Substrate failure
Substrate failure turns up in 2 fields: adhesives, waterproofing. Here is what it means to the people in each one.
Substrate failure in adhesives
A joint failure where the bonded material itself breaks before the adhesive or the interface gives.
Substrate failure shows as torn wood fibers, ripped paper, delaminated composite plies, broken plastic or pulled-off paint layers. In most cases it means the bond is stronger than the part, which is the goal for wood, paper and packaging joints. On painted or plated parts it can reveal that the coating, not the adhesive, is the weak link.
"Coupon broke in the composite, first ply came away with the adhesive, so it's substrate failure, not the glue."
Substrate failure in waterproofing
The joint face itself breaking, so sealant comes away with a skin of concrete, coating or EIFS still attached.
When the sealant and its bond are stronger than the surface, the surface gives instead: weak concrete skin, poorly bonded paint, EIFS base coat or crumbling mortar tears off with the bead. It means the sealant modulus is too high for the substrate or the surface was weak to begin with. Low-modulus sealants and removing weak surface layers fix it.
"Bead came off with a layer of paint on the back, that's substrate failure, the coating's the weak link."