Aging of Composite Aircraft Structures
Dr. John Tomblin, Executive Director, NIAR and Sam Bloomfield Distinguished Professor of Aerospace Engineering
Lamia Salah, Senior Research Engineer, Manager, Fatigue and Fracture Laboratory
As more commercial and military airplanes are flying beyond their original design life, it has become necessary to answer questions about their continued airworthiness and structural integrity. Most of the aging aircraft studies have focused on metallic structures, but with the increasing use of composites in primary aircraft structures, it is crucial to address aging concerns for composite components.
The B-737-200 graphite/epoxy stabilizer was developed by The Boeing Company as part of the NASA Aircraft Energy Efficient (ACEE) program initiated in 1975. The purpose was to challenge aircraft manufacturers to redesign existing aircraft components using graphite/epoxy composites. Boeing manufactured 5 ½ shipsets, and received FAA Type Certification in August 1982.
The Beechcraft Starship program was officially launched in 1982. The objective was to produce the most advanced turboprop business airplane feasible at the time and to promote the use of composites in a business aircraft. The first Starship was flown Feb. 15, 1986. The second joined the test flight program in June 1986, and the third was ready for flight in the early spring of 1987. In the course of a two-year flight test program, these aircraft flew nearly 2,000 hours. The Starship achieved FAA Type Certification on June 14, 1987.
These aircraft are examples of two common structural arrangements for composite primary structure. The B-737-200 was mainly solid laminate construction; the Starship was primarily honeycomb sandwich construction. These aircraft are flying test beds for composite structural aging effects. They each flew for more than a decade and thousands of flight hours; this provides real life examples of the aging of composite structures. It is fortunate that these structures are available to examine for aging effects. This program uses these retired aircraft structures to determine the effects.
The primary program objective is to evaluate the aging effects on composite aircraft structures. It involves the investigation of two aircraft structures, a decommissioned Boeing 737 stabilizer that had a commercial service history of 18 years and a Beechcraft Starship with 12 years of service. The proposed research is divided into small sub-tasks to understand the aging mechanisms of the structures which includes the following:
- Conduct non-destructive inspection to identify defects induced during manufacture or service
- Investigate the structures for cracks, delaminations, damages, repair and if applicable bond integrity
- Identify possible changes in mechanical properties and resin chemistry
- Identify material degradation due to heat, humidity, ultraviolet (UV) radiation, oxidation, etc.
- Evaluate bearing conditions around holes and fasteners
- Investigate possible bearing failures or delaminations around the holes
- Evaluate effectiveness of repairs
The B737 composite horizontal stabilizer, shown in figures 1 and 2, consists of a co-cured skin and stiffener panel, 191 inches long and 50.5 inches wide at the root with stringers spaced 3.85 inches apart. Bolted titanium spar lugs—consisting of two titanium plates bonded and bolted externally to a pre-cured graphite-epoxy spar—were used to fasten the stabilizer to the fuselage center section.. Honeycomb ribs were used for simplicity in terms of tooling, fabrication and cost. The composite design yielded an average weight savings of approximately 21.6 percent with respect to the metal configuration or a final weight of approximately 206 lbs.
Figure (1) B-737 stabilizer configuration. Figure (2) B-737 composite stabilizer after 18 years of service. The Beechcraft pressure cabin is a sandwich construction consisting of only two parts, a right- and left-hand side bonded and riveted at the center section along the top and the bottom centerlines. The main wing is also a sandwich construction with no spanwise stringers, three spars and five ribs, as shown in Figure 3.
Figure (3). Beechcraft Starship aft and fwd wing cross sections. Figure (4). Beechcraft Starship Overall Dimensions (Courtesy of Beechcraft) The initial teardown of the B-737-200 horizontal stabilizer has shown little effects of the long service experience. The first mechanical and physical property tests have shown little difference in the values after 18 years and 52,000 flight hours. The mechanical and physical property testing of the stabilizer continues and, in conjunction with information gained from the Starship teardown and testing, the evaluation team will assess service performance and durability of aged composite structures.
