Dual vacuum stressing technique for holographic nondestructive testing of honeycomb sandwich panels

Dual vacuUm stressing technique for holographic nondestructive testing of honeycomb sandwich panels M.V. Rao, R. Samuel and K. Ramesh Holographic nondestructive testing (HNDT) was carried out to identify the debond areas in flat honeycomb sandwich test coupons and large panels made of aluminium facesheet and different film adhesives ( Redux 31 2, 31 2-L and 31 2-UL). The details of the development and establishment of a new (dual) vacuum stressing technique for H N D T of honeycomb panels are presented. It is found that the detectability of debonds in honeycomb panels using the dual vacuum stressing technique is independent of the facesheet material and film adhesive system used for manufacturing the panels.

Keywords: holographic N DT, debond detection, honeycomb sandwich panels, dual vacuum stressing technique

Spacecraft structural components such as deck plates, shells, solar panel substrates, antenna dishes, etc are made of honeycomb sandwich construction because of its light weight and high stiffness characteristics. Debonds in these honeycomb sandwich components are known to occur either during the fabrication process or testing. Detection of debonds in honeycomb sandwich components is vital to assess the integrity of spacecraft. Holographic non-destructive testing (HNDT), because of its high displacement sensitivity (0.3/~m), whole field and. non-contact nature, has been found suitable and useful for the inspection of honeycomb sandwich panels[1-4]. Marchant[1], Enderberg et al t2] and Querido [3] reported H N D T works on honeycomb panels made of carbon-fibre-reinforced plastic (CFRP) facesheet and ultralight film adhesive ( 100 g m - z). These HNDT results using the thermal stressing technique were good because honeycomb panels made of CFRP facesheEts have low thermal conductivity. Vest t4] has reported that both thermal and vibration stressing techniques are normally used for H N D T of honeycomb panels. H N D T studies at our centre 5 on fiat honeycomb sandwich test coupons and large panels have shown that the facesheet material, film adhesive system and size of the honeycomb panel affect the detectability of debonds by thermal and vibration stressing techniques. The test coupons (size 200 mm x 200 mm) were made of aluminium alloy/CFRP facesheet and different film adhesives of various weight values (CIBA Redux 312, 312-L and 312-UL with weight values of 300, 200 and 100 g m -z respectively). HNDT results on test coupons have shown that the thermal stressing technique may be used for honeycomb panels made of

aluminium facesheet with thick film adhesive only or panels made of C F R P facesheets. Also, the detectability of debonds in test coupons by the vibration stressing technique is independent of the facesheet material and film adhesive system. However, HNDT studies on large honeycomb panels (size 1500 mm x 1600 mm) made of aluminium facesheet and the light and ultralight film adhesives Redux 312-L and 312-UL using thermal and vibration techniques have shown poor results. In the case of thermal stressing, the fast heat transfer through the less filleting available (at the interface of the facesheet and the honeycomb core cell wall) in light and ultralight film adhesives affects the detectability of the debonds. In the vibration stressing technique, exciting a large honeycomb panel at higher frequencies is difficult because of the inherently higher damping and attenuation in the panels. Another disadvantage of vibration stressing is that one has to observe carefully in the real-time hologram and scan through the required frequency range (approx 100 Hz to 100 kHz) to identify various sizes of debonds. Hence there is a need for an efficient stressing method for HNDT of honeycomb panels. HNDT work using single and dual vacuum stressing techniques with external window attachments has shown very good results for both test coupons (size 200 mm x 200 mm) and larger honeycomb panels (size 290 mm x 1100 mm and 1500 mm x 1600 mm) [61. At the same time, similar work was reported by Rubayi and Liew[7] using the single vacuum stressing technique for testing a composite

0308-91 26/90/050267-04 © 1990 Butterworth-Heinemann Ltd NDT International Volume 23 Number 5 October 1990

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Holographic set-up A standard off-axis reference beam arrangement (Figure 1) with a 50 mW helium-neon laser was used for the holographic set-up. All the holograms were recorded using a thermoplastic recording camera.

Single vacuum stressing technique The set-up for the single vacuum stressing technique is shown in Figure 2. The transparent window was rigidly fixed to the holographic table. The honeycomb test coupon (Figure 3) with simulated debonds (adhesive missing) of various sizes (24, 18 and 12 mm diameter) was attached to the window by applying an initial vacuum of about 200 mm of mercury. A double exposure hologram was recorded after applying a differential vacuum of 50 mm of mercury (Figure 4). The debonds were seen along with annular rings due to the overall diaphragmtype deformation of the panel. The smaller debonds of size 18 and 12 mm diameter were seen only as kinks along with the annular ring background.

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at the rear side of the same panel and interconnecting the vacuum line to this as shown in Figure 5. A double exposure hologram was taken after applying a differential vacuum level of 50 m m of mercury. Figure 6 displays the hologram corresponding to the dual vacuum stressing technique where all the debonds, including the smallest, were seen very clearly. As expected, the annular rings were not observed since the vacuum became equal on both sides of the panel. In order to check the sensitivity of this technique, a test coupon made of aluminium facesheet and film adhesive Redux 312-L with four debonds (adhesive missing) of equal size (24 m m diameter) was tested by the dual vacuum stressing technique. A

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Fig. 6 Hologram for dual vacuum stressing (AP=50 mm Hg, Debonds 24, 18, 12 mm diameter)

double exposure hologram was taken after applying a differential vacuum of 10 m m of mercury and is shown in Figure 7. The debonds were seen very clearly and this proved that the debond size could be calibrated against the differential vacuum fol" the observation of the various debond sizes.

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independent of the film adhesive system and the facesheet material used for manufacturing the panels.

Acknowledgements The valuable suggestions given by Dr P.S. Nair, Head, Structures Division during discussions are sincerely acknowledged. The encouragement given by Mr A.V. Patki, Group Director, Mechanical Systems Group for this work is acknowledged.

References

Fig. 7 Hologram for dual vacuum stressing (AP=10 mm Hg, Debonds 24 mm diameter)

Conclusions The dual vacuum stressing technique is found to be very effective for H N D T of honeycomb panels. The debond sizes can easily be calibrated in terms of the applied differential vacuum. It is found that the detectability of debonds in honeycomb panels using this technique is

1 Marehaut,M.J.'Holographic interferometryfor CFRP honeycomb panels' RAE-TR-73192 (1973) 2 Enderherg,N.P. et al'Establishment of non-destructive inspection techniques for honeycomb sandwich with carbon fibre composite face sheets by means of holographic methods' ESTEC Contract 1957/73 (1978) 3 Querido, R.J. 'New applications of holographic non-destructive testing of advanced composite materials in aerospace construction' 11th Worm Conf on N D T (3-8 November 1985) pp 461-468 4 Vest, C.M. 'Status and future of holographic non-destructive evaluation' SPIE 349 (1982) pp 186-198 5 Rao, M.V. and Samuel, R. 'Effect of film adhesive thickness on HNDT of honeycombsandwich panels' SEM Fall Conf(1988) 6 Rao,M.V.and Samuel,R. 'Holographicinspectionoflargehoneycomb sandwich panels by vacuum stressing technique' Doc. ISAC-31-9005-05-02 (May 1990) 7 Ruhayi, N.A. and Liew, S.H. 'Vacuum stressing technique for compositelaminate inspection by optical method' Exp Tech (March 1989)

Authors The authors are in the Structures Division, ISRO Satellite Centre, Bangalore, India.

Paper received 29 March 1990. Revised 1 August 1990

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