FEA - failure location in tensile test
FEA - failure location in tensile test
(OP)
Hello FE Gurus,Please help me to understand the failure mechanism in FE. Lets consider the simple 1-D tensile test. During actual test,the sample starts necking arround the location where the material is weak within the gauge length( may be porosity or some other material functions). How this can be compared in FE? In FE we use same element property for the entire gauge length.Theoriticaly all element must get same stess and strain wihtin gauge length until the weakest element is identified, and then the necking should occur. My question is how this weakest element is identified in FE?
thanks,
puli
thanks,
puli





RE: FEA - failure location in tensile test
corus
RE: FEA - failure location in tensile test
RE: FEA - failure location in tensile test
Thanks,but my question is not about simulating the necking behaviour,rather about deciding the location of necking/failure. Since the cross section of gauge length is uniform,all element with in the gauge must get same stress & strain.Then how come suddenly few elements in one location started necking in FE?
Thanks again,
puli888
RE: FEA - failure location in tensile test
I think your question is related to bifurcation problem. I agree with you for single homogeneous element test there should not be a bifurcation, unless there's a numerical rounding error, which (i think) may lead to 'somehow' wrong result. However, when you consider higher more elements for the test (i.e. plane strain compression, tensile test) and with proper constitutive equation, I think there might be a bifurcation (or shear band). As far as I know this is not a must, and this shear band depends on the constitutive model that is used, i.e. the elastic and or perfectly plastic (i think) won't give this kind of result. Model that may give this result is the ones consider softening behaviour (what i know is hypoplastic model since i'm studying about it).
I am also interested in when does bifurcation start to occur.
Note: Please don't believe my opinion 100%. Nice posting.
RE: FEA - failure location in tensile test
RE: FEA - failure location in tensile test
I understand the physical test very well and agree with you 100% on your explanation. But my question is about the FE simulation. How the failure/necking location is predicted in tensile test(FEA)?...
Thx,
Puli888
RE: FEA - failure location in tensile test
RE: FEA - failure location in tensile test
as you noted the real failure location is due to a randomly distributed imperfection within the material. Therefore i don't think you can predict the precise failure location prior to the test. The shank of the test piece is considered to be all the same material, so i'd expect that FEA would predict failure close to the ends, where the test section transitions into the loading fixture.
i think the most you can hope to predict is that the failure load is higher than the standard properties, since these are "knocked down" to ensure that most of the material is stronger than the handbook values.
if you wanted to, i guess you could include a spherical inclusion (0.001" - 0.002" diameter) in your model, which would initiate failure.