ABSTRACT
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.
The major subfields of
acoustics are classified below,
PHYSICAL ACOUSTICS
|
BIOLOGICAL ACOUSTICS
|
ACOUSTICAL ENGINEERING
|
|
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INTRODUCTION TO ACOUSTIC EMISSION TESTING
Acoustic
Emission (AE) refers to the generation of transient elastic waves produced by a
sudden redistribution of stress in a material. When a structure is subjected to
an external stimulus (change in pressure, load, or temperature), localized
sources trigger the release of energy, in the form of stress waves, which
propagate to the surface and are recorded by sensors. With the right equipment
and setup, motions on the order of picometers (10 -12 m) can be identified.
Sources of AE vary from natural events like earthquakes and rockbursts to the
initiation and growth of cracks, slip and dislocation movements, melting,
twinning, and phase transformations in metals. In composites, matrix cracking
and fiber breakage and debonding contribute to acoustic emissions. AE’s have
also been measured and recorded in polymers, wood, and concrete, among other
materials.
Detection and
analysis of AE signals can supply valuable information regarding the origin and
importance of a discontinuity in a material. Because of the versatility of
Acoustic Emission Testing (AET), it has many industrial applications (e.g.
assessing structural integrity, detecting flaws, testing for leaks, or
monitoring weld quality) and is used extensively as a research tool.
Acoustic Emission
is unlike most other nondestructive testing (NDT) techniques in two regards.
The first difference pertains to the origin of the signal. Instead of supplying
energy to the object under examination, AET simply listens for the energy
released by the object. AE tests are often performed on structures while in
operation, as this provides adequate loading for propagating defects and
triggering acoustic emissions.
The second
difference is that AET deals with dynamic processes, or changes, in a material.
This is particularly meaningful because only active features (e.g. crack
growth) are highlighted. The ability to discern between developing and stagnant
defects is significant. However, it is possible for flaws to go undetected
altogether if the loading is not high enough to cause an acoustic event.
Furthermore, AE testing usually provides an immediate indication relating to
the strength or risk of failure of a component. Other advantages of AET include
fast and complete volumetric inspection using multiple sensors, permanent
sensor mounting for process control, and no need to disassemble and clean a
specimen.
Unfortunately,
AE systems can only qualitatively gauge how much damage is contained in a
structure. In order to obtain quantitative results about size, depth, and
overall acceptability of a part, other NDT methods (often ultrasonic testing)
are necessary. Another drawback of AE stems from loud service environments
which contribute extraneous noise to the signals. For successful applications,
signal discrimination and noise reduction are crucial.
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.
THEORY
- ACOUSTIC WAVES
WAVE
PROPAGATION
A primitive wave released at the AE source is
illustrated in the figure right. The displacement waveform is a step-like
function corresponding to the permanent change associated with the source
process. The analogous velocity and stress waveforms are essentially
pulse-like. The width and height of the primitive pulse depend on the dynamics
of the source process. Source processes such as microscopic crack jumps and
precipitate fractures are usually completed in a fraction of a microsecond or a
few microseconds, which explains why the pulse is short in duration. The
amplitude and energy of the primitive pulse vary over an enormous range from
submicroscopic dislocation movements to gross crack jumps.
Waves radiates from the
source in all directions, often having a strong directionality depending on the
nature of the source process, as shown in the second figure. Rapid movement is
necessary if a sizeable amount of the elastic energy liberated during
deformation is to appear as an acoustic emission.
As these primitive
waves travel through a material, their form is changed considerably. Elastic
wave source and elastic wave motion theories are being investigated to
determine the complicated relationship between the AE source pulse and the
corresponding movement at the detection site. The ultimate goal of studies of
the interaction between elastic waves and material structure is to accurately
develop a description of the source event from the output signal of a distant
sensor.
However, most
materials-oriented researchers and NDT inspectors are not concerned with the
intricate knowledge of each source event. Instead, they are primarily
interested in the broader, statistical aspects of AE. Because of this, they
prefer to use narrow band (resonant) sensors which detect only a small portion
of the broadband of frequencies emitted by an AE. These sensors are capable of
measuring hundreds of signals each second, in contrast to the more expensive
high-fidelity sensors used in source function analysis. More information on
sensors will be discussed later in the Equipment section.
The signal that is
detected by a sensor is a combination of many parts of the waveform initially
emitted. Acoustic emission source motion is completed in a few millionths of a
second. As the AE leaves the source, the waveform travels in a spherically
spreading pattern and is reflected off the boundaries of the object. Signals
that are in phase with each other as they reach the sensor produce constructive
interference which usually results in the highest peak of the waveform being
detected. The typical time interval from when an AE wave reflects around the
test piece (repeatedly exciting the sensor) until it decays, ranges from the
order of 100 microseconds in a highly damped, nonmetallic material to tens of
milliseconds in a lightly damped metallic material.
ATTENUATION
The
intensity of an AE signal detected by a sensor is considerably lower than the
intensity that would have been observed in the close proximity of the source.
This is due to attenuation. There are three main causes of attenuation,
beginning with geometric spreading. As an AE spreads from its source in a
plate-like material, its amplitude decays by 30% every time it doubles its
distance from the source. In three-dimensional structures, the signal decays on
the order of 50%. This can be traced back to the simple conservation of energy.
Another cause of attenuation is material damping, as alluded to in the previous
paragraph. While an AE wave passes through a material, its elastic and kinetic
energies are absorbed and converted into heat. The third cause of attenuation
is wave scattering. Geometric discontinuities (e.g. twin boundaries,
nonmetallic inclusions, or grain boundaries) and structural boundaries both
reflect some of the wave energy that was initially transmitted.
Measurements of the
effects of attenuation on an AE signal can be performed with a simple apparatus
known as a Hsu-Nielson Source. This consists of a mechanical pencil with either
0.3 or 0.5 mm 2H lead that is passed through a cone-shaped Teflon shoe designed
to place the lead in contact with the surface of a material at a 30 degree
angle. When the pencil lead is pressed and broken against the material, it
creates a small, local deformation that is relieved in the form of a stress
wave, similar to the type of AE signal produced by a crack. By using this
method, simulated AE sources can be created at various sites on a structure to
determine the optimal position for the placement of sensors and to ensure that
all areas of interest are within the detection range of the sensor or sensors.
WAVE
MODE AND VELOCITY
As mentioned earlier, using AE inspection in conjunction with
other NDE techniques can be an effective method in gauging the location and
nature of defects. Since source locations are determined by the time required
for the wave to travel through the material to a sensor, it is important that
the velocity of the propagating waves be accurately calculated. This is not an
easy task since wave propagation depends on the material in question and the
wave mode being detected. For many applications, Lamb waves are of primary
concern because they are able to give the best indication of wave propagation
from a source whose distance from the sensor is larger than the thickness of
the material. For additional information on Lamb waves, see the wave mode page
in the Ultrasonic Inspection section.
AE BARKHAUSEN TECHNIQUES
BARKHAUSEN
EFFECT
The Barkhausen
effect refers to the sudden change in size of ferromagnetic domains that occur
during magnetization or demagnetization. During magnetization, favorably
oriented domains develop at the cost of less favorably oriented domains. These
two factors result in minute jumps of magnetization when a ferromagnetic sample
(e.g. iron) is exposed to an increasing magnetic field (see figure). Domain
wall motion itself is determined by many factors like microstructure, grain
boundaries, inclusions, and stress and strain. By the same token, the
Barkhausen effect is too a function of stress and strain.
BARKHAUSEN
NOISE
Barkhausen noise
can be heard if a coil of wire is wrapped around the sample undergoing
magnetization. Abrupt movements in the magnetic field produce spiking current
pulses in the coil. When amplified, the clicks can be compared to Rice Krispies
or the crumbling a candy wrapper. The amount of Barkhausen noise is influenced
by material imperfections and dislocations and is likewise dependent on the
mechanical properties of a material. Currently, materials exposed to high
energy particles (nuclear reactors) or cyclic mechanical stresses (pipelines)
are available for nondestructive evaluation using Barkhausen noise, one of the
many branches of AE testing.
APPLICATIONS
Acoustic
emission is a very versatile, non-invasive way to gather information about a
material or structure. Acoustic Emission testing (AET) is be applied to
inspect and monitor pipelines, pressure vessels, storage tanks, bridges, aircraft,
and bucket trucks, and a variety of composite and ceramic components. It is
also used in process control applications such as monitoring welding processes.
A few examples of AET applications follow.
WELD
MONITORING
During the welding
process, temperature changes induce stresses between the weld and the base
metal. These stresses are often relieved by heat treating the weld.
However, in some cases tempering the weld is not possible and minor cracking
occurs. Amazingly, cracking can continue for up to 10 days after the weld has
been completed. Using stainless steel welds with known inclusions and
accelerometers for detection purposes and background noise monitoring, it was
found by W. D. Jolly (1969) that low level signals and more sizeable bursts
were related to the growth of microfissures and larger cracks respectively.
ASTM E 749-96 is a standard practice of AE monitoring of continuous welding.
BUCKET
TRUCK (CHERRY PICKERS) INTEGRITY EVALUATION
Accidents,
overloads and fatigue can all occur when operating bucket trucks or other
aerial equipment. If a mechanical or structural defect is ignored, serious
injury or fatality can result. In 1976, the Georgia Power Company pioneered the
aerial manlift device inspection. Testing by independent labs and electrical
utilities followed. Although originally intended to examine only the boom
sections, the method is now used for inspecting the pedestal, pins, and various
other components. Normally, the AE tests are second in a chain of inspections
which start with visual checks. If necessary, follow-up tests take the form of
magnetic particle, dye penetrant, or ultrasonic inspections. Experienced
personnel can perform five to ten tests per day, saving valuable time and money
along the way. ASTM F914 governs the procedures for examining insulated aerial
personnel devices.
GAS TRAILER TUBES
Acoustic
emission testing on pressurized jumbo tube trailers was authorized by the
Department of Transportation in 1983. Instead of using hydrostatic retesting,
where tubes must be removed from service and disassembled, AET allows for in
situ testing. A 10% over-pressurization is performed at a normal filling
station with AE sensors attached to the tubes at each end. A multichannel
acoustic system is used to detection and mapped source locations. Suspect
locations are further evaluated using ultrasonic inspection, and when defects
are confirmed the tube is removed from use. AET can detect subcritical
flaws whereas hydrostatic testing cannot detect cracks until they cause rupture
of the tube. Because of the high stresses in the circumferential direction of
the tubes, tests are geared toward finding longitudinal fatigue cracks.
BRIDGES
Bridges contain
many welds, joints and connections, and a combination of load and environmental
factors heavily influence damage mechanisms such as fatigue cracking and metal
thinning due to corrosion. Bridges receive a visual inspection about
every two years and when damage is detected, the bridge is either shut down,
its weight capacity is lowered, or it is singled out for more frequent
monitoring. Acoustic Emission is increasingly being used for bridge
monitoring applications because it can continuously gather data and detect
changes that may be due to damage without requiring lane closures or bridge
shutdown. In fact, traffic flow is commonly used to load or stress the bridge
for the AE testing.
AEROSPACE
STRUCTURES
Most aerospace structures consist of complex
assemblies of components that have been design to carry significant loads while
being as light as possible. This combination of requirements leads to
many parts that can tolerate only a minor amount of damage before failing.
This fact makes detection of damage extremely important but components are
often packed tightly together making access for inspections difficult.
AET has found applications in monitoring the health of aerospace structures
because sensors can be attached in easily accessed areas that are remotely
located from damage prone sites. AET has been used in laboratory
structural tests, as well as in flight test applications. NASA's Wing
Leading Edge Impact Detection System is partially based on AE technology.
The image to the right shows a technician applying AE transducers on the inside
of the Space Shuttle Discovery wing structure. The impact detection
system was developed to alert NASA officials to events such as the
sprayed-on-foam insulation impact that damaged the Space Shuttle Columbia's
wing leading edge during launch and lead to its breakup on reentry to the
Earth's atmosphere.
The following research material gives details of few
applications of acoustic emission technique.
APPLICATION IN MATERIAL RESEARCH
Acoustic emission technique finds one of its largest application
fields in material research. Examples are the detection of the point of damage
initiation and the rate of damage evolution under mechanical loading (tensile,
bending, fatigue, creep),the study of phase transformation, detection of
coating wear, etc.
The emitted stress waves can be detected
by coupling piezo-electric sensors to the structure or material under study. By
analyzing the quantity and properties of acoustic emission signals information
is obtained about the processes that are active in the material under study.
Generally acoustic emission technique can
be used to obtain information of the microstructural changes that are occurring
in any loaded material.
APPLICATION IN STRUCTURAL INTEGRITY MONITORING.
The main area where the acoustic emission
technique has found practical, industrial applications is in the field of
structural integrity monitoring. By equipping a loaded, safety critical
structure with a number of piezo electric sensors, information can be gathered
about the evolution of damage in the structure during its service life.
Nowadays, wireless acoustic emission sensor networks are used for structural
health monitoring in civil engineering.
CONCLUSION
Because the physical process of acoustic emission occurs in a wide
variety of materials and under a large range of loading conditions, the
technique offers great potential for use as a continuous monitoring technique.
Due to its inherent advantages as compared to other techniques, it should
always be preferred when continuous detection is required.
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