AFRL and Boeing have developed an innovative structural inspection technique for testing bonded structures. This technique provides materials engineers a first-time, validated, nondestructive capability to test for weak adhesive bonds and determine the minimum strength of adhesively bonded aerospace structures. The new method will enable aircraft designers to use bonded structures in a wider range of applications, and industry analysts predict it will provide both a 25% reduction in fabrication and assembly time and a 75% reduction in life-cycle costs.

Among the advantages that bonded structures offer aerospace weapon systems designers are lower acquisition costs, lower life-cycle costs (due to the elimination of fasteners), and smoother airflow over aerodynamic surfaces. Bonded structures also weigh less than conventional fabrications, resulting in increased range, speed, payload, and loiter capability for advanced weapon systems. The reduced structural weight is of particular importance to systems designed for long-range strike; persistent intelligence, surveillance, and reconnaissance missions; and multirole, all-environment mobility.

Technician uses the laser-based bond inspection device to evaluate a bonded composite joint in the vertical position. (photo courtesy of LSP Technologies)

AFRL engineers launched the Composites Affordability Initiative (CAI) to focus design and manufacturing efforts on the use of bonded and integrated structures to reduce costs and increase structural efficiency in aircraft structures. Designers regularly employ bonded structures for a variety of secondary applications; however, several factors have previously inhibited their use in primary structures. One of the major inhibitors has been the lack of a nondestructive technique for assessing the strength of bonded joints. Boeing engineers led the CAI team's quality assurance technology effort, which subsequently produced the new laser bond inspection technique (patent pending).

Boeing researchers used high-peak-power, short-pulse-length laser excitation to generate stress waves in graphite epoxy, composite-to-composite structures (see figure). By interpreting the waves as they move through the structure, technicians can discriminate between weak and strong adhesive bonds in these structures. Using the laser-based bond inspection technique, they can also identify variations in surface preparation techniques, levels of surface contamination, and changes in paste adhesive mixing. The research team conducted over 3,000 laser stress wave experiments to validate the technique's repeatability and reliability in discriminating between weak and strong adhesive joints. This inspection technique has potential as a cost-effective method for ensuring that a bonded joint has a minimum load-carrying capacity immediately after its manufacture and that the joint can maintain this capacity throughout its service life.

With the help of two Small Business Innovation Research contracts, LSP Technologies, Inc., is pursuing further development of the laser bond inspection device to enable its use on the production floor. The successful introduction of this technology into standard manufacturing processes will enable the confident use of adhesively bonded structures in a variety of high performance applications.

Mr. Paul B. Hauwiller (Anteon Corporation), of the Air Force Research Laboratory's Materials and Manufacturing Directorate, wrote this article. For more information, contact TECH CONNECT at (800) 203-6451 or place a request at http://www.afrl.af.mil/techconn_index.asp . Reference document ML-H-05-08.