We investigated how fast cracks grow in recycled titanium material compared to the conventionally used steel material for application in offshore wind turbine towers. We performed tension and cyclic tests on notched samples, that were either un-corroded or pre-corroded and measured the crack growth with a high-speed camera. We found that the steel had slower crack growth rate than the titanium in both corrosion conditions, the titanium had no change in crack growth rate between the un-corroded and pre-corroded and that the steel had a slower crack growth rate in pre-corroded samples compared to the un-corroded samples.
Using rolled S355 steel and recycled field assisted sintered technology (FAST) Ti-6Al-4V, compact tension (CT) samples were cut from the billets and polished and half where then placed in a saltwater environment for three weeks. We coated the samples in a speckled pattern and then used a high-speed camera to do digital image correlation (DIC) on the sample surface. Then we performed monotonic and cyclic tension to measure the toughness and the fatigue crack growth rate with the camera. The steel alloy had better toughness and slower fatigue crack growth rate compared to the titanium alloy for both corrosion conditions, this is because of the higher ductility and the ability to blunt the crack tip.
The titanium alloy had no change in the toughness or fatigue crack growth rate when comparing the corrosion conditions suggesting that the titanium did not interact with the salt water. The steel alloy had higher toughness and slower crack growth rate in the corroded condition compared to the un-corroded condition this was due to the oxide at the notch growing out from the surface and binding from each side, effectively reducing the notch length. This is a form of crack healing but is not relevant to in-service condition as, in our experiment, there was no load applied during corrosion and an in-service wind turbine would be in a stress-corrosion cracking conditions.
These results show that the titanium material would not be useful in the application for wind turbine monopiles, but as there was no change in the properties comparing before and after corrosion, this material could be used where high strength and good corrosion resistance, i.e. thin components, are required.
This project was completed at the University of Alberta in Canada, with Prof James Hogan and his team at the CDAM group. Everyone was very friendly and welcoming, helping me get my experiments set up and settled into the city. The project was only 12 weeks and if I had more time to complete the project, I think more analysis and experimental work could have been completed, which would have made the scientific outcome more rigorous. Alberta is a beautiful place with the Rocky Mountains not far from the university. I visited the Rockies and saw the water which is exceptionally blue due to the minerals dissolved from the rocks, this make it an excellent place to go for hikes and bike rides. It was a wonderful opportunity to travel to Canada, work with exceptional scientists from all around the world and then have a lovely holiday after I completed the project.
Patrick Curran, 2020 Cohort
This scheme was funded by the UKRI - Mitacs globalink doctoral exchange program
Patrick is a final year PhD student studying fatigue in titanium at the University of Manchester. His project involves lots of material characterisation and computer simulations to try and predict how a material is going to fail. In his spare time he enjoys being in the countryside, and going for hikes or bike rides, which was one of the reasons he enjoyed his trip to Canada so much.
