Find load at fracture for beam 1

[McDesign25]

Is it possible to predict the load at fracture for a simple FEA beam with some certainty using linear FEA package? For example, I am interested in finding the bracketed range of forces that could produce a failure of a cantilevered beam that is fixed on one end and a concentrated force on the other end. However, I only have use of a linear static FEA package, and was wondering if this was adequate to determine the force at failure based on the von mises stresses and looking at the material properities, stress v. strain curve. If I could achieve this with a simple cantilevered model, could the same results be applied to a more complex FEA model. The material is a plastic polymer with a 70% elongation at failure, 6% at yield and has a limited linear range up to 1% elongation, before it becomes non-linear. Is there any technical papers or rules of thumb for estimating the breaking force assuming the loading is applied slowly, i.e. the same rate as the material property data? Thanks.

 

[fkmeyers]

My initial comment would be no but I did find something at

http://www.nenastran.com/newnoran/conferencePaper.php.

Check out the paper on "Numerical Validation and Application of the Neuber-Formula in FEA-Analysis." Personally I would use a full non-linear run with large displacement/rotation effects and a non-linear stress-strain curve. If all you have is linear then I would look at Neuber. von Mises is not going to tell you anything more than roughly when yielding will start but after that everything becomes non-linear. With cantilever beams there could be a large amount of displacement and motion which would affect load paths. Large displacement effects become important if the displacements are more than one beam depth.

 

[Kwan]

My approach to this problem would be to use plastic bending. This is a hand analysis method. I've never used it for polymers so I'm not sure if it would be too helpful.

Another approach would be to use you linear FEM with iterations. Apply a small load, output the deformation, now use this as a starting point for the next run. Be sure to apply the new material stiffness for each run. Examine the extreme fiber stress compared to the rupture allowable of the material. (von mises is for yield as Fk mentioned).

Now, depending on the shape of your beam, the failure may be stability of the compression side of the beam. Even a square shape would probably fail in block compression well before material rupture.

If this is a usual analysis that you perform, a nonlinear FEM program with physical testing would be helpful. A testing program would be the best way to verify your eventual method.

 

[GBor]

Good Paper, Frank,

I would have agreed with your initial thoughts.

Garland