How Ansys LS-DYNA Calculates and D3PLOT Processes Contact "Stresses"

How Ansys LS-DYNA calculates and D3PLOT processes contact stresses

IMPORTANT:

The "stresses" on interfaces will always be lower than the true values in the elements, and they will also be spread over a wider area.

This is because of the way the penalty force and contouring algorithms work. Consider the following example (see the three figures below)

a)	A single node (say on the surface B side) touches part of the A surface. (A and B sides are inter-changeable in this example.)
b)	The reaction forces on the A side are distributed among the four nodes on the A side segment.

c) Contact stress is then computed over the sphere of influence of each node on the A surface, and stress is assigned to each contact segment.

d) Contouring which, in D3PLOT averages over elements, spreads the stress over a further region of contact segments.

Thus the contact at a single point has generated an apparent stress spread over twenty five elements - clearly this is not correct. To get round this you can:

Plot "force" components (eg FM_FORCE_MAGNITUDE ) which omit step (c) above, so force is spread only over nine segments in this example. Plot force vectors ( VECTOR command) which will only show force arrows at the four nodes on the A side segment.

However in real examples the problem is less severe: the genuine contact is usually spread over several segments and it is only around the borders of the contact region that stresses spread out too far.

Nevertheless contact "stresses" should not be treated as more than approximate contact pressures, and in particular they must not be expected to be the same as (true) stress in the underlying elements.