S. Dixon

The detection of magnetic phase transitions in single crystals of erbium and erbium-thulium by electromagnetic acoustic transducers

Measurements of ultrasonic shear wave velocity as a function of temperature and applied magnetic field have been performed on single crystals of Er and . Electromagnetic acoustic transducers (EMATs) are used to generate and detect the in-plane radially polarized shear wave. The shear wave is propagated down the c axis of the hexagonal close packed single crystal samples. The EMAT acoustic coupling efficiency has been observed to increase close to the magnetic phase transition temperatures, showing the dominance of the magnetoelastic interactions over the applied-field-dependent Lorentz force mechanism in these samples. Measurement of the EMAT acoustic coupling efficiency is identified as a promising technique for identifying magnetic phase transitions. Because the ultrasound measurement technique is a non-contact one, problems associated with acoustic couplants are avoided.

Laser based ultrasound using different wavelengths for the inspection of composite materials

This paper investigates damage free ultrasound generation on Carbon Fiber Reinforced Composites using a range of different lasers: TEA CO2 (10.6 μm), Nd:YAG (1064 nm) and a XeCl excimer laser (308 nm). It is essential that no damage is caused to the components and this restricts generation to the thermoelastic regime, where it is due to rapid thermal expansion. For this reason, the laser damage thresholds of the samples are presented and compared.

Laser based ultrasound generation efficiency in carbon fiber reinforced composites

This paper compares the generation efficiencies and damage thresholds in carbon-fiber reinforced composites using a Nd:YAG laser (1.06 μm) and a TEA CO2 laser (10.6 μm). It is essential to remain within the thermoelastic regime in order to prevent damage. The energy is absorbed within the optical absorption depth, the temperature rise is affected by the laser wavelength and pulse duration. Results from a Nd:YAG laser with an Optical Parametric Oscillator tunable up to 4 μm are also reported.

Non-contact ultrasonic generation on thin metal sheets using a plasma igniter

This paper describes some preliminary results obtained using a plasma igniter and compares it in a couple of applications to both electromagnetic (pulsed coil) and laser based sources. The application of the plasma igniter ultrasonic generation source is limited for reasons that will be explained, but its relatively low cost means that it does merit consideration in some cases.

A Non-Contact Laser-Emat System for Crack and Hole Detection in Metal Plates

The use of non-contact ultrasonic techniques can have distinct advantages over conventional contact methods, allowing more rapid and practical scanning without suffering from variations introduced by an acoustic couplant. Non-contact ultrasonic testing can also be used for inspecting components on a production line. The system described here uses separate ultrasonic generation and detection techniques. Longitudinal waves are generated in a sample by means of a pulsed laser, and waves scattered from defects within the sample are used to identify the presence and location of simulated surface breaking cracks and side drilled holes. The longitudinal waves are detected using an electromagnetic acoustic transducer (EMAT) on the same side as the ultrasonic generation point, located coaxial to the generation laser beam.

A Novel Non-Contact Ultrasonic System for Liquid Level Measurement in Moving Metallic Containers

The majority of conventional liquid level monitors in industry operate using a gamma-ray absorption technique. Gamma-rays with energies utilised in these machines are strongly absorbed by water/liquids. In a γ -ray measurement a collimated beam is momentarily exposed to the container that is to be measured, with some of the beam below and some above the nominal liquid level. The γ-rays are detected using a scintillation tube after passing through the container and its contents. Thus if more of the γ -ray beam is absorbed by a higher liquid level, the counts drop, and vice-versa. The type of machine is in the main a pass/fail monitor — that is it passes a can if the liquid level is above a predetermined height, and rejects it if it is not. There are several problems associated with the use of γ -ray based systems, most notably environmental considerations, the fact they are potentially hazardous and subject to strict legislation.

Laser Ultrasound in Single Crystal Silicon

Laser generated ultrasound has been quite extensively studied in single crystal silicon [1,3]. Off-epicentral waveforms have been measured in plate like samples, these are quite complex as silicon is quite strongly anisotropic with one of the shear waves exhibiting cusps. Cusps form due to severe beam steering in some anisotropic materials, Any ultrasonic experiment actually measures the wave surface (represents energy transport) and not the velocity surface [4]. The normal to the slowness surface represents the direction of energy transport, hence it is a useful representation when considering beam steering effects. Materials with incurved slowness surfaces exhibit cusps. Even in isotropic materials laser generated off-epicentral waveforms are complex, both longitudinal and shear waves have radial and tangential components of displacement. At angles greater than the critical angle the head wave appears and the shear wave drastically changes form.

The Detection of Magnetic Phase Transitions in ER-TM by Electromagnetic Acoustic Transducers

The phases in magnetic materials can be studied ultrasonically. Magnetic phase transition points can be determined from discontinuities in ultrasonic velocity as a function of temperature. Conventionally this is done using piezoelectric transducers (normally quartz) pulsed with a tone burst generator. Very accurate velocity measurements are required as the change in velocity is typically 1 part in 104 [1]. A common problem experienced with contacting transducers is the fracturing of the acoustic couplant bond at low temperatures. Non-contacting acoustic techniques have no problems with bond failure, electromagnetic acoustic transducers (EMATs) can be used for measurements on rare earth magnetic materials.

A Novel Laser Ultrasound Source and its Implementation in the Drinks Canning Industry

The majority of conventional liquid level monitors in the drinks canning industry operate using a gamma-ray absorption technique. Gamma-rays with energies utilised in these machines are strongly absorbed by water/liquids. In a gamma ray measurement a collimated beam is momentarily exposed to the can that is to be measured, with some of the beam below and some above the nominal liquid level. The gamma-rays are detected using a scintillation tube after passing through the can and its contents. This is shown in the schematic diagram of fig.1. Thus if more of the gamma-ray beam is absorbed by a higher liquid level, the counts drop, and vice-versa. The type of machine is in the main a pass/fail monitor — that is it passes a can if the liquid level is above a predetermined height, and rejects it if it is not. There are several problems associated with the use of gamma ray based systems, most notably environmental considerations, the fact they are potentially hazardous and the strict legislation required.