In order to evaluate the mechanical behavior of joints in a double-lap adhesively bonded assemblage by means of strain gages located at the outer surfaces of adherends, this study takes into account the non-linear beavior of the bond. This behavior may be caused by joint unevenness, which produces some plastic adaptation at the onset of pulling, or by the opening of microcracks. Since the measurement profile along the sample keeps the same shape as the analytic solution, at least up to the threshold of flaw propagation, the model is based on the classical theory that the adherends are solicitated in traction and the adhesives comport in simple shear but with a sliding coefficient (ratio of shear modulus to joint thickness) that varies along the overlap. A quasi-linear approximation of the solution permits a numerical estimation of shear modulus, which allows an assessment of adhesive mechanical properties and double-lap joint quality.