ISO 6721-3:2021

Overview

ISO 6721-3:2021 specifies a resonance-curve flexural vibration method to determine dynamic mechanical properties of plastics. The standard defines how to measure the flexural complex modulus (E*f) and damping characteristics (loss factor, tan δ) for homogeneous plastics and laminated systems used for acoustic insulation (e.g., metal sheets coated with damping plastics or sandwich constructions). Tests are typically performed as a function of temperature and frequency to characterise viscoelastic behaviour.

Key topics and technical requirements

• Test principle: Forced bending vibrations are applied and the resonance curve is recorded to derive storage modulus (E′f), loss modulus (E″f) and loss factor (tan δ).
• Frequency range: Excitation between about 10 Hz and 1 000 Hz; multiple vibrational orders may be used to vary frequency.
• Measurement limits: Storage modulus values above approximately 0.5 MPa are within the recommended range; the method is suitable for loss factors up to around tan δ ≈ 0.1.
• Oscillation mode: Uses oscillation mode III as defined in ISO 6721-1; the measured modulus is designated E*f.
• Specimen mounting:
  • Method A - vertical mounting, upper end clamped (suitable for most plastics, including softer materials).
  • Method B - horizontal suspension by fine fibres at vibrational nodes (suitable for rigid, dimensionally stable specimens and laminated systems).
• Apparatus: Electromagnetic exciter and detector, temperature-controlled enclosure, two small steel plates bonded to specimen ends, and tracking filters to minimize noise.
• Instrumentation tolerances: Exciter amplitude and detector sensitivity must vary by no more than 0.5% across a single resonance peak; frequency resolution ≥ 0.1%.
• Specimen requirements: Rectangular bars with specified preparation and dimension-measurement procedures; density measurement methods referenced to ISO 1183 series.

Applications and users

ISO 6721-3 is used to:

• Characterise viscoelastic and damping properties of plastics and laminated damping systems for acoustic insulation.
• Support R&D and material selection in industries where vibration and sound damping are critical (automotive, aerospace, building acoustics, consumer electronics).
• Provide quality control data and acceptance criteria for damping layers, coated sheets, and sandwich panels.
• Inform finite-element models and component design where temperature- and frequency-dependent mechanical data improve accuracy.

Typical users: materials testing laboratories, polymer scientists, product developers, QA engineers, acoustic/materials consultants.

Related standards

• ISO 6721-1 - Plastics: Determination of dynamic mechanical properties - General principles.
• ISO 1183-1 / 1183-2 / 1183-3 - Methods for determining the density of non-cellular plastics.
• Other parts of the ISO 6721 series for complementary dynamic-mechanical test methods.

Keywords: ISO 6721-3, flexural vibration, resonance-curve method, dynamic mechanical properties, flexural complex modulus, loss factor, damping, plastics testing, vibration testing, temperature-dependent modulus.