When a bonded joint comes apart, the way it separated tells you why. Adhesive failure and cohesive failure look similar at a glance but point to completely different root causes, and reading the fracture surface correctly is the fastest route to a fix. What Adhesive Failure Looks Like Adhesive failure occurs at the interface between the adhesive and the substrate. The adhesive releases cleanly from one surface, leaving that face bare while the full adhesive layer stays on the opposite part. It means the bond to at least one substrate never developed properly. Common causes: Surface contamination. Oils, mold release, fingerprints, or oxide layers block molecular contact. Even a monolayer of silicone can drop peel strength by more than half. Inadequate surface preparation. Smooth, low-energy plastics such as polypropylene need abrasion, plasma, or a primer before an adhesive can wet them. Wrong adhesive for the substrate. A chemistry that bonds glass well may have no affinity for a fluoropolymer or a powder-coated panel. Poor wetting. An adhesive that is too viscous, applied too cold, or cured before it could flow will not make full contact with the surface texture. What Cohesive Failure Looks Like Cohesive failure happens inside the adhesive layer itself. Both substrate faces are left coated with adhesive residue, and the split runs through the middle of the bond line. The adhesive gripped both parts successfully, but the material could not carry the load applied to it. Common causes: Incomplete cure. A UV adhesive that received too low a dose stays soft in the core and tears under stress. Dose is intensity multiplied by exposure time, and every adhesive has a published minimum. Aging and environmental attack. Heat, humidity, and light slowly break polymer chains. Hydrolysis and oxidation both reduce cohesive strength over months or years. Overload. The joint was simply asked to carry more force, or more concentrated stress, than the cured adhesive is rated for. Voids and bubbles. Entrapped air concentrates stress and gives a crack an easy path through the layer. Reading the Fracture Surface Set the two separated parts side by side under good light: One clean face, one fully coated face: adhesive failure. Investigate cleaning and surface preparation first. Both faces coated with a torn, fibrous residue: cohesive failure. Check cure completeness and whether the joint is overloaded. A mix across the bond area: a partial or transitional failure, often meaning marginal surface prep combined with marginal cure. Treat both. The substrate itself broke: substrate failure, which usually means the adhesive and the bond were stronger than the part. This is often an acceptable result. Fixing Adhesive Failure Start with the surface. Solvent-wipe with a clean, lint-free cloth and a fast-evaporating solvent such as isopropyl alcohol, changing the cloth often so you are removing contamination rather than spreading it. Abrade low-energy plastics and check surface energy with dyne pens if wetting is marginal. Confirm the adhesive is rated for both substrates, and consider a primer or plasma step for difficult materials. Choosing the…