I'm trying to calculate the Interfacial Shear Strength (ISS) of carbon nanotubes with polymeric matrices with the next formula:

ISS=E / PI*R*L^2

where E is the pull-out energy, R the nanotube radius and L its lenght. The problem I'm having is with the order of magnitude of calculated ISS: I'm obtaining values of 50000 MPa and the reported values are about 50-200 MPa.

I'm using E=413 Kcal/mol (converted to Joules) with R=6,7 A and L=45A (converted to metres). This value of energy is in the order of other reported pull-out energies in kcal/mol, so I don´t know where I'm doing the mistake when calculating ISS.