Density-based methods

1. SIMP

\[E = E_{min} + (E_{max} - E_{min}) \rho ^ p\]

2. RAMP

\[E = \frac{\rho}{(1 + 8. * (1. - \rho)) * E_{max}}\]

3. code

if method =='SIMP':
    C = rho_e**3
else: # for RAMP method
    C = rho_e/(1 + 8. * (1. - rho_e))