Measurement of Elastic Constants at Low Temperatures by Means of High Frequency Ultrasonic Waves

Abstract Anisotropic composite materials have been extensively utilized in mechanical, automotive, aerospace and other engineering areas due to high strength-to-weight ratio, superb corrosion resistance, and exceptional thermal performance. As the use of composite materials increases, determination of material properties, mechanical analysis and failure of the structure become important for the design of composite structure. In particular, the fatigue failure is important to ensure that structures can survive in harsh environmental conditions. Despite technical advances, fatigue failure and the monitoring and prediction of component life remain major problems. In general, cyclic loadings cause the accumulation of micro-damage in the structure and material properties degrade as the number of loading cycles increases. Repeated subfailure loading cycles cause eventual fatigue failure as the material strength and stiffness fall below the applied stress level. Hence, the stiffness degradation measurement can be a good indication for damage evaluation. The elastic characterization of composite material using mechanical testing, however, is complex, destructive, and not all the elastic constants can be determined. In this work, an in-situ method to non-destructively determine the elastic constants will be studied based on the time of flight measurement of ultrasonic waves. This method will be validated on an isotropic metal sheet and a transversely isotropic composite plate.

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High Frequency Ultrasonic Wave Propagation in Anisotropic Materials

10.26686/wgtn.16985206 ◽

2021 ◽

Author(s):

◽

Andrew Paul Dawson

Keyword(s):

Wave Propagation ◽

Ultrasonic Wave ◽

High Frequency ◽

Guided Wave ◽

Ultrasonic Waves ◽

Tissue Characterisation ◽

Ultrasonic Wave Propagation ◽

Matching Layer ◽

Fibrous Tissues ◽

Wave Modes

<p>The influence of highly regular, anisotropic, microstructured materials on high frequency ultrasonic wave propagation was investigated in this work. Microstructure, often only treated as a source of scattering, significantly influences high frequency ultrasonic waves, resulting in unexpected guided wave modes. Tissues, such as skin or muscle, are treated as homogeneous by current medical ultrasound systems, but actually consist of highly anisotropic micron-sized fibres. As these systems increase towards 100 MHz, these fibres will significantly influence propagating waves leading to guided wave modes. The effect of these modes on image quality must be considered. However, before studies can be undertaken on fibrous tissues, wave propagation in more ideal structures must be first understood. After the construction of a suitable high frequency ultrasound experimental system, finite element modelling and experimental characterisation of high frequency (20-200 MHz) ultrasonic waves in ideal, collinear, nanostructured alumina was carried out. These results revealed interesting waveguiding phenomena, and also identified the potential and significant advantages of using a microstructured material as an alternative acoustic matching layer in ultrasonic transducer design. Tailorable acoustic impedances were achieved from 4-17 MRayl, covering the impedance range of 7-12 MRayl most commonly required by transducer matching layers. Attenuation coefficients as low as 3.5 dBmm-1 were measured at 100 MHz, which is excellent when compared with 500 dBmm-1 that was measured for a state of the art loaded epoxy matching layer at the same frequency. Reception of ultrasound without the restriction of critical angles was also achieved, and no dispersion was observed in these structures (unlike current matching layers) until at least 200 MHz. In addition, to make a significant step forward towards high frequency tissue characterisation, novel microstructured poly(vinyl alcohol) tissue-mimicking phantoms were also developed. These phantoms possessed acoustic and microstructural properties representative of fibrous tissues, much more realistic than currently used homogeneous phantoms. The attenuation coefficient measured along the direction of PVA alignment in an example phantom was 8 dBmm-1 at 30 MHz, in excellent agreement with healthy human myocardium. This method will allow the fabrication of more realistic and repeatable phantoms for future high frequency tissue characterisation studies.</p>

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High-frequency guided ultrasonic waves to monitor corrosion thickness loss

10.1063/1.4974574 ◽

2017 ◽

Cited By ~ 4

Author(s):

Paul Fromme ◽

Fabian Bernhard ◽

Bernard Masserey

Keyword(s):

High Frequency ◽

Ultrasonic Waves ◽

Thickness Loss ◽

Guided Ultrasonic Waves

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The thermal conductivity of Ni 2 + doped KMgF 3 in a magnetic field at low temperatures: the effect of single ions and coupled pairs

Proceedings of the Royal Society of London Series A - Mathematical and Physical Sciences ◽

10.1098/rspa.1970.0061 ◽

1970 ◽

Vol 315 (1523) ◽

pp. 551-568 ◽

Cited By ~ 7

Keyword(s):

Magnetic Field ◽

Thermal Conductivity ◽

Elastic Constants ◽

Low Temperatures ◽

Coupling Constants ◽

Thermal Resistivity ◽

Paramagnetic Resonance ◽

Spin Lattice ◽

Average Spin ◽

Lattice Coupling

The thermal conductivity between 0.4 and 4.2 K and in magnetic fields up to 50 kOe of KMgF 3 doped with Ni 2+ has been measured. The results are analysed to give values of the average spin-lattice coupling constants ( x Sl ) for the Ni 2+ ion. These are in agreement with values calculated using the magneto-elastic constants (GX1 and 6r44) derived from acoustic paramagnetic resonance (a.p.r.) experiments. Below IK the thermal resistivity as a function of magnetic field shows a number of anomalies, for which possible causes are discussed; it is concluded that they result from phonon interactions with exchange-coupled pairs of Ni 2+ ions. Such pairs are also observed in a.p.r. experiments.

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