MES Drop Test and Plastic Deformation

[aifric]

Dear all,

I am trying to simulate the impact between two bumpers which are made from a ductile material. The bumpers are incorporated within a simple spring suspension and collide when the suspension undergoes a drop test. The suspension means that the bumpers collide in one direction only but for ease of analysis I have omitted the suspension from my model. The aim of the analysis is to observe the plastic deformation incurred by both bumpers upon impact so that I can compare it to what I have observed in reality. I am working on a micron scale. My problem is that my model shows no plastic deformation and I don't know what to change?

My current model has: one bumper fixed at the base and an initial velocity which corresponds to the height of the drop test is applied to the other bumper which is fixed in every direction except translation in the drop direction. I defined both bumpers as bricks. In element definition I chose Plastic - von Mises with isotropic hardening and large displacement. I defined my own material as it was not available in the library but this meant that I did not have data for either damping nor strain hardening modulus. In analysis parameters I applied gravity in the correct direction  and applied a multiplier of 1 to the load curve at time zero and greatly increased the capture rate in order to capture the collision. And finally I defined a surface to surface contact between the two colliding surfaces. One is a concave and one is a convex surface so I chose a Point to Surface contact and high speed contact.

ANy help would be greatly appreciated!

[John_Holtz]

Hi aifric,

You mentioned what results you did not get (no plasticity), but you did not state what results you did get. Is the stress higher than the yield strength?

If the stress is above the yield, then the part must be yielding. How are you determining whether the model is yielding? Are you checking for permanent deformation after the rebound? Looking for the plasticity result? (From the Help page "Results > Results Environment > Results Menu > Nonlinear Results Menu", the section about the plasticity results indicates what needs to be done on the Element Definition dialog to get the plasticity results output. Also, I recall that there are two options for turning on the output. Try the other option of one of them does not produce results.)

If the stress is below the yield, then the question becomes what is wrong in the simulation. Material properties? Mesh too coarse?

[zhuangs]

John has provided lots of technical details.  Here I would like to add a method to directly make the model show plasticity.

First, please go to "Results" window and check the maximum von Mises stress in those contact steps (hope you still keep the resutls).  Let's suppose that the maximum von Mises stress (VMstress_max) =10000.

Second, go back to "FEA Editor" window and "Edit material".  Select "Edit Properties".  You should have set "Modulus of Elasticity" and "Poisson's Ratio".  Let's suppose that Modulus of Elasticity (E) =3000000.  Now let's set "Strain Hardening modulus" = 0.1*E =300000, and set "Yield strength" = 0.5*VMstress_max=5000.  And then click "OK" to save the material data.

Third, rerun the analysis using the new material properties.

-Shoubing