An Insider's Look At The Aging Aircraft Conference

The Decomposition Of Composites

The annual Aging Aircraft Conference (AAC) is the premier exposition on leading edge technology and research in the critical area of aging commercial and military aircraft. The 2007 AAC, held April 16-19, attracted over 1,000 leading scientists, designers, engineers and program managers for government, industry and academia, who unveiled their new technologies, techniques and findings.

This time around, the focus widened further than corrosion, cracking and wiring issues and looked at where the industry thinks it is in coping with the new generation of composite structures. With the experience of a tail-fin loss on American Airlines Flight AA587 and a rudder separation on Air Transat Flight 961, uncertainty about the long-term structural integrity of composite build still prevails in some quarters.

The USAF expressed confidence in its systems-engineering approach to durability, damage tolerance and battle damage repair in hybrid airframe structures. The wider aviation community confidently anticipated that the incidence of ramp damage would not decrease in future. Therefore, the subject of composite airframe downtime and timely damage repair schemes was naturally uppermost in the CFO mindset.

Specialist anisotropic patching is the main consideration. The repair patch is considered as a repaired plate sitting under a combined loading of in-plane shearing and stretching, as well as out-of-plane bending. Load carriage around the boundary of an elliptical plate of varying aspect ratio is apparently the determinant of an adequately sized repair scheme.

Analytical modeling for determining if repairs are indeed required was the other consideration. It's accepted that bent, torn or deeply scratched metal must be repaired, but that's a comparatively easy exercise in metal-cutting, -bashing and riveting. However, with composites, of equal concern is the blunt force trauma situation, particularly the unreported or unnoticed ding that may leave no evident surface damage.

The SAE Commercial Aircraft Composite Repair Committee's charter to establish industry consensus on standards for composite materials inspection, maintenance and repair was covered.

Coupon-level testing, proceeding through scale-up testing to full-scale curved panel tests of sandwich composite structures program has to date used a blend of comprehensive testing projects and analytical methods. FAA is developing a specialist understanding of (and baseline data for) the residual strength of sandwich composite structures damaged in service or found to have manufacturing defects.

With the looming introduction of the all-composite Boeing 787 "Dreamliner", testing full-scale fuselage panel specimens under conditions representative of those seen by an aircraft in actual operations requires the capability of applying pressurization, axial, hoop, and shear loads.

The sandwich structural configuration in an airframe experiences in-plane loads, while damaged regions may experience localized out-of-plane bending and bulging due to pressurization loading. During testing to destruction, acoustic emission methods have been used to monitor for damage growth in real-time and served as an early warning for imminent failure.

Nondestructive inspection methods are used to scan for non-visual damage. These techniques include pulsed thermography and computer-aided tap testing. Composite fuselages are typically without intermediate stiffeners or frames to contain damage growth after a catastrophic event, such as an uncontained turbine blade-spit. Further validating test data and projections from coupon and element research is needed to provide an assessment of sandwich damage tolerance and design principles for use in full-scale composite fuselages.

In-service damage aside, as more composite components are being certified and used on primary structure, there's a need to investigate the aging characteristics of these composite structures.

The methodology includes: changes in mechanical properties using coupon- and element-level testing; degradation in physical properties and resin chemistry; structural cracks, delaminations, disbonds, impact damage, repair, and bond integrity; the longevity of repairs, material deterioration due to fluid contamination, heat, humidity, pressure altitude, ultraviolet (UV) radiation, and oxidation; integrity of load-bearing environments around holes and fasteners; possible bearing failures or delaminations around the holes; and possible changes in fracture toughness due to aging (moisture exposure, heat, UV, etc.). A fleet of demobilized Beechcraft Starships is used in this study.

A separate research program is assessing the effects of different variables on the static and repeated load performance of scarf repairs applied to composite laminate and sandwich structures, especially when a faulty process has been implemented and remained detected by Non Destructive Inspection). Taper-sanded or scarf repairs are currently the preferred method for repairing thick composite structures to restore the load-carrying capability to its as-designed level.