2- Acoustic Emissions Flashcards by Akmal Mughal

,,,,,,, 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

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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

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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

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The ,,,,,,method detects, locates, identifies, and displays flaw data for the stressed object the
moment the flaw is created

AE test

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flaws can not be retested by the ,,,,, method

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Almost all materials produce acoustic emissions when

they are stressed beyond their ,,,,, to ,,,,,,

normal design ranges

final failure

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first practical use of AE occurred in about ,,,,,, bc

6500

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the father of modern AE testing was ,,,,,, of Germany

Josef Kaiser

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A short, transient AE event is produced by a very fast release of elastic energy for a specific ,,,,, movement

dislocation

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The ,,,,,,,, is the source of the elastic wave that propagates in ,,,,,,directions and cannot be stopped

local dislocation

all

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On flat surfaces, the wave propagates as ,,,,,, around the source

concentric circles

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amplitude of the concentric waves is attenuated with ,,,,,, time and distance

increasing

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The maximum distance where an AE event can be detected depends on ,,,,,,,,

material properties,
geometry of the test object,
its content and
environment

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On ,,,,,, or ,,,,, surfaces, AE events can be detected at ,,,,,, meters, which is a great advantage for this method

flat
cylindrical
several

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With tank bottom testing, waves are analyzed that propagate through the liquid from the source to the ,,,,,,

tank wall

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Without liquid in the tank, the waves would be greatly ,,,

attenuated

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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

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AE waves reach the sensors with certain delays that depend on the position of the ,,,,,

AE source

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The position of the source can be calculated using the

,,,,,,,. This is known as,,,,,,

different arrival times

“location calculation.”

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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

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Material properties and environmental conditions influence the start of the release of ,,,,, (i.e., the start of crack formation).

elastic energy

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There are two types of AE signals,,,,,,

transient ( or bursts)

continuous

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starting and ending points of the transient signal are clearly differentiated from ,,,,,,

background noise

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With continuous AE signals, ,,,,, and ,,,,, variations can be seen but the signal ,,,, thereby appearing continuous

amplitude
frequency
never ends

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,,,,,,, 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