def sigma_cr_plate(
        thickness,
        width,
        psi=None
):
    # Docstring
    """
    Critical stress of a plate.

    Calculates the critical stress for a simply supported plate.

    Parameters
    ----------
    thickness : float
        [mm] Plate thickness
    width : float
        [mm] Plate width
    psi : float, optional
        [_] Ratio of the min over max stress for a linear distribution,
        (sigma_min / sigma_max)
        Default = 1, which implies a uniform distribution

    Returns
    -------
    float
        [MPa] Plate critical stress

    Notes
    -----
    To be extended to include cantilever plate (outstand members)

    References
    ----------
    .. [1] Eurocode 3: Design of steel structures - Part 1-5: Plated structural elements. Brussels: CEN, 2005.

    """
    # Convert inputs to floats
    thickness, width = float(thickness), float(width)

    # Default value for psi
    if psi is None:
        psi = 1.
    else:
        psi = float(psi)

    # Calculate kapa_sigma
    k_sigma = 8.2 / (1.05 + psi)

    # Elastic critical stress acc. to EN3-1-5 Annex A
    sigma_e = 190000 * (thickness / width) ** 2
    sigma_cr = sigma_e * k_sigma

    # Return value
    return sigma_cr