ISO 6721-9:2019

Overview

ISO 6721-9:2019 specifies a sonic-pulse propagation method to determine the dynamic mechanical properties of plastics - specifically the tensile storage modulus (E') of polymers by measuring longitudinal sonic pulse velocity. The method targets discrete frequencies typically between 3 kHz and 10 kHz and is suitable for materials with storage moduli from 0.01 GPa to 200 GPa and loss factors below 0.1 (around 10 kHz). It is part of the ISO 6721 series on dynamic mechanical properties.

Key topics and technical requirements

• Principle: determine longitudinal wave velocity v from the transit time of a single longitudinal sonic pulse and calculate tensile storage modulus by E' = ρ·v² (ρ = density).
• Frequency range: nominal 3–10 kHz (piezoelectric transducers with matching resonant frequency).
• Specimen types and dimensions:
  • Thin films or fine fibres preferred.
  • Typical specimen sizes: lengths 200–500 mm; widths 1–10 mm; thickness/diameter 0.1–1 mm.
  • Example specimens: tapes 300 × 5 × 0.1 mm or fibres 300 mm × 0.1 mm (diameter).
• Apparatus and measurement:
  • Two piezoelectric transducers (transmitting and receiving) aligned longitudinally; one transducer movable for distances approx. 50–500 mm.
  • Distance L measured to ±0.5% accuracy; arrival time to ±0.5 µs.
  • Velocity obtained from slope ΔL/Δt via linear regression of at least three separation distances.
• Material limits and considerations:
  • Suitable for materials with loss factor < 0.1; higher loss introduces amplitude decay and velocity errors.
  • Not generally suitable for cellular plastics, composite plastics, or multilayer products.
• Calculation and reporting: follow ISO 6721-1 for general principles; density must be measured per ISO 1183 methods. Report E' and v to three significant figures.

Applications and who uses it

• Used by materials scientists, polymer testing laboratories, R&D engineers, and quality control teams to:
  • Characterize dynamic tensile stiffness of thin films, fibres, and long specimens.
  • Support material selection for high-frequency mechanical or acoustic applications.
  • Provide input data for simulation, product design, or acoustic/material compatibility studies.
• Also relevant to manufacturers of test instrumentation (sonic transducers, arrival-time electronics) and ISO-conformant testing services.

Related standards

• ISO 6721-1 - General principles for dynamic mechanical property measurement.
• ISO 1183-1 / ISO 1183-2 / ISO 1183-3 - Methods for determining density of non-cellular plastics (required for E' calculation).
• Other parts of the ISO 6721 series for complementary dynamic mechanical testing methods.

Keywords: ISO 6721-9, tensile vibration, sonic-pulse propagation method, dynamic mechanical properties, tensile storage modulus E', longitudinal wave velocity, polymer testing.