Fatigue strength and singular stresses in connections between machinery components
Backgroud:
Wind energy is growing fast. Many new wind turbines are installed all over the world with increasing dimensions, currently up to 126 meter rotor diameter and masses of several hundreds of tonnes. New components are required in the development of new wind turbine types and sizes. The Product Development service offered to customers by the product group Wind Energy of MECAL fulfils this demand, through design and analysis of wind turbine components.
Wind turbines include many connections between machinery components, which are often bolted connections. The fatigue strength of the components and bolts are evaluated by means of stress calculations using 2D or 3D FEM calculations. In these projects the fatigue and ultimate strength of components is determined by combining stresses from a finite element analysis with simulated wind turbine loads. In-house developed MECALl software is available for these strength calculations and is used for product certification. This approach is often straight forward and gives realistic material stresses.
However, in the contact surface between the machinery components, unrealistic high stresses can occur at the outer contours of the components, e.g. at the inner and outer diameter of a flange. At these positions there is a sudden, stepwise, transition of material which gives a sudden stiffness change in the FEM model. This sudden increase of stiffness results to singular stress results. One of the causes is the (unrealistic) high influence of the element mesh in the FEM model at the resulting stresses.
The reality will be different. The outer contours of the components will include chamfers and rounding’s (often not modelled in FEM) that reduce the singularity. In addition some plastic deformation will occur in high stressed areas which will reduce the stresses also. Furthermore local gapping in the connection will take away the occurrence of tensile stresses.
The singular points are often ignored in the strength calculation. This is motivated by reasons like mentioned above
Purpose:
Detailed FEM calculations, including geometrical details, non-linear material, contact and load history effects are necessary for a better understanding and validation of the above mentioned phenomena.
Outline:
The required actions to be taken during the project are:
- Training in software programs, theory;
- Detailed calculations, investigations;
- Evaluation of output, making proposal;
- Documentation.
Preferably you study a master in mechanical engineering. You will work amongst a team of engineers that are assessing and designing wind turbine components. Some of these engineers will be involved in this project for assistance. Your role is to provide a better understanding of the phenomena occurring between components, possibly resulting in improved calculation methods for future strength evaluation projects.
Type of project:
Final thesis, Master level.
Coordinator / for more information on the subject:
Bart Visser, Ir, Chief engineer, +31 (50) 3110 996, b.visser@mecal.eu
Product group and location:
Wind Energy, Product Development, Enschede or Groningen
To apply for the project:
Mr Dick van Haare, Recruitment, +31 (53) 4821 400, d.vanhaare@mecal.eu