Learning Forum

[zayeem fazili]

Dear members,

I am eager to understand SN curve of a material test. If a material is tested (axial fatigue test) the resultant mean SN curve  shows 50 % probablity of survival/failure as accepted by many researchers. Or it depends on the tensile stregth of material. If tensile stregth value coresponds to 95 % Probability of survival as per the standards. In that case would the resultand SN curve be also 90 % probability of survival not the 50 % survival

[peteroznewman]

”a material is tested (axial fatigue test) the resultant mean SN curve shows 50 % probablity of survival/failure.” 

I don’t know why you tacked on the phrase “as accepted by many researchers”, what do you mean by that?

You have to also say the stress ratio for the axial fatigue test. Was the stress varying between tensile and zero, or was the stress varying between tension and compression?  The latter has a zero mean stress.

Tensile strength is based on a one-time load event, so is a totally different number than the SN curve data.

Tensile strength is measured using many samples.  There will be variation in the values of those samples. There will be a mean strength, but no one uses that for design purposes. The data for all samples is analyzed to come up with an Allowable Strength value. Different standards are used to come up with that value. B-Basis is a strength value at which only 10 in 100 specimens will fail with a 95% confidence level. As more specimens are tested a higher value of strength can be used as a valid allowable.

Fatigue testing requires multiple samples to be tested at each stress level. Also, the cycles must be run until the sample breaks. This makes gathering data to create an SN curve very expensive.

[zayeem fazili]

Hi thanks for your reply.

 

The stress ration is 0.1 in my case. I tested 55 samples of different notches at same nominal load. As nothes are different the samples will fail at different levels. My understanding was the resultant SN curve which I get should be considered 50 % Probability of survival / failure.

Now the tensile strength for the specimen  (Al 2618 T6) would be 420 MPa which coresponds to 97.5 % probability of survival as per FKM standard. The resultant SN curve I  got, should it be treated as 50 % probability of failure or 2.5 % as tensile stregth values corespond to 97.5 % probability of survival for the material. I know if I would have used mean tensile stregth value then 50 % probability of survival/failure for SN curve is justified or there is no need to draw relation between probability of static tensile strength value used and the SN curve generated ?