Mechanical behaviour of laminated hybrid adhesive point connections when exposed to humidity conditions

The use of structural glass has increased lately, due to the higher demands for transparency in modern architecture. Therefore, the role of glass changes from a mere transparent function (used in windows only) to a function in which the glass is contributing to the bearing structure as well. Because of the fragile nature of glass, it is important to understand its behaviour. Due to the manufacturing process, the strength of glass is decreased severely by small cracks that are present on the surface. Also, glass cannot redistribute stress peaks very well. Stress peaks tend to occur near the connection between two (glass) components, making this a critical part in glass design. A common way to joint glass elements is by a bolted connection. However, this type of connection does not seem suitable for glass design. Due to the drilling process extra cracks will occur, causing a reduced material resistance, and at the edge of the holes large stress intensifications arise. Adhesive connections are more in line with the above stated problems of glass design. No drilling is needed, so no extra cracks will occur. Also, forces are transferred over the entire surface of the connection, leading to smaller stress peaks in the glass. However, adhesive connections know risks of their own. Problems are (1) the sensitivity of some adhesives to aging affects like humidity, (2) the strongly non-linear behaviour of the adhesive which causes stress peaks in the connection, and (3) the complex manufacturing process of the joint, because some adhesives will liquefy and squeeze out during the lamination process. In this thesis a concept for an adhesive connection is proposed that addresses these issues. The concept proposes the use of two adhesives in order to reach the following effect. The first adhesive is a rigid ionomer adhesive (SG) that is very sensitive for humidity and the second adhesive is a flexible silicone adhesive (TSSA) which is not sensitive to aging effects. The first adhesive is used in the centre of the connection and the second adhesive forms an external ring to protect the SG against moisture exposure. Also, the non-linear behaviour of the rigid adhesive is leading to stress peaks at the edge of the connection. By placing a less rigid adhesive at the edge, the overall stress distribution is more evenly divided. The manufacturing process is enhanced because the TSSA remains solid during the lamination process, so it can be used as a mould for the SG that will liquefy. The concept is called a hybrid adhesive connection. The concept is tested for a circular point connection exposed to tensile loading. This is done by means of an experimental and numerical analysis. The experimental analysis focusses on the deterioration of the hybrid circular connection after exposure to either immersion or 100% relative humidity. The numerical analysis concentrates on the stress distribution of the proposed connection. The following conclusions can be drawn from this investigation. In the manufacturing process of the experimental analysis, also the TSSA is squeezed out due to its softness. Therefore, extra measures should be taken to prevent this, which means that the manufacturing process of hybrid connections is not improved in comparison to SG connections. Deterioration of the connection due to humidity is still visible for a hybrid connection containing a ring of TSSA with a width of 5mm. This effect is less visible when a width of 10mm TSSA is used. Unfortunately, due to lamination problems, the samples suffered from poor SG bonding, which makes it difficult to give substantial conclusions on the matter. The numerical analysis gave more clear results. The E-modulus of TSSA is very low in comparison to that of SG. Therefore, the contribution of the TSSA to the bearing capacity of the connection is negligible, so the stress distribution is not enhanced. On the other hand, due to the external ring of TSSA, the stress peaks in the connection are moved inwards, from the perimeter of the connection to the perimeter of the SG centre. This is a positive effect with regard to humidity exposure. The strength of the hybrid circular point connection in comparison to the TSSA connections, is only slightly increased, but the stiffness of the hybrid connection surely is enhanced. Treats of the hybrid connection might be the risk of air bubbles in between the SG and TSSA and the absence of the whitening effect in the TSSA.Civil Engineering and GeosciencesStructural EngineeringBuilding Engineerin