Constitutive law of adhesive layer measured with mixed mode bending test

• New theory for measuring mixed mode constitutive law of adhesive layer.
• In situ SEM observation of the tensile/shear failure mechanism.
• Difference in mode I/II response was caused by anisotropy generated by shear banding.

New analytical theory is presented in this paper to measure the mixed mode constitutive law of adhesive bonding. The theory is on the basis of the J-integral theory and the mixed mode bending test. The fracture energy and the mode I/II constitutive law components were obtained at different mode mixity ratios. A comprehensive discussion is carried out with focus on the plastic yielding and initiation of localized damage. In situ SEM was used to analyse the tension/shear deformation mechanisms. The mixed mode fracture energy is found to increase significantly as the mode mixity ratio increased. The tension stress started to decrease after plastic yielding, while the shear stress kept increasing until the emergence of localized damage. This difference was attributed to the anisotropy of the adhesive material, caused by shear banding. The plastic deformation consisted of cavitation caused by tension and shear banding caused by shear. The present method is verified with numerical simulations. The constitutive law measured with the present method can be used to develop new cohesive zone models, and may produce a more accurate analysis of adhesive bonded structures than existing methods do.

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