Predicting Peak Load in Tension

[metalman8357]

This is probably a very simple question but bear with me as I am a beginner with FEA. I have a steel part that I'm applying a simple tensile load to and I'd like FEA to predict the maximum load before fracture. I'm using Autodesk Simulation 2014 with a nonlinear solver. I apply a load, but all I'm seeing is the von mises stress which is good for predicting where the part yields, but does not tell me the peak load that it will take before fracture.

Thanks,
M

 

[rb1957]

sigh ...

you're using technical terms, probably inappropriately ...

when you say "fracture" i don't think you mean damage tolerance, or stress intensity, but rather static failure, ie the stress on the part = ftu.

now, you know the stress in the part due to the applied load, and you know ftu. the piece that i think you should know is that your FEA is probably linear (ie stress is directly proportional to load).

that should be enough for you to figure it out ...

Quando Omni Flunkus Moritati

 

[btrueblood]

"Autodesk Simulation 2014 with a nonlinear solver"

What material model are you using? Did you verify the model you are using by trying to model a tensile specimen and reproducing its stress-strain curve to failure?

 

[rb1957]

sigh (at myself this time) ... i should have noted you're using a NL solver ... so (to coin a phrase) if the load doesn't break it, you must increase the load ... no? the accuracy of the model prediction will depend heavily on the accuracy of the post-yield part of the stress/strain curve.

Quando Omni Flunkus Moritati

 

[metalman8357]

I drafted a simple 3D tensile specimen and a ran a nonlinear static analysis on it using autodesk simulation mechanical. The material I selected is ASTM A441 with a yield strength of 46ksi. I calculated that my tensile sample needs to be loaded to 2875lbf to acheive this yield stress, and so I applied this load to one end. To my amazement, when I ran the simulation it perfectly predicted the stress at 46.122 ksi.

Now for my question...I want my model to able to predict the ultimate tensile strength of the sample. If I assume a tensile strength of 66ksi for the material and apply a corresponding load of 4125lbf, the von mises stress shows 77ksi in the material not the expected 66ksi. I'm using the Von Mises Isotropic Hardening plasticity material element, and I'm using a tetrahedryl mesh. Any reason why I cant seem to predict the stress in the material after it has yielded?

 

[rb1957]

is it showing true stress (ie accounting for the loss of area, due to necking) or engineering stress (based on the original area) ?

ftu is a uni-axial stress. von Mises will include the effect of the lateral principal stresses ... look at max principal (rather than vM).

Quando Omni Flunkus Moritati

 

[inline6]

are you really interested in fracture as the failure mode? or just gross yielding and permanent deformation?

a non linear elasto-plastic analysis can be used and you can monitor plastics strain until they reach the strain that would be at fracture or some other value that is deemed "failure".

the stress vs strain should is often input using true stress and strain for the better solvers.

it is worth noting that the true stress at fracture of a ductile unaxial specimen is often higher than the UTS because of necking and poisson's ratio presenting a smaller cross sectional area for the apllied load. this is why true stress and strain are needed for accurate high plasticity simulations

 

[jhardy1]

Does your non-linear solver handle non-linear materiel properties (stress-strain curves, including yield, strain hardening, etc), or only non-linear geometric behaviour (P-Delta, etc)?

http://julianh72.blogspot.com