def j2k(j, E, nu, plane_stress=True):
    """
    Convert fracture 
    
    Parameters
    ----------
    j: float (in N/mm)
    E: float
        Young's modulus in GPa.
    nu: float
        Poisson's ratio
    plane_stress: bool
        True for plane stress (default) or False for plane strain condition.
    
    Returns
    -------
    K : float
        Units are MPa m^0.5.
    """
    
    if plane_stress:
        E = E / (1 - nu ** 2)
        
    return (j * E) ** 0.5