Transcript The Concept of Structural Health Monitoring

‫ האדריכלים והאקדמאים‬,‫לשכת המהנדסים‬
‫במקצועות הטכנולוגיים בישראל‬
‫ ענף בדיקות לא הורסות‬- ‫אגודת מהנדסי מכונות‬
‫יום עיון השנתי בנושא פליטה אקוסטית‬
Acoustic Emission Apparatus
and
Data Acquisition
February 24, 2011
Zohar Elman, Dr. Boris Muravin
This presentation for acoustic emission
education purposes only
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Outline
Apparatus
Sensors
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AE Sensor Design
Piezoelctricity Effect and Piezoelectric Manufacturing Process
Modes of Vibration and Temperature Effect
Frequency response
Modification of AE Waves by Medium and Sensor
Aperture Effect
Resonant Sensors
Wide Band Sensors
Differential Sensors
Capacitive Sensors
Laser Interferometer Sensors
Couplants, Bonds and Sensitivity
Installation of Sensors on Structures, Waveguides
Preamplifiers and Cables
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Preamplifiers
Frequency Filters
Important Aspects of Preamplifier Performance
Coaxial Cables/Connectors
Other Cables
Electric Properties of Cables/Impedance Matching
Sensor Calibration
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Primary Calibration
Reciprocity
Reproducibility
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Outline
Data Acquisition
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Digital Signal Proccesing
AE Data Acquisition Devices and Block Diagram
AE Systems and Signal Noise Generator
AE Digital Signal Processing Features
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Hit Based Features
Time Driven Features
Combination of Features
HDT,PDT and HLT
Frequency Filters
Thresholds Used in AE
Background Noise
Waveform Based Processing
Burst and Continues Signals
E 1316: Standard Terminology for Nondestructive Examinations
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Apparatus
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AE Sensor Design
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An AE sensor consists of several parts:
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A piezoelectric ceramic element with electrodes on each face.
One electrode is connected to an electric ground, the other to a signal lead.
A backing material behind the element is designed to minimize reflections back to
the element and to damp the signal around the resonance frequency.
The case provides an integrated mechanical package and may also serve as a
shield to minimize electromagnetic interference.
Sensors may have an internal preamplifier (integral sensors).
Typical AE sensor mounted on a test object.
http://www.ndt-ed.org/EducationResources/CommunityCollege/Other%20Methods/AE/AE_Equipment.htm
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AE Sensor Design
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Two principal dimensions associated with AE sensors are the piezoelectric
element’s thickness and diameter.
Element thickness controls the frequencies at which the sensor has the
highest electrical output (sensitivity).
Active element diameter defines the area over which the sensor
averages surface motion.
The piezoelectric and elastic constants of the piezo also effects the
resonance frequency.
Case
Preamplifier
Electrical
Lead
Piezoelectric
element
Couplant layer
Wear Plate
Regular AE Sensor
vs. an
Integrated AE Sensor, Piezoelectric
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element and preamplifier.
Piezoelectricity Effect
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Acoustic Emission (AE) sensors usually use piezoelectric elements for
transduction.
The element is coupled to the test item’s surface, so that dynamic surface
motion propagates into the piezoelectric element.
The dynamic strain produced in the element produces a voltage-vs.-time
signal as the sensor’s output.
Fig. 1: When pulled, compressed or twisted, and electric charge is generated. Fig. 2: Piezoelectric element under
The reverse is also true, when applying an electric charge to the element.
compression/tensile stress.
http://greencleen.wordpress.com/2010/09/22/talk-powered-cell-phones/
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http://www.newelectronics.co.uk/article/Defa
ult.aspx?articleid=22782&img=1
Piezoelectric Crystal Manufacturing
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Principal steps in a piezoelectric crystal fabrication:
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Powder Mixing- Fine powders of the component metal oxides are mixed in
specific proportions.
Calcination- The mixed powder is heated to a temperature of about 1000°C in
order to remove volatile elements and create thermal decomposition.
Pressing- The powder is then pressed in the desired shape of element.
Firing- The element is then sintered according to a specific time and temperature
program.
Electroding- Electrodes are attached to the desired faces of the element
Poling- The element is then subjected to a strong direct current electric field just
below the curie temperature
Before (left), during (middle) and after (right) poling.
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Modes of Vibration of Piezoelectric Element
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Temperature Effect on AE sensors
Relation between temperatures and piezoelectric
characteristics
Effect of Currie Temperature:
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At a certain temperature, Curie Temperature, piezoelectric ceramics undergo
permanent change and loose there piezoelectricity.
Piezoelectric ceramics have been used successfully within 500c of their cuie
temperature.
Testing limitation are encountered in environments with high temperature due to
loss in piezoelectric characteristics.
Effect of Fluctuating Temperature:
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Special problems are encountered when sensors encounter fluctuating
temperatures.
Piezoelectric ceramics consist of domains which a regions which the electric
polarization is in one direction.
Temperature changes can cause some of these domains to flip, resulting in a
spurious electric signal that is not easily distinguished to an AE event.
Sensors should be allowed o reach thermal equilibrium before data is taken at
different temperatures
Single crystal piezoelectric (quartz) are recommended for application with
fluctuating temperatures orMore
the inuse
of wave guides.
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Frequency Response
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The majority of emission testing is based on the processing of signals with a
frequency range from 30kHz to 1MHz.
Attenuation of the wave motion increases rapidly with frequency, and for
materials with high attenuation, it is necessary to sense lower frequencies
to detect AE hits.
For materials with low attenuation, the background noise will be higher, so
AE hits will be easier to detect at high frequencies.
AE sensors can be designed to sense a portion of the whole frequency
spectrum by changing the piezoelectric dimensions, which account for the
popularity of this transduction mechanism.
Fig. 6: typical Voltage vs. Time and Power [dB] vs. Frequency spectrum.
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Aperture Effect
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When the displaced surface is a sine wave, then there are occasions when
one or more full wavelength will match the diameter of the piezoelectric
element.
When this occurs, the average movements may give a zero output.
This effect, called the Aperture Effect, has been carefully measured and
theoretically modeled.
For an element larger than the wavelengths of interest, the sensitivity will
vary with the properties of the test material, depending on frequency and on
direction of wave propagation.
Because of these complications, it is recommended that the element’s
diameter will be as small as other constraints allow.
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Resonant Sensors
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These sensors have one or more preferred frequencies of oscillation governed by crystal
size and shape.
Enables to make select a suitable trade-off between the desired detection range and the
noise environment.
Higher amplitude output in response to broadband excitation.
Low fidelity: Output is not similar to the motion of the original wave.
Most practical AE testing employs resonant type sensors that are more sensitive and less
costly than wideband sensors.
In practice, the vast majority of AE testing is done with sensors that are resonant at about
150 kHz.
Sensor response is determined by the :
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Piezoelectric crystal
The way the element is backed and mounted inside the senor housing
Coupling and the mounting of the sensor.
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Wide Band Sensors
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Wideband sensors are typically used in research applications or other
applications where a high fidelity AE response is required.
Frequency response I relatively smooth and flat.
High fidelity: Good reproduction of original wave motion.
In research applications, wideband AE sensors are useful where frequency
analysis of the AE signal is required.
Helps determine the predominant frequency band of AE sources for noise
discrimination and selection of a suitable lower cost, general purpose AE
sensor.
In high fidelity applications, various AE wave modes can be detected using
wideband sensors, providing more information about the AE source.
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Capacitive Sensor
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Used mainly in laboratories.
Flat frequency response.
Capacitance is a property that exists between any two conductive surfaces
within some reasonable proximity.
Changes in the distance between the surfaces changes the capacitance.
It is this change of capacitance that capacitive sensors use to indicate
changes in position of a target.
Capacitive sensors are basically position measuring devices.
Their outputs always indicate the size of the gap between the sensor's
sensing surface and the target.
When the probe is stationary, any changes in the output are directly
interpreted as changes in position of the target.
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Laser Optical Interferometer
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In recent years the laser based ultrasonic (LBU) method has been developed to
detect flaws in materials, which consists of two techniques:
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The generation of ultrasonic waves by laser.
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The detection of surface motion by laser interferometer.
Advantages:
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Non-contact measurement is practicable. Can be accurately calibrated.
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Measurements are possible in hostile environments.
Disadvantages:
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Most laser interferometers require a reflective surface for sufficient sensitivity.
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Handling or setting-up a laser interferometer equipment is not very easy.
Fig. : Methods of AE Interferometer.
Fig. : Interferometer used for AE and displacement measuring.
http://www.zygo.com/?/met/applications/positionangle/
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Differential Acoustic Emission Sensor
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Differential sensors are identical to general purpose sensors in physical,
electrical and response characteristics, except that two signal leads
(instead of one as for general purpose sensors) are brought out to attach to
a differential pre-amplifier.
By using a differential preamplifier, common mode noise is eliminated,
resulting in a lower noise output from the preamplifier, and a higher
electrical noise rejection in difficult and noisy environments.
Noise improvements in the range of 2dB or higher can be expected using a
differential sensor and preamplifier over a single ended general purpose
sensor.
Differential sensors are used in environments where very low level AE
signals need to be processed and is also very applicable in high noise
environments.
Differential sensors are slightly higher in price than their general purpose
sensor counterparts.
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Couplants and Bonds
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The purpose of the couplant is to provide a good acoustic path from the test material to the
sensor.
The mounting has a significant effect on the performance of the sensor (sensitivity and
frequency band).
Optimum and reproducible detection of AE requires both appropriate sensor-mounting
methods and procedures (see ASTM E-650)
Mounting Methods:
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Compression (Use of mechanical force, couplant is strongly advised)
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Bonding (direct attachment with adhesive that also acts as the couplant).
Virtually any fluid will act as a good couplant. Fluid will not transmit shear waves.
It is necessary for the couplant to have chemical compatibility, to fully wet the surface but not
to corrode it.
Mounting Requirements
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Sensor Selection (size, sensitivity, frequency response environmental and material
compatibility)
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Structure Preparation (mechanical preparation and cleaning)
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Couplant or Bonding Agent Selection to suite with the environment as well as acoustical
conductivity.
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General mounting techniques (amount, selection of couplant and mounting fixture)
After mounting verification of sensor sensitivity should be verified (standard requirement)
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Installation of Sensors on Structures
Type of installation and choice of couplant material is defined by a specifics of
application.
 Glue (superglue type) is commonly used for piping inspections.
 Magnets usually used to hold sensors on metal pressure vessels. Grease
and oil then used as a couplant.
 Bands used for mechanical attachment of sensors in long term applications.
 Waveguides (welded or mechanically attached) used in high temperature
applications.
 Rolling sensors are used for inspection rotating structures.
 Special lead (Pb) blankets used to protect sensors in nuclear industry.
Sensor attached with magnet
Pb blanket in nuclear applications
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Waveguide
Rolling sensor
produces by PAC
Preamplifier
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The function of an preamplifiers is to increase the strength of the input
signal.
 Amplifying and frequency filtering are the two components of signal
conditioning.
 The typical AE amplifieir is a linear, voltage amplifier with the property:
Output Voltage= Input Voltage x Gain
Vo(t) = G x Vi(t)
 Gain - the ratio of output voltage to input voltage- can also be expressed
in decibels B). The decibel is a logarithmic unit:
dB = 20 logG
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Preamplifiers
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To bring the voltage to a higher level, so that any electromagnetic noise
picked up on the long cable will have relatively less effect.
To provide circuitry that can deliver the signal down long lengths of cable
with Minimum loss.
Frequency filters are often package as an integral part of the preamplifier.
Amplifiers in the main system:
 To bring the signal to the desired level for measurement. This gain can be
either a fixed feature of the system, or an operator-controlled test variable.
Preamplifier 60 dB
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Important aspects of amplifier performance
Noise
 The generation of electrical “Johnson“ noise is a thermodynamic process associated
with all resistive component.
 The actual input resistance at the preamp is chosen to optimize the
signal-to-noise ratio.
 This noise is somewhat influenced by the bandwidth of the filters used. The narrower
the Bandwidth, the less the noise.
 Theoretically it is possible to reduce the level of noise, by decreasing the temperature
of the resistor, although not always practical.
Dynamic Range
This is simply the range from the smallest manageable signal to the largest manageable
signal.
 In other words, the range from the electronic noise level to the saturation level.
 Depending on just how you specify its measurement, it is on the order of 80dB for AE
systems.
 Note that the 16-bit digitization process used in today's DSP-based systems has a
nominal dynamic range of 96dB( each bit gives a factor of 2, or 6dB); thus, it can give
an adequate rendering all the way from the highest signals down to electronic noise
and AE signals just emerging from it.
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Coaxial Cables / Connectors
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The cables function is to transmit an electric signal from the source to the
main load and to connect points together electrically.
For AE work, coaxial cable is almost universal because of its superior
electrical shielding.
RG-58 is most commonly used due to its sturdiness with BNC connectors.
BNC (Bayonet Neill-Concelman) connector is a common type of RF
connector used for the coaxial cable.
The connector is the most vulnerable point.
As a rule cables should be handled carefully.
Fig. : BNC Connector
Fig. : RG-58 Coaxial Cable.
http://techgenie.com/wp-content/uploads/Coaxial-Cable.jpg
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Other Cable Types
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There are a number of not so common cable types
used for AE such as Twisted Pair cables and Optical
Fiber cables.
Twisted Pair cable consists of a pair of insulated wires
twisted together.
Cable twisting helps to reduce noise pickup from
outside sources.
Optical Fiber cable is a cable containing one or more
optical fibers.
Optical fiber refers to the medium associated with the
transmission of information as light pulses along a
glass or plastic strand or fiber.
Optical fiber carries much more information than
conventional copper wire and is in general not subject
to electromagnetic interference.
The glass fiber requires more protection within an
outer cable than copper.
Fig. : Multi-Twisted Pair cable.
http://wapedia.mobi/en/Twisted_pair
Fig. : Optical Fiber cable.
http://www.easychinasupply.com/produc
ttrade/last_pt_sel/512876.html
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Electrical Properties of Cables /
Impedance Matching
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While using long cables in AE applications, one must recognize their nature
as long transmission lines, carrying an electrical wave with a definite speed
(somewhat less than the speed of light).
The specified impedance of a coaxial cable (e.g. 50 ohms for RG-58 or RG174) refers to the voltage/current ratio in a transmission line.
This property becomes important only in great lengths of cable.
Impedance - the total opposition that a circuit presents to the flow of an
alternating current.
When the cable is connected into a load, it is advisable to have impedance
matching between load and cable so there is no reflection.
The resistance of a cable (proportional to its length) will cause loss of signal
as the voltage is divided between the cable and the load.
The resistance of cables used in AE work is very small, a few ohms at the
most.
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Primary Calibration
Step Function Force Calibration - ASTM Standard: E1106-86
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The step function force calibration is based on the fact that a known and
measureable input displacement can be generated on the surface of the test block.
The units of calibration are output voltage per unit mechanical input (displacement,
velocity acceleration).
A step function force input initiates an elastic wave that propagates through the
test block.
Given step function source, the free displacement of the test block can be
calculated by elastic theory (transfer function of the test block).
The displacement is also measured by a capacitive transducer with a known
absolute sensitivity (wide band).
It is essential that the theoretical displacement and the capacitive measurement
agree.
The calibration reveals the frequency response of a sensor to waves at a surface.
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Primary Calibration
Step Function Force Calibration - ASTM Standard: E1106-86
Calibration Results
D(fm) = U(fm) / S(fm)
U(fm) - FFT of sensor under test
S(fm) - FFT of capacitive sensor
rm - magnitude of D(fm) = |D(fm)|
A - Sensitivity of the reference sensor in V/m.
In absolute units: So(fm)= A*rm
FFT of capacitive sensor
FFT of sensor under test
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D(fm) = U(fm) / S(fm)
Reproducibility of AE Sensor Response
ASTM Standard: E976
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Intended to provide a reliable and precise way of comparing a set of sensors and
telling whether an individual sensor’s sensitivity is degrading during its service life.
More economical and simple than primary or secondary calibration.
Recommended for routinely checking the sensitivity of AE sensors
This method is not an absolute calibration technique.
The essential elements of apparatus for this method are:
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The AE sensor under test
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The test block (acrylic rod, steel block, nonresonant blocks).
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A signal source (Pencil lead break, pulse generator, white noise generator,
sweep generator, gas jet)
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Measuring and recording equipment
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Data Acquisition
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Digital Signal Processing
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Today, AE systems use digital signal
processing techniques with low costs
per channel and more improved real time
performance.
AE systems have been expanded to
include time-based or continuous
features, hit-based based time domain
features, frequency and combination
based features.
The basic system is composed of one or
more AE transducers connected to
a processor.
A computer stores al AE features and
combines them with process and
control inputs to form outputs that can be
printed, replayed or analyzed.
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Fig. : Typical AE system block diagram.
ASNT- Acoustic Emission Testing Handbook, Vol 6.
AE Data Acquisition Devices
Example of AE device parameters:
 16 bit, 10 MHz A/D converter.
 Maximum signal amplitude 100 dB AE.
 4 High Pass filters for each channel with
a range from 10 kHz to 200 kHz (under
software control).
 4 Low Pass filters for each channel with
a range from 100 kHz to 2.1 MHz (under
software control).
 32 bit Digital Signal Processor.
 1 MB DSP and Waveform buffer.
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AE Systems
Handheld AE- Computerized instrument for AE testing applications.
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Due to its portable nature, the system can be used in any remote application.
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It can perform traditional AE feature extraction based AE signal processing, as
well as advanced waveform based acquisition and processing.
Wireless Remote Monitoring- This monitoring mode allows remote inspection.
Handheld measuring and processing system.
Wireless monitoring system.
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Signal / Noise Generator
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Noise generator produce AE signals necessary to verify the correct
operation of AE sensors, preamplifiers and AE systems.
Can also be used as a pulse or waveform generator for Acousto-Ultrasonics
or guided wave applications.
According to the desired application, the noise generator signals output can
be modified by amplitude, frequency range and other features.
Noise generator can also produce white noise for calibrating purposes.
Typical Noise Generator.
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Hit Based Features
Features that are collected form the waveform:
 Peak amplitude - The maximum of AE signal.
dB=20log10(Vmax/1µvolt)-preamlifier gain
 Energy – Integral of the rectified voltage signal over the duration of the AE hit.
 Duration – The time from the first threshold crossing to the end of the last threshold crossing.
 Counts – The number of AE signal exceeds threshold.
 Average Frequency –Determines the average frequency in kHz over the entire AE hit.
A. F 
A E counts
[ kH z ]
D uration
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Rise time - The time from the first threshold crossing to the maximum amplitude.
Count rate - Number of counts per time
unit.
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Time Driven Features
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The time driven AE features are collected and recorded on a timed basis,
referred to as the time driven data rate.
Time driven data report on the changing of background or continuous AE
activity during time, showing trends, leaking or faulty components in
continuous processes.
Typical time driven features include root mean square, average signal level,
external parametrics and absolute energy.
Absolute energy is also a valuable parameter because it provides a
summation of energy over time, independently of hit activity.
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Detecting the Signal: The Hit Based Process
Starting the Hit
Start of Hit
Starting the Hit
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Threshold setting is used to adjust the sensitivity
The first crossing of the threshold starts the hit.
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Hit Definition Time (HDT)
Long HDT
Short HDT
Voltage
Threshold
Hit 1
Short HDT
Hit 2
Time
Long HDT
Hit 1
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Peak Definition Time (PDT) and Hit Lockout Time (HLT)
Peak Definition Time :
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The function of the Peak Definition Time (PDT) is to enable determination of the time of
the true peak (risetime).
Hit Lockout Time :
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A lockout time which starts at the end of the hit during which the system does not
respond to threshold crossing .
Used to inhibit the measurement of reflections and late arriving signals.
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Thresholds Used in AE
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Threshold Level - The setting of an instrument that causes it to register only
those changes in response greater or less than a specified magnitude.
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Floating Threshold - Any threshold with amplitude established by a time
average measure of the input signal.
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System Examination Threshold - The electronic instrument threshold which
determines which data will be detected.
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Voltage Threshold - A voltage level on an electronic comparator such that
signals with amplitudes larger than this level will be recognized. The voltage
threshold may be user adjustable, fixed, or automatic floating.
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Background Noise
Background Noise: Signals produced by causes other than acoustic emission and are not relevant to the purpose of the
test
Types of noise:
 Hydraulic noise –Cavitations, turbulent flows, boiling of fluids and leaks.
 Mechanical noise –Movement of mechanical parts in contact with the structure e.g. fretting of pressure vessels
against their supports caused by elastic expansion under pressure.
 Cyclic noise – Repetitive noise such as that from reciprocating or rotating machinery.
 Electro-magnetic noise.
Control of noise sources:
 Rise Time Discriminator – There is significant difference between rise time of mechanical noise and acoustic
emission.
 Frequency Discriminator – The frequency of mechanical noise is usually lower than an acoustic emission burst
from cracks.
 Floating Threshold or Smart Threshold – Varies with time as a function of noise output. Used to distinguish
between the background noise and acoustic emission events under conditions of high, varying background noise.
 Master – Slave Technique – Master sensor are mounted near the area of interest and are surrounded by slave or
guard sensors. The guard sensors eliminate noise that are generated from outside the area of interest.
150
100
Floating
threshold
Amplitude
50
0
-50
-100
-150
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0
200
400
600
800
Time
1000
1200
1400
1600
1800
FREQUENCY FILTERS
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Frequency filters are used to reduce low-frequency mechanical noise and highfrequency electronic noise.
Recommended frequency range for AE work was 100-300kHz.
This range was high enough to escape most mechanical noise, but low enough to
be able to detect AE at a great enough distance for most practical work.
Frequency filters come in high pass, low pass and band pass.
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Using Frequency Filters
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Consider filters in both preamplifier and main instrument. Together the define the pass band.
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The larger the Db/octave roll-off rate, the more effective the filter.
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Integral-preamp sensors have filters but they do not have a high roll-off rate.
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Example of software switchable filters (MISTRAS, DiSP etc) have software
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Normal ranges:
high pass: 10,20,100,200 kHz
low pass: 100,200,400,1200 kHz
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Using Frequency Filters
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Set the high pass filter based on noise and detection range requirements.
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Use 100 kHz for general purpose
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200 kHz for high noise situations
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20 kHz for long-rate testing in quiet situation and or in highly attenuating materials
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10 kHz in extreme or research cases.
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Set the high-frequency end to get adequate bandwidth.
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Consider requirements for frequency analysis or waveform capture.
To prevent “aliasing”, the low pass filter must be less than the sampling rate.
The top end of the calculated frequency spectrum will be at one half
of the sampling frequency.
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Verify that the preamplifier pass band is compatible with the pass band of the
main instrument and in the analysis.
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E 1316: Standard Terminology for Nondestructive Examinations
Scope: This standard defines the terminology used in the Nondestructive Testing standards. These nondestructive
testing (NDT) methods include: acoustic emission, electromagnetic testing, gamma- and X radiology, leak testing,
liquid penetrant testing, magnetic particle testing, neutron radiology and gauging, ultrasonic testing, and other
technical methods.
Section A: Common NDT Terms
Nondestructive Testing - the development and application of
technical methods to examine materials or components
in ways that do not impair future usefulness and
serviceability in order to detect, locate, measure and
evaluate flaws; to assess integrity, properties and
composition; and to measure geometrical characteristics.
Indication - the response or evidence from a nondestructive
examination.
Interpretation - the determination of whether indications are
relevant or nonrelevant.
relevant indication - an NDT indication that is caused by a
condition or type of discontinuity that requires
evaluation.
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Evaluation - determination of whether a relevant indication is
cause to accept or to reject a material or component.
E 1316: Standard Terminology for Nondestructive Examinations
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acoustic emission (AE)—the class of phenomena whereby transient elastic waves are generated by
the rapid release of energy from localized sources within a material, or the transient waves so
generated.
AE rms —the rectified, time averaged AE signal, measured on a linear scale and reported in volts.
attenuation —the decrease in AE amplitude per unit distance, normally expressed in dB per unit
length.
count, acoustic emission (emission count) —the number of times the acoustic emission signal
exceeds a preset threshold during any selected portion of a test.
energy, acoustic emission signal—the energy contained in an acoustic emission signal, which is
evaluated as the integral of the volt-squared function over time.
effective velocity — velocity calculated on the basis of arrival times and propagation distances
determined by artificial AE generation; used for computed location.
floating threshold—any threshold with amplitude established by a time average measure of the
input signal.
hit—the detection and measurement of an AE signal on a channel.
location, continuous AE signal —a method of location based on continuous AE signals, as opposed
to hit or difference in arrival time location methods.
sensor, acoustic emission—a detection device, generally piezoelectric, that transforms the particle
motion produced by an elastic wave into an electrical signal.
signal strength—the measured area of the rectified AE signal with units proportional to volt-sec.
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