A composite material joined with a curable phenolic resin adhesive, with the phenolic resin adhesive comprising a stiffening agent precursor, and with the stiffening agent precursor selected to react with reaction by-products of the phenolic resin adhesive during curing to produce a reaction product stiffening agent in a cured bonding layer that is detectable by ultrasound, resins comprising the stiffening agent precursor, bonding layers comprising the reaction product stiffening agent, and methods for making the composite material joints and inspecting the composite material joints are disclosed.