Recent content by afreiden

No, you can only use small strain "linear elasticity" if the material is undergoing small elastic strains. If it's a polymer or something that is undergoing large elastic strains, then you have to use a hyperelastic relation. In rate form, this would be called "hypo"-elasticity.

I didn't even know an analytical expression existed for the orthotropic case, but after I saw your post I went in search of it and found it... http://maeresearch.ucsd.edu/~vlubarda/research/pdfpapers/JOMMS-08.pdf (Eq. 39 or Eq. 48). Yikes! I can't imagine anyone has written an...

I had the same question when I was first introduced to the UL approach and objective rates... In a high-fidelity finite element analysis (e.x. a mesh of continuum elements in ADINA or ABAQUS), two things need to happen: 1) In order for the elements to talk to each other, they need a common...

Strains are nice because we can physically see them! Credit for example #1 in the figure goes to www.utsv.net. That example by itself is pretty convincing to me that the Green strain matrix is invariant to rigid body rotation and the Almansi strain is not. But to double-check, I created...

For a given deformation, the #'s in the 3x3 Almansi matrix would change under a rigid body rotation. OTOH, the #'s in the Green matrix would not change their values. I interpret that to mean that the Almansi matrix does not "factor out" rotations, but I think we're talking semantics because...

A lot of the confusion in this thread is due to the wording in the OP. Instead of "linear strain" and "nonlinear strain," you should say "small strain" and "large strain." The word "nonlinear" refers to the constitutive relation between stress and strain, and in this thread is specifically...

Chestermiller's first response is great. I wanted to make one additional comment regarding "design." Strength of materials equations for beams, columns, etc., are still used extensively in the field of structural engineering. Structural engineers design the skeletons of skyscrapers using...

I assume that T_R is the First Piola-Kirchhoff Stress (PK1), T is the Cauchy Stress, F is the Deformation Gradient, J is the determinant of F, and W is the Strain Energy Density... T_R=\frac{\partial W}{\partial F} is another way of saying that the PK1 Stress and the Deformation Gradient are...

Are your simulations dynamic (e.x. applying the indentation over a time of 5 seconds)? I assume the answer is yes, in which case perhaps the timestep you're using for implicit is too large, which is causing error in your results. I'm not that familiar with ANSYS, but usually software with...

Your images are still broken. You say that this test setup has been successfully used before, but were those previous materials as soft as yours?

In the same way that 3D elasticity tells you G, based on E and \nu, plasticity (von Mises) tells you that the material will "yield" in pure shear at a value of \tau_y, which is known, based on your known uniaxial yield stress, \sigma_y. This value is: \tau=\frac{\sigma_y}{\sqrt{3}} Again, the...

Like this? OR like a truss... like the last example here?

FEM can do anything you want to do. Car crash test simulations are done with high-fidelity FEM (8-noded "solid" elements) using such tools as ABAQUS, ANSYS, LS-DYNA. Structural engineers model the beams and columns of high rise buildings (or perhaps more relevant for you: the bars of a bridge...

So, beyond 5% strain, your material is no longer elastic? Your plots are showing 70% strain, so I assume that is mostly plastic behavior. The Mooney Rivlin model is an elastic model. Are you asking if you can simulate the response of some structure composed of this material (thin membrane, as...

The former