Weak fillet bond failure
IL Blog
From the photograph, there appears to be weak fillet bond failure where the core disbonds from the adhesive fillets. The adhesive fillets are formed during the initial bonding process, and they are essential for the strength of the bond. There is a conference paper on this:
Davis, M.J., Chester, R.J., Perl, D.R., Pomerleau, E., Vallerand, M., Honeycomb Bond and Core Durability Issues; Experiences within CREDP Nations, Aging Aircraft Conference, Williamsberg, VA, Aug 31-Sep 02 1998.
If you google that, there is a copy on my web site.
Essentially, the core simply separates from the adhesive layer leaving very little damage to the core or the adhesive layer. Typically once the core to adhesive bond is degraded, the flat-wise tensile strength (the strength of the core out-of-plane) falls to about 10% of original manufacture. This has been the cause of a significant number of rudder faiiures on one type of military aircraft. It is caused by water entering into the honeycomb panel itself, not just into the fuel tank, but actually penetrating the space between the inner and outer skins of the panel. This can only occur if there is a defect (such as a disbond of an edge member or a panel penetration which may have been repaired) which must occur first to enable the water to enter the core.
This type of failure should not occur if the water can be kept out of the core, so it is important to find where the water is getting in, not just to repair the part.
Regards
Blakmax