Investing in fatigue

Fatigue is the weakening of a material caused by cyclic loading that results localized structural damage and the growth of cracks. Once a fatigue crack has initiated, it will grow a small amount with each loading cycle. The crack will continue to grow until it reaches a critical size. A lot of structures and equipment are subjected to repetitive loading and likely to develop cracks or failures. Hence we want to calculated the fatigue damage as accurate as possible. At the moment specific interest is the simulation and calculation of fatigue lifetime estimations for welded steel constructions. Our calculations are mostly performed with industry wide accepted standards like DNV-GL or FKM.

The subject of fatigue is not always intuitive or transparent. Users from academia and from engineering sectors, standards and commercial fatigue solvers have many different available calculation methods. And, we do not always have the material data at hand for each specific case. Let alone large benchmarks and/or the resources to acquire the best known calculation method for the case. Therefore, we are sponsoring and involved in for example several PHD projects at the TU Delft like the 4D-Fatigue project and it’s follow-up SCReen.

We believe these are excellent opportunities to further develop our physical background and calculations tools. We believe that through our direct cooperation with the universities, knowledge transfer and the implementation of the program results in our tools are guaranteed. Actually, we expect that new criterions will be validated on short-term which may lead to increasing the confidence level and improvement of the fatigue lifetime estimate accuracy in our analyses. And if so, these calculation methods will be implemented within the SDC Verifier software so that we can all benefit from this.

Also, just a headsup for everyone, many of our specimens failed at compression side of the joint. Fatigue failure due to pure loading in compression is very much possible and even can be very similar to pure tension! This is something that many engineers seem to wipe of the table instantly, for example by only calculating with maximum principle stresses in the FE model, so watch out.

From our own tests it is has become even more clear that fatigue calculations are not straight forward. General thoughts do not always match results from practice. And also widely accepted and used standards and fatigue calculation software can easily miss an accurate fatigue life estimation of a construction.