fatigue load scaling problem

  • Last Post 12 June 2018
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keyurckp posted this 12 June 2018

here i have applied a load of 500 N Here i have applied a scaling of 250

  • Now my doubt is when i apply a load of above type exactly how it is going to act on the body
  • 1st - the amplitude of the load remains 250 N and mean becomes 500 N.
  • 2nd - it will be a fluctuating load of 750 N.
  • 3rd - the loads will be multiplied like 500*250 as amplitude with mean to be zero.  

      or there is some other way 

  • My second doubt is " The scale factor is applied after the stresses have been collapsed from a tensor into a scalar. Thus any multiaxial stress collapse methods that are sensitive to the sign (Von-Mises, Maximum Shear, Maximum Principal) may not give the same answer had the scale factor been applied to the environment load itself." i have read it in ansys help can anyone explain the above lines with example.

          Thank you

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peteroznewman posted this 12 June 2018

Your first question, the 3rd bullet is the answer. The load will be multiplied by the scale factor to determine the fatigue life.

I always use a scale factor of 1 and apply the actual load to the structure, so I don't worry about that statement.


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keyurckp posted this 12 June 2018

For the second point sir my basic doubt is that, is the theory of fatigue only applicable to uni axial stress case or there is much more to understand from the second question?

keyurckp posted this 12 June 2018

Sir i had posted one more question related to history data in fatigue input it would be a great if you can help me out of it.

peteroznewman posted this 12 June 2018

Crack growth, which leads to a fatigue failure, requires tensile stress to open the crack. Compressive stress closes the crack and it doesn't grow. The problem with the von Mises scalar value of stress is that tensile and compressive stress are both positive and come out the same, so 10^5 cycles of compressive stress will do no damage, while 10^5 cycles of tensile stress can lead to fatigue failure. That is why Maximum Principal Stress is a better scalar value of stress, it captures the tension side of the cycle, while Minimum Principal Stress captures the compression side of the cycle and those values can come out negative.