ISO 6721-8:2019

Overview

ISO 6721-8:2019 - "Plastics - Determination of dynamic mechanical properties - Part 8: Longitudinal and shear vibration - Wave‑propagation method" specifies an ultrasonic wave propagation method to determine the storage components of the longitudinal complex modulus (L*) and shear complex modulus (G*) of polymers. The standard targets discrete ultrasonic frequencies (typically 0.5 MHz to 5 MHz) and is intended for materials with storage moduli from 0.01 GPa to 200 GPa and loss factors below 0.1 (around 1 MHz).

Key topics and technical requirements

• Measurement principle: determine longitudinal and transverse wave velocities in a specimen and its density; compute storage moduli as density × velocity².
• Frequency & material limits: typical frequencies 0.5–5 MHz; suitable for low‑loss materials (loss factor < 0.1). High-loss materials introduce waveform distortion and require methods outside this document.
• Two methods described:
  • Method A - Immersion method: transmitter and receiver in a liquid bath; specimen mounted on a turntable to vary incidence and excite longitudinal and refracted shear waves.
  • Method B - Transducer contact method: specimen sandwiched between transmitting and receiving transducers using longitudinal or shear polarized transducer pairs and coupling fluid.
• Apparatus and instrumentation: ultrasonic transducers (short pulse, ~3–4 cycles), transit‑time measurement equipment (e.g., calibrated oscilloscope or timing electronics), turntable for angle control, coupling fluids, temperature measurement and control.
• Specimens and preparation: small specimen capability (typically 50 mm × 20 mm × 5 mm) or small regions of sheets; procedures for measuring specimen dimensions, density (references to ISO 1183 series), conditioning and test atmosphere.
• Results and limitations: procedures for calculating L* and G*, treating anisotropy/homogeneity, reporting format, and stated precision. The method emphasizes bulk wave propagation (wavelength ≪ transverse specimen dimensions).

Applications and users

ISO 6721-8 is useful for:

• Materials scientists and polymer R&D studying viscoelastic properties at ultrasonic frequencies.
• Industrial testing and quality control labs assessing stiffness (storage modulus) and directional properties (anisotropy/homogeneity) in plastics, composites and sheets.
• Acoustic/materials testing service providers and equipment manufacturers developing ultrasonic modulus measurement systems.
  Typical applications include material characterization, comparative testing, validation of simulation inputs, and nondestructive evaluation of small or localized specimen regions.

Related standards

• ISO 6721-1 - General principles for dynamic mechanical properties (series overview).
• ISO 1183-1 / -2 / -3 - Methods for determining density of non‑cellular plastics (referenced for specimen density measurement).
• ISO 6721 series (other parts) for complementary test methods and reporting.

Keywords: ISO 6721-8:2019, plastics, dynamic mechanical properties, ultrasonic wave propagation, longitudinal modulus, shear modulus, storage modulus, immersion method, transducer contact method, ultrasonic testing.