Full definition
The two fundamental failure modes of an adhesive joint, with critically different root causes and corrective actions. Adhesive failure: the bond separates cleanly at the interface between the adhesive and the substrate — one surface retains all the adhesive, the other surface is clean. Root causes: inadequate surface preparation (the #1 cause — oil, dirt, oxidation, or mold release preventing adhesive wetting), wrong adhesive for the substrate, insufficient adhesive coverage, or premature loading before full cure. Corrective action: improve surface preparation (degrease, abrade, prime). Cohesive failure: the adhesive itself fractures internally — adhesive residue remains on both surfaces. Root causes: adhesive too weak for the load (under-specified), adhesive layer too thick (stress concentration), temperature exceeding adhesive capability, or manufacturing defect (improper mix ratio, air bubbles). Cohesive failure actually indicates that the adhesive-to-substrate bond was stronger than the adhesive itself — a well-bonded joint. Corrective action: use a stronger adhesive, increase bond area, or reduce load. Mixed mode: combination of adhesive and cohesive zones — indicates partially adequate surface preparation. Per ASTM D5573 for failure mode classification in structural adhesive joints.