Modeling bolt bearing deformations with Pinching4

[castonguay.pierre]

Hello,

A fellow student and I are trying to model the bearing deformations of a bolt hole using the Pinching4 uniAxial material. The behavior is essentially bi-linear, such that points 2, 3, and 4 are on the same line in the plastic range. Here is what we are trying to model, broken down into steps to help make it clearer:

1. A bolt is in a standard round hole.

2. Displacements to the right are imposed. The material will “yield” and ovalization of the hole will take place.

3. Imposed displacements are reversed (imposed to the left). The bolt loses contact with any material and the force drops to zero because the bolt is now free to move in the hole until it hits the left side of the hole. Graphically, we’ve hit the plastic range, the load has dropped straight down to zero and remained zero until we are back to the origin (point (0,0 )).

4. We are back where we started. Repeat step 2, but with imposed displacements toward the left.


With Pinching4, we’ve been able to model this, except for the unloading part in #3. We’d like the load to drop straight to zero, but it drops at a slope approximately equal to the initial elastic slope.

We’ve tried many different combinations of gammaK, gammaD, gammaF, gammaE, and uForce but haven’t been able to change the slope of the unloading stiffness to what we want (a vertical line). We believe the answer lies in the definition, or calculation, of the “ * ”, or deformation value, of the first point after unloading (gray line) in Figure 1 (Definition of Pinching4 Uniaxial Material Model) of the Pinching4 material documentation.

How is this point calculated? How could we possibly force this “unloading line” to be become vertical until it hits the x-axis?

Also, there is no information about the contact author.

Any help is greatly appreciated!

[silvia]

have you tried hysteretic, there is a factor to control the unloading, it might work if you use a negative value

[castonguay.pierre]

Thanks, Silvia!

We've looked at it, but we don't see how we'd be able to assign zero stiffness between the time the bolt goes from one end to the other. We'd have to pass through (0,0), or the origin, every time we cycle. So far, we don't see how we'd be able to achieve this with Hysteretic.

However... we figured out to change our unloading line to become vertical: we placed our first point very close to the y-axis (small deformation). Since the unloading slope is more or less equal to the "elastic" slope, it is now almost vertical... and we don't exactly need an elastic range in this behavior, since the elastic behavior will be taking place in the brace member.

We'd like to show you the actual response we have so far using Pinching4 so you'd get a clearer idea of what we're trying to achieve, but we don't know how to add images to posts on here...

Thanks!

[silvia]

you can just put a link to your own server, or there are free image hosting sites.