Acoustic Emission Technique
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29-12-2010, 04:25 PM
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Acoustic emission technique (AET) has been extensively researched and found to be one of the most of the promising techniques for condition monitoring of machine tools. AE is non-intrusive, simple in operation and gives fast dynamic response.
Acoustic Emission Technique (AET) is relatively recent entry in the field of NDE which has particularly shown a very high potential for material characterization and damage assessment in conventional as well as non – conventional materials. Due to its complementary non – destructive evaluation methods, it is utilized for a wide range of applications.
Acoustic Emission (AE) is defined as the class of phenomenon where transient elastic waves are generated by the rapid release of energy from localized sources within a material, or the transient elastic waves so generated. In other words, AE refers to the stress waves generated by dynamic processes in materials. Emission occurs as a release of a series of short impulsive energy packets. The energy thus released travels as a spherical wave front and can be picked from the surface of a material using highly sensitive transducers, (usually electro mechanical type).
This paper discusses the applicability of AET for monitoring defects in materials,structures,etc while in operation. This prevents sudden failure of a machine tool,material or a structure.
INTRODUCTION TO ACOUSTICS.
Acoustics is the interdisciplinary science that deals with the study of sound, ultrasound and infrasound (all mechanical waves in gases, liquids, and solids). A scientist who works in the field of acoustics is an acoustician. The application of acoustics in technology is called acoustical engineering.
The science of acoustics spreads across so many facets of our society—music, medicine, architecture, industrial production, warfare and more. Art, craft, science and technology have provoked one another to advance the whole, as in many other fields of knowledge.
The word "acoustic" is derived from the Greek word (akoustikos), meaning "for hearing” or “ready to hear".
The study of acoustics revolves around the generation, propagation and reception of mechanical waves and vibrations.
THEORY - AE SOURCES
As mentioned in the Introduction, acoustic emissions can result from the initiation and growth of cracks, slip and dislocation movements, twinning, or phase transformations in metals. In any case, AE’s originate with stress. When a stress is exerted on a material, a strain in induced in the material as well. Depending on the magnitude of the stress and the properties of the material, an object may return to its original dimensions or be permanently deformed after the stress is removed. These two conditions are known as elastic and plastic deformation, respectively.
The most detectible acoustic emissions take place when a loaded material undergoes plastic deformation or when a material is loaded at or near its yield stress. On the microscopic level, as plastic deformation occurs, atomic planes slip past each other through the movement of dislocations. These atomic-scale deformations release energy in the form of elastic waves which “can be thought of as naturally generated ultrasound” traveling through the object. When cracks exist in a metal, the stress levels present in front of the crack tip can be several times higher than the surrounding area. Therefore, AE activity will also be observed when the material ahead of the crack tip undergoes plastic deformation (micro-yielding).
Two sources of fatigue cracks also cause AE’s. The first source is emissive particles (e.g. nonmetallic inclusions) at the origin of the crack tip. Since these particles are less ductile than the surrounding material, they tend to break more easily when the metal is strained, resulting in an AE signal. The second source is the propagation of the crack tip that occurs through the movement of dislocations and small-scale cleavage produced by triaxial stresses.
The amount of energy released by an acoustic emission and the amplitude of the waveform are related to the magnitude and velocity of the source event. The amplitude of the emission is proportional to the velocity of crack propagation and the amount of surface area created. Large, discrete crack jumps will produce larger AE signals than cracks that propagate slowly over the same distance.
Detection and conversion of these elastic waves to electrical signals is the basis of AE testing. Analysis of these signals yield valuable information regarding the origin and importance of a discontinuity in a material. As discussed in the following section, specialized equipment is necessary to detect the wave energy and decipher which signals are meaningful.
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