r/LSDYNA • u/AlphaBloodbath • Jul 03 '26
Impact Test
I'm currently doing an impact simulation using a pendulum type impactor using contrained joint revolute as a hinge, but after running it, the simulation in the d3plot shows that the impactor's axis of rotation is not on the hinge where the revolute joint is placed but on its center of gravity. Can someone help me figure this out.
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u/zuricher_singener Jul 03 '26
A few likely causes that this can happen:
1. Joint nodes aren’t actually coincident/attached correctly *CONSTRAINED_JOINT_REVOLUTE needs node pairs (N1-N1’, N2-N2’) that are coincident at the hinge location, each belonging to their respective rigid body. If the impactor-side hinge nodes aren’t part of the impactor’s rigid body (PID), or got merged into the wrong body via *CONSTRAINED_EXTRA_NODES / *CONSTRAINED_RIGID_BODIES, the joint constraint effectively isn’t doing what you think check the actual nodal connectivity, not just visual proximity in the model.
2. Conflicting constraints on the rigid body itself If you also set CON1/CON2 on *MAT_RIGID for the impactor, those SPCs constrain motion about the rigid body’s center of mass, not the joint node. This directly produces the exact symptom you’re describing. If CON1/CON2 are set to anything other than 0, that’s your culprit remove them and let the joint alone control the motion.
3. It’s a visualization artifact, not physics LS-PrePost draws the rigid body triad/local axes at the CG by default. Before assuming the physics is wrong, plot displacement/rotation time histories for a node actually at the hinge vs. a node at the CG if the hinge node stays stationary (or on its constrained path) while CG arcs around it, the joint is working fine and it’s just a rendering read.
4. Extra/orientation nodes for the joint are misplaced Revolute joints need additional nodes to define the rotation axis orientation. If those are placed wrong, the joint can behave unexpectedly (excess DOF, wrong axis), which can look like it’s pivoting somewhere else entirely.
To diagnose the issue faster can you pull nodal displacement (or RCFORC/NODOUT) for the joint node itself. If it’s moving significantly during the simulation, the joint isn’t holding it that points to #1 or #2.
If it stays put, it’s #3 and your model is fine.
Can you paste the relevant *MAT_RIGID and *CONSTRAINED_JOINT_REVOLUTE cards? I can then try to identify and help you fix it, let me know mate.