EVALUATION OF DAMAGE IN FOAM SANDWICH COMPONENTS FOR AIRCRAFTS
Anuradha Dwarakanath*, C. R. L. Murthy*, S. Viswanath**, K. Dwarakanath**, R. M. V. G. K. Rao ***
*Department of Aerospace Engineering, IISc. Bangalore 560 012
** Structures Division, NAL, Bangalore
***F R P Division, N A L Bangalore 560 017
Keywords: Foam Sandwich, Acoustic Impact, Fokker Bond, Delaminations, Aerospace
ABSTRACT Foam sandwich construction is a light weight construction used for aerospace applications for example Trainer Aircraft. The panels or components are fabricated through molding process with foam as filler and either metal or FRP as facesheets. As the surface debonds and defects are very common phenomena which are detrimental to the performance of the structure, it is essential that they are nondestructively detected and evaluated. In fact, conventional and established methods such as ultrasonics and radiography can be utilised at fabrication stage itself. However, for field use, for example helicopter rotor blades it becomes imperative that the techniques and equipment should be simple, portable and reliable. Research carried over the past several years have brought out the utility of vibrational methods such as cointap and Fokker bond in this regard. However, as measurements with these methods are not absolute, it often becomes necessary to develop reference standards and conduct a large number of experiments for a reliable procedure yielding repeatable results. In the current set of investigations foam sandwich panels and some components of an aircraft comprising of two layer GRPF face sheets with polyurethane foam as filler were examined for detection of debonds and defects by Fokker Bond and Acoustic Impact Technique (AIT) (woodpecker). Known defects were introduced in the panels in terms of Teflon inserts and foam removal in addition to edge delaminations by inserting a teflon film and removing it after curing. Two such panels were subjected to acoustic impact tests and analysis was carried out in both time and frequency domains. These panels were ultrasonically scanned to obtain C-SCAN images as reference to evaluate AIT results. Further Fokker Bond testing was carried out on the same panels as also some critical joints on the actual component. The results obtained from these tests are presented and discussed in this paper.
Publication Source: Trends in NDE Science & Technology; Proceedings of the 14th World Conference on Non-Destructive Testing, New Delhi, 8-13 December 1996.Vol. 2, pages 523 - 528
Publisher: Ashgate Publishing Company
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