code: def get_yield_stress(self, n):
          
          # TODO: root finding could be more robust
          comp = root(self.get_stability_criteria, -1, args=n)
          tens = root(self.get_stability_criteria, 1, args=n)
          return (comp.x, tens.x)
docs: Gets the yield stress for a given direction
  
          Args:
              n (3x1 array-like): direction for which to find the
                  yield stress