At last I can respond
Hi John
Sorry about the delay. Things are still hectic, so I have extracted some information for a conference paper I am currently drafting. Sorry about the length. I have broken it into two messages. My concern is for the continuing airworthines of metal bonds. Here goes.
There are three forms of bond failure:
Cohesion failure usually occurs through the plane of the carrier cloth, which is the weakest plane in an effective bond because of the reduced surface area caused by the presence of the carrier cloth. The surface is rough and often slightly milky in appearance due to shear hackles formed by the failure. Bonds which fail by cohesion exhibit high strength. The causes of cohesion failure include design deficiencies such as inadequate overlap length, thermal stresses or peel stresses, however cohesion failure can also occur from the presence of voids which reduce the available bond overlap length below a critical size.
Adhesion bond failures occur at the interface between the adhesive and the adherend, with residual adhesive remaining at any location on one surface only. The chemical bonds at the interface become weaker than the adhesive strength at the plane of the carrier cloth. The surface of the adhesive is smooth and often replicates surface features from the adherend. Adhesion failures exhibit low strength and may occur with no applied load if degradation of the interface is complete. Causes of adhesion failure include contamination during manufacture, the use of out-of-life adhesive, or inadequate temperature control during production, however such cases should be eliminated by quality assurance tests. The remaining cause is interfacial degradation in service.
Mixed-mode failure exhibits some cohesion failure and some adhesion failure. This is because the interface is partially degraded. In effect mixed-mode failure is just an adhesion failure waiting to occur. The failure exhibits areas of smooth failure as well as areas which are rough. The strength of adhesive bonds exhibiting mixed-mode failure is lower than the cohesion failure strength. The strength of the bond and the proportion of surface smoothness or roughness depend upon the level of degradation of the bond interface.
Continued...