Andrew Doyle/LONDON
LOCKHEED MARTIN Tactical Aircraft Systems has performed an ultrasonic inspection of a large, complex, contoured composite component, by linking the inspection device to a computer-aided-design data file and generating a false external "surface".
Components with complex external shapes are notoriously difficult to inspect ultrasonically, because of the lack of parallel surfaces. The process depends on reflecting sound waves between the surfaces of a part and back to the source.
The company claims that it achieved an industry "first" while inspecting a horizontal-stabiliser pivot shaft for the Lockheed Martin/Boeing F-22 next-generation fighter aircraft.
The part hollow, 2.9m-long and made from graphite epoxy, was inspected using an LK UT90 ultrasonic scanning system.
To overcome the lack of parallel surfaces, the LK UT90's motor drive was linked to the CATIA-generated false external surface.
According to Mike Duncan, Lockheed Martin's manager of advanced programmes - quality, this meant that the inspection device remained at the optimum angle, in relation to the external surface of the part, to ensure that the ultrasonic sound waves were reflected from the inner surface of the hollow part and back to the device.
As a consequence, the device was not perpendicular to the external surface of the part. Therefore, says Duncan, the false surface was also required to compensate for the fact that sound waves were defracted as they crossed the external surface of the part.
Using the technique, the machine was able to perform a single-sided inspection of the pivot shaft while maintaining accuracies of 0.5° on three axes.
Although the process is vital to the F-22 programme, Duncan believes that it is unlikely to find applications in the civil sector. "You don't normally see composites this thick on an aircraft," he says.
The pivot shaft is manufactured by Magna, Utah-based Alliant Techsystems, and consists of 490 plies of composite tape which together measure up to 50mm thick at certain points.
Lockheed Martin, Boeing and Pratt & Whitney, are producing parts for the first F-22 engineering and manufacturing development aircraft, which is scheduled to be flown in 1997.
Source: Flight International
