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Questions about SN curve creation

Question asked by Wayne Jamieson on Apr 7, 2017

Hi guys. I have some questions regarding the creation of fatigue SN curves.


Note: the curve I’m defining is based on BS7608 class B with a stress amplitude of 100MPa at 1e7 cycles and I have calculated the stresses at cycles ranging from 1 to 1e9 (for completeness).


  1. There is an LR= -1 option for fully reversed load cases; do I half the curve stresses (e.g. 50MPa not 100MPa) when using this option?
  2. There is an LR=0 option for zero based load cases; do I use the full stress amplitude (e.g. 100MPa) when using this option?
  3. If I’m working with a load ratio fatigue case e.g. LR=0.43; do I keep the stress amplitude at 100MPa, input LR=0.43 and let the software figure out what to do?


These are probably very silly questions but we’re only now starting to assess the fatigue module and the SN curve definition part of the documentation is somewhat… lacking.


Thanks in advance.