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 implement both multimaterial mode contact and crack contact. First, normals are needed to find the component of the velocity in the approaching direction, which is used to detect contact. Second, the normals are needed implement contact mechanics 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.

Multimaterial Normal Vector Options

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. NairnMPM has implemented new methods that seem to work better. No one method works for all problems, which is why multiple options are available. This section describes the available options.

Each Material's Gradient

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

Explicit Crack Normal Vector

Found from crack path and often more accurate the multimaterial mode such as for interfaces.