Class UserTic

Another example for coulomb friction

const c = new arch.UserTic()

// We assume a distribution of friction and cohesion
// Arrays frictionArray and cohesionArray are given for triangles

// tract is the traction before convertion to displacement
// id    is the id of the triangle for the considered surface
c.setFunction( (tract, id) => {
     const μ = frictionArray[id]
     const C = cohesionArray[id]
     const σ = tract[0]
     const τ = μ*σ + C
     const t = Math.sqrt(tract[1]**2 + tract[2]**2)

     if (t >= τ) {
         const δ = τ/t
         tract[1] -= δ*tract[1]
         tract[2] -= δ*tract[2]
     } else {
         tract[1] = tract[2] = 0 // stick
     }

     return tract
})

surface.addConstraint(c)