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.
In qualification reports, a failure mode percentage is usually expected next to the strength value, estimated by eye on both halves.
Also heard as
- cohesion failure
- CF
Do not mix it up with
- Adhesive failure: Adhesive failure separates at the interface and leaves one surface clean, pointing to a surface problem.
Related terms
Adhesive failure
A break at the interface between adhesive and part, leaving one surface clean or nearly clean.
Substrate failure
A joint failure where the bonded material itself breaks before the adhesive or the interface gives.
Lap shear
The standard strength test for adhesives, pulling apart a single overlap joint and reporting load over bond area.
Fiber tear
Paper or wood fibers torn out and stuck to the adhesive when a bond is pulled apart, showing the bond beat the substrate.