Professor Zeljko Domazet, Dean of Engineering of the University of Split in Croatia, recently visited the Wessex Institute to discuss a number of collaborative projects. During his visit, he gave a Seminar on “Fatigue Failures and Repairs in Industry”.
WIT has a number of initiatives with the University of Split, including the recent publication of several volumes on Electromagnetics and the organisation of two important Conferences in that city, one on Computational Methods in Electromagnetics (ELECTROCOMP) and the other on Boundary Element Methods (BEM). A Master programme on Electromagnetic Compatibility has also been launched with the two Institutions collaborating in the training and supervision of the participants.
In his lecture, Zeljko outlined several case studies of fatigue failure in industry and how the problems have been resolved.
The steps to successfully resolving the problems are, in the first place, an adequate analysis, followed by damage repairs to determine the best possible procedure. The component residual life should also be taken into account and, finally, a control procedure should be set up.
Zeljko described some interesting studies, including structural cracks in the gears of a large cement kiln. The case was studied with finite elements and it was determined that the stresses were relatively small. It was difficult to understand why the cracks occurred until they found that there were many impurities deposited in the gears which gave rise to large stress concentrations.
Another interesting application was in the case of naval engineering in which the brackets supporting the main engine started to develop cracks. The problem seemed to have been due to a sudden change of rigidity and a design leading to including inaccuracies during manufacturing of the supports leading to large stress concentrations. The fatigue life was reduced to a few weeks rather than years! The solution was to redesign the support.
The third case presented by Zeljko was the problem of structural cracks in the legs of a large crane. They tried to repair them by welding but their reoccurrence led to the carrying out of a full study, including a finite element model. A suggested change in design reduced the stress concentration but the new design could not be implemented immediately due to the financial penalties that would result if work was delayed in the ship yard. A series of temporary reinforcements were built and the cracks were monitored using strain gauges.
The conclusion is that although all fatigue cases are different, it is important to be aware of the details, as in other ways to remember that “the devil is the detail!”.