Fatigue Analysis MSC Patran

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22September2011TLM CAE – Dr. Onur KIRLI ZF confidential ©2011 2

FE-Based Fatigue Analysis Procedurewith MSC Patran 2010 and MSC Fatigue

FIN stands for ‘Fatigue INformation’ file

FES stands for ‘Finite Element Stress’ results

.op2 and .xdb / .t16 / .fil and .odb / .rst / .D3plot


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Create an outer skin shell group


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This skin shell group will be “region” at material inputs


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The analysis type offered depends on the FE results available.

For linear static analysis, the options include Strain life EN

(initiation) and Stress life SN, along with various multiaxial methods.

Random Vibration Fatigue is available if PSD results are used, and

Seamweld and Spotweld options are also available.

For general analysis performing an EN approach analysis first and

then decide whether a multiaxial approach is required following an

inspection of the biaxiality plots.

SN is not preferred for most FE analysis because it is invalid in

regions of elasticplastic stress such as those adjacent to notches

and holes.

Initiation is used at comparisons.


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There is a choice of Element or Nodal results. ‘Element’ resultoption was recommended for shell elements as this yields the

most accurate stresses.

For linear analyses, (or nonlinear where material hardening is

not considered), there’s no difference between taking stressor strain. However, if you wish to include nonlinear material

behaviour in the FE analysis, you should use the Strain results

here, and select EP Input (elasticplastic).

Node option is used at comparisons.


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Fatigue is influenced by the residual stress field in the

component and the mean stress of the cyclic hysteresis

loop. Several methods are available to account for this,

the default is SWT as the most popular method.

Elasticplastic correction can be overridden if a nonlinear analysis of material hardening is carried out in

the FE analysis.

Standard EN material properties are applicable only for

uniaxial stress states. Where stresses are proportionalbiaxial (e.g. plane strain, torsion, etc.) a correction is

required. Software recommend using the Hoffmann

Seeger method for all analyses. If the stress state is

nonproportional, we must use a more complex fatigue

analysis.

S-W-T, Neuber, Material Parameter isused at comparisons.


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The FE results file contains 6 component stresses or

strains for each element. These pertain to the three

axial and three shear components. Principal stresses or

stress invariants (like VonMises) can be obtained from

the components. This selection allows the user to pick

which stress property to use for the fatigue analysis. In

general, the Abs. Max Principal stress should be used

as this yields the best fatigue results.

Critical Plane is used at comparisons.


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1

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Selecting Load Case


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1

2 3

Selecting Time History

5

4Loading can be in the form of constantamplitude or variable amplitude.

Constant amplitude loads are entereddirectly using Wave creation. Variableloads are represented by a time signalfile in either nCode DAC or MTS

RPCIII format. (ASCII files can betranslated if required).


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Convert Time History File or Header lines to skip = 4

BEGINNKANALNAME = ['F']

EINHEIT = ['N']

ENDE300000

30000

30000030000

=>

1

2


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Multiple Event Input fromDuty Cycle

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2


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1

FAT_DC_WAIT = Yes for Multiple Event Solution


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3


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Marked inputs are enough for E-N Analysis


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1 2

3

Region is the group created at slide 4, Layer isthe side of the shell which have highest stress.


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By calculating normals in MSC.Fatigue, the results are expressed as surfaceresolved stresses, meaning the two major principal stresses lie in the plane of thesurface with the third principal stress being zero (normal to the surface). This isimportant for models with solid elements especially given that 99% of cracksinitiate on the surface. The main reason that we need surface resolved stresses isfor the biaxiality analysis to properly calculate the biaxiality ratio which will bediscussed later in this example. Without surface resolved stresses it would bedifficult, if not impossible, to assess the multiaxial stress state of the component.


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8Solution and Result Plot


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Fatigue Analysis MSC Patran