RIGID MOTION in Explicit analysis not enforced

[plafortune]

Hi,

I have the following set-up for an Explicit dynamic simulation: 

- Part 1 has a rigid material

- Part 2 is flexible and is held in place with a Ridid Body Connector

- Part 1 is controlled by a RIGID MOTION. Vertical displacement is 3 mm (+y), while translation in other directions are fixed to 0

- The RIGID MOTION has its respective "Transient Table Data"

The deformed geometry looks like this: 

Part 1 does go up by the expected amount, but when its contacting part 2, it "rebounds" on it. RIGID MOTION is suppose to be ZERO along X and Z, but apparently those rigid motions are NOT enforced to be 0.

Any suggestion (model attached)? 

Thanks 

Pierre 

[plafortune]

Hi, 

I solved it by adding a constraint in the same direction, in the same way that we do in implicit to control a displacement: 

I didn't add it before, because the help says that it's not necessary: 

"Enforced motion in an explicit analysis must not have a constraint that fixes the geometry in the same direction at the same location. (This is not the behavior in other analysis types.)"

Thanks 

Pierre

[John_Holtz · Accepted solution]

Hi Pierre,

A rigid body is reduced to a single node located at the center of mass. The rigid motion (and constraint that was applied later) is also applied at the center of mass node during the analysis. The center of mass node can translate and rotate. It may be hard to see, but the result of the analysis without the rotational constraint does show that the center of mass node is controlled in X, Y, and Z motion. What you saw in the original setup was the rigid body rotating about the center of mass.

Your solution was that you applied a rotational constraint to the rigid body. The rotation constraint prevented the center of mass node from rotating which in turn prevents the rigid body from rotating. (A nonrigid solid cannot have a node constrained in rotation because the mathematics do not calculate the rotation of a solid.)

To go one step further, if the X or Z Rigid Motion had included a nonzero value, the Tx and Tz constraint would have prevented the Rigid Motion from moving the part.

John

[plafortune]

Thanks John...that makes a lot of sense. 

Pierre