AbstractThe mechanisms of resin controlled failure in adhesive joints and composites (delamination and transverse cracking) are examined. An in‐situ failure model based on the fracture mechanics principles is applied here to describe the failure processes involved. The model centers on the crack tip plastic zone developed in the thin resin layer between the fibers or the adherends. The plastic zone in the resin layer is heavily influenced by a dominant slow varying stress distribution, approximated to ber−m/2dependent withm≪ 1 (ris the distance from the crack tip). The adhesive or composite fracture toughnessG*ICcan then be expressed as a function of several resin properties of comparable importance: modulusE, yield stress σy, resinGICand residual stress. The relative significance of the resin properties on the adhesive or composite fracture is discussed. The effects of temperature, loading rate, and resin toughening on such failure as a result of the corresponding variations in resin properties are also add