S/N diagram and endurance limit

[azzt]

Hi

I'm having a bit of a problem with the ordinate of a S/N diagram, it says that it is maximum stress, which I find a bit strange. I can understand this if the stress ratio =min/max =-1 then the mean stress will be zero and hence the alternating stress amplitude will be max stress / 2.

However if the stress ratio does not equal -1 then one can have a mean and alternating stress that satisfies the value of maximum stress but not the stress ratio.

I'm sure this is trivial, so apologies for being pig ignorant.

Thanks in advance.

allan

 

[TVP]

allan,

I don't quite understand your example, but hopefully I can be of some assistance. When there is a non-zero mean stress (R = 0.2, 0.6, etc.), it means that all of the stress components are positive. One can plot an S-N diagram using any of the following as the ordinate value:

maximum stress
minimum stress
stress amplitude
stress range

The link below has a brief explanation of the relevant parameters:

http://www.public.iastate.edu/~e_m.424/Fatigue.pdf

 

[metengr]

TVP is correct, for the S-N diagram or stress-life approach, the ordinate can be expressed by any one of the stress parameters mentioned above.

 

[metengr]

allan;
Perhaps this may help you. Basic fatigue data using the stress-life approach is collected in the laboratory using completely reversed alternating stresses, zero mean cyclic stress.

In most applications where mean stress is influential, you need to use either a master diagram or constant life diagram. Of course an alternative approach if you do not have either diagram is to use one of several empirical relationships that relate failure under non-zero mean stress to failure at the same life under zero mean cyclic stresses.

 

[azzt]

Hi

Thanks for all the replies. I appreciate your time.

I managed to sort myself out by finding more data and plotting it on a Haigh diagram which makes more sense. I believe my misunderstanding was that I initially only had data for the condition R=0.1 and I could not see how this could be useful. I obtained more dta at R=-1 and R=0.5 and discovered the Haigh diagram. All is now clear

thanks

allan