Characterising the acoustoplastic effect in ultrasonically assisted forming of metals

Lead Research Organisation: University of Glasgow
Department Name: School of Engineering

Abstract

The plastic deformation of metals is a key process in many industrial forming operations, from forging critical aerospace components to deep-drawing of aluminium cans. The superposition of high-power ultrasonic excitation on the static loading of resonant forming tools has been demonstrated to reduce the forming force required during plastic deformation of metals, offering opportunities for a significant increase in process speed and reduction in demand on energy resources.

The effect of ultrasonics on metal deformation is known as acoustoplasticity and was first described in the 1950's. Since then several hypotheses have been proposed to attempt to explain the phenomenon, but there is still no universally accepted explanation, and this hinders the optimal design of ultrasonically assisted forming equipment and wide application of the technology.

The aim of the project is to apply new experimental mechanics measurement techniques to gain insights into the acoustoplastic effect. The project objectives are:

1. Design an experimental test rig to enable ultrasonically assisted tension tests of different metal specimens, characterising the evolving strain, stress and thermal fields using an ultra-high speed camera for digital image correlation alongside thermography;

2. Design an experimental test rig that allows ultrasonic excitation to be accommodated in an x-ray synchrotron to characterise the evolving microstructure of specimens simultaneously subjected to static and ultrasonic loading.

The experiments in both objectives are completely novel and can offer the potential to conclusively measure and define acoustoplasticity.

The project aligns closely to the EPSRC Manufacturing the Future theme, progressing towards innovations in high-power ultrasonics applied to manufacturing technologies. Products that rely on the deformation of metals are pervasive and significant commercial and energy resource benefits can be gained from the application of ultrasonically excited tools. This project will move this research field towards increasing confidence in the uptake of ultrasonically assisted forming technologies.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509668/1 01/10/2016 30/09/2021
1805527 Studentship EP/N509668/1 03/10/2016 31/03/2020 Colin Frances Souza
 
Description The plastic deformation of metals is key in many industrial forming processes, from forging critical aerospace components to deep-drawing aluminium cans. It has been demonstrated that high-power ultrasonics can reduce the force required to cause and maintain yielding during plastic deformation of metals, offering opportunities for significant increases in process speed and reduction in demand on energy resources. The effect, which was first observed in the 1950's, is known as acoustoplasticity.

Even after decades of research, the fundamental nature of acoustoplasticity is still contested. Whilst many researchers link ultrasonic excitation with a real change in intrinsic material behaviour, a similar number refute conclusions of this nature and point to stress superposition, suggesting that the actual peak stress within the specimen has not been measured correctly. This is due in part to the inconsistency in experimental setups and instrumentation used. Consequently, of fundamental importance is that the measurements made with piezoelectric force transducers are interpreted correctly.
We have developed an enhanced test apparatus which can easily be modelled in a numerical simulation. Surface velocity measurements, which are better established and trusted than the piezo-electric force measurements, are being used to calibrate the numerical model, allowing an alternative route to finding the forces within the test specimen, as well as throughout the test apparatus.
In addition to this a strain gauge acquisition system has been developed which can accept the high frequency signal from a strain gauge applied the surface of the specimen. This provides a direct measurement of dynamic elastic strain (and, by calculation, force) within the specimen itself.
The combination of these methods permits an assessment of confidence in the force measurement, and hence in the validity of the phenomenon of acoustoplasticity.

In addition to evaluating force, we have developed a system capable of imaging the plastic strain field on our vibrating specimen. An industrial USB camera was combined with a high-speed strobe to provide images which were subsequently processed using Digital Image Correlation, producing the full-field strain map over the test specimen. This allows the study of the effect ultrasonic excitation has over the length of the specimen.
Exploitation Route The immediate impact of this research is on the current state-of-the-art in measuring mechanical properties related to the acoustoplastic effect. Established methods have proven inadequate; the process developed in this work points to a method which accounts for the inhomogeneous and inertial nature of the ultrasonic tensile test. The enhanced test apparatus and method for capturing the full-field strain could be used to re-assess acoustoplastic constitutive models, which were originally developed using inferior test methods.
Longer term it is hoped that these methods could be used to develop tooling capable of delivering the benefits in metal forming that previous research seems to promise.
Sectors Aerospace, Defence and Marine,Manufacturing, including Industrial Biotechology

 
Description IEEE-IUS 2019 Outreach event 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Schools
Results and Impact Science outreach event linked to IEEE International Ultrasonics Symposium, held in Glasgow Science Centre, had several benches of interactive exhibits. I manned the stall demonstrating acoustic levitation with polystyrene balls. Something like ten schools brought groups of children; certainly visited by pupils in the low hundreds. A very busy day and the demonstrations were very popular, facinating some and starting conversation on working in engineering and science, careers in the same, as well as in the science and technology behind the demonstration.
Year(s) Of Engagement Activity 2019