ORIGINAL PAPER Long-term durability of CFRP foam core sandwich structures M. John • R. Schlimper • M. Rinker • T. Wagner • A. Roth • R. Scha ¨uble Received: 29 March 2011 / Revised: 12 August 2011 / Accepted: 12 August 2011 Ó Deutsches Zentrum fu ¨r Luft- und Raumfahrt e.V. 2011 Abstract The work presented in this article is directed towards the application of CFRP foam core sandwich structures as primary structures in commercial aviation. With closed cell rigid foams, it is possible to produce comparatively low priced high-integral sandwich compo- nents having a complex geometry in terms of a curved and a variable lateral cut. Sandwich structures are offering a good bending stiffness and strength to weight ratio. Thus, they are suited for using in structures which are at risk to fail by buckling (Herrmann et al. Sandwich structures 7: advancing with sandwich structures and materials, 2005). The investigations are focused on a CFRP sandwich structure with polymethacrylimide (PMI) foam core, named ROHACELL Ò RIST. Besides good structural sta- bility at thermal conditions, the foam is characterized by a good strength and stiffness to weight ratio (Seibert, Rein- forced Plast 50(1):44–48, 2006). Primary structures in aircraft applications are exposed to a superposition of in-service loads and environmental conditions. The typical working loads in combination with environmental condi- tions were investigated. The structure needs a sizing with respect to large temperature changes and influences of humidity. Thus, the time, temperature, and moisture dependency of the mechanical behavior were studied for the single components of the structure and for the com- posite itself. Therefore, Finite Element Models on macro- scopic level were built with reference to the experiments. For each in-service case, the residual stresses arising during manufacturing have to be regarded and quantified (John et al. ECCM14, 2010). During manufacturing, the sand- wich structure is cured at 180°C. Due to the different stiffnesses and coefficients of thermal expansion of the foam and the CFRP face sheets, residual stresses are induced by cooling down to service temperature. Among others, some tests were made at laterally closed CFRP sandwich structures with a storage time up to half a year at certain climate conditions. The aging process is not only controlled by external conditions, but also by a rear- rangement of molecules, for example, the relaxation behavior of the PMI foam (Gutwinski et al. Verb- undwerkstoffe. Wiley, Weinheim, 2009). Another question of the long-term behavior of the CFRP foam core sandwich structure is the characteristic of delamination of the face sheets from the inner core after an impact has occurred. To describe the crack growth behavior of the sandwich structure fracture mechanical principles can be used esti- mating the damage tolerance. Sandwich specimens with initial damage were loaded up to 3 million mechanical load cycles. Keywords CFRP foam core sandwich  Long-term durability  Residual stress  Relaxation 1 Introduction Sandwich structures are widely used in applications with stringent lightweight requirements. Foam core sandwich This paper is based on a presentation at the German Aerospace Congress, September 27–29, 2011, Bremen, Germany. M. John (&)  R. Schlimper  M. Rinker  T. Wagner  R. Scha ¨uble Fraunhofer Institute for Mechanics of Materials IWM, Walter-Huelse-Strasse 1, 06120 Halle/Saale, Germany e-mail: marianne.john@iwmh.fraunhofer.de A. Roth Evonik Ro ¨hm GmbH, Kirschenallee, 64293 Darmstadt, Germany 123 CEAS Aeronaut J DOI 10.1007/s13272-011-0028-2