Acoustic emission techniques (AET) are emerging as a powerful tool for NDT of plant components such as pressure vessels, pipes, welds etc. Acoustic emission (AE) is defined as the phenomenon whereby transient elastic waves are generated by the rapid release of energy from localized sources like places of transient relaxation of stress and strain fields. Fracture, plastic deformation, crack initiation and growth are a few examples of the phenomenon resulting in AE.
AET as a technique for monitoring and evaluating structural integrity is superior to other techniques because of its capacity for: (1) continuous monitoring (2) inspection of complete volume of the component (3) providing advance warning and (4) detection and location of any crack initiation and propagation and system leaks. The basis of AET is the work of Josef Kaiser.
Acoustic emission testing is based on the fact that solid materials emit sound, or acoustic emissions, when they are mechanically or thermally stressed to the point where deformation or fracturing occurs. Almost all materials produce an acoustic emission when they are stressed to failure. AET detects and analyses minute AE signals generated by discontinuities in material under a stimulus such as stress, temperature etc.
AE testing is based on the Kaiser Effect which encompasses two important discoveries. The first one states that materials emit minute pulses of elastic energy when placed under stress. The second discovery was that once a given load was applied and acoustic emission from that load had ceased, no further emission would occur until the previous stress level was exceeded, even if the load was removed and reapplied later. Kaiser effect can be time dependent for materials with elastic aging.
Depending on the energy released, two types of AE are observed: (1) continuous and (2) burst. Continuous emission is characterized by low amplitude emissions, which vary with AE activity. In metals and alloys this type of emission occurs during plastic deformation and fluid leakage. Burst emissions are characterized by short duration (10 micro seconds to a few milliseconds) and high amplitude pulses due to discrete release of strain energy. This type of emission occurs during crack initiation and propagation and stress corrosion cracking.
Acoustic emission test equipment consists of transducers, preamplifiers, single channel signal processing instruments, multichannel signal processing systems, distributive and microprocessor controlled systems, AE weld, and leak and bearing monitors. In large pressure vessels it is necessary not only to detect AE signals but also locate their sources. This is accomplished by suitably placing several sensors over the surface of the pressure vessel by spacing them appropriately. The time of arrival of signals to the various sensor locations is monitored. Since sound has a high velocity and the sensors are placed close together in a steel vessel, time resolutions in microsecond range must be made in order to locate the source of signals to within less than 25mm. The common ways in which AE signals can be processed are:
Counting: Ringdown counts are the number of times the signal crosses a threshold level, set for eliminating background noise. Ringdown counts, Ringdown count rates are used to process AE signals.
Energy Analysis: used for continuous and burst type emissions.
Amplitude Analysis: used to characterize emissions from different processes.
Frequency Analysis: used to identify different types of failures.
Advanced signal analysis concepts, such as pattern recognition or spectral analysis can be used.
When the discontinuity approaches critical levels, the AE count rate increases markedly, thus giving a warning for impending instability and failure of the component. AE evaluation of structures depends on the ability to detect weak signals in a noisy electrical and mechanical environment. This is achieved by the use of proper instrumentation to discriminate between wanted and unwanted signals. The major impetus for development AE systems is the desire for safe operation of pressure vessels especially in nuclear-power plants.
Acoustic emission testing is especially used for testing large pressure vessels, storage tanks and piping systems. In the past this type of industrial equipment had to be shutdown, emptied and decontaminated. These processes apart from being expensive can take several days for completion. After decontamination it is usually considered safe for an inspector to enter the equipment to take X-rays, make visual examination or ultrasonic measurements.
With AE technology, chemical and petrochemical reactors can be monitored as they are started up. Storage tanks and vessels can be monitored as they are filled up. Acoustic emissions can be used to proof test and determine the structural integrity of fiberglass or metallic systems. The following table lists other applications of AE technique:
Table 1: Acoustic Emission Applications
Buried pipeline monitoring
Nuclear plant monitoring
Detection of leaking valves
Offshore platform testing
Fatigue mechanics problems
General weld monitoring
Steel lamination detection
Fracture Mechanics problems
Stress corrosion detection
Laser material processing
Tool wear monitoring
In flight AE monitoring
Glass reinforced plastic tank (GRP) testing
Human knee joint implants
In order to validate the application of AET for a given system, a number of simulation experiments are carried out by introducing flaws into similar structure/component or on laboratory specimens and monitoring AE response during the growth of these flaws. The defects detected by AET have been confirmed by other NDT methods like ultrasonic testing, magnetic particle testing etc. AE inspection is extremely sensitive compared to other familiar NDT methods. The minimum detectable crack size for ultrasonic, radiography and eddy current testing is 0.5 mm if ideal conditions are met for each method. AET on the other hand can detect crack growth of the order of 25 microns.