywords: Sandwich structure; Foam; Core material; Rohacell; Flexural testing; Failure envelope; Analytical model 1. Introduction Lightweight sandwich constructions are frequently used in aeronautical, marine and construction appli- cations. These structural elements consist of two stiff facings (metallic or composite), a low density core (honeycomb or foamed material) and an adhesive layer between the core and each facing. A schematic of a sand- wich structure, with geometrical definitions, is shown in Fig. 1. By use of this type of structure, an increase in part thickness (by increasing core dimensions) can yield a significant increase in structural stiffness while not gre- atly increasing weight [1]. The overall flexural rigidity, * Corresponding author. Tel.: +1-972-4-832-6050; fax: +1- 972-4-822-6836. E-mail address: daphneb@alum.mit.edu (D. Benderly). 0142-9418/$ - see front matter  2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0142-9418(03)00061-8 Fig. 1. Schematic of sandwich structure. assuming that the modulus of the facings is much greater than the modulus of the core, is given by [2]: (EI) eq = E f bt 3 6 + E c bc 3 12 + E f bt(c + t) 2 2  E f btc 2 2 (1)