Electromechanical response of polypropylene ferroelectret transducers under application of high-voltage pulses was measured by laser Doppler vibrometry and compared with results of ultrasonic through-air transmission between two ferroelectret transducers. The electromechanical response was completely explained by piezoelectric and electrostrictive effects. The electrostrictive effect dominates at high voltages and provides significant enlargement of the transducer constant, up to factor of 2.5. The induced strain of 1.7% was achieved at −2000 V. The nonlinear ultrasonic transmission was shown to be well described by the piezoelectric and electrostrictive response of transmitter, except in the range of high negative exciting voltages where some limitation of the transmitted signal was observed. This limitation seems not to be a fundamental one and does not abolish the advantages of high-voltage excitation of polypropylene ferroelectret transducers.

The human factors approach relies on understanding the properties of human capability and limitations under various conditions and the application of that knowledge in designing and developing safe systems. Following the principles of the MTO (Man Technology Organisation) approach, emphasis should be given to the way people interact with technical as well as organisational systems. A model describing human factor influences in relation to the performance shaping factors and their effect on manual ultrasonic inspection performance had been built and a part of it empirically tested. The experimental task involved repeated inspection of 18 defects according to the standard procedure under no, middle and high time pressure. Stress coping strategies, the mental workload of the task, stress reaction and organisational factors have been measured. The results have shown that time pressure, mental workload and experience influence the quality of the inspection performance. Organisational factors and their influence on the inspection results were rated as important by the operators. However, further research is necessary into the effects of stress.

Viscoelastic properties of cellular polypropylene ferroelectrets (PP FEs) were studied at low frequencies (0.3–33 Hz) by dynamic mechanical analysis and at high frequencies (250 kHz) by laser Doppler vibrometry. Relaxation behavior of the in-plane Young's modulus (Y11′ ∼ 1500 MPa at room temperature) was observed and attributed to the viscoelastic response of polypropylene matrix. The out-of-plane Young's modulus is very small (Y33′ ≈ 0.1 MPa) at low frequencies, frequency- and stress-dependent, evidencing nonlinear viscoelastic response of PP FEs. The high-frequency mechanical response of PP FEs is shown to be linear viscoelastic with Y33′ ≈ 0.8 MPa. It is described by thickness vibration mode and modeled as a damped harmonic oscillator with one degree of freedom. Frequency dependence of Y33* in the large dynamic strain regime is described by the broad Cole-Cole relaxation with a mean frequency in kHz range attributed to the dynamics of the air flow between partially closed air-filled voids in PP FEs. Switching-off the relaxation contribution causes dynamic crossover from the nonlinear viscoelastic regime at low frequencies to the linear viscoelastic regime at high frequencies. In the small strain regime, contribution of the air flow seems to be insignificant and the power-law response, attributed to the mechanics of polypropylene cell walls and closed air voids, dominates in a broad frequency range. Mechanical relaxation caused by the air flow mechanism takes place in the sound and ultrasound frequency range (10 Hz–1 MHz) and, therefore, should be taken into account in ultrasonic applications of the PP FEs deal with strong exciting or receiving signals.

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The extent of damage caused by impacts in fibre reinforced composites depends on the energy of the impacts, on the velocity and the shape of the impacting body, on the material and structure of the composite and on the geometry of the structure. Here, mainly the thickness of the component is essential. The non-destructive evaluation of these damages can be carried out using both ultrasound and active thermography methods. A comparison of the detection sensitivity of these methods for the different damages is carried out in this paper depending on the fibre composite material used (CFRP and GFRP), the thickness of the material and the impact energy. The NDT methods used after the damage are supplemented by thermographic measurements with high temporal resolution, which were already recorded during the impact.