def stress_model(strain, modulus):
    """
    Returns the linear estimate of the stress-strain curve using the strain and estimated modulus.

    Used for fitting data with scipy.

    Parameters
    ----------
    strain : array-like
        The array of experimental strain values, unitless (or with cancelled
        units, such as mm/mm).
    modulus : float
        The estimated elastic modulus for the data, with units of GPa (Pa * 10^9).

    Returns
    -------
    array-like
        The estimated stress data following the linear model, with units of Pa.

    """

    return strain * modulus * 1e9