Fatigue calculations and damage calculations with complex loads
In large mechanisms and in mechanisms with long service-life, criteria like transportability, cost-price, dynamic loading and size-restrictions often lead to fatigue-critical designs. Huygens has developed extensive expertise in fatigue assessment.
Fatigue calculations can be done analytically for normalized situations. The figure shows an excerpt of a shaft-fatigue calculation in MathCAD, according to DIN 743.
On the other end of the spectrum, regarding the complexity of fatigue calculations, lie situations where loads vary in intensity and in direction in complex patterns. This is, for example, the case in a tidal turbine where fatigue is assessed by adding damage of a set of recurring cycles (time-series), incorporating cycles stemming from rotation of the blades in a non-uniform flow, due to surface-waves, varying flow-strength, and non-uniform flow, in turn attributed to the boundary layer effects on the sea-floor.
These types of challenges can be tackled with normal stress projections on planes at each node of the mesh, rainflow counting and a full damage assessment (typically through Palmgren-Miner).
The figure shows a water turbine, shows the detailed meshing necessary at locations of stress-concentrations, shows an example of optimal local coordinates on each point of the mesh for cumulative fatigue analysis, and shows cumulative damage projected on a part of the surface resulting from a complex load pattern.
