potted sandwich

[fredpalmieri]

does anyone have experience with foamed adhesive potted graphite epoxy/aluminum honeycomb sandwich structures? the particular case involves a horizontal sandwich panel made to connect to another vertical sandwich panel by using a tongue and groove type mechanism. The tongues of the horizontal panel are spaced periodically along its length, penetrating through one face sheet of the perpendicular vertical panel. The honeycomb core is removed in the vicinity of the tongues both from the horizontal panel as well as the vertical panel and both the tongues and the surrounding slots in the panel are potted to effect a bond. Under thermal cycling, cracks developed in the bond. I would not normally predict such an event to occur. Would anyone know why? Of course, the thermal expansion coefficient of the potting is very large, but we use potted inserts all the time without problems developing.

 

[crbeebe]

I have a theory, but need more info on the configuration. I'm having a hard time visualizing how the tongue penetrates the facesheet of the vertical panel...is the tongue vertically oriented? Does the tongue insert into the edge of the vertical panel? Does the tongue penetrate the horizontal panel facesheet?

 

[Compositepro]

It sounds like you have more of a mortise and tennon type of joint than a tounge and groove. The cracking is probably occuring because by replacing the honeycomb with potting compound you have not only increased the CTE though the thickness of the sandwich panel you have also greatly increased its modulus or rigidity.

The sandwich panel tennon is expanding in thickness as it warms due to the rigid potting compound. This is a relatively large area. This is resisted by the aluminum skin in the in the mortise which is a far smaller bond area. Also the high shrinkage potting compound is inside a large, low-shrinkage aluminum hole. After a heat cure the stresses are relieved by cracks.

Potted inserts are low shrinkage parts surrounded by a high shrinkage compound. A disbond would not show as a visible crack.