Fracture toughness of adhesive joints. I. Relationship between strain energy release rates in three different fracture modes and adhesive strengths

The three strain energy release rates, GIC, GIIC, and GIIIC, of adhesive joints can be attributed to their ability to resist crack propagation of solids in the adhesive layer. The dependencies of GIC, GIIC, and GIIIC on crack lengths for various adhesive joints were determined using the double‐cantilever beam specimen by a compliance method. The two types of adhesive strengths, i.e., adhesive tensile strength and adhesive shear strength, corresponding to GIC and GIIC, respectively, were carried out at room temperature and 65% RH with a crosshead speed of 10 mm/min. The GIC, GIIC, and GIIIC were dependent upon crack length and had constant values irrespective of geometric parameters of the specimen over the crack length of five times adherend thickness, 0.65 (= crack length over half a length of span) and eight times adherend thickness, respectively. In the region of the crack length, we determined the following increasing order of fracture toughness: GIC < GIIIC < GIIC. A positive correlation was found between adhesive tensile strength and GIC. A significant relation between adhesive shear strength and GIIC was not found in this work. Further studies are needed to clarify the relation between adhesive shear strength and GIIC with general adhesives. © 1994 John Wiley & Sons, Inc.