An Introduction to Acoustic Emission

Definition

Acoustic Emission (AE) refers to generation of transient elastic waves during rapid release of energy from localised sources within a material. The source of these emissions in metals is closely associated with the dislocation movement accompanying plastic deformation and with the initiation and extension of cracks in a structure under stress. Other sources of AE are: melting, phase transformation, thermal stresses, cool down cracking and stress build up, twinning, fiber breakage and fiber-matrix debonding in composites.

AE Technique

The AE technique (AET) is based on the detection and conversion of high frequency elastic waves emanating from the source to electrical signals. This is accomplished by directly coupling piezoelectric transducers on the surface of the structure under test and loading the structure. The output of the piezoelectric sensors (during stimulus) is amplified through a low-noise preamplifier, filtered to remove any extraneous noise and further processed by suitable electronics. AET can non-destructively predict early failure of structures. Further, a whole structure can be monitored from a few locations and while the structure is in operation. AET is widely used in industries for detection of faults or leakage in pressure vessels, tanks, and piping systems and also for on-line monitoring welding and corrosion. The difference between AET and other non-destructive testing (NDT) techniques is that AET detects activities inside materials, while other techniques attempt to examine the internal structures of materials by sending and receiving some form of energy.

Types of AE

Acoustic emissions are broadly classified into two major types namely, continuous type and burst type. The waveform of continuous type AE signal is similar to Gaussian random noise, but the amplitude varies with acoustic emission activity. In metals and alloys, this form of emission is considered to be associated with the motion of dislocations. Burst type emissions are short duration pulses and are associated with discrete release of high amplitude strain energy. In metals, the burst type emissions are generated by twinning, micro yielding, development of cracks.

Kaiser Effect

Plastic deformation is the primary source of AE in loaded metallic structures. An important feature affecting the AE during deformation of a material is ‘Kaiser Effect’, which states that additional AE occurs only when the stress level exceeds previous stress level. A similar effect for composites is termed as 'Falicity effect'.

AE Parameters

Various parameters used in AET include: AE burst, threshold, ring down count, cumulative counts, event duration, peak amplitude, rise time, energy and rms voltage etc. Typical AE system consists of signal detection, amplification & enhancement, data acquisition, processing and analysis units.

Sensors / Soure Location Identification

The most commonly used sensors are resonance type piezoelectric transducers with proper couplant. In some applications where sensors cannot be fixed directly, waveguides are used. Sensors are calibrated for frequency response and sensitivity before any application. The AE technique captures the parameters and correlates with the defect formation and failures. When more than one sensors is used, AE source can be located based by measuring the signal’s arrival time to each sensor. By comparing the signal’s arrival time at different sensors, the source location can be calculated through triangulation and other methods. AE sources are usually classified based on activity and intensity. A source is considered to be active if its event count continues to increase with stimulus. A source is considered to be critically active if the rate of change of its count or emission rate consistently increases with increasing stimulation.

AET Advantages

AE testing is a powerful aid to materials testing and the study of deformation, fatigue crack growth, fracture, oxidation and corrosion. It gives an immediate indication of the response and behaviour of a material under stress, intimately connected with strength, damage and failure. A major advantage of AE testing is that it does not require access to the whole examination area. In large structures / vessels permanent sensors can be mounted for periodic inspection for leak detection and structural integrity monitoring. Typical advantages of AE technique include: high sensitivity, early and rapid detection of defects, leaks, cracks etc., on-line monitoring, location of defective regions, minimisation of plant downtime for inspection, no need for scanning the whole structural surface and minor disturbance of insulation.

AET Limitations

On the negative side, AET requires stimulus. AE technique can only qualitatively estimate the damage and predict how long the components will last. So, other NDT methods are still needed for thorough examinations and for obtaining quantitative information. Plant environments are usually very noisy and the AE signals are usually very weak. This situation calls for incorporation of signal discrimination and noise reduction methods. In this regard, signal processing and frequency domain analysis are expected to improve the situation.

A few Typical Applications

• Detection and location of leak paths in end-shield of reactors (frequency analysis)
• Identification of leaking pressure tube in reactors
• Condition monitoring of 17 m Horton sphere during hydro testing (24 sensors)
• On-line monitoring of welding process and fuel end-cap welds
• Monitoring stress corrosion cracking, fatigue crack growth
• Studying plastic deformation behaviour and fracture of SS304, SS316, Inconel, PE-16 etc
• Monitoring of oxidation process and spalling behaviour of metals and alloys