Early this year a small joint industry project was conducted on the evaluation of embedded composite-steel insert connections. These innovative ‘hybrid’ connections consist of a steel plate, potentially with spikes added for additional locking, embedded in a composite. A potential benefit of this configuration is that e.g. composite ship skin panels can be joined using standard welding techniques which is expected to be beneficial for the parties that would be interested in using composites.
The goal of this evaluation project was to investigate the influence of welding-induced heat on the composite-steel connection. Welds were applied on different distances from the composite-steel connection, resulting in different temperature peaks at the connection.
The project was initiated by Airborne Marine and co-ordinated by Netherlands Maritime Technology.
The participants of the project and their main activities were:
Da Vinci College Duurzaamheidsfabriek
Knowledge centre WMC
Manufacturing of test panels
Water jet cutting/milling into specimens
Research on pre-treatment and primer
Testing of specimens
A limited number of test panels was manufactured by vacuum infusion. The specimens were symmetric, with on both sides a steel plate inserted in glass fibre reinforced thermoset Epoxy-Glass composite. At one side a weld was applied on the steel insert at a certain distance from the composite (the ‘hot’ side), and at the other side the steel insert was used for clamping purpose only (‘cold’ side). The embedded length of the steel at the clamping/cold side was larger than the embedded length at the welded/hot side. Thus, failure of the insert was forced to occur at the welded side.
The results of the test campaign indicate that there is an influence of welding distance on the connection strength.
However, in order to obtain conclusive results, further optimization of the test coupons is necessary and a larger test programme is required. Furthermore, this programme was limited to quasi-static loading, whereas fatigue loading of such connections is of interest for the foreseen application. The project partners are currently investigating further opportunities for joining forces in this interesting and relevant topic.
In the view of the parties that have taken part of this initial screening, the promising element is two-fold:
- It has been proven in a tidal energy project that this type of relatively large embedded inserts can transfer high loads.
- When composite parts can be fitted with such elements and offered as ‘a finished product’ this only requires standard welding expertise at the party that is aiming for better performances by lower weight, less (or no) corrosion and/or improved fatigue resistance.