Calibration and Traceability

Acoustic emission (AE) can be defined as the spontaneous release of elastic energy by a material when it undergoes deformation. It refers to sounds, in the form of acoustic and ultrasonic energy, emitted from either a process or material which can provide further information about that process or material. Examples of use of AE cover a wide spectrum, from monitoring crack growth in large pressure vessels, storage tanks, and large structures such as bridges and aircraft, to studying tool wear and compositions of liquids.

What are the advantages of AE and how can it be used?

AE differs from other non-destructive testing (NDT) techniques in two main respects: firstly, the energy is released from within the test object itself, rather than being caused by the intentional injection of an acoustic signal into the component under test, as occurs with conventional ultrasonic nondestructive testing; and secondly, AE is capable of detecting dynamic processes associated with the degradation of structures, such as crack growth or plastic deformation. AE makes use of the fact that localised discontinuities will appear long before a structure fails. This gives the maintenance engineer a very useful tool and can prevent the need for unnecessary and expensive early shut-down of plant, as well as providing the means to minimize expensive, and often dangerous, failures.

Another advantage of using AE for structural monitoring is that the test can cover large areas of the structure in one go, and is relatively quick to perform particularly if the AE detecting sensors remain in-situ and are not re-mounted for each test. It can also avoid the need for detailed examination of all parts of a structure, including areas where access is extremely difficult and may be impossible without some stripping down of the structure itself.

The stress waves measured at the surface of the structure depend largely on the nature of the structure and on the propagation path of the waves themselves. Generally the term "acoustic emission", refers to the stress waves that arrive at the surface within an approximate frequency range of 100 kHz to 1 MHz. These frequencies correlate to microscopic events that often produce displacements at the surface of a material of the order of picometres.

Although AE has been in use for many years, the last ten years has seen increased computing power enable advances in signal processing so that the reliability of the AE technology is now recognised by a wider range of industries.

One of the main problem areas for AE is the lack of traceability of the measurements to fundamental units. This means that results cannot be transferred even between like structures to compare performance. Calibration of AE sensors is possible, and the need for quality control at the manufacturing stage is increasing the need for such calibrations. But, measuring the sensitivity of an AE sensor is a long way from actually producing traceable measurements. For calibration the sensor is coupled to a standard test block and the sensitivity is obtained in response to a particular wave mode - a useful technique for comparing the sensitivity of sensors. However, as soon as the sensor is used in the real world and attached to a complex structure the medium plays a large role in the output of the sensor. The calibration of the sensor therefore effectively becomes invalid, for several reasons - many more wave modes exist, the structure material may be different to that of the test block and the coupling of the sensor to the structure rather than the test block is known to be an important issue.

Acoustic emission - a new area for NPL

Although much research has been performed into acoustic emission (AE) over the years, there is currently no centre of excellence within Europe where issues of calibration and traceability of measurement to fundamental units have been vigorously pursued. Within the DTI programme, NPL have recently started a programme of work to consider these aspects of AE.

NPL research on AE

NPL research centres on the following key areas:
      
  considering a possible methodology for the calibration of acoustic emission sensors used for the detection and location of discrete microfracture events that accompany stable crack growth in metals, due to fatigue and stress corrosion
        
  considering possibilities for a 'system calibration' which would effectively consider a test structure and the attached AE sensor as one. An 'in-situ' calibration of the system with a known AE reference source, quantified in standard units, would then enable the real AE event to be characterised by comparison with the reference source. Possible transfer sources and a method for their calibration are being considered, to provide a traceable link for AE measurements
        
  use of modeling techniques to understand the interaction of a reference source and the structure, the propagation of wave modes within the structure and their interaction with the sensor
        
  investigating the feasibility of use of time reversal techniques in AE
        
  keeping AE users informed of progress and giving them opportunities to trial new methods when appropriate

Calibration of AE sensors
NPL has completed a collaborative project with Lloyd's Register of Shipping and Airbus UK to propose a possible methodology to measure the out-of-plane sensitivity of AE sensors used specifically for detection of crack growth in metals, when they are exposed to compressional wave modes. A full pdf version of the report of the work (reference CMAM 82 ) can be obtained from the NPL web site.

This information is supplied by courtesy of the National Physical Laboratory, Teddington, England