Mechanical APDL Structural Analysis Guide

14.3.2. Establish the Size, Fatigue Material Properties, and Locations

Define the following data:

• Maximum number of locations, events, and loadings


• Fatigue material properties


• Stress locations and stress concentration factors (SCFs)

1. Define the maximum number of stress locations, events, and loadings.

By default, your fatigue evaluation can consider up to five nodal locations, ten events, and three

loadings within an event. You can use the following option to establish larger dimensions (that

is, allow more locations, events, or loadings), if necessary.

Command(s):FTSIZE

GUI: Main Menu> General Postproc> Fatigue> Size Settings

2. Define material fatigue properties.

In order to calculate usage factors, and to include the effect of simplified elastic-plastic compu-

tations, you must define material fatigue properties. The material properties of interest in a fa-

tigue evaluation are:

• The S-N curve, a curve of alternating stress intensity ((Smax - Smin)/2) versus allowable number of
  cycles. The ASME S-N curves already account for maximum mean stress effects. You should adjust
  your S-N curve to account for mean-stress effects, if necessary. If you do not input an S-N curve,
  alternating stress intensities will be listed in decreasing order for all possible combinations of
  stress conditions, but no usage factors will be calculated.
  Command(s):FP
  GUI: Main Menu> General Postproc> Fatigue> Property Table> S-N Table


• The Sm-T curve, a curve of design stress-intensity value versus temperature. This curve is needed
  if you want the program to detect whether or not the nominal stress range has gone plastic.
  Command(s):FP
  GUI: Main Menu> General Postproc> Fatigue> Property Table> Sm_T Table


• Elastic-plastic material parameters M and N (strain hardening exponents). These parameters are
  required only if you desire simplified elastic-plastic code calculations. These parameters' values
  can be obtained from the ASME Code.
  Command(s):FP
  GUI: Main Menu> General Postproc> Fatigue> Property Table> Elas-plas Par

The following example illustrat es the use of the FP command to input material fatigue

properties:

! Define the S-N table:

FP,1,10,30,100,300,1000,10000! Allowable Cycles, N

FP,7,100000,1000000! "

FP,21,650,390,240,161,109,59! Alternating Stress-

FP,27,37,26! Intensity Range, S, ksi

! Define the Sm-T table:

FP,41,100,200,300,400,500,600! Temperature,°F

FP,47,650,700,750,800! "

FP,51,20,20,20,18.7,17.4,16.4! "Design Stress-Intensity

FP,57,16.1,15.9,15.5,15.1! Value", Sm (=2/3*Sy or

! 1/3 *Su), ksi

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Evaluating Fatigue