Adherent preparations, curing process play important roles in imparting the adhesion and cohesion strengths respectively to the bond. Generally in metallic honeycomb sandwich structure's fabrication process, the adherents are prepared for good adhesion characteristics through fine emerying, degreasing/ultrasonic cleaning and chemical cleaning. Similarly proper processing of adhesive results in good cohesion strength of the bond. Hence process variation in both adherent preparation and curing cycle will effect the final bond strength of the sandwich.

It is very essential to inspect the adherents for their good adhesion characteristics and final bonded sandwich products for their cohesion quality. Various NDT techniques are in vogue to evaluate the quality of bond after it is cured. But most of them are of qualitative type. Further to inspect the adhesion quality of the bond, no NDT tool is available. Hence it is imperative to inspect the adherents at an intermittent stage for checking their adhesion quality and the final product for the cohesion quality.

This paper presents various NDT techniques adapted for the metallic sandwich products of Honeycomb Laboratory, VSSC. For characterization and inspection of aluminum adherents for their adhesion quality evaluation, extensive use of Fokker contamination tester (FCT), contact angle analyser, scanning electron microscope (SEM) was carried out. For cohesive quality evaluation quantitatively, extensive use of Fokker bond tester (FBT) was made, which is based on ultrasonic resonance impedance principle. Other NDT techniques adapted, were X-ray radiography, holography, etc., which are useful in locating defects like voids, debonds, core damages, integrity of inserts bonds, etc. NDT studies are conducted for both flat and curved sandwich structures, which will be finally used in aerospace products of launch vehicles.