|Quoting Klaus (Reply 1):|
How do you know how far the composite structure is actually damaged? Do you cut out pieces and subject them to microscopic and chemical testing?
There are a handful of established non-destructive inspection methods for composite laminates. Most maintenance checks only specify a general visual inspection for damaged fibers on the surface of the laminate. A more detailed first check is a "sonic tap test", where a trained mechanic will tap a heavy metal coin against the component, listening for any change in sound across the part where there may be damage. Where there is large delamination it is very easy to tell by the tap test (a good laminate will have a sharp tapping sound, a delaminated part will make more of a dull thud). Boeing's inspection manuals (at least for the 737, 757, and 767, possibly others too) provide guidelines for a proper tap-test, down to the required size and weight of the "coin" used to tap the part.
Where damage is suspected, inspectors can utilize thermal, ultrasonic, or X-Ray inspection. Thermographic inspection is used where moisture ingression is suspected. The part is heated in an oven, then brought out to cool at room temperature. Composite structure will cool rapidly, but trapped moisture will remain hot and show up on a thermal camera.
Ultrasonic is the best method to find exact damage. A tool is used which emits a sonic pulse and measures the return echo (very similar to SONAR, but on a smaller scale). Changes in the return pattern can indicate voids, defects, or damage.
I'm sure there are other inspection methods available, but these methods have been in place for decades to inspect composite components. The 787 requires inspection on a much larger scale, so Boeing may have developed entirely new procedures.
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