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A recent issue of had an interesting exchange in its letters column. A member expressed concern that though he hadn't heard of any in-flight failures of fiberglass sailplanes, he was nevertheless concerned based on the following facts:

* Fiberglass and other composite construction is highly dependent on good workmanship. Very small separations, areas of uncured resin, or bubbles will drastically reduce the structural strength of a composite member.


* Most epoxy construction is subject to degradation by ultraviolet light. Enough ultraviolet is present in sunlight to cause degradation.

* The strength of composite structures is temperature-dependent. Many epoxies lose significant strength at temperatures around 200 degrees F. This temperature can be achieved inside a sailplane trailer on a summer day or inside a wing on an assembled ship. Repeated exposure to elevated temperatures can accelerate the natural aging process of composites.

* No wholly reliable method of determining the ultimate strength of composite structures exists, except for destructive testing. Furthermore, the only way of detecting many potential failure sites is by use of very expensive industrial x-ray equipment and a skilled technician to interpret the pictures.

He points out that "...a new technology may improve our certainty of the structural integrity of our aircraft. It is called acoustic emission testing. In this process, several microphones are attached to the structure in question. The structure is then loaded to about half its expected design load. A computer and spectrum analyzer listen through the microphones for the characteristic sounds emitted by potential failure sites. The weak point is located by timing the arrival of the acoustic signals at each microphone. The severity of the defect is determined by the number and magnitude of acoustic emissions received.

The telephone company used this method to test the integrity of "cherry-picker" booms. Since a wing spar failure may result in a fall much greater than the height of a telephone pole, it behooves us to look into the applicability of acoustic emission testing to aircraft construction and maintenance. The equipment is not inexpensive, but aren't our lives worth it?"

You can order a PDF or printed copy of QuickTalk #5 by using the Q-talk Back Issue Order Page.