Blade bolted connection analysis

Wind & Water Structural Analysis
1

Hello,

I’m looking for standards/analysis/design documentation for the bolted connection (t-bolts) from the blade to the pitch bearing. My purpose is to develop a fatigue test load.

Thanks for the help

2

I would do the following (I believe this is the standard method)

  1. build FE model of blade root section, bolts, hub and any other components you believe will influence the stress distribution
    (a) bolts may be represented either by solids or beams
    (b) a conservative simplification would be to define them with constant cross-section, comprising the nominal area of cross section of the ‘T’-bolts - this is conservative because the stiffness is on the high side
    (c) create suitable boundary conditions between beam elements and solid elements. For instance a ‘T’-bolt might be ‘hinged’ freely about one degree of freedom but cantilevered with respect to the orthogonal degree of freedom
  2. create appropriate material property-sets for blade root (isotropic approximation to fibre-glass), bolts, hub (cast iron?)
  3. take account of residual stresses in the bolt due to friction in the bolt threads using a standard method (e.g. VDI 2230)
  4. apply loads at a point coincident with the point at which the loads were extracted from the simplified beam element model (in FAST)
  5. apply extreme loading combination/ calculate extreme structural margins
  6. extract the relationship between applied loads and stresses in the bolts (at various locations) - unit load relationships
  7. build an analytical model of a single segment of the bolted connection (e.g. see VDI 2230 for a method)
  8. test that the maximum loading state during normal operation does not exceed the limits of linear behaviour of the bolted connection (e.g. connection does not open during normal power production in normal conditions)
  9. include the unit load relationship from 6 above to take account of bending, which is not accounted for in the VDI2230
  10. convert load time series to stress time series and calculate fatigue damage using Miner’s rule and fatigue properties for bolt (e.g. detail category 36* from EN 1993-1-9)
3

Hi Mark,

Thank you for the detailed reply. The project I was working on is currently over, and I just used a simplified model of one section using VDI 2230. In a sense I just determined the joint stiffness. This was to develop a hypothetical damage equivalent load for a bolt with an embedded sensor.

However; in the future I would look into more details as you have mentioned.