2- Acoustic Emissions Flashcards
,,,,,,, testing is based on the fact that solid
materials emit sonic or ultrasonic acoustic emissions when they are mechanically or thermally stressed to the point where deformation or fracturing occurs
Acoustic emission
Acoustic emission testing is based on the fact that solid materials emit ,,,,, or ,,,, emissions when they are mechanically or thermally ,,,,,, to the point where deformation or fracturing occurs
sonic
ultrasonic acoustic
stressed
During plastic deformation, dislocations move through the material’s crystal lattice structure producing ,,,,,,,AE signals, which can be measured only over short distances under laboratory conditions
low-amplitude
The ,,,,,,method detects, locates, identifies, and displays flaw data for the stressed object the
moment the flaw is created
AE test
flaws can not be retested by the ,,,,, method
AE
Almost all materials produce acoustic emissions when
they are stressed beyond their ,,,,, to ,,,,,,
normal design ranges
final failure
first practical use of AE occurred in about ,,,,,, bc
6500
the father of modern AE testing was ,,,,,, of Germany
Josef Kaiser
A short, transient AE event is produced by a very fast release of elastic energy for a specific ,,,,, movement
dislocation
The ,,,,,,,, is the source of the elastic wave that propagates in ,,,,,,directions and cannot be stopped
local dislocation
all
On flat surfaces, the wave propagates as ,,,,,, around the source
concentric circles
amplitude of the concentric waves is attenuated with ,,,,,, time and distance
increasing
The maximum distance where an AE event can be detected depends on ,,,,,,,,
material properties,
geometry of the test object,
its content and
environment
On ,,,,,, or ,,,,, surfaces, AE events can be detected at ,,,,,, meters, which is a great advantage for this method
flat
cylindrical
several
With tank bottom testing, waves are analyzed that propagate through the liquid from the source to the ,,,,,,
tank wall
Without liquid in the tank, the waves would be greatly ,,,
attenuated
liquid-filled pipelines where the maximum distance AE events can be detected is ,,,,,, than in gas-filled tubes because the AE signal is attenuated ,,,,, in the liquid
volume than in thin-walled tubing
longer
less
AE waves reach the sensors with certain delays that depend on the position of the ,,,,,
AE source
The position of the source can be calculated using the
,,,,,,,. This is known as,,,,,,
different arrival times
“location calculation.”
AE testing can be a process chain consisting of the following
steps:
- Test object and application of load
- Source mechanisms(releasing elastic energy)
- Wave propagation (from the source to the sensor)
- Sensors:
- Acquisition of measurement data
- Display of measurement data
- Evaluation of the display
Material properties and environmental conditions influence the start of the release of ,,,,, (i.e., the start of crack formation).
elastic energy
There are two types of AE signals,,,,,,
transient ( or bursts)
continuous
starting and ending points of the transient signal are clearly differentiated from ,,,,,,
background noise
With continuous AE signals, ,,,,, and ,,,,, variations can be seen but the signal ,,,, thereby appearing continuous
amplitude
frequency
never ends
,,,,,,, filters are used to minimize background noise while enhancing the AE signal.
band pass
Digital systems sample the AE signal every ,,,, or ,,,,, times a second
100ns
10 million
time interval between first threshold crossing and peak
amplitude is known as
Rise Time
time interval between first and last threshold crossing
Signal Duration
RMS
Root mean square
AE bursts are produced by both ,,,,and ,,,,
defects
background noise
,,,,, is one of the most important burst features
Peak amplitude
In most cases, bursts with less than ,,,,, crossings and durations ,,,,, are regarded as unwanted noise signals
three threshold
less than 3 ms
,,,,, usually indicate electrical noise peaks if they arrive at all channels at ,,,,,
Very short signals
the same time
Coupling agents are crucial to the quality of ,,,,,
sensor coupling
coupling agent provide good ,,,,, between the ,,,,, and the ,,,,,,of the test object
acoustic contact
sensor and surface
Care must be taken to assure couplant is selected so
that it is acceptable at the normal ,,,, and does not ,,,, the test object’s surface
test temperature
corrode
A variety of ,,,,,, are available for industrial application
silicone greases (coupling agent)
coupling layer should be made as ,,,,, as possible by firmly pressing the sensor ,,,, the test object’s surface
thin
against
Quality of coupling must be verified through ,,,,
pencil lead break or automatic coupling test
,,,,, type of sensors convert ,,,,,, into ,,,,,signals
Piezoelectric
mechanical waves
electrical AE
Piezoelectric sensor are ,,,, and more ,,,, than other sensor techniques such as ,,,,,,
hardy
sensitive
capacitive, electrodynamic, or laser-optical.
When testing metal vessels for integrity, frequencies between ,,,, and,,,, are often used
100 Hz
300kHz
At frequencies range 100 Hz to 300kHz, the sensors have a resonance of about ,,,,,
150kHz
Piezoelectric sensors, cover the range of 150 to 300kHz with a variation of sensitivity of about ,,,,
6dB
The resonant frequency indirectly determines the,,,,, of the sensor
spatial range
High-frequency sound attenuates ,,,,, so it has a shorter ,,,,,.
faster
detection distance
Background noises coming from longer distances
typically consist of frequency components below ,,,,,
100kHz
For testing tank bottoms, sensors with a high sensitivity down to ,,,,,,are required because these signals must run for ,,,,,,.
25kHz
long distances
Sensors often contain built-in ,,,,, and are attached to ,,,,, using magnetic holding devices
pre-amlifiers
magnetic test objects
Typically, the ,,,,, power supply
for the preamplifiers is fed through the signal cable and can have a length of several hundred meters
28 VDC
The sensitivity of piezoelectric sensors can be as high as ,,,,,,
1000V/mm
a displacement of 0.1 picometer (pm) generates a ,,,,,,signal that can be easily distinguished from electrical noise, which is about ,,,,,,
100 mVpk
10 mVpk
For comparison purposes, the atomic radii of the elements are in the range of 150pm with manganese at ,,,,, and lead at ,,,,
112pm
175pm
Types of AE sensors are; S..... A.... D.... AD..
Single
Compact Single
Differential
Compact Differential
,,,,,,,, sensors are typically used when there is a degradation of the AE signal by ,,,,, or ,,,,,, interferences.
Differential
electrostatic
electromagnetic
When testing magnetic materials, sensors are usually attached to the test object using ,,,,
magnetic holders
When nonmagnetic objects are tested, ,,,,,, may be used for sensor hold-down
elastic ties, tape, clamps, and glue
cable connects the sensor with the preamplifier and should not be longer than ,,,,, because of the capacitance load on the sensor
1.2m
The ,,,,,, inside the channel is used to eliminate noise sources and matches the measurement chain to the requirements of the application
frequency filter
The ,,,, to ,,,, filter is used for tank bottom tests where leakage and corrosion may be problems
20 to 100kHz
The ,,,,to ,,,,, filter is used for integrity studies of
metallic components.
100 to 300kHz
Filters with frequencies above ,,,,, can be used where there are short distances between sensors
300kHz
,,,,,, converters are used to digitize the filtered AE signals.
A/D ( analog to digital )
A continuous sampling rate of ,,,, or ,,,,,measurement values per second per channel is used for feature extraction, which is processed in real time.
10MHz or 10 million
FPGAs
field programmable gate arrays
Optional transient recorders provide a way to store large amounts of information about complex ,,,,
AE waves
The transient recorder is an optional addition to the
channel ,,,,
plug-in
The ,,,,temporarily stores data when the CPU is busy with other tasks and cannot accept more data
data buffer
The difference distance between a defect source and other sensors equals the ,,,,,
arrival time difference times the sound velocity
The defect location calculation is based on the ,,,,,, of the AE signal propagating from its source to different sensors
arrival time differences
AE wave is propagating in ,,, from its source and arrives at different sensors at different times.
concentric circles
This time delay is proportional to the,,,,
distance between the sensor and the source
,,,,,, is a mathematical way to determine the point density within a certain area, marking those areas of high point density by colored rectangles or circles
Clustering
Advantages of AE testing;
-Pneumatic pressure testing can minimize potential overdesign of tank support systems.
- during pressurization provides in-depth information
beyond the statements—failure, leakage, or noticeable deformation
during test.
- the pressure system can be tested within standard operating conditions and acceptable temperature ranges
AE Testing Advantages by Operating Medium: (Gas or Liquid);
- Minimizes downtime caused by removal of the tank working medium if this medium is used for pressurization
- Minimizes expensive drying operations following tank de-inventorying (as required with hydrotesting).
- Minimizes de-inventorying water and subsequent decontamination expenses.
- Minimizes the problem of catalysts becoming unstable in chemical reactors during de-inventorying operations.
- AE testing can detect corrosion and minimize or eliminate the need for internal inspection.
AE Testing advantages Compared to Other NDT Methods;
- monitors the dynamic reaction of the test object passively, without intervention, when the load is applied.
- In most cases, the AE method can detect sources at several meters distance from the sensor
- AE provides 100% pressurized wall monitoring
- AE provides real-time monitoring of the growth of unknown defects at a given load, even remotely by data transmission.
- It monitors a structure under all operating conditions.
AE Testing Limitations;
- Defects that do not grow or move cannot be detected
- The only defects that can be detected without exceeding the highest preceding load are defects that are already active at the actual load level and
endangering the component - Evaluation criteria do not exist in the form of commonly accessible data; the rating of AE results is dependent on the knowledge and experience
of the service provider. - AE testing is sensitive to process noise exceeding the detection threshold. Raising the threshold level decreases test sensitivity. Above a certain noise level, AE testing is no longer efficient.
AE Testing Location Errors;
- A different wave mode than the assumed one determines the arrival time.
- A wave takes a different propagation path than assumed by the algorithm.
• Multiple waves that overlap at the sensor.
• Sources emit signals in such quick succession that there is not enough time
for the signals in the structure to decay. Therefore they do not represent
a new hit
AE transducers are state-of-the-art piezoelectric sensors operating at frequencies from ,,,,, to ,,,,,,
20 KHz to 1.5 megahertz (MHz)
some AE transducer sensors are capable of reliable operation at temperatures as high as ,,,,, centigrade
550°C
A ,,,,, airborne sensor is used for remote leak monitoring applications.
200kHz
,,,,, testing is a powerful method for examining the behavior of materials deforming under stress
Acoustic emission (AE)
,,,,,,can be defined as a transient static wave generated by the rapid release of energy within a material
Acoustic emission (AE)
AE sensors are low-noise detectors that operate in the ultrasonic frequency range of ,,, to ,,,,
10kHz to 2MHz
Through AE Testing, Physical motions as small as ,,,, can be detected
1 X 10 minus 12 ( pm)
AE sensors can hear the breaking of a ,,,, of metal or a ,,,,,, in a fiber-reinforced composite material
single grain
single fiber
AE technology provides an ,,,,, system to prevent catastrophic failures
early warning
AE is also used for a wide range of process applications such as ,,,, and a variety of
,,,, processes
leak detection, particle impacts, electrical discharges,
friction-related process
ISO and ASTM define acoustic emission as the class of phenomena that generates transient elastic waves by ,,,,
rapid release of energy from localized sources
The CEN document restricts the acoustic emission term to the transient elastic waves generated by ,,,,,
the release of energy or by a process.
The ASTM defines the “Felicity effect” as ,,,,,
the presence of an acoustic emission at a fixed sensitivity level, at stress levels below those applied previously
Three types of compressive elastic waves generated in solids are;
- primary waves, or P-Waves
- shear waves, secondary waves, or S-waves
- surface wave, Raleigh wave, or R-wave
Impact-echo testing relies mainly on the propagation and reflection of ,,,,
P-waves.
For normal concrete, P-wave speed varies from about ,,, to ,,,,
3000 to 5500m/s.
The relationship among wave speed, frequency, and wavelength is given by Eq.
C = f x λ (lambda)
where;
C = speed of the stress wave
f = frequency in kHz
λ = wavelength in m
