def allowable_stress_unity_check(sigma, allowable, df=0.72):
    """Calculate the allowable stress unity check value.

    Refs: https://en.wikipedia.org/wiki/Permissible_stress_design
    https://en.wikipedia.org/wiki/Factor_of_safety

    :param sigma: applied stress level to be checked.
    :type sigma: float
    :param allowable: the allowable stress, e.g. the yield stress.
    :type allowable: float
    :param df: the design factor.
    :type df: float
    :returns: unity check value.
    :rtype: float    
    """
    return sigma / allowable * df