NORMAS DE INGRESO Y EGRESO DEL PERSONAL, VEHÍCULOS, MATERIALES Y OTROS

5.3.3.2.1 General

(1) Depending on the required computational accu- racy, fatigue analysis by calculation may be performed with the aid of one of the following three procedures: – by using stress-time series and damage accumula-

tion to register the complex interaction between the external loadings and the structural responses as accurately as possible, or

– by using stress spectra and damage accumulation. The superposition of the various load effects shall include the worst physical meaningful combina- tion.

– with equivalent constant-range spectra as a simpli- fied form of the fatigue analysis. Here the equiva- lent constant-range spectra shall be used in accor- dance with Section 4.B.2.3.

(2) The procedure and the applied loads shall be documented adequately (see Appendix 5.A.5).

(3) Tower and foundation shall be verified accord- ing to EN 1993-1-9:2005.

(4) If not defined otherwise in referenced standards, the influence of the mean stress shall be considered in accordance with Section 5.3.3.4.

(5) For complex components subjected to com- bined loading (see Fig. 5.3.1), adequate procedures for the hot-spot localization shall be applied. In general, stress-time series shall be used and the entire compo- nent must be analysed.

5.3.3.2.2 Simplified fatigue analysis

(1) For the simplified fatigue analysis, which is generally applied when considering safety margins by stress reserve verification (e.g. comparison of plant variants with different rotor diameters), equivalent constant-range spectra can be used. In the following, it will be assumed that the elaboration of equivalent constant-range spectra on the basis of the Palm- gren/Miner method is already known. Explanations on this method can be taken from e.g. [5.2].

(2) When generating the equivalent constant-range spectrum, the slope parameter of the S/N curve corre-

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sponding to the material used shall be applied. The decisive slope parameter of the design S/N curve is given in Section 5.3.3.5.

(3) In this and the other analysis approaches, the partial safety factor γM shall be applied in relation to

the criteria given in Table 5.3.1.

(4) For the stress superposition in the case of multi- axial stress conditions, see Section 5.3.3.2.4.

(5) When using the simplified fatigue analysis for considering safety margins, it shall be observed that the assumed reference load cycle number generally does not correspond to the assumed design lifetime of the component.

(6) Reducing influences on the fatigue resistance (such as probability of survival Pü, surface influence

etc.) shall be taken into account analogously to the determination of the S/N curves according to Section 5.3.3.5.

Note:

By way of example, instances for the application of the partial safety factor γM are named in Table 5.3.2 for fatigue analyses which normally apply according to the criteria listed in Table 5.3.1 for the force- and moment-transmitting components of a wind turbine to be considered here.

Table 5.3.1 Partial safety factor γM for fatigue verification

Inspection and accessibility Component failure results in destruction of wind turbine or endangers people Component failure results in wind turbine

failure or consequential damage

Component failure results in interruption

of operation Periodic monitoring and

maintenance; good accessibility

1.15 1.0 1.0 Periodic monitoring and

maintenance; poor accessibility

1.25 1.15 1.0

Table 5.3.2 Example for the partial safety factor γM

Penetrations for reinforcing steel in the foundation section

Cannot be inspected γM=1.25

Bearing collar of the rotor shaft Cannot be inspected without removing the shaft

γM =1.25

Planet carrier of main gearbox Cannot be inspected; failure can lead to destruction of wind turbine

γM =1.25

Bolted connection of hub/rotor shaft (multiple bolt connection)

A single bolt failure in a multiple bolt connection can be detected before com- plete failure of the connection

γM =1.15

Fixture for control cabinets Fixture of accumulators

Operation of wind turbine will be inter- rupted

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5.3.3.2.3 Damage calculation

(1) The execution of fatigue verifications via dam- age accumulation will hereunder be assumed to be known. Explanations of this method may for instance be taken from [5.1], [5.2].

(2) When working out a damage accumulation, all stress ranges Δσi due to operational loads in accor-

dance with Chapter 4 shall, as a matter of principle, be used in conjunction with their associated stress cycle numbers ni. The damage sum D from the fatigue

strength calculation is dependent on the material, type of loading and structural geometry. The damage sum may not exceed the following values:

D ≤ 1

In case of welded machinery components that are subjected to variable amplitude loading, the damage sum may not exceed

D ≤ 0.5

e.g. when using the Palmgren/Miner linear damage accumulation hypothesis:

D =

∑

i

ni / Ni ≤ Dadmi

where:

ni = number of stress cycles in one bin of stress

ranges

Ni = number of tolerable stress cycles in one bin of

stress ranges

(3) The number of tolerable stress cycles Ni is here

the permissible number of stress cycles of the relevant S/N curve for the stress range Δσi · γM .

(4) The partial safety factor γM is given in Table

5.3.1.

(5) For the damage accumulation, the design S/N curves given in the paragraphs that follow (see Section 5.3.3.5) and the equivalent stresses described in Sec- tion 5.3.3.3 shall be used.

(6) For stress superposition in the case of multi- axial stress conditions, see Section 5.3.3.2.4.

5.3.3.2.4 Notes on the superposition of multi-axial stress conditions

(1) For multi-axially stressed components (see Fig. 5.3.1), it is necessary to consider the complex stress conditions in a realistic manner and to prepare them for the damage accumulation calculation in a physi- cally meaningful manner. For this, the relevant time series of the fatigue loads are applied in accordance with Chapter 4.

Periodical periodicalNon-

Cyclic Cycle forms

Single loading Combined loading

Proportional Non-proportional Synchronous (in phase) Out of phase Different frequencies Uniaxial stress state Multi-axial stress state 1 -1 0 g(t) π 2π ωt a) Sinusoidal 1 -1 0 π 2π _ωt b) Trapezoidal 1 -1 0 π 2π _ωt c) Triangular σ σ t t t t+T Principal directions σ2 σy_τ yx τxy σx σ1 ϕ = constant ϕ = varying ϕ t t t t e.g.. σ τ F F

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(2) When analysing multi-axial stresses, it is rec- ommended that the dominating (damage-relevant) stress distribution or stress combination be estab- lished for the critical regions via consideration of the principal stresses and principal-stress directions. Oc- casionally, the presence of a dominating load compo- nent, or the combination of load components, may lead to a stress condition that is close to uni-axial. In such cases, this may allow a possible simplification that is appropriate for the problem.

(3) If the nominal stress approach is chosen for the assessment of welded joints and if normal and shear stresses occur simultaneously, their combined effect shall be considered in accordance with [5.1] or [5.3]. (4) If the structural stress approach is chosen for the assessment of welded joints, principal stresses shall be analysed. In cases where the direction vector of the principal stress is approximately in line with the perpendicular to the weld seam and does not change significantly over time, this stress may be used in combination with the fatigue resistance values in accordance with [5.1] or [5.3]. If the direction vector varies significantly, the other principal stresses need to be analysed as well. Their combined effect shall be considered in accordance with [5.3].

(5) The applicable procedure depends on the mate- rial, the type of loading and structural geometry, and shall be defined in consultation with GL.