4
Chapter 1
Guidance for calculation of Stress Concentration Factors in the web
fillet radii of crankshafts by utilizing Finite Element Method
■ Section 1
Definition of Stress Concentration Factors in crankshaft fillets
1.1 General
1.1.1 The objective of the analysis is to develop Finite Element Method (FEM) calculated figures as an alternative to
the analytically calculated Stress Concentration Factors (SCF) for the crankshaft fillets. The analytical method is based
on empirical formulae developed from strain gauge measurements of various crank geometries, and therefore the
application of these formulae is limited to those geometries.
1.1.2 The SCFs calculated according to the rules of this document are defined as the ratio of stresses calculated by
FEM to nominal stresses in both journal and pin fillets. When used in connection with the present method in the
applicable Rules or the alternative methods, von Mises stresses shall be calculated for bending, and principal stresses
for torsion care should be taken to avoid mixing equivalent (von Mises) stresses and principal stresses.
1.1.3 This guidance, as well as that contained in Ch 2, Guidance for evaluation of Fatigue Tests, is valid for both solid
cranks and semi built cranks, except journal fillets.
1.1.4 The analysis is to be conducted as linear elastic FE analysis, and unit loads of appropriate magnitude are to be
applied for all load cases.
1.1.5 The calculation of SCF at the oil bores is not covered by this chapter.
1.1.6 It is advised that the element accuracy of the FE solver in use is checked, e.g. by modelling a simple geometry
and comparing the stresses obtained by FEM with the analytical solution for pure bending and torsion.
1.1.7 Boundary Element Method (BEM) may be used instead of FEM.
■
Section 2
Model requirements
2.1 General
2.1.1 The basic recommendations and perceptions for building the FE-model are presented in Ch 1, 2.2 Element
mesh recommendations. It is obligatory for the final FE-model to fulfill the requirement in Ch 1, 2.4 Element mesh quality
criteria.
2.2 Element mesh recommendations
2.2.1 In order to fulfil the mesh quality criteria it is advised that the FE model for the evaluation of Stress
Concentration Factors be constructed according to the following recommendations:
• The model consists of one complete crank, from the main bearing centerline to the opposite side main bearing
centerline.
• Element types used in the vicinity of the fillets:
• 10 node tetrahedral elements;
• 8 node hexahedral elements;
• 20 node hexahedral elements.
• Mesh properties in fillet radii. The following applies to ±90 degrees in circumferential direction from the crank plane.
• Maximum element size a=r/4 through the entire fillet as well as in the circumferential direction. When using 20 node
hexahedral elements, the element size in the circumferential direction may be extended up to 5a. In the case of
multi-radii fillet, r is the local fillet radius. (If 8 node hexahedral elements are used even smaller element size is
required to meet the quality criteria.)
• Recommended manner for element size in fillet depth direction:
• First layer thickness equal to element size of a;
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