# How to get tension force of a cohesive contact

Hi,

I use cohFrictMa with cohesion as particle material to do triaxial compression simulation. During the simulation, the contact force in normal direction could be compression and tension.

I only know using i.phys.

Thanks,

Leonard

## Question information

- Language:
- English Edit question

- Status:
- Solved

- For:
- Yade Edit question

- Assignee:
- No assignee Edit question

- Solved by:
- Jan Stránský

- Solved:
- 2020-10-21

- Last query:
- 2020-10-21

- Last reply:
- 2020-10-20

Jan Stránský (honzik) said : | #1 |

Hello,

try i.phys.

The dot product of a vector "F" and unit vector "n" is a projection of F on the direction of n, i.e. the "oriented length" in the "n" direction.

The magnitude should be the same as i.phys.

cheers

Jan

Jérôme Duriez (jduriez) said : | #2 |

In case you would have this in mind, note that CohFrictMat does not really use two forces, one in tension and one in compression, but just one that can be tensile in nature up to some threshold = https:/

Leonard (z2521899293) said : | #3 |

Hi Jan,

I test your idea with two particles which works really fine!

I just have a question: For example, for one interaction in the model, i.phys.

I have this question because in my impression, the direction of contact normal unit vector "n" is arbitrary, not always orientates to the direction of compression.

Thanks

Leonard

Leonard (z2521899293) said : | #4 |

Hi Jérôme,

Yes, thanks for your reminder!

Leonard

Jan Stránský (honzik) said : | #5 |

Yes, the same sign w.r.t. tension/compression of i.phys.

There is no "direction of compression" (recall action-reaction law).

The direction of contact normal (if it is from body A to body B or from B to A) is arbitrary, more precisely chosen by collider "randomly" (i.e. non-trivially).

But, the value of internal force (tension/

So if you swapped body1 and body2 of the contact (i.geom.normal would be negated), i.phys.normalForce would need to be negated, too, to preserve the consistency of formula [1]

cheers

Jan

[1] https:/

Leonard (z2521899293) said : | #6 |

Thanks Jan Stránský, that solved my question.

Jérôme Duriez (jduriez) said : | #7 |

You can also realize that geom.normal is from i.id1 to i.id2 [1] (whether i.id1 is b.id = 10 or 20 in a 10-20 contact being indeed collider-

This insures the given sign for the dot product, in accordance with the repulsive (compression) or attractive (tension) nature of the force.

[1] https:/

[2] https:/