ISO 6721-3:2021 pdf download – Plastics — Determination of dynamic mechanical properties — Flexural vibration — Resonance- curve method. 6.2 Shape and dimensions Specimens shall be rectangular bars or strips thick enough to give sufficient bending stiffness, which is critical for the resonance frequency. On the other hand, the thickness shall be sufficiently small when compared to the wavelength of the bending vibration. The specimen thickness shall also be limited to avoid effects due to shear deformation and rotatory inertia if accurate values of E ′ are required. Length-to-thickness ratios of less than 50 shall be avoided if values of...