Returns the effective elastic constants
        from the elastic tensor expansion.

        Args:
            strain (Strain or 3x3 array-like): strain condition
                under which to calculate the effective constants
            order (int): order of the ecs to be returned

def get_effective_ecs(self, strain, order=2):
        """
        Returns the effective elastic constants
        from the elastic tensor expansion.

        Args:
            strain (Strain or 3x3 array-like): strain condition
                under which to calculate the effective constants
            order (int): order of the ecs to be returned
        """
        ec_sum = 0
        for n, ecs in enumerate(self[order-2:]):
            ec_sum += ecs.einsum_sequence([strain] * n) / factorial(n)
        return ec_sum