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Development program to certify composite doubler repair technique for commercial aircraft

Description: Commercial airframes exceeding 20 service years often develop crack and corrosion flaws. Bonded composite doublers offer a cost effective method to safely extend aircraft lives. The Federal Aircraft Authority (FAA) has completed a project to introduce composite doubler repair technology to the commercial aircraft industry. Instead of riveting steel or aluminum plates for repair, a single composite doubler may be bonded to the damaged structure. Adhesive bonding eliminates stress concentrations caused by fastener holes. Composites are readily formed into complex shapes for repairing irregular components. Also, composite doublers can be tailored to meet specific anisotropy needs, eliminating structural stiffening in directions other than those required. Other advantages include corrosion resistance, a high strength-to-weight ratio, and potential time savings in installation. One phase of this study developed general methodologies and test programs to ensure proper performance of the technique. A second phase focused on reinforcement of an L-1011 door frame, and encompassed all lifetime tasks such as design, analysis, installation, and nondestructive inspection. This paper overviews the project and details the activities conducted to gain FAA approval for composite doubler use. Structural tests evaluated the damage tolerance and fatigue performance of composite doublers while finite element models were generated to study doubler design issues. Nondestructive inspection procedures were developed and validated using full-scale test articles. Installation dry-runs demonstrated the viability of applying composite doublers in hangar environments. The project`s documentation package was used to support installation of a Boron-Epoxy composite repair on a Delta Air Lines L-1011 aircraft. A second product of the results is a Lockheed Service Bulletin which allows the door corner composite doubler to be installed on all L-1011 aircraft. 9 refs., 10 figs., 2 tabs.
Date: July 1, 1997
Creator: Roach, D.P.
Partner: UNT Libraries Government Documents Department

Ultrasonic inspection technique for composite doubler/aluminum skin bond integrity for aircraft

Description: As part of the FAA`s National Aging Aircraft Research Program to foster new technologies for civil aircraft maintenance and repair, use of bonded composite doublers on metal aircraft structures has been advanced. Research and validation of such doubler applications on US certified commercial aircraft has begun. A specific composite application to assess the capabilities of composite doublers was chosen on a L-1011 aircraft for reinforcement of the comer of a cargo door frame where a boron-epoxy repair patch of up to 72 plies was installed. A primary inspection requirement for these doublers is the identification of disbonds between the composite laminate and the aluminum parent material. This paper describes the development of an ultrasonic pulse echo technique using a modified immersion focus transducer where a robust signal amplitude signature of the composite aluminum interface is obtained to characterize the condition of the bond. Example waveforms and C-scan images are shown to illustrate the ultrasonic response for various transducer configurations using a boron-epoxy aluminum skin calibration test sample where disbonds and delaminations were built-in. The modified focus transducer is compatible with portable ultrasonic scanning systems that utilize the weeper or dripless bubbler technologies when an ultrasonic inspection of the boron-epoxy composite doublers installed on aircraft is implemented.
Date: February 1, 1998
Creator: Gieske, J.H.; Roach, D.P. & Walkington, P.D.
Partner: UNT Libraries Government Documents Department

Results from FAA program to validate bonded composite doublers for commercial aviation use

Description: The number of commercial airframes exceeding twenty years of service continues to grow. In addition, Service Life Extension Programs are attempting to extend the {open_quotes}economic{close_quotes} service life of commercial airframes to thirty years. The use of bonded composites may offer the airframe manufacturers and aircraft maintenance facilities a cost effective method to extend the lives of their aircraft. The Federal Aviation Administration Assurance NDI Validation Center (AANC) to validate the use of bonded composite doublers on commercial aircraft.
Date: September 1, 1997
Creator: Roach, D.P.
Partner: UNT Libraries Government Documents Department

Development of an underwater spin facility for combined environment testing

Description: In response to a request from the US Government, Sandia National Laboratories has developed an instrumentation system to monitor the conditions along an underwater, rotating drive shaft. It was desired to study the structural integrity and signal acquisition capabilities of the Shaft Instrumentation System (SIS) in an environment which closely simulates the actual deployment conditions. In this manner, the SIS response to ill-defined conditions, such as flow field turbulence or temperature fluctuations, could be determined. An Underwater Spin Facility was developed in order to verify the operation of the instrumentation and telemetric data acquisition system in a combined environment of external pressure, transient axial loads and centrifugal force. The main components of the Underwater Spin Facility are a large, five foot diameter pressure vessel, a dynamically sealed shaft, a drive train assembly and a shaker table interface which is used to apply the axial loads. This paper presents a detailed description of the design of the Underwater Spin Facility. It also discusses the SIS certification test program in order to demonstrate the successful performance of the Underwater Spin Facility. 8 refs., 10 figs.
Date: January 1, 1991
Creator: Roach, D. P. & Nusser, M. A.
Partner: UNT Libraries Government Documents Department

Experimental and analytical program to determine strains in 737 LAP splice joints subjected to normal fuselage pressurization loads

Description: The Federal Aviation Administration Technical Center (FAATC) has initiated several research projects to assess the structural integrity of the aging commercial aircraft fleet. One area of research involves the understanding of a phenomenon known as ``Widespread Fatigue Damage`` or WFD, which refers to a type of multiple element cracking that degrades the damage tolerance capability of an aircraft structure. Research on WFD has been performed both experimentally and analytically including finite element modeling of fuselage lap splice joints by the Volpe Center. Fuselage pressurization tests have also been conducted at the FAA`s Airworthiness Assurance NDI Validation Center (AANC) to obtain strain gage data from select locations on the FAA/AANC 737 Transport Aircraft Test Bed. One-hundred strain channels were used to monitor five different lap splice bays including the fuselage skin and substructure elements. These test results have been used to evaluate the accuracy of the analytical models and to support general aircraft analysis efforts. This paper documents the strain fields measured during the AANC tests and successfully correlates the results with analytical predictions.
Date: February 1, 1996
Creator: Roach, D.P. & Jeong, D.Y.
Partner: UNT Libraries Government Documents Department

Overview of composite projects at the FAA Airworthiness Assurance Validation Center

Description: The Airworthiness Assurance NDI Validation Center (AANC) was established by the Federal Aviation Administration (FAA) William J. Hughes Technical Center at Sandia National Laboratories in 1991 to support nondestructive inspection (NDI) technology development and assessment. The evaluations are done using a variety of characterized test specimens and test beds including entire transport and commuter aircraft. Although the initial work at the Center concentrated on metallic structure, the FAA has more recently expanded the AANC`s charter to include projects directed at composite repair and inspection. The three projects briefly described in this paper are: (1) the validation and technology transfer of a thermographic techniques for composite inspection, (2) the development of generic composite laminate and honeycomb calibration reference standards, and (3) the certification of the use of boron epoxy doubler on a Lockheed L-1011.
Date: March 1, 1997
Creator: Shurtleff, W.W.; Roach, D.P. & Valley, M.T.
Partner: UNT Libraries Government Documents Department