In the field of fatigue testing in laboratory conditions, the common practice is uniaxial testing (i.e. in tension). The real-life loads on structures are usually not limited to one direction but contain multiaxial components. For this kind of testing, the ship and offshore structures department of Delft University of Technology developed a machine that is capable of exciting stresses in a specimen that resembles real life. A preliminary study has been conducted for the feasibility of using the acoustic emission method for monitoring fatigue on this multiaxial testing machine and in specific on a tubular double-side welded T-joint in offshore applications. This method is a tool to help to understand the fatigue progress in the material during testing.
Fatigue is a process by which damage is caused under cyclic loading below the yield stress. This phenomenon initiates on a microscale were dislocations accumulate and grow into a crack. The crack can grow until the structural integrity fails in complete rupture. The fatigue lifetime may be predicted; this is done with the help of uniaxial fatigue models. For multiaxial fatigue these models are not accurate and therefore a multi axial fatigue model should be developed. The crux here is to do these experiments to gain knowledge of the multiaxial fatigue to develop an accurate model for multiaxial fatigue.
The acoustic emission method is a non-destructive evaluation method that can help to determine the condition of a structural component. The acoustic emission signals have features that contain information on what is going on in the material. Feature extraction can also give insight to the fatigue lifetime. In addition, the acoustic emission signals can be used for localization of the damage. The localization calculation is done by comparing different times of arrivals and with this information the source can be localized. If this localization and tracking of the fatigue crack can be achieved on the multiaxial testing machine the new prediction models that are developed at the university can be more rigorously evaluated.
Combining the knowledge of fatigue and acoustic emission method, the following main research question is formulated: “Is it possible to detect a fatigue crack with the acoustic emission method, and how can the crack length and position be estimated in the tubular welded T-joint specimen multiaxial loading?”
Before the main question could be tackled, the background noise of the testing machine is measured for possible interference for future test. The second sub-question is: What features of the acoustic emission signal can be acquired and processed robustly for the tubular specimen? The last sub-question is: What are the essential features of the acoustic emission signals released by fatigue cracks under multiaxial loading? This last question could not be answered because there were no multiaxial experiments during the duration of this project. However, the methodology presented here as shown on a uniaxial case is believed to be directly applicable for multiaxial loading cases as well
Three experiments were performed of which one uniaxial test was used for detailed acoustic emission analysis. The data was analyzed with localization of the fatigue crack in the specimen. The localization of the crack and tracking its position is shown to be possible when the data is of sufficient quality. The difference in the estimate of the crack size by acoustic emission and the measured crack length turned out to be smaller than 10% when the crack reached its maximum length.