The influence of loading rate on the mode III fracture properties of adhesively bonded composites

The Mode III interlaminar fracture properties, G_IIIc, of an adhesively-bonded glass/epoxy composite are investigated over a wide range of crosshead displacement rates, using the edge crack torsion (ECT) test geometry. The ECT test fixture has been modified to conduct impact testing on these bonded materials. Tests on all of the samples highlighted a significant crack length dependency with the value of G_IIIc increasing rapidly with increasing crack length. Tests were also undertaken on the plain glass reinforced epoxy over an equally wide range of crosshead displacement rates. For a given crack length, the interlaminar fracture toughness of the adhesively-bonded system was superior to that offered by the plain composite, an effect that is attributed to the presence of significant crack-tip blunting within the adhesive layer. The interlaminar fracture toughness of the composite and the adhesively-bonded system remained roughly constant over the range of crosshead displacement rates considered here, suggesting that these systems do not exhibit any significant rate-sensitive fracture behavior. Finally, the crack tip loading conditions are verified by conducting an FEA analysis of the ECT specimen. Here, it was shown that Mode III loading predominates at the center of the test specimen, whereas regions of Mode II loading were observed close to the test supports.