Non-Destructive Testing & The Link Between Environmental Degradation & Mechanical Properties of Composite Honeycomb Panels

Following the in-service failure of a Canadian CF-18 Hornet rudder, a fleet wide inspection was performed using Infrared Thermography. This resulted in several rudders being removed from service due to excessive indications of moisture ingress. The United States Navy, Spanish Air Force and the Canadian Forces have reported occurrences of CF-18 honeycomb core bond line failures. The cause of the disbonding is not completely understood, but is believed to be a result of moisture ingress through the laminate bondline and sealant. Moisture has been detected in various honeycomb components including trailing edge flaps, ailerons and especially rudders where large deposits of water have been found in the honeycomb core cells. Moisture ingress into these sandwich structures is believed to occur by one of two methods, via diffusion though the bulk adhesive layer or by wicking along the adhesive-adherend interface. Once the moisture gains access to the honeycomb cells, it leads to corrosion, bond degradation and eventual failure of the component. Current inspection of these components is difficult with conventional techniques such as CScan and Radiography which typically require long down times or component removal. Thermography has been used in recent efforts as an alternative to conventional methods, allowing for in-situ inspection and little-to-no aircraft down time. However, both for Thermography and conventional NDT techniques, the question still remains as to the exact nature of the indications and whether or not the component remains serviceable. To make this connection, an effort has been put forth to link these NDT indications to the mechanical properties and bond-line condition of these bonded honeycomb components. Testing has been performed on a series of CF-18 honeycomb rudders in an attempt to develop a correlation between and the degraded mechanical properties and various types of NDT inspection indications, providing for a better method for assessments of serviceability of composite honeycomb panels.