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4.3 Stress Method

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The Stress Method drop-down menu is for the user to select applicable stress type as the basis of fatigue life calculations.  The user is expected to be familiar with relevant design code being used and be able to choose appropriate stress type.  The stress method depends on which design code is going to be used.  The Decision Matrix button next to the Stress Method drop-down menu provides a decision making matrix that helps the user make a appropriate selection.

 

 

Press the Decision Matrix button, the Decision Matrix dialog window will pop up:

 

 

Click on one of the radio buttons to make a selection, and click OK button to close the dialog.  The Decision Matrix dialog and the Stress Method drop-down menu perform the same function; the user can make the selection from either way.

 

The Matake criterion is not included in the decision matrix; it will be activated where the shear stress + normal stress combination or 6-component stresses exist.

 

The mean stress criteria are not included in the decision matrix; they will be activated where applicable.

 

4.3.1 Shear stress

 

Shear stress series for EN 12952-3, EN13445-3 unwelded components, ASME Section VIII Division 3 smooth bar fatigue curves, and ASME Section III Division Appendices.

 

4.3.2 Shear stress +  normal stress  

 

Shear stress series and normal stress series for EN 12952-3, EN13445-3 unwelded components, ASME Section VIII Division 3 smooth bar fatigue curves, and ASME Section III Division Appendices.

 

Shear stresses and normal stresses are entered under separate tabs.  Number of shear stresses and number of normal stresses must be equal.

 

 

4.3.3 Stress intensity    

 

Shear intensity series for EN 12952-3, EN13445-3 unwelded components, ASME Section VIII Division 3 smooth bar fatigue curves, and ASME Section III Division Appendices.

 

4.3.4 von Mises stress 

 

von Mises stress series for Basquin model, EN13445-3 welded and unwelded components, and ASME Section VIII Division 2 smooth bar fatigue curves.

 

4.3.5 Principal stress 

 

Principal stress series for Basquin model, and EN13445-3 welded and bolted components.

 

4.3.6 Membrane + bending stress 

 

Membrane stress series and bending stress series for fatigue assessment of welds in ASME Section VIII Division 2 and Division 3.  If component crack face pressure needs to be considered, add the pressure values to membrane stress values. The crack face pressure should be specified if the maximum value of the membrane plus bending stress used in the analysis occurs on a surface that is exposed to the fluid pressure. A conservative approach is to always specify the crack face pressure.  The crack face pressure is based on the actual or operating pressure defined in the loading time history.

 

 

Membrane stresses and bending stresses are entered under separate tabs.  Number of membrane stresses and number of bending stresses must be equal.

 

4.3.7 Membrane + bending + shear stress 

 

Membrane stress series, bending stress series and shear stress series for fatigue assessment of welds in ASME Section VIII Division 2 and Division 3.  If component crack face pressure needs to be considered, add the pressure values to membrane stress values. The crack face pressure should be specified if the maximum value of the membrane plus bending stress used in the analysis occurs on a surface that is exposed to the fluid pressure. A conservative approach is to always specify the crack face pressure.  The crack face pressure is based on the actual or operating pressure defined in the loading time history.

 

The shear stress series is for multiaxial fatigue when the structural shear stress range is not negligible.

 

 

Membrane stresses, bending stresses, and shear stresses are entered under separate tabs.  Number of membrane stresses, number of bending stresses, and number of shear stresses must be equal.

 

4.3.8 6-component stresses 

 

In continuum mechanics, stress at a point can be expressed in nine stress vectors, but because the stress tensor in equilibrium is a symmetric tensor, there are only six independent components to the stress tensor.  The 6-component stresses in the order of σxyzxyyzxz represent the stress at a point.  In the counter of the interface, every 6 components are counted as one stress.  The component stresses can be manually written, but are often more conveniently generated by FEA programs and imported to the input editor.

 

This approach can be used in Basquin model, EN 12952-3, EN13445-3 unwelded, weilded and bolted components, ASME Section VIII Division 2 and Division 3 smooth bar fatigue curves, and ASME Section III Division Appendices.

 

4.3.9 6-component stresses + 6-component plastic strains 

 

For ASME Section VIII Division 2 smooth bar inelastic analysis, enter 6-component stresses and 6-component plastic strains under separate tabs.

 

 

The 6-component stresses in the order of σxyzxyyzxz represent the stress at a point.  In the counter of the interface, every 6 components are counted as one stress.  The component stresses can be manually written, but are often more conveniently generated by FEA programs and imported to the input editor.

 

Likewise, The 6-component plastic strains in the order of εx,εy,εz,γxy,γyz,γxz represent the stress at a point.  In the counter of the interface, every 6 components are counted as one strain.

 

Number of stresses and number of strains must be equal.

 

4.3.10 6 component stresses + 6 component total strains 

 

For Basquin-Coffin-Manson (BCM) model, enter 6-component stresses and 6-component total strains under separate tabs.

 

 

The 6-component stresses in the order of σxyzxyyzxz represent the stress at a point.  In the counter of the interface, every 6 components are counted as one stress.  The component stresses can be manually written, but are often more conveniently generated by FEA programs and imported to the input editor.

 

Likewise, The 6-component total strains in the order of εx,εy,εz,γxy,γyz,γxz represent the stress at a point.  In the counter of the interface, every 6 components are counted as one strain. 

 

Number of stresses and number of strains must be equal.

 

4.3.11 Stress +  strain (BCM model)

 

For Basquin-Coffin-Manson (BCM) model, stresses and strains are entered under separate tabs.  The strains in BCM model are total strains.

 

 

Number of stresses and number of strains must be equal.

 

4.3.12 6 component strains (KBM model) 

 

In Kandil-Brown-Miller (KBM) model, enter 6-component strains in the order of εx,εy,εz,γxy,γyz,γxz.  In the counter of the interface, every 6 components are counted as one strain.  The component strains can be manually written, but are often more conveniently generated by FEA programs and imported to the input editor.

 

4.3.13 Shear strain + normal strain (KBM model) 

 

In Kandil-Brown-Miller (KBM) model, shear strains and normal strains are entered under separate tabs. 

 

 

Number of shear strains and number of normal strains must be equal.

 

4.3.14 Shear strain + normal stress (FS model) 

 

In Fatemi-Socie (FS) model, shear strains and normal stresses are entered under separate tabs. 

 

 

Number of shear strains and number of normal stresses must be equal.

 

 

 

 

 

 

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