Surface Normals
Both multimaterial mode MPM and explicit cracks need to calculate surface normals. These normals are a crucial step in accurate contact physics calculations.
Introduction
The figure shows a surface between two materials (a and b) or between two sides of a crack (above and below). The arrow shows a vector normal to the surface oriented from side a to side b. Finding this normal accurately is a key component of implementing both multimaterial mode contact and crack contact. First, normals are needed to find the component of the velocity in the approaching direction and the separation in the opneing direction, both of which are used to detect contact. Second, the normals are needed to implement contact mechanics such as to model frictional contact or imperfect interfaces.
Practical experience using multimaterial mode MPM simulations has revealed that accuracy of results is very sensitive to the method used to find the normals. In fact, when a multimaterial mode simulation has numerical issues, the causes of those issues is almost always connected to problems in finding surface normals. See below for the various options in multimaterial mode MPM to finding normals. Numerous options are needed because no on option works for all problems.
Simulations that use explicit cracks, have far fewer issues with normals. The normals are still crucial, but because the crack geometry is tracked, the crack normals are more easily calculated. See below for some details on crack surface normals.
Multimaterial Normal Vector Options
NairnMPM has implemented several methods for finding normals in multimaterial mode contact. No one method works for all problems, which is why multiple options are needed. This section describes the available options.
Each Material's Gradient
When multimaterial mode MPM was initially developed, contact was handled separately for each material's velocity field and the normal vector was found from that material's mass gradient.[1] Show picture two materials with two normals
Maximum Gradient
Show full vs partial
Average Gradient
Show top with two edges
Maximum Volume
Just describe
Custom Specified Normal
Developer flags and maybe more options int he future
Rigid Material Bias Option
Volume Screening
Explicit Crack Normal Vector
Found from crack path and often more accurate the multimaterial mode such as for interfaces.
References
- ↑ S. G. Bardenhagen, J. E. Guilkey, K. M. Roessig, J. U. Brackbill, W. M. Witzel, and J. C. Foster, "An Improved Contact Algorithm for the Material Point Method and Application to Stress Propagation in Granular Material," Computer Modeling in Engineering & Sciences, 2, 509-522 (2001).