@classmethod
def from_ratios(cls, Rxy=1, Ryz=1):
    """Return isochoric ``DefGrad`` tensor with axial stretches defined by strain ratios.
        Default is identity tensor.

        Keyword Args:
          Rxy, Ryz (float): strain ratios

        Example:
          >>> F = DefGrad.from_ratios(Rxy=2, Ryz=3)
          >>> F
          DefGrad:
          [[2.28942849 0.         0.        ]
           [0.         1.14471424 0.        ]
           [0.         0.         0.38157141]]

        """
    assert Rxy >= 1, 'Rxy must be greater than or equal to 1.'
    assert Ryz >= 1, 'Ryz must be greater than or equal to 1.'
    y = (Ryz / Rxy) ** (1 / 3)
    return cls.from_comp(xx=y * Rxy, yy=y, zz=y / Ryz)
