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5.5 ASME Sec VIII Div 2

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If ASME Sec VIII Div 2 is selected in the Fatigue model/code drop-down menu in Project Setup dialog box in 5.1 Data Entry and Project Setup, the ASME Sec VIII Div 2 button  on the toolbar becomes activated. 


Choose Input>ASME Sec VIII Div 2 or click ASME Sec VIII Div 2 button.  The ASME Sec VIII Div 2 Data Input dialog box appears. 



5.5.1 Selecting Material


Select a material from Spec No. drop-down menu.



5.5.2 Stress Analysis



If stresses and strains are imported from an ANSYS .rst file, choose Elastic or Elastic-Plastic from the Stress Analysis group box.  When Elastic-Plastic is chosen, the plastic strains should be available in ANSYS .rst file.  If stresses or strains are manually inputted, the Stress Analysis group box is for information only; there is no need for user's interference.  If von Mises stresses or 6-component stresses are entered, the Elastic radio button is automatically checked.  If 6-component stresses plus 6-component strains are entered, the Elastic-Plastic radio button is automatically checked.


5.5.3 Vessel Class



Choose Class 1 or Class 2 from Vessel Class group box.


Class 1 Vessel – a vessel that is designed using the allowable stresses from Section II, Part D, Subpart 1, Table 2A or Table 2B.


Class 2 Vessel – a vessel that is designed using the allowable stresses from Section II, Part D, Subpart 1, Table 5A or Table 5B.


5.5.4 Weld Surface Fatigue-Strength-Reduction Factor



The default value of weld surface fatigue-strength-reduction factor Kf is 1.0.  To change Kf value, click Change button and the following dialog window displays.



Select Kf value per quality value and definition by clicking corresponding radio button.  Click OK button to confirm and return to previous window.


5.5.5 Sn : Sp Ratio



Sn = primary + secondary stress (Pm + PL + Pb + Q)


Sp = primary + secondary + peak stress (Pm + PL + Pb + Q + F)


It is required that entered stresses be Sp so that Sn is derived from Sp by Sn:Sp ratio (<= 1.0). Leaving Sn:Sp ratio = 1.0 is conservative.


5.5.6 Selecting Fatigue Curve



The radio button is automatically selected by the program according to stress types the user entered; there is no user's interference required.  The Use welded joint fatigue curve is selected when Membrane + bending stress or Membrane + bending + shear stress were manually entered.  In all other cases, the Use smooth bar fatigue curve is selected. Use smooth bar fatigue curve - Fatigue analysis performed through direct interpretation of the smooth bar fatigue curve.


The program automatically select a fatigue curve according to the user selected material.  The suggested fatigue is displayed in the Selected fatigue curve information box.



If you are not satisfied with the selection, click Change button to select another fatigue curve from the following pop-up dialog window.



Click OK button to confirm and return to previous window.  Click Cancel button to ignore the change.


Click Plot fatigue curve button.  The Fatigue Curve window appears.  You can zoom, print or export the chart, or change the styles of the chart by clicking the buttons on the tool bar.  Click X button on the upper-right corner to close the window.  The following is an example. Use welded joint fatigue curve


An equivalent structural stress range parameter is used to evaluate the fatigue damage for results obtained from a linear elastic stress analysis. The controlling stress for the fatigue evaluation is the structural stress that is a function of the membrane and bending stresses normal to the hypothetical crack plane.


Fatigue cracks at pressure vessel welds are typically located at the toe of a weld. For as‐welded and weld joints subject to post weld heat treatment, the expected orientation of a fatigue crack is along the weld toe in the through‐thickness direction, and the structural stress normal to the expected crack is the stress measure used to correlate fatigue life data. For fillet welded components, fatigue cracking may occur at the toe of the fillet weld or the weld throat, and both locations shall be considered in the assessment. 




The welded joint design fatigue curves can be used to evaluate welded joints for the following materials and associated temperature limits:


(a) Carbon, Low Alloy, Series 4XX, High Alloy, and High Tensile Strength Steels for temperatures not exceeding 371°C (700°F)


(b) Series 3XX High Alloy Steels, Nickel–Chromium–Iron Alloy, Nickel–Iron–Chromium Alloy, and Nickel–Copper Alloy for temperatures not exceeding 427°C (800°F)


(c) Wrought 70 Copper–Nickel for temperatures not exceeding 371°C (700°F)


(d) Nickel–Chromium–Molybdenum–Iron, Alloys X, G, C-4, and C-276 for temperatures not exceeding 427°C (800°F)


(e) Aluminum Alloys


The Use welded joint fatigue curve is selected when Membrane + bending stress or Membrane + bending + shear stress were manually entered.



Click Edit button to edit weld fatigue assessment data in the following pop-up dialog window.



(1) Cyclic stress-strain curve data.  The cyclic stress-strain curve describes the material model for material nonlinearity.  From the drop-down menu select the appropriate material so that the material parameters nCSS and KCSS can be retrieved by the program.


(2) Vessel wall thickness.  Enter the thickness to account for size effect for fatigue analysis.


(3) Statistical basis for coefficients for welded joint fatigue curves.  From the drop-down menu choose a statistical basis so that the coefficients C and h for welded joint fatigue curves can be retrieved by the program.  The lower 99% Prediction Interval (–3σ) shall be used for design unless otherwise agreed to by the Owner‐User and the Manufacturer.


(4) Fatigue improvement.  If a fatigue improvement method is performed that exceeds the fabrication requirements of the Code, then a fatigue improvement factor, fI , may be applied. The fatigue improvement factor fI is calculated by the program if Burr grinding or TIG dressing or Hammer peening radio button is checked.  If fatigue improvement is not considered, check No improvement (fI = 1.0) radio button.


(5) Environmental modification factor fE.  The design fatigue cycles may be modified to account for the effects of environment other than dry ambient air that may cause corrosion or subcritical crack propogation. The environmental modification factor, fE, is typically a function of the fluid environment, loading frequency, temperature, and material variables such as grain size and chemical composition. A value of fE = 4.0 shall be used unless there is specific information to justify an alternate value based on the severity of the material/environmental interaction. A value of fE = 1.0 may be used for dry ambient air. The environmental modification factor, fE, shall be specified in the User’s Design Specification.


(6) Weld quality.  If a defect exists at the toe of a weld that can be characterized as a crack‐like flaw, i.e., undercut, and this defect exceeds the value permitted by the Code, then a reduction in fatigue life shall be calculated.  Check Crack-like flaw exists radio button and enter the depth of the crack‐like flaw a at the weld toe. The calculation is valid only when a/t ≤ 0.1.  If defect is not considered, check Crack-like flaw does not exist radio button.


Click OK button to confirm and return to previous window.


Plotting of fatigue curve for welded joints is not available.


When completed, click OK button to confirm and return to the main window.






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