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5.6 ASME Sec VIII Div 3

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Fatigue analysis is mandatory for Division 3 vessels.  It is the pressure vessel User’s responsibility to provide, or cause to be provided, information in sufficient detail so an analysis for cyclic operation can be carried out.

If ASME Sec VIII Div 3 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 3 button  on the toolbar becomes activated. 

 

Choose Input>ASME Sec VIII Div 3 or click ASME Sec VIII Div 3 button.  The ASME Sec VIII Div 3 Data Input dialog box appears. 

 

 

5.6.1 Selecting Material

 

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

 

 

5.6.2 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.

 

5.6.3 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.

 

5.5.6.1 Use smooth bar fatigue curve 

 

Fatigue analysis performed through direct interpretation of the smooth bar fatigue curve.

 

Click Smooth bar correction factors button to edit fatigue strength reduction factor, roughness factor, and vessel mean stress type in the following pop-up dialog window.

 

 

The fatigue strength reduction factor Kf is a stress intensification factor which accounts for the effect of a local structural discontinuity (stress concentration) on the fatigue strength.  If local effect is already accounted for in the inputted stresses, choose Kf = 1.0; for V-type threads on bolts, choose Kf = 4.0 for cut threads and Kf = 3.0 for rolled threads.  If other value of Kf is to be used, click Manual input radio button and enter a Kf value.

 

The Roughness factor group box is for roughness factor calculation.  The roughness factor Kr is calculated according to the value of either average surface roughness Ra or maximum surface roughness Rmax.  Note that the following fatigue curves have specific surface roughness applicabilities:

 

Figure KD-320.1 and Figure KD-320.4, applicable for an average surface roughness of 19Ra μin. (0.5Ra μm) or a maximum surface roughness of 59Rmax μin (1.5Rmax μm).

 

Figure KD-320.2 and Figure KD-320.3, the influence of the surface roughness is included in the curve, i.e., Kr = 1.0; therefore, a surface roughness factor need not be applied. In these cases, enter Ra = 1 μin. (enter Ra = 0.11 μm. in metric unit system), or enter Rmax = 1 μin. (enter Rmax = 0.1 μm. in metric unit system).

 

Autofrettage is one of several processes that can be used to produce favorable residual stresses in thick‐walled pressure vessels. The theory of autofrettage is based on the fact that the stress in a thick‐walled cylindrical vessel is higher at the bore than at the outside surface for a given internal pressure. If such a vessel is subjected to a continuously increasing pressure, the entire vessel will deform elastically until some pressure is reached at which the material at the bore begins to plastically deform. As the pressure continues to increase, the boundary at which material begins to yield moves from the bore through the vessel wall until it reaches the outer wall, causing plastic collapse [see KD-210(e)(6)]. In the process of autofrettage, the pressure is increased from the point of first yielding at the bore to a pressure that will place the elastic–plastic interface at the desired radius. The removal of this pressure then produces compressive residual tangential stress at the bore and tensile residual tangential stress at the outer wall.

 

For autofrettaged vessels or nonautofrettaged vessel with compressive mean stress, check Autofrettaged or nonautofrettaged with compressive mean stress radio button; For nonautofrettaged vessels with tensile mean stress, check Nonautofrettaged with tensile mean stress radio button.

 

Click OK button to confirm and return to previous window.

 

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.

 

 

5.5.6.2 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. 

 

NOTES:

 

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, and High Tensile Strength Steels for temperatures not exceeding 700°F (371°C)

 

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

 

(c) Wrought 70 Copper‐Nickel for temperatures not exceeding 450°F (232°C)

 

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

 

(e) Aluminum Alloys for temperatures not exceeding 225°F (107°C)

 

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

 

 

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

 

 

(1) Cyclic stress-strain curve data.  From the drop-down menu select the appropriate material so that the material constants 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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