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A fracture investigation was conducted on a modified epoxy adhesive (Metlbond 1113-2). Initially various adhesive fracture strength and energy tests and the mechanisms of mechanical and chemical adhesion were discussed. A critical study was then made of the sensitivity of the strain energy release rate·to specimen preparation. The rate and time dependence of the adhesive were then investigated using the TDGB specimen. The fractured surfaces were also studied using scanning electron microscopy to determine the mechanisms of failure. / Master of Science

A novel concept, the fracture efficiency, is proposed in this dissertation as a performance index of fracture specimen designs for adhesive bonds. Evaluated based on a simple quantity, the fracture efficiency parameter, the fracture efficiency represents how much crack driving force a certain specimen design can produce along the bondline for a given maximum non-singular stress in the adherends. In this dissertation, a study of membrane peeling specimens shows that various existing test geometries have little difference in the fracture efficiency. Furthermore, the study shows that it is unlikely to develop new n1embrane peeling tests with high fracture efficiency. Therefore, gross inelastic energy dissipation in the film specimen may occur when using the membrane peeling tests, and may result in large errors in the film adhesion measurement when only elastic material response is considered in the analysis. As a result, the use of a backing material for film adhesion measurement is suggested because of the significant increase in the fracture efficiency when a proper backing material is used. Besides the use as one of the guidelines for fracture specimen designs, it is also demonstrated in this dissertation that the fracture efficiency parameter can also be used to help one determine the validity of a measured bond fracture strength. The study of fracture efficiency leads to new analyses of several existing fracture test geometries, and contributes to a better understanding of the standard blister, island blister, peninsula blister, peel, and cracked lap shear tests. It is found that the island blister and peninsula blister tests can produce very high energy release rates at low pressures by using a small island radius or a small peninsula width. However, these so called "high energy release rate" tests also induce localized high stress concentrations near the debond front and therefore, yielding or rupturing may still unavoidable in such specimens. The significant localized inelastic effect on the measurement of the bond fracture strength is also demonstrated in the peninsula blister experiment for an adhesive tape specimen subjected to a relatively low pressure load. Another important conclusion drawn from the analyses of various membrane peeling tests is that the existing membrane blister tests are special cases of the peel test if compared very near the debond front. Besides analyses for membrane peeling specimens, this dissertation also reports a new closed form solution for the cracked lap shear specimen using a geometrically nonlinear beam-column approach. Excellent agreement for energy release rates has been shown between the closed form solution and geometrically nonlinear finite element analyses. The study also discusses how to design a cracked lap shear specimen of constant energy release rate and high fracture efficiency. / Ph. D.

The strength of materials solution for a new bonded cantilever beam test specimen to determine adhesive shear properties is reviewed and discussed. A parametric analysis for the adhesive shear stress and for the end deflection reveals the specimen dimensions required for reliable bonded adhesive shear properties determination. Recommendations are provided for conducting reproducible tests. A pure and quasi-uniform shear test for stiff adhesives is proposed. Analytical solutions are compared with Finite Element solutions from VISTA and NOVA for the stresses in the adhesive. It appears that the assumption of pure shear is nearly valid even for very stiff and/or very thick adhesives. In order to increase the end point deformations for stiff adhesives, a modified specimen is proposed. Three-dimensional effects through the thickness of the adhesive layer are studied with the program ABAQUS. Experiments were performed using the two methods derived from theory and good correlation between theory and experiment were obtained with some restrictions. For both methods, experimental results underlined the need for defining proper specimen geometry prior to testing. Simple numerical codes are proposed to facilitate this purpose. / Master of Science

This study developed and analyzed a modification of the blister test permitting nearly constant strain energy release rate testing of adhesive bonds. The work consisted of three parts; (1) development of the testing technique to evaluate strain energy release rate and to record the time dependent nature of the fracture process, (2) numerical analysis of the constrained blister test to determine the applicability of an approximate solution for several materials, and (3) development of an. analytical technique to evaluate the strain energy release rate for relatively stiff specimens. / Master of Science

A strength of materials type solution is obtained for the shear stress state in the adhesive layer of a bonded cantilever beam subjected to an end load. The shear stress is constant through the thickness of the adhesive layer and varies from zero at the fixed end to a maximum value at the free end. This maximum value can, under certain conditions, be calculated from knowledge of the load and the beam geometry only. The adhesive's shear modulus can then be determined from a measurement of the shear strain in the adhesive layer. An expression for the beam deflection is also obtained. It contains a coefficient of adhesion which is potentially useful to evaluate surface treatments or other factors leading to different states of adhesion. Fracture mechanics application of the specimen, nonlinear and viscoelastic adhesive behavior are briefly mentioned. / Master of Science