Section 2 Model requirements

2.1 General

2.1.1 The basic recommendations and assumptions for building of the FE-model are presented in Ch 4, 2.2 Element mesh recommendations. The final FE-model must meet one of the criteria in Ch 4, 2.4 Element mesh quality criteria.

2.2 Element mesh recommendations

2.2.1 For the mesh quality criteria to be met, construction of the FE model for the evaluation of stress concentration factors according to the following recommendations is advised:

The model consists of one complete crank, from the main bearing centreline to the opposite side’s main bearing centreline.
The following element types are used in the vicinity of the outlets:
- 10-node tetrahedral elements;
- 8-node hexahedral elements;
- 20-node hexahedral elements.
The following mesh properties for the oil bore outlet are used:
- Maximum element size a = r / 4 through the entire outlet fillet as well as in the bore direction (if 8-node hexahedral elements are used, even smaller elements are required for meeting of the quality criterion).
- Recommended manner for element size in the fillet depth direction:
  - First layer’s thickness equal to element size of a;
  - Second layer’s thickness equal to element size of 2a;
  - Third -layer thickness equal to element size of 3a.
In general, the rest of the crank should be suitable for numeric stability of the solver.
Drillings and holes for weight reduction must be modelled.

2.2.2 Submodeling may be used as long as the software requirements are fulfilled.

2.3 Material

2.3.1 Pt 5, Ch 2, 3 Crankshaft Design of the Rules and Regulations for the Classification of Ships, July 2022 does not consider material properties such as Young’s modulus (E) and Poisson’s ratio (ν). In the FE analysis, these material parameters are required, as strain is primarily calculated and stress is derived from strain through the use of Young’s modulus and Poisson’s ratio. Reliable values for material parameters must be used, either as quoted in the literature or measured from representative material samples.

2.3.2 For steel the following is advised:

2.4 Element mesh quality criteria

2.4.1 If the actual element mesh does not fulfil any of the following criteria in the area examined for SCF evaluation, a second calculation, with a finer mesh is to be performed.

2.4.2 Principal stresses criterion: The quality of the mesh should be assured through checking of the stress component normal to the surface of the oil bore outlet radius. With principal stresses σ₁, σ₂ and σ₃ the following criterion must be met:

2.4.3 Averaged/unaveraged stress criterion: The criterion is based on observation of the discontinuity of stress results over elements at the fillet for the calculation of the SCF:

Unaveraged nodal stress results calculated from each element connected to a node i should differ by less than 5 per cent from the 100 per cent averaged nodal stress results at this node i at the location examined.