ISO 6721-8:2019

ISO 6721-8:2019 pdf free.Plastics – Determination of dynamic mechanical properties -Longitudinal and shear vibration – Wave-propagation method.
Measurements are made of the velocity of longitudinal and transverse acoustic waves in a specimen and the specimen density. The frequency of the wave is chosen so that its wavelength in the specimen is significantly less than the specimen dimensions in a plane transverse to the direction of wave propagation. The wave then propagates as a bulk wave. The longitudinal and shear storage moduli are given by the product of the material density and the square of the longitudinal and the shear wave velocities respectively.
1’wo methods are described in this document for measuring wave velocities. In the immersion method, the specimen intercepts a beam of longitudinal acoustic wave pulses passing between a transmitting and receiving transducer in a bath of a suitable liquid. At normal incidence, longitudinal wave pulses are excited in the specimen. As the angle of incidence is increased, the amplitude of the longitudinal refracted wave decreases and a refracted transverse (shear) wave is generated. Longitudinal and transverse wave velocities are deduced from measurements of differences in pulse transit times with and without the specimen in the beam and a knowledge of the velocity of sound in the liquid.
In the transducer contact method, the specimen is sandwiched between two transducers, one launching and the other receiving acoustic wave pulses. For the determination of longitudinal and transverse wave velocities, transducer pairs having longitudinal and transverse polarisations, respectively, are used. Wave velocities are again obtained from measurements of differences in pulse transit times with and without the specimen in the beam.
Figure 1 shows, schematically, suitable apparatus for measuring velocity by an immersion method. Two ultrasonic transducers are mounted coaxially in a bath containing a liquid, one acts as a transmitter T of longitudinal ultrasonic wave pulses and the other as a receiver R. The transmitter is driven by a series of high-voltage, short-duration electrical pulses from the transducer drive unit. A pulse repetition interval of about 1 rns is satisfactory. Acoustic pulses launched by the transmitter travel through the liquid and the specimen and are detected by the receiving transducer. The specimen is mounted on a turntable, located between the transducers T and R, such that the angle of incidence of the acoustic beam can be varied and measured to ± 0,5°. The specimen can be removed from the beam. The receiving transducer is connected to electronic equipment that will enable measurement of the difference in the arrival times of pulses received with and without the specimen in the beam. An oscilloscope, whose time base is accurately calibrated and triggered by the transducer drive unit, is suitable for this purpose.
The receiving transducer may be replaced by a reflecting surface, such as a metal block, positioned normal to the axis of the transmitter as shown in Figure 2. The transmitting transducer is now used to detect the beam of pulses reflected back through the liquid and the specimen and is connected to the transit-time measuring equipment.
This test arrangement may be more appropriate lithe specimen is only available as a thin sheet since the transit time in the specimen is twice that obtained using the transmitter and receiver arrangement.ISO 6721-8 pdf download.

Leave a Reply

Your email address will not be published. Required fields are marked *