Calculating Reserve factor based on Non-Linear Material FEA

[AeroStructAnalyst]

Hello all,
How do you determine your reserve factor from a non-linear FEA (material)? Generally, most non-linear FEA I have come across tend to use a non-linear material true stress-strain curve for non-linear FEA (material). And there after extract the Maximum Strains, derive the 'equivalent Stress' using the True Stress-Strain curve and then work out the Reserve Factor by dividing the Ultimate Stress of the material by the 'Equivalent Stress'

However, is it conservative to use the max strain from the material non-linear, then derive your equivalent stress and compare it to your ultimate stress to derive your Reserve Factor? I am also aware that some use "Reduction of Area" or "Percentage elongation" to work out their Reserve Factor.

Secondly, when you enter your true Stress-Strain curve in your non-linear analysis, for load cases that results in Strains higher than the Maximum Strain specified in the true Stress-Strain cruve, how does the FEA workout the resulting Stress? I asked because most recently, I came across a non-linear FEA where the Strains were in excess of 16%, however the true stress-Strain curve used in the analysis had a Max Strain of about 7%, yet the V-Mises Stress were derived for those high strains. Does this mean that the FEA tool simply linearly extrapolated the inputed True Stress-Strain curve to infinity?

Your response is very much appreciated.

 

[rb1957]

i think if you want to find the true RF for a non-linear case, then you need to factor your load untill you get to failure.

 

[AeroStructAnalyst]

Thank rb1957,
I quite understand that, but then, how do you define your failure strain? is it overly conservative to assume this to be the strain at Ultimate Stress for a ductile material?

 

[rb1957]

materials have a maximum strain and a maximum stress defined, no?

i read your post as saying if my model has a maximum/critical strain of 0.0x, which is equivalent to a material stress of yy0 ksi from the material stress/strain curve (though doesn't your FEA output a stress experiences for the strain output ?), then is the RF = uts/yy0 ?

and my answer is no, i don't think so; i think you have to factor up your applied loads untill the critical stress (determined from the critical strain if necessary) = uts.

is your critical strain axial (ie x- or y-) or multi-axial (something like von mises or principal) ?