Acoustic emission measurements on metal foams

https://doi.org/10.1016/j.jallcom.2003.10.094Get rights and content

Abstract

The acoustic emission (AE) response during compression and indentation of different kinds of aluminum foam was recorded and evaluated with respect to the various stages of the deformation process. During compression continuous AE response was measured, while during indentation a response consisting of individual AE events was monitored. Studying these individual signals during indentation permits to study the changes in deformation mechanisms during compression.

Introduction

Cellular materials surrounding us possess potential for use in light-weight structures (like wood) or in energy absorption in transport industry or in packaging (like polymer foams) [1]. Recently, a novel cellular material called metal foam has been fabricated. It offers unique combination of properties since it is stiff, light, fire retardant and good energy absorber. Due to these properties metal foams find numerous applications, e.g. in transport and aerospace industries [2]. However, despite of applications, the deformation mechanisms in these materials have not yet been fully understood.

The acoustic emission (AE) stems from transient elastic waves generated within the material due to sudden localised structure changes. AE responds to collective dislocation motion and damage processes [3] and therefore yields information on the dynamic processes involved in deformation of the material. Consequently, AE is a promising technique to investigate deformation processes in metal foams. Up to now, a correlation between the stress–strain behaviour of the foams with different composition of solid material and AE response was only revealed in [4].

In this paper, the AE response during indentation and compression of Al-based metal foams was investigated. With this technique the onset of the plastic deformation can be measured during deformation. Since during indentation the deformation is localised in a small volume, the AE response can be connected to micromechanical deformation process.

Section snippets

Experimental procedure

Two kinds of metal foams were investigated: an aluminium–silicon foam (AlSi10) and Alporas foam (AlCa5Ti3). The aluminium–silicon foam was fabricated by powder metallurgy. Aluminium alloy powder, containing 10 wt.% silicon was mixed together with the blowing agent, titanium hydride and then continuously hot extruded into a foamable precursor. The precursor was foamed up in steel moulds to form panels, which have two solid skins. These skins were removed before deformation. With this method

Indentation

Discrete AE signals were monitored during indentation on Alporas foams of different relative densities. On Fig. 1 a typical stress-displacement curve can be seen up to 4 mm of displacement. The scatter in the deformation curve is rather large, approximately 15%, since the diameter was smaller than six times the average cell diameter [7]. Nevertheless, common characteristics of the stress-displacement curves are found, after the quasi-linear stage the stress increases slowly and the deformation

Conclusions

The AE response during indentation and compression of Al-based metallic foams was examined with respect to the various stages of the deformation process. Indentation was found to be an appropriate tool for investigating the AE response during deformation since discrete AE signals were measured. Two types of AE signals could be distinguished according to the rising time of the amplitude of the AE signal. It indicates basically two different modes of deformation: fracture and plastic yield.

AE

Acknowledgements

The authors thank to F. Simančı́k and N. Babcsán for providing samples. This work is a part of the Research Program MSM113200002 that is financed by the Ministry of Education, Youth and Sports of the Czech Republic. A support from the Grant Agency of Czech Republic (Grant 103/01/1058) and a support within the framework of the collaboration between the Eötvös Loránd University and the Charles University are also appreciated. Financial support of the Hungarian Scientific Research Fund (OTKA)

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