FFS Forum: Acoustic Emission Testing – The Least Understood Inspection Technique (Part 1 of 2)

Editor’s Note: This is the first of a two-part column on Acoustic Emission Testing (AET). It presents a brief overview of the basic principles of AET and how it works. Part two of this series, published in the May/June 2021 issue of Inspectioneering Journal, addresses some of the advantages and disadvantages of AET and discusses a few examples of its typical applications.

Introduction

Fitness-for-service (FFS) assessments and inspection are like two sides of the same coin. FFS assessments usually begin with some issue discovered in an inspection and often end with a recommendation for some regular inspection plan to monitor the condition over time or cycles. For many components and systems, especially where damage progression can’t be analytically predicted, regular in- service inspection becomes an integral part of the FFS process. With this in mind, I’d like to use this issue of the Fitness-for-Service Forum to try to provide a little better understanding of one particular inspection method: Acoustic Emission Testing.

What is Acoustic Emission Testing (AET)?

People have known about acoustic emission for a long time.

Audible acoustic emission (AE) from materials was noticed long ago when early potters relied on audible sounds to identify damaged ceramics.  The first known observations in metals are thought to have been by early tinsmiths who noticed that tin produces audible emission during plastic deformation, so called “tin cry”.  The earliest written reference to tin cry was in the eighth century, but since the earliest pottery appeared around 6,500 BC and earliest tin from at least 2500 BC, it’s fair to say that primitive knowledge of AE in metals has been around for thousands of years ^[1].  Of course, AE, as we know it today, doesn’t rely on audible sounds – it employs much higher frequencies.  But before we go too deep, let’s start with a definition.

I found what I thought was a pretty good definition in a 1984 paper published by the National Bureau of Standards.  It reads:

“Acoustic emission (AE) is the name given to the transient mechanical waves spontaneously generated by abrupt localized changes of strain within a body. Dislocation motion and crack growth are the mechanisms by which these strain changes occur during growth of flaws in materials; even minute crack propagation or plastic deformation results in elastic waves which can cause surface motion of a body. This surface motion is sometimes of sufficient amplitude to be detected by sensors (transducers) attached to the surface; the sensors convert a mechanical disturbance to a voltage-time waveform” ^[2].

An earthquake is probably the most understandable example of acoustic emission.  In an earthquake, the sudden release of a dislocation in the earth’s crust results in propagation of a compression wave that eventually works its way to the surface.  At the surface, the compression wave manifests as movement of the surface, shaking buildings and cars and creating havoc.  

This is more than an analogy.  The earliest scientific work in acoustic emission was presented by Professor Fuyuhiko Kishinouye at the Earthquake Research Institute conference in Japan in 1933.  Professor Kishinouye induced fracture in wood and measured the acoustic emission as a means of studying fracture of the earth’s crust in earthquakes ^[3].