This study deals with improving the performance of bonded structures, particularly for marine applications. In such applications, it is important to analyze the influence of aging on the non-linear behavior of the adhesive in an assembly. The standardized thick adherend shear test (TAST), often used to analyze the full stress-strain curve under shear loading, characteristically has large edge effects. Crack propagation close to the substrate-adhesive interfaces is observed and thus can lead to incorrect analysis of the behavior of the adhesive. This paper presents the development of a modified TAST fixture which gives a more homogeneous adhesive stress state in the adhesive joint and strongly limits edge effects in order to obtain reliable experimental results. A modular assembly using a re-usable support fixture and small samples with machined beaks, which can be cut out of bonded plates or bonded bars, is proposed in order to analyze the behavior of aging bonded assemblies. An optimization of this fixture was introduced, based on numerical analysis of the stress distributions in the adhesive joint taking the influence of the fixture into account. Some experimental results obtained using a prototype of the proposed modified TAST fixture are presented in order to validate the possibilities of this sort of experimental fixture. (c) Koninklijke Brill NV, Leiden, 2009