Dr. Ronald Krueger, Associate Research Fellow at the National Institute of Aerospace (NIA), co-organized and attended a joint CMH-17-EASA-FAAWorkshop on Sandwich Disbonding held at the Technical University of Denmark (DTU) in Copenhagen, July 9-12, 2019. He gave a presentation CMH-17 Disbond Growth Team: Coordinated efforts to Address Sandwich Face Sheet/Core Disbondingand led the following discussion. He also presented High Fidelity Modeling of Face Sheet/Core Disbondingon behalf of Nelson Carvalho who could not attend the workshop. Profs. Leif Carlsson (Florida Atlantic University) and George Kardomateas (Georgia Tech) also presented results from the FAA funded project Study of Damage Modes in Lightweight Sandwich Structures Using Analysis and Testing.

During the workshop, an international group of 35 experts discussed face sheet/core disbonding in honeycomb sandwich constructions, which poses a threat to the structural integrity of a sandwich component but can be controlled through design, including damage tolerance principles, and maintenance procedures. This damage mode is of particular interest to the certification authorities such as the FAA and EASA since it was evident in several in-service occurrences, such as a rudder structural failure. Additionally, other control surface challenges, including lost strength and fluid ingression, have had relationships with disbonding. Past studies funded by the FAA and EASA have focused on face sheet/core disbonding which can lead to damage propagation caused by internal pressure changes in the core due to ground-air-ground cycles.

Sandwich structures are also common in rotorcraft and general aviation that do not fly at high altitudes.  Further, sandwich monocoque structures have failed in a way that demonstrated the need to pay particular attention to disbond damage modes in single load path primary structure applications.

Future composite structure applications–for example, composite sandwich construction of the fuselage of business jets that experience higher altitudes than transport aircraft and new urban mobility vertical takeoff & landing aircraft products–are also driving a need to understand the phenomenon of disbond growth under generalized load conditions including advanced maneuvers and gust conditions.

To identify, describe and address the phenomenon, the Sandwich Disbond Growth Team was established as a group of experts within the CMH-17 Disbond and Delamination Task Group in 2011 following a similar workshop held at the National Institute of Aerospace. The Composite Materials Handbook (CMH-17)creates, publishes and maintains proven, reliable engineering information and standards, subjected to thorough technical review, to support the development and use of composite materials and structures. Currently, the group is working to identify sandwich structural design details where sandwich disbonds initiate and other real structure scenarios, which may promote growth.  The goals of such studies are to evaluate potential sandwich disbonding and establish design guidelines that eliminate the growth potential or, at least, minimize the effect on structural integrity. The ultimate goal is to develop and provide tools to minimize sandwich disbonding and help mitigate any safety-related concerns.

Stakeholders in this activity are the certification authorities, original equipment manufacturers, suppliers, universities as well as government research facilities and associated contractors.