In this report, adhesive strength criteria are investigated experimentally and analytically. The cylindrical butt joint specimen and the round bar butt joint specimen with various thickness (30~1000μm) of adhesive layer are subjected to various combined tensile and torsion loadings in the experiments. Apparent critical fracture loci are obtained in the stress plane composed by the average tensile stress σand the maximum shear stress max  for various adhesive thicknesses, which are approximated

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... to ellipses and the lengths of axis of these ellipses in bothσand max  directions decrease linearly with increasing adhesive thickness in log-scale. Effects of adhesive thickness on the critical stress is larger inσdirections than in max  direction. Principal stress, principal strain and von Mises stress distributions at around the adhesive interface edge of these specimens under the critical load are obtained by the elasto-plastic finite element analysis using MARC. As a result, the mean value of von Mises stress distribution at the singularity area which is 0.4 % long of adhesive layer thickness from the interface edge, is found to be the most dominant factor in determining the adhesive strength. Also, it is found that the critical Mises stress as the strength of adhesive joints decreases linearly with increasing adhesive thickness in log-scale. The critical stress value can be obtained directly as the critical von Mises stress at the interface edge from a torsion test of a round bar butt joint specimen and a corresponding numerical stress analysis.