Research aims to improve joint connection technologies for lightweight and high-strength structures

According to a new research team, led by professor Yukihiro Matsumoto from Toyohashi University of Technology’s Department of Architecture and Civil Engineering and the Electronics-Inspired Interdisciplinary Research Institute (EIIRIS), proposed and demonstrated a method for improving the dynamic behaviour of bolted connections using Glass Fiber Reinforced Polymer (GFRP) materials.

The findings of the research were published in the journal ‘Polymer Composites’.

As pultruded GFRP materials used in construction applications have reinforced fibre directions aligned along the axis of the member axis, bolted connections are fragile and at a disadvantage in design.

The research team demonstrated that a noticeable increase in bearing strength can be realised by pasting a thin GFRP plate, which uses a multiaxial base material moulded by vacuum-assisted resin transfer moulding which enables high-quality moulding, to the bolted connection.

The research team also demonstrated that this method could improve the fragile fracture behaviour of the joint connections. The results of this study will enable safer, more secure and lighter building structures with longer lifespans to be designed.

Because of its lightweight and high strength, applications for Fiber Reinforced Polymer (FRP) are increasing, such as its use in the repair and reinforcement of existing buildings, pedestrian bridges and floodgates and it is expected that they will be used in emergency repairs of structures and the structure of buildings in the future. The pultrusion method, one of the methods for producing FRP members, can produce long members with ultra-high productivity.

It is a commonly used moulding method for FRP architectural members. However, because the pultrusion method generally places many reinforced fibres, which ensure the strength and stiffness of FRP materials, in the direction of pultrusion (along the longitudinal axis of the member), it is known to show local damage and brittle fractures around bolt holes when connections are made using bolts, etc. Therefore, care should be taken regarding this fracture behaviour.

Therefore, the research team has performed research to minimise the increase in weight and production costs and to improve the dynamic behaviour of bolted connections by using vacuum-assisted resin transfer moulding, which is used to make parts of ships and blades of wind turbines made of FRP, and by pasting a GFRP plate that is several millimetres thick and has multiple fibre directions.

By reinforcing the necessary areas with only the necessary amount of GFRP, they demonstrated in their experiments that the connection strength of fibre-reinforced polymer can be increased significantly without losing the productivity or light-weight properties of FRP. In addition, based on the experimental results and existing design formulas, the research team also proposed a design formula for when their proposed connection reinforcing method is used and successfully provided data that can be applied in design.

The research team leader, Professor Yukihiro Matsumoto, said; “As the name suggests, a fibre-reinforced polymer is a material that requires the good use of fibres. We have reached this idea by observing the fracture behaviours of connections through previous experiments and collecting information on various FRP moulding methods. Though it is simple, what we needed was only the pasting of a thin plate to the connection. I am surprised that good effects were observed by carefully considering fibre direction. I think the useful and general-purpose results were obtained due to the enthusiasm of the PhD students in the determination of experimental variables and the development of an experiment plan.”

Going forward, the research team will demonstrate the effectiveness of their proposed connection method through connection experiments assuming construction structures as well as experiments and analyses of full-scale specimens and will promote the development of these reinforced connections, aiming for their application as a reinforcement method for existing FRP structures.