ULTRASOUND DETECTION AND EMISSION TECHNIQUES: APPLICATION TO THE STUDY OF FIRST-ORDER PHASE TRANSITIONS

The technique of Acoustic Emission has been used widely in non-destructive testing, but also has applications in measurements of first order phase transitions in single crystals. When a material is subjected to an external stimulus, such as the application of stress or changing the temperature, energy can be released in the form of elastic waves, measurable as acoustic emission. This can be due to, for example, a structural phase transition, or the formation or motion of defects (as used in non-destructive testing). These elastic waves will travel through the sample, reflecting off its sides, and can be detected at the surface. Most workers in non-destructive testing are interested mainly in the detection of sufficient events to be of significance to indicate formation of a defect. However, an understanding of the statistics of the events can have many benefits.The group at the University of Barcelona are pioneers in studying acoustic emission in martensitic-type structural phase transitions, where a crystal will change its structure as the stress and/or temperature is changed, and in understanding the characteristic avalanche phenomena observed in such transitions. We will improve on the experimental understanding and the techniques required to detect the acoustic emission. Modelling will be performed of the wave propagation within the complex sample geometries required for use in the stress rig, to develop an algorithm for locating the sources of acoustic emission, and to identify optimal transducer positions for detection of AE events. Non-contact techniques will be investigated for improving measurements, for example electromagnetic acoustic transducers and also laser based detection. Laser detection will allow smaller measurement spot-size and greater spatial resolution, and confirmation of modelling results. Laser generation of ultrasound will also be investigated; a recent study has shown that the application of ultrasonic pulses can reduce the energy barriers to hysteresis in some structural transitions in NiMnGa single crystals. We will look at generating the ultrasound pulses using non-contact methods, such as laser generation, to increase spatial resolution and simplify measurements.Samples studied will include CuZnAl and FePd, which are known to have a martensitic phase transition and will be provided by the group of Prof. Vives, and magnetic systems such as NiMnGa will be investigated to study magnetic phase transitions. This proposal covers research which is highly beneficial to both groups and which will lead to much further collaboration, as well as being of interest to other researchers. This research is not possible without a combination of expertise in theoretical models of acoustic emission, modelling of ultrasound and non-contact ultrasonics.

The research within this proposal has the potential to bring benefits to several diverse areas, as well as a potential to improve the use of acoustic emission (AE) in industry, increasing the safety and management of plant inspection. The main beneficiaries will be academic, however this work has the potential to impact a wide variety of research areas. - Research groups at Warwick and Barcelona. This work will enable significant steps forward for each research group, through combining theoretical and experimental techniques to create a fuller understanding of measurements around first order phase transitions. - Groups performing research on avalanche effects, through the increased understanding and new experimental techniques developed during the project. - Extension to other disciplines, such as work on AE in crack formation, with the potential to look for similar avalanche patterns and understand how they're related. The research impact is not wholly linked to academia, through links to non-destructive testing in industry. Furthermore, outreach activities undertaken by members of the Warwick group (such as demonstrations and talks for schools) will benefit from having results from fundamental physics research through to applied work such as crack detection. The improved understanding of the AE events and verification of theoretical models will be highly beneficial for academics working in this and related areas, and knowledge may be applicable to other events such as those detected during crack formation. This has the potential to bring benefits to UK industry, through improving the understanding of AE in crack formation, and in particular rapidly growing defects. An improved understanding will help to improve safety and management of testing. This is a short project, but the experiments and techniques developed will allow verification of the models. Techniques, understanding and industrial applications are likely to require further development, and later funding will be sought for this. The project will involve several people, and development of skills is an essential part of the proposal. - The visiting research fellow will learn new experimental techniques and improve his skills in this area - The group at Warwick will gain a greater understanding of the theory and effects involved in first order phase transitions, which will be highly beneficial for later research, and in particular the measurements of ultrasound around magnetic phase transitions - The work will involve postdoctoral RAs and PhD students, and allow them to improve their range of skills whilst assisting in experiments and performing modelling of ultrasonic propagation in complex geometries - A pair of final year undergraduate project students will be involved in low-temperature ultrasonic measurements during this project, and will benefit from the expertise which Prof. Vives will bring to the group. The impact described above will require dissemination of the knowledge gained, through extending collaboration, training and through publication of the work. Prof. Vives currently has potential collaborations with researchers at Imperial College (Professor Christensen), the University of Cambridge (Prof. Salje and Drs Moya and Perez-Reche), as well as the Computational Mathematics Group in the Warwick Mathematics Institute, who he has previously worked with on the European MULTIMAT project. Interaction with these groups will take place by the means of seminars, discussions etc, and potential collaborations with the Warwick Group will be sought. Dissemination to a wider academic audience will take place through publications in high-impact scientific journals, and through presenting the work at conferences. Dissemination of research of potential interest to industry will be done through the links between the Warwick group and the EPSRC funded Research Centre in Non-Destructive Evaluation (RCNDE).