Calculates the poisson ratio for a specific direction
        relative to a second, orthogonal direction

        Args:
            n (3-d vector): principal direction
            m (3-d vector): secondary direction orthogonal to n
            tol (float): tolerance for testing of orthogonality

def directional_poisson_ratio(self, n, m, tol=1e-8):
        """
        Calculates the poisson ratio for a specific direction
        relative to a second, orthogonal direction

        Args:
            n (3-d vector): principal direction
            m (3-d vector): secondary direction orthogonal to n
            tol (float): tolerance for testing of orthogonality
        """
        n, m = get_uvec(n), get_uvec(m)
        if not np.abs(np.dot(n, m)) < tol:
            raise ValueError("n and m must be orthogonal")
        v = self.compliance_tensor.einsum_sequence([n]*2 + [m]*2)
        v *= -1 / self.compliance_tensor.einsum_sequence([n]*4)
        return v