ISO 11093-8:2017 specifies a method for the determination of the flexural modulus by using experimentally measured natural frequencies in the free-free mode of transverse vibration of cylindrical paper and board cores, which meet the following criteria:

- internal diameter:

50 mm to 350 mm;

- minimum wall thickness:

0,02 × internal diameter or not less than 2,0 mm;

- minimum length of core:

8 × internal diameter.

NOTE For the determination of the flexural modulus by the three-point method, see ISO 11093‑7.