We report an evolution of an all-digital ultrasonic pulse technique for measurements of elastic constants of solids. An unambiguous analytical procedure is described for determining the correct time delay of echoes without any need for actual echo overlap. We also provide a simple procedure for making corrections for transducer-bond-induced phase shifts. The precision of a measurement made with this system at ambient temperature exceeds one part in 107 without the use of mixers, gates, time delays, and other complications normally associated with such measurements.

http://absimage.aps.org/image/MAR06/MWS_MAR06-2005-000667.pdf

Digital ultrasonic pulse-echo overlap system and algorithm for unambiguous determination of pulse transit time

The speed and attenuation of ultrasound propagation can be used to determine material properties and identify phase transitions. Standard ultrasonic contact techniques are not always convenient due to the necessity of using couplant; however, recently reliable non-contact ultrasonic techniques involving electromagnetic generation and detection of ultrasound with electromagnetic acoustic transducers (EMATs) have been developed for use on electrically conducting and/or magnetic materials. We present a detailed study of magnetic phase transitions in a single crystal sample of Gd64Sc36 magnetic alloy using contact and non-contact ultrasonic techniques for two orientations of external magnetic field. Phase diagrams are constructed based on measurements of elastic constant C33, the attenuation and the efficiency of generation when using an EMAT. The EMATs are shown to provide additional information related to the magnetic phase transitions in the studied sample, and results identify a conical helix phase in Gd64Sc36 in the magnetic field orientation .

/pdf/magnetic-phase-transitions-in-gd-64-sc-36-studied-using-non-54x1xxrpsr.pdf

Magnetic phase transitions in Gd 64 Sc 36 studied using non-contact ultrasonics

This thesis is concerned with the detection of surface-breaking defects, such
as stress corrosion cracking, using an ultrasonic scanning approach in which a laser
source and detector are scanned over the near-field of a defect. Large increases
in the amplitude and frequency content of an incident ultrasonic wave are present
when either the source or the detector is very close to the defect, leading to a phenomenon
known as ultrasonic near-field enhancement. The extent of the ultrasonic
enhancement varies with defect characteristics such as defect depth and angle to the
surface.
Ultrasonic enhancement is observed in both experiment and finite element
simulations using Rayleigh waves for both scanning laser detection and scanning
laser source methods. The near-field enhancement is shown to vary as a function of
the angle of the defect to the horizontal for Rayleigh wave enhancements, allowing
the positioning and characterisation of artificial angled defects that are similar to
rolling contact fatigue defects in railtrack. The mechanisms behind the near-field
enhancement of Rayleigh waves at angled defects are identified, and this aids in the
understanding of the behaviour of ultrasound as it interacts with surface-breaking
defects.
Ultrasonic enhancements are also reported to be present in individual Lamb
wave modes for interactions with artificial open-mouthed defects in thin plates,
which are similar to the open end of stress corrosion defects. The mechanisms behind
both the scanning laser detection and scanning laser source enhancements are
identified and used to explain the variation in the enhancement as a function of
increasing defect severity. Positioning of these defects is also achieved by identification
of the enhancement location.
Finally, the scanning laser technique is applied to real stress-driven defects,
and both scanning approaches are shown to be capable of detecting partially-closed
defects in a variety of sample geometries. The position, geometric alignment and an
estimate of the defect depth are obtained for real defects in thin plates, pipework
sections and in irregularly shaped engine components.

A study on the near-field interactions of ultrasonic surface waves with surface-breaking defects

Ultrasonic testing using contacting transducers such as quartz or PZT is well established. However, standard measurement techniques used require physical contact of the sample and ultrasonic transducer and some sort of couplant between the two. With this configuration there is a possibility of damaging the sample, transducer or bond during testing, thermal cycling, or removal of the transducer. We present results taken using recent advances in non-contact methods of ultrasound generation and detection using electromagnetic acoustic transducers (EMATs), which offer some significant benefits over contact ultrasonic techniques. Circumventing the need for couplant removes the possibility of contaminating the system, which is an issue for some material property measurements, and allows easier measurements over a wider range of temperatures. An automated data analysis system has been developed which allows the velocity of sound in the sample, and hence the elastic constants, to be determined to a high accuracy. This technique is illustrated using measurements of the alloy Gd64Sc36.

Non-contact Ultrasonic Measurements of the Elastic Constants of Magnetic Materials

Measurement of elastic constants is crucial for engineering aspects of predicting the behavior of materials under load as well as structural health monitoring of material degradation. Ultrasonic velocity measurement for material properties has been broadly used as a nondestructive evaluation method for material characterization. In particular, pulse-echo method has been extensively utilized as it is not only simple but also effective when only one side of the inspected objects is accessible. However, the conventional technique in this approach measures longitudinal and shear waves individually to obtain their velocities. This produces a set of two data for each measurement. This paper proposes a simultaneous sensing system of longitudinal waves and shear waves for elastic constant measurement. The proposed system senses both these waves simultaneously as a single overlapped signal, which is then analyzed to calculate both the ultrasonic velocities for obtaining elastic constants. Therefore, this system requires just half the number of data to obtain elastic constants compared to the conventional individual measurement. The results of the proposed simultaneous measurement had smaller standard deviations than those in the individual measurement. These results validate that the proposed approach improves the efficiency and reliability of ultrasonic elastic constant measurement by reducing the complexity of the measurement system, its operating procedures, and the number of data.

/pdf/measurement-of-elastic-constants-by-simultaneously-sensing-3k88zyt79x.pdf

Measurement of Elastic Constants by Simultaneously Sensing Longitudinal and Shear Waves as an Overlapped Signal

The use of ultrasound for measuring elastic constants and phase transitions is well established. Standard measurements use piezoelectric transducers requiring couplant and contact with the sample. Recently, non-destructive testing (NDT) has seen an increase in the use of non-contact ultrasonic techniques, for example electromagnetic acoustic transducers (EMATs) and laser ultrasound, due to their many benefits. For measurements of single crystals over a range of temperatures non-contact techniques could also bring many benefits. These techniques do not require couplant, and hence do not suffer from breaking of the bond between transducer and sample during thermal cycling, and will potentially lead to a simpler and more adaptable measurement system with lower risk of sample damage. We present recent work adapting EMAT advances from NDT to measurements of single crystals at cryogenic temperatures and illustrate this with measurements of magnetic phase transitions in Gd64Sc36 using both contact and non-contact transducers. We discuss the measurement techniques implemented to overcome noise problems, and a digital pulse-echo-overlap technique, using data analysis in the frequency domain to measure the velocity.

/pdf/measuring-elastic-constants-using-non-contact-ultrasonic-9aq5v3k6cf.pdf

Measuring elastic constants using non-contact ultrasonic techniques

http://iopscience.iop.org/article/10.1088/1742-6596/286/1/012050/pdf

Non-contact ultrasonic measurements of the elastic constants of magnetic materials

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Non-contact Ultrasonic Measurements of the Elastic Constants of Magnetic Materials

Measuring elastic constants using non-contact ultrasonic techniques

Non-contact ultrasonic measurements of the elastic constants of magnetic materials