|Acoustic Emission: Heeding the Warning Sounds from Materials
by Mark F. Carlos
Most materials and structures emit energy in the form of mechanical vibrations (acoustic emission) as a result of sudden change or movement. This is usually due to a defect-related phenomena, such as cracking or plastic deformation. These acoustic emissions propagate from the source, throughout the structure. The technique of electronically listening to these acoustic emissions is used worldwide, detecting and locating defects as they occur, across the entire monitored area, providing early warning of pending failure, in a timely and cost effective manner.
All types of structures including bridges, buildings, pressure vessels, pipelines, airplanes, ships, petrochemical and production processes, undergo continuous loading and stressing. On bridges and buildings, traffic loads and environment (e.g., temperature and wind) exert formidable stress. On pipelines and vessels, the process itself, both temperature and pressure, supply the stress. In production processes, machines apply stresses to materials as they are being formed, shaped and joined (e.g., in welding). These stresses eventually cause defect growth (e.g., cracking) in weaker or fatigued areas of the structure. Acoustic emission (AE) is unique to all other nondestructive test (NDT) methods in that it detects the defect growth, in real time, as it is occurring. It is of great importance to have such a nondestructive test method that can detect and locate flaws as early as possible. As a result, the structure can be repaired or replaced long before a catastrophe occurs, thereby preventing loss of life, environmental damage, and costly repairs.
Definition and Description of the Acoustic Emission Generation Process
Acoustic emission is defined in E 1316, Terminology for Nondestructive Examinations, as the class of phenomena whereby transient elastic waves are generated by the rapid release of energy from localized sources within a material. Although this definition sounds rather complex, it can easily be explained with the aid of Figure 1, which shows a material cracking under stress. The actual cracking process emits acoustic waves that emanate in an omni-directional manner from the source. An acoustic emission sensor (usually piezo-electric based) in contact with the material being monitored, detects the mechanical shock wave and converts the very low displacement, high frequency mechanical wave, into an electronic signal that is amplified by a preamplifier and processed by the AE instrument. As can be seen in Figure 1, stress plays an important role in the AE generation process. In many AE applications, stress is automatically applied by the process itself (e.g., pipelines), and in others, the stress is applied by an externally induced force. The key however, is that the stresses being applied are nondestructive, that is, they are well below the expected defect tolerance of the material.
Typically, AE systems operate in a range of 1 kHz to 2 MHz or greater in frequency. The lower frequency limit is imposed by background noises such as friction, outside impacts, or process generated signals, that tend to mask acoustic emission. The upper frequency limit is imposed by attenuation, which tends to limit the range of detection of acoustic emission signals. A critical part of the AE application process is the selection of a suitable frequency range for AE detection and signal processing. It must be above the non-AE related background noises, while providing the necessary detection range (distance/frequency) and sensitivity to AE related signals. This is accomplished through the selection of AE sensors (that operate in various narrow-band or wide-band frequency ranges) and electronic signal filtering.
Acoustic emission is generally transient in nature, occurring in discrete bursts. AE systems process these bursts as AE hits by analyzing various aspects of the waveforms associated with each hit, one at a time. Figure 2 shows an AE hit waveform and a few of the AE features that are processed by the AE system. Time of hit, rise time, AE amplitude, AE counts, duration, frequency content, and even the waveform itself are key AE features that can be analyzed to help identify the source of AE as noise- or defect-related.
Acoustic Emission System Description
AE systems come in many varieties. They range from simple single-channel, single purpose devices to complex multi-channel, multiprocessing systems. A simplified block diagram of an AE system is shown in Figure 3. The basic AE system consists of one or more AE sensors, and a preamplifier (per channel) that is connected to an AE processor. The processor receives signals from the AE sensors/preamplifiers as well as signals from external sensors or control inputs, which might be following the process or the stress (or load) being applied to the materials under test.
The job of the AE processor is to process these inputs together to form outputs indicative of the activity detected and correlated to the process or stress measured. These outputs can be a pass/fail signal for control purposes, or might be an indicator or set of graphical outputs. Outputs can illustrate trending, or the relationship of AE to the load or stress on the structure. The display might show plots of AE locations on a structure with a cluster analysis to help the operator determine areas of concern.
Location of Acoustic Emission Sources
One of the key advantages of AE over other nondestructive test methods is the fact that it detects signals in real-time that are emanating from the materials themselves. An AE stress wave travels from the source through the structure and can be detected by an AE sensor placed on the structure. The sensor may be located some distance from the source (subject to attenuation) and still detect the signal. If multiple sensors are placed on the same structure, it is then possible to determine the location of the source by analyzing the time difference of arrivals to each sensor and processing triangulation calculations.
To determine location in one dimension (a line), two sensor arrivals are required as depicted in Figure 4. To determine location in two dimensions (over a surface or plane) three sensor arrivals are required (Figure 5). To determine location in three dimensions, a minimum of four sensor arrivals are required. The severity of a source can be analyzed by monitoring multiple AE events in the same area and applying a cluster analysis as shown in Figure 6. Source location is an extremely powerful tool in AE analysis and can be used to monitor a relatively large structure with a minimum number of sensors. This is a tremendous advantage in the case of vessels, especially when they are insulated, since very few access holes are needed for placement of AE sensors to determine structural integrity of the vessel. All other NDT techniques require all the insulation to be removed for full inspection, making them much more expensive than AE examination techniques.
Some Acoustic Emission Applications
The following is intended to provide examples of some successful applications of AE, and is by no means a complete list.
Crack Detection: One of the oldest and most successful applications of acoustic emission has been in manufacturing, with the detection of cracking in various materials during bonding, forming or pressing operations. AE systems are interfaced to programmable controllers and are gated to monitor for cracking, only during the high-stress point of the process. When a crack occurs, the AE system provides a failure output for part rejection. Many AE crack detection systems are operating today providing continual monitoring and real time inspection.
Weld Analysis: AE has been successfully applied to welding applications for many years. AE systems have addressed solutions to real time weld quality analysis and weld control, including resistance spot welding (alternating current and direct current), metal-inert gas/tungsten-inert gas welding, electron beam welding, laser welding, and friction welding.
Every burst of AE received from the welding process has a logical source and meaning. By focusing in on the expected occurrence of a given weld phenomena through time, frequency and feature discrimination, this can be isolated and used as a control or feedback on the process assuring ultimate control of the welding process. AE has been able to detect oxidation burn-off, nugget formation, hot cracking, expulsion and post weld cracking. Each of these phenomena are very important in weld quality analysis and feedback control for decision making in stopping the weld at the appropriate time.
Vessel Inspection: One of the most successful applications of AE is in vessel inspection for the petrochemical industry in both metal and FRP vessels and spheres. Vessel testing is performed every day by many different service companies around the world. Sensors are placed on the vessels in arrays to monitor the entire pressure boundary. The vessel is then subjected to pressures typically 10 percent above previous operating levels (well below the vessel pressure rating) with the test pressures being subjected in a pressure rise-hold-rise-hold fashion to peak stress, while monitoring the AE activity during each of these pressurization segments. AE examination of vessels is the most sensitive and cost-effective method for vessel inspection in the world today and adheres to standards set by ASTM and the American Society of Mechanical Engineers (using MONPAC technology).
Leak Detection: In leak detection, the instrumentation detects the AE signal that is generated from the turbulent or cavitational flow through a crack, valve, seal or orifice. Acoustic energy is transmitted through the fluid, through air or the structure to a piezo electric sensor. The signal is then processed, filtered and compared to a leak profile then located using triangulation techniques. Existing installations include monitoring of pipelines in utility and petrochemical plants, as well as leak detection in boilers, vessels, and through valves.
These systems offer the capability to connect and monitor multiple sensors throughout a plant. The systems can be operated in a standalone mode, interfaced to programmable controllers, or tied into plant-wide distributed control systems. They also offer the ability to plot plant piping and vessel drawings on CRT monitors that can pinpoint leak locations.
Nuclear Lift Rig Examination: The use of AE has led to savings estimated at US$5 million in the nuclear industry in a series of tests on pressure water reactors in-containment lift rigs that are used during refueling and inspection outages. The internal lift rig in particular is often contaminated in service so that inspection with the magnetic particle method becomes difficult and costly. Since the AE examination requires contact only for mounting and removing sensors, it can be accomplished with much less radiation exposure. The examination runs in parallel with other refueling activities, monitoring the rigs during critical lifts, without impinging on the critical path. The AE evaluation techniques were integrated successfully with follow-up inspections to give realistic, confirmed diagnoses of structural conditions.
Pulp and Paper Industry: Over the last 15 years, AE has become an increasingly useful and accepted method of nondestructive evaluation within the paper industry. AE applications within the paper industry have exposed cracks in rotating pressure vessels, such as steam-heated Yankee and paper machine dryers, and in rotating equipment, such as felt rolls, reel spools, calendar rolls, and suction rolls. Rejectable defects have also been detected in machine structural components, such as swing arms. AE has uncovered the delamination of bonded materials, including thermal spray metal coatings and rubber roll covers. AE is accepted by major insurance underwriters and is considered a replacement examination method for hydrostatic testing of pressure vessels in fitness for service examinations. Consensus bodies from the paper industry have recommended AE testing by NDT personnel experienced with the vessels and materials to be examined.
Acoustic emission is an elastic stress wave generated by the rapid release of energy within a material. What this really means is that materials talk when they are in trouble. Plastic deformation, crack initiation and crack growth, whether from fatigue or corrosion all give rise to AE. A structure can be monitored in real-time to provide warning of impending failure. All materials including metals, composites, ceramics, FRP structures, rocks, and wood, produce AE. Unlike almost all other NDT methods, the energy that is converted to AE signals comes from the material itself. As a result, the AE technique is sensitive to growing defects. Since the AE signals from defects radiate throughout the structure, relatively few AE sensors can detect and qualify defects over a large area. AE is a world-recognized technique. Personnel qualifications per national and international organizations are available and are required in order to perform AE examinations. AE is incorporated into many ASTM standards, ASME and other codes. //
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