def brittle_coulumb_fos(s1, s3, Sut, Suc):
    """
    Computes the brittle coulumb factor of safety
    :param s1:  the first principle stress in kpsi
    :param s3:  the second principle stess in kpsi
    :param Sut: Ultimate tensile
    :param Suc: Ultimate compression
    :return:
    """

    if s1 >= 0 and s3 >= 0:
        n = Sut/s1
        return n
    elif s1 >= 0 and s3 < 0:
        n_inv = (s1/Sut) - (s3/Suc)
        n = 1/n_inv
        return n
    elif s1 < 0 and s3 < 0:
        n = -Suc/s3
        return n