Compared to state of the art

Composite material patching is a very promising method for repairing and/or reinforcing steel structures. Composite patches prevent crack growth and extend the lifetime of the repaired structure. A composite patch works as a crack arrestor by decreasing the stress in the area of the crack tip in the case of cracked structures. A part of the applied load is transferred from the base plate through an adhesive layer to the composite patch, thus reducing the stress levels in the substrate.

Composite patching has proven its effectiveness and cost benefits by its application in the aerospace industry for several years now, since there are already several thousands of operating patches in various aluminium aircraft structural parts. However, there are several fundamental differences between the aerospace applications and bridge/marine/offshore steel applications, which dictate a separate approach and investigation of the problem. These differences include the different stiffness of the base metal (stiffer steel versus the more similar to composites flexible aluminium), the completely different geometries involved (significantly thicker plating and larger beams in steel structures), the different loading cases and the different operating and environmental conditions. Furthermore, there are big differences between what is widely accepted as normal repair cost in an aerospace structure and its steel marine/bridge counterpart.

Composite patch repairs and/or reinforcements overcome many, if not all the aforementioned disadvantages of the traditional methods.

• They do not involve hot works in any way and, therefore, existing deadweight loading or proximity to explosive environments has no particular consequences.
• Patches can be applied directly on corroded steel members by performing a simple surface preparation, thus removing the need for replacement.
• They can be completed faster
• They exhibit good fatigue resistance
• They do not cause stress concentrations
• They result in low added weight

All these innovations reduce significantly the cost and the time of the repair or reinforcement.

However, there are some parameters of the procedure that still need further investigation, the most crucial among them being the uncertainty about the long-term performance of a steel-to-composite adhesive joint. The entire metal-composite system is subject to influence by external environmental parameters and there is lack of knowledge in the current literature regarding the evaluation of composite materials from this perspective. The scope of the work proposed here includes not only evaluation of the composite patches themselves, but an evaluation of the repaired system as a whole and its continued performance over time as it is subjected to external environmental conditions. The definition of proper patch designs and proper patch application and monitoring procedures that result in a long-lasting patch repair/reinforcement will enable us to consider these repairs as permanent, as is already the case with the aerospace industry.