FAA's CECAM begins Aging of Composite Structures study
The U.S. Federal Aviation Admin.'s (FAA) Center of Excellence for Composites and Advanced Materials (CECAM) at Wichtia State University's (WSU) National Institiute for Aviation Research (NIAR) has initiated a study of the effects of aging on aricraft structures. Data generated by the program will provide a better understanding of the aging of composite aircraft structures. Ultimately, the FAA will use the data to assess the efficacy of its current/emerging certification methods. It also will be used to issue policy pertaining to usage of composites with respect to aging factors.
The Aging of Composite Aircraft Structures study will be conducted by principal investigators Dr. John Tomblin, executive director, NIAR; Sam Bloomfield, WSU distinguished professor of aerospace engineering; and Lamia Salah, senior research engineer and manager, Fatigue and Fracture Laboratory.
CECAM's study overview notes that as more commercial and military airplanes fly beyond their original design life, the FAA has found it necessary to answer questions about their continued airworthiness and structural integrity. Most aging aircraft studies, however, have focused on metal structures. The increasing use of composites in primary aircraft structures makes it crucial to address aging concerns for composite components, CECAM maintains.
Objectives
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 subtasks to understand the aging mechanisms of the structures which includes the following:
- Conduct nondestructive 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.
Background
The B-737-200 carbon/epoxy stabilizer was developed by The Boeing Co. (Seattle, Wash.) 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 carbon fiber/epoxy composites. Boeing manufactured five-and-one-half 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 is mainly solid laminate construction; the Starship is primarily honeycomb sandwich construction. These aircraft are flying test beds for composite structural aging effects, says CECAM. Each flew for more than a decade, accruing thousands of flight hours.
Test articles
The B737 composite horizontal stabilizer consists of a cocured skin and stiffener panel, 191-inches/485-cm long and 50.5-inches/128-cm wide at the root with stringers spaced 3.85 inches/9.78 cm apart. Bolted titanium spar lugs — consisting of two titanium plates bonded and bolted externally to a pre-cured carbon/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 lb/93.4 kg.
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 also is a sandwich construction with no spanwise stringers, three spars and five ribs.
Although work on the study has just begun, investigators have observed little visual evidence of degradatory effects.
Initial observations
According to CECAM, the initial teardown of the B-737-200 horizontal stabilizer and early 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.
