Update Maxwell model for consistency

README.md

@@ -30,7 +30,7 @@ F = N k \delta
 \delta_{i+1} = sgn \left( x_{i+1} - x_{i} + \delta_{i} \right) \min \left( \left| x_{i+1} - x_{i} + \delta_{i} \right|, \Delta \right)
 ```
 ```math
-\Delta = \frac{\mu_c}{k}
+\Delta = \frac{N \mu_c}{k}
 ```
 - Generalized Maxwell Slip Model
 ```math

src/friction.f90

@@ -594,9 +594,9 @@ pure module function lg_get_state_var_count(this) result(rst)
         !!
         !! The signle-element, Maxwell model is defined as follows.
         !!
-        !! $$ F = N k \delta $$
+        !! $$ F = k \delta $$
         !! $$ \delta_{i+1} = sgn \left( x_{i+1} - x_{i} + \delta_{i} \right) \min \left( \left| x_{i+1} - x_{i} + \delta_{i} \right|, \Delta \right) $$
-        !! $$ \Delta = \frac{\mu_c}{k} $$
+        !! $$ \Delta = \frac{N \mu_c}{k} $$
         !!
         !! where:
         !!    

src/friction_maxwell.f90

@@ -9,10 +9,10 @@ module function mx_eval(this, t, x, dxdt, nrm, svars) result(rst)
 
     real(real64) :: s, d, delta
 
-    delta = this%friction_coefficient / this%stiffness
+    delta = nrm * this%friction_coefficient / this%stiffness
     s = x - this%x_prev + this%d_prev
     d = sign(1.0d0, s) * min(abs(s), delta)
-    rst = nrm * this%stiffness * d
+    rst = this%stiffness * d
 
     this%d_prev = d
     this%x_prev = x

test/friction_model_tests.f90

@@ -118,9 +118,9 @@ function test_maxwell() result(rst)
     mdl%friction_coefficient = coeff
 
     ! Compute the actual solution
-    delta = mdl%friction_coefficient / mdl%stiffness
+    delta = normal * mdl%friction_coefficient / mdl%stiffness
     sdelta = min(abs(pos), delta) * sign(1.0d0, pos)
-    ans = normal * mdl%stiffness * sdelta
+    ans = mdl%stiffness * sdelta
 
     ! Test
     f = mdl%evaluate(0.0d0, pos, 0.0d0, normal)