Cohesive failure
Cohesive failure turns up in 3 fields: thermal spray, adhesives, waterproofing. Here is what it means to the people in each one.
Cohesive failure in thermal spray
A bond test break inside the coating, with coating left on both halves of the test specimen.
Failure mode in which the coating breaks through its own thickness, between splats or layers, so both halves of the test show coating. It means the bond to the substrate was stronger than the coating's internal strength. Low cohesive strength points to unmelts, oxides, porosity or dusting between passes. In a two-layer system it is reported by which layer failed.
"Cohesive failure in the zirconia at 3,500 psi, that's normal for this top coat, it passes."
Cohesive failure in adhesives
A break that runs through the adhesive itself, leaving adhesive on both parts.
When a joint fails cohesively, the adhesive was the weakest link and the interfaces held, which usually means the surface prep did its job. Engineers often specify that a set share of the fracture surface, such as 80 to 100 percent, must be cohesive. Very low cohesive strength can point to undercure, off-ratio mixing, contamination of the adhesive or exposure above Tg.
"Failure's 100 percent cohesive, adhesive on both coupons, so the prep's good. We just need a stronger glue."
Cohesive failure in waterproofing
A sealant tearing apart within itself while staying stuck to both joint faces.
Cohesive failure in service often points to joints that are too narrow for the movement, three-sided adhesion, sealant that is too thick, weathering or reversion. In a pull test, though, cohesive tearing is the desired result, because it proves adhesion is stronger than the sealant. Inspectors note whether a split runs down the center of the bead or near a face.
"Bead split right down the middle, cohesive failure, the joint's moving more than that sealant's class."