Mechanical Behaviour of Transparent Structural Silicone Adhesive Laminated Connections under Monotonic and Cyclic Loading

Abstract

Transparency and translucency are essential features of modern architecture. Glass products have been widely used in facade applications and recently their use has expanded to the load-bearing structure due their large in-plane compressive strength. However, the fragile nature of glass requires special treatment and attention to detail to avoid fracture caused by tensile stresses. Connections between glass components are very critical to the structural integrity of the system. Bolted connections, which are often used to transfer forces between glass elements, require drilling the glass. This process may cause additional flaws and stress intensifications around the holes and thus reduces the bearing capacity of glass elements. Adhesive connections offer an alternative approach for the connecting joints and enable a more uniform distribution of stresses. Laminated connections have recently been developed that combine high strength and transparency. This work focuses on the Transparent Structural Silicone Adhesive (TSSA), produced by Dow Corning, that is used for the realization of laminated connections. TSSA connections have been used in several projects worldwide; however, the hyperelastic and viscoelastic nature of the material has not been fully investigated. In this work, the mechanical response of TSSA laminated connections under static and cyclic loading is investigated by means of experimental, analytical and numerical studies.

Firstly, the shear behaviour of TSSA laminated circular connections is characterized by means of monotonic and cyclic loading tests at different frequencies. The hysteretic behaviour of the material under loading cycles, which is caused due to its viscoelastic nature, is analyzed. The adhesive exhibits significant stress-softening under repeated cycles that becomes more severe as the maximum load increases. Energy dissipation analysis is conducted to understand the nature of this phenomenon with the aim to simplify the cyclic behaviour of the adhesive for modeling purposes.

Secondly, TSSA laminated circular connections are subjected to monotonic and cyclic tensile loading of increasing maximum load. The development of the whitening phenomenon is studied for both cases. The stress level when whitening appears for the first time and the way it propagates to the adhesive surface show some consistency both for the cases of static and cyclic loading. The occurrence of the stress-softening phenomenon is also recorded, in order to observe whether it appears within the working limit of the connection.

Thirdly, the deformation behaviour of the adhesive is described analytically based on hyperelastic prediction models. Conventional test set-ups, such as uniaxial and biaxial tension tests, are combined with the simple shear test results obtained within the framework of this thesis, for the material characterization of TSSA. The hyperelastic material parameters are calibrated by a simultaneous multi-experiment-data-fit based on the nonlinear least squares optimization method. The softening behaviour observed in shear tests is modeled based on a simplified pseudo-elastic damage model, which is supported by most finite element software. A first attempt is also made to model the actual softening response of the adhesive. A less conservative approach proposed by Guo, also based on the theory of pseudo-elasticity, proved to give a good approximation of the actual cyclic response of the adhesive.

Finally, TSSA laminated connections on the edge of the glass are experimentally and numerically investigated. The edge bonded specimens are tested in shear and the stress distribution of the adhesive is analyzed by means of a three-dimensional finite element model. The distribution of stresses in the adhesive is non-linear showing significant stress peaks towards the free edges of the adhesive. A parametric study is conducted to relate the magnitude of the stress peaks with the eccentricity of the applied load.