hr. J. Solidv S~rucrures Vol. 34, No. 4, pp. 509-525, 1997 Copyright 0 1996 Ekvier Science Ltd Printed in Great Britain. All rights reserved PII: zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFED SOO2&7683(%)00026-1 002&7683/W $17.00 + .%I USING THE FRACTURE EFFICIENCY TO COMPARE ADHESION TESTS YEH-HUNG LAIt and DAVID A. DILLARDS Engineering Science and Mechanics Department, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0219, U.S.A. (Received 29 October 1994; in revisedform 29 January 1996) Abstract-This paper reports the use of a novel concept, the fracture efficiency, to evaluate adhesion tests by deciding whether a particular test would be more or less likely to cause gross yielding or rupturing in the adherend/coating before the fracture condition is satisfied. Because of the technical importance and the difficulty in obtaining meaningful debond energies for coatings, much of the paper is devoted to the coating adhesion measurement problem. The fracture efficiency of the general coating delamination contiguration is investigated and its theoretical limit is determined. The study suggests that it is unlikely to devise a new test with a significantly higher fracture efficiency than the existing tests. New experimental or analytical techniques considering the inelastic energy dissipation are needed for coating adhesion measurement. In addition to the fracture efficiency, the fracture mode mixity of the general coating delamination problem is also investigated. The con- ditions which may result in the contact of crack surfaces near the crack tip region are identified. The preferred configurations which would tend to induce interfacial delamination are also discussed. Finally, from the viewpoint of fracture efficiency, guidelines are given to aid in the selection of appropriate test geometries for coating adhesion measurement. Copyright 0 1996 Elsevier Science Ltd. INTRODUCTION A large number of fracture tests for measuring the fracture strength of adhesively bonded assemblies have been developed over the years. These tests may be conveniently grouped into three categories : laminated beam tests, blister/peel tests, and miscellaneous tests. The laminated beam tests consist of adherends which are loaded so that they behave like beams. This category includes the double cantilever beam tests of various loading modes, end notched flexure, end notched cantilever, mixed mode flexure, and four point bend tests. Blister/peel tests consist of an adherend (or a coating) bonded (or adhering) to a substrate which usually has a much higher rigidity than the peeled adherend. This category includes peel tests of various peel angles and blister tests such as Dannenberg’s blister test, the standard circular blister, constrained blister, strip blister, island blister, and peninsula blister tests. When the bending stiffness of the thin adherend is negligible, and it behaves as a membrane loaded in tension, the debonding of the adherend is like the peeling of a membrane from the substrate. Therefore, this type of blister/peel test is called a “membrane peeling tests” in this paper. Other fracture tests not belonging to the previous two categories may be grouped together as miscellaneous tests, including the compact tension, compact shear, cone, Brazil nut sandwiches, indentation tests, etc. Excellent sources with references to most of these tests can be found in Brinson (1990). One condition for successful fracture testing is to induce debonding without rupturing the adherend or the coating. When the specimen is debonded, analysis methods are then applied to determine the debond energy. Most of the analyses reported for the current fracture tests are based on the assumption that the adherends or coatings are loaded within the elastic range. These elastic analyses become invalid when general yielding occurs in the adherend. Therefore, the accuracy of using these elastic analyses depends on the extent of yielding. Only a small fraction of the literature for adhesive fracture tests has addressed the analyses of fracture specimens with general yielding. Chang et al. (1972) derived the energy t Present address: Eastman Kodak Company, Rochester, New York 14650-2116, U.S.A. $ To whom correspondence should be addressed. 509