In January 2010 a consortium of 15 parties from eight European countries have kick started a three year European funded project on Composite Patch Repair for Marine and Civil Engineering Infrastructure Applications - COPATCH. This is a novel effective repair and/or reinforcement method for large steel structures with defects. Two basic steel structural types will be dealt with in this work, namely marine structures (mainly ships) and steel civil engineering structures (e.g. bridges).
Many of the major problems that these structures face during their operational life are common, with fatigue playing a predominant role among them. Repeated loading is a very common loading condition for both these structures, which, in areas of stress concentrations, leads to the initiation and growth of fatigue cracks. These cracks, if not detected and properly repaired in time, can grow to critical lengths and result in catastrophic structural failures.
An additional structural problem of either marine or other large steel structures is corrosion, particularly for vessels and structures designed with thinner plating due to the use of higher-strength steels. Corroded plating or beams jeopardise the strength and stiffness of the structure and measures have to be taken to reinstate the original structural characteristics. Besides the above two major defects of the steel structures, there is often the need for upgrading the strength and/or the stiffness of a structure, enabling it to face new loading conditions or helping in mitigating initial design deficiencies.
The traditional ways to deal with such problems are well known. Fatigue cracks in welds or panels usually involve renewal of part of the weld, welding in the cracks in a panel or replacement of the whole parent panel itself. The standard way to face heavy corrosion is also the replacement of the corroded structural members, whereas the use of bolted or welded doubler plates seemed to be the only way until now to reinforce a structural part. There are circumstances, however, when these repair or reinforcement approaches are either time-consuming and costly, or simply impossible to be followed.
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
The consortium is currently planning to invite interested stakeholders to follow the project activities, within the framework of a relevant stakeholders’ forum.