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Laser Inspection Determines Strength of Bonded Structures

by Dr. John Russell
AFRL, Materials and Manufacturing Directorate

Accomplishment: A breakthrough has been achieved in structural inspection techniques. For the first time, the ability to test for weak bonds and nondestructively determine a minimum strength of adhesively bonded aerospace structures has been proven. This method enables a wider use of bonded structures to achieve up to 25 percent fabrication/assembly and 75 percent life-cycle cost reductions.

Inspection of a bonded composite joint
A structural inspection is performed on a bonded composite joint in the vertical position. (Photo courtesy of LSP Technologies)

Payoff: Bonded structures offer many advantages to aerospace weapon systems: 1) lower acquisition costs; 2) lower life-cycle costs due to the elimination of fasteners; 3) enable smoother airflow over aero surfaces; and 4) enable lighter weight structures, which translates into increased range, speed, payload or loiter for advanced weapon systems requiring a low structural weight fraction, such as long-range strike, persistent ISR, and multi-role, all-environment mobility.

Inspection head device
The top view of the inspection head device. (Photo courtesy of LSP Technologies)

Background: The Air Force Research Laboratory’s (AFRL) Materials and Manufacturing Directorate (ML) and the Air Vehicles Directorate initiated the Composites Affordability Initiative (CAI), which focused efforts on reducing costs and increasing structural efficiency through bonded and integrated structures. Bonded structures are used for a variety of secondary applications; however, several factors have inhibited the use of efficient bonded primary structures. One major hurdle has been the lack of a nondestructive technique to assess the strength of a bonded joint. Boeing, a CAI team member, led the quality assurance technology effort and has developed a laser bond inspection technique (patent pending).

High peak-power, short pulse-length laser excitation can generate stress waves that can be used to discriminate between weak and strong bonds in graphite-epoxy composite-to-composite structures. The technique can identify variations in surface preparation techniques, levels of surface contamination, and/or changes in paste adhesive mixing. In more than 3000 laser stress-wave experiments, this approach has been found to be repeatable and reliable in detecting weak versus strong joints. Such an approach offers a potentially cost-effective method to be certain of a minimum predetermined load-carrying capability of a bonded joint after manufacture or in-service.

A production floor laser bond inspection device is being developed and optimized in a Small Business Innovative Research program with LSP Technologies sponsored by AFRL/ML.

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Spring 2006
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