Significance and Use
5.1 Controlled stimulation, that is, the application of mechanical or thermal load, can generate AE from flawed areas of the structure. Sources may include flaw growth, oxide fracture, crack face stiction and release on load application, and crack face rubbing.
5.2 The load range above normal service (peak) load is used to propagate fatigue cracks in the plastically strained region ahead of the crack tip. Crack propagation may not be a reliable source of AE, depending on the alloy and microstructure, the amount (rate) of crack extension, and possibility of brittle fracture in a segment of crack extension.
5.3 Load increases resulting in significant ductile tearing may produce less emission than expected for the amount of crack growth. Processes that result in more brittle cleavage fractures are more detectable and produce more emission for smaller amounts of flaw growth. These include corrosion fatigue and stress corrosion cracking modes of flaw growth, and would also be more likely in cast or welded structures than in fabricated (forged, rolled, or extruded) structures. Distributed defect structures such as hydrogen embrittlement, or creep cavitation in high temperature steels, may also produce significant emission without evidence of an existing crack-like flaw.
5.4 Application and relaxation of load can produce secondary mechanically-induced emission that is not related to flaw extension. This includes crack face stiction release on loading—usually evidenced by emission at the same rising load value regardless of peak load; or crack face rubbing on load release as the fracture surfaces come back together.
5.5 The load rate can be a significant concern as instrumentation can become saturated with AE activity. The ability to differentiate real data from background noise can be compromised.
5.6 Background noise must be fully investigated and minimized before any AE monitoring can begin.
Abstract
This practice provides guidelines for acoustic emission (AE) examination or monitoring of structures, such as pressure vessels, piping systems, or other structures that can be stressed by mechanical or thermal means. The basic functions of an AE monitoring system are to detect, locate, and classify emission sources. Other methods of nondestructive testing (NDT) may be used to further evaluate the significance of acoustic emission sources. Acoustic emission examination of a structure usually requires application of a mechanical or thermal stimulus. Such stimulation produces changes in the stresses in the structure. During stimulation of a structure, AE from discontinuities (such as cracks and inclusions) and from other areas of stress concentration, or from other acoustic sources (such as leaks, loose parts, and structural motion) can be detected by an instrumentation system, using sensors which, when stimulated by stress waves, generate electrical signals. Annual calibration and verification of pressure transducer, AE sensors, preamplifiers, signal processor, and AE electronic waveform generator should be performed.
Scope
1.1 This practice provides guidelines for acoustic emission (AE) monitoring of structures, such as pressure vessels, piping systems, or other structures that can be stressed by mechanical or thermal means.
1.2 The basic functions of an AE monitoring system are to detect, locate, and classify emission sources. Other methods of nondestructive testing (NDT) may be used to further evaluate the significance of reported acoustic emission sources.
1.3 Units—The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each system are not necessarily exact equivalents; therefore, to ensure conformance with the standard, each system shall be used independently of the other, and values from the two systems shall not be combined.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.5 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
