The acousto elastical stress measurement - a new procedure for the geotechnical on-line monitoring

IBJ Technology / Structural Health Monitoring – Real Time Stress Measurement
The acousto-elastical stress measurement - a new procedure for the
geotechnical on-line monitoring
F.-M. Jäger
With the new procedure for the permanent monitoring by the acousto-elastical
stress measurement changes in the structures can be recognized in time into civil
engineerings. The world-wide only on-line procedure permits the measurement of
loads and stress situations directly in the building. With ultrasonic the stress in the
building directly and in real time is seized. All changes can be transferred
immediately on-line by Internet or radio. The sensors are brought either directly into
the building or later attached to endangered places. Design features such as carriers
or bridge bearings can be supervised particularly simply. The sensors are constantly
at or active in the building. The simple structure and the small size permit
comprehensive application at all buildings from steel or concrete. Into all engineering
structures can be measured static loads and tensions and small dynamic changes. In
the procedure many important civil engineerings could be supervised world-wide. The
costs of such a monitoring are small. The collapse of buildings accompanies with a
measurable change of the tensions and loads. These are correlated with the world-wide
available stove data of seismic events and examined for plausibility. Thus also a
monitoring of buildings of all kinds on damage is possible by disasters (earthquake,
ground slips, mudslide etc.).
1. Physical fundamentals of the acousto-elastical measurement
Contrary to the stress analysis of construction units, where generally the change of
speed of the transversals and longitudinal waves is seized and evaluated, in situ
stress measurement regarded here uses only the change of the speed of the
longitudinal waves within the thickness of a measuring body. Past direct
measurements of the speed of sound in rocks or concrete are unsuitable for
regulations of the stress ratios. Rock anisotropies, tears etc. affect saliently these
measurements. Particularly different contents of pore waters make such
measurements with difficulty comparable and unsuitable for a monitoring [Huang et

al. 2001]. The instrumentation influence of changing porosities and/or dampness
contents can lie the far over stress-dependent portion of the measuring effect.
For the broad use of the measurement of speeds of ultrasonic waves from there the
influence of changing rock parameters must be if possible excluded. The new
beginning for the evaluation of the acousto-elastical effect is based on the use of
measuring bodies made of metal in the inhomogenous and anisotropic items under
test. These new applications of the acousto-elastical effect for the interests of the
geotechnics are described by several relevant patent specifications [Jäger,
2005,2006,2007,2008,2009]. The measured variable is in all applications the running
time of an ultrasonic impulse in a homogeneous measuring body, for example made
of metal. Favourable way is this measuring body for more-axial receivers a metal
cube or for in-axial receivers a metal plate with several or a PVDF foil for each
tension direction. The force application takes place on the measuring cube and/or on
the metal plate and concomitantly via the PVDF foil. The force application changes
also the mechanical stress in the measuring body. Since this mechanical stress is not
directly measurable, one must select either the detour over a mechanical size or over
further directly dependent variables. The ultrasonic speed is like that one, from the
mechanical stress, dependent variable. However still further factors of influence exist:
· For the measuring instrument practically as factors of influence (material
constants), which can be accepted constantly: the modulus of elasticity , the
density and the Poisson number n.
· The most important variable measured variable, the temperature, which over
other material-specific parameters the speed of sound directly (thermal
dependence on c) or indirectly affects (thermal coefficient of expansion a).
Contrary to liquids and gases the speed of sound c in the solid body hangs of the
modulus of elasticity off. In addition, there is here besides a dependence on the
density the solid body. For longitudinal waves in a long staff with a diameter
smaller than the wavelength, under neglect, is valid for the lateral contraction:
( 1 )
For transverse waves arises:

( 2 )
with the shear modulus .
For the homogeneous and isotropic solids regarded here simplified without roll-direction-
controlled constants are regarded here. Thus the speed of sound does not
depend on the direction of propagation. The speed of sound then additionally still
depends on the transverse contraction ratio (Poisson number) n:
( 3 )
this is valid for a longitudinal wave. For a transverse wave arises:
( 4 )
Ultrasonic waves have a frequency range of over 20 kHz. The transverse contraction
ratio one calls also Poisson number and is defined as follows:
( 5 )
with the change of diameter and length variation the body.
As measured variable for an embedded measuring body no mechanical measured
variable is available.
Interference-freely and without
influence of the item under test
however the running time is
measurable, which (in the
broadest sense) is in reverse
proportional to the mechanical
stress in the measuring body.
Fig.1:The Acousto-elastical Effect

The acousto-elastic effect describes the influence of tensile stress on the speeds of
ultrasonic waves in the measuring body. The out spreading speeds is described
thereby in the following form, in that the material density, which elasticity and shear
modulus (flexible constant of IITH order) as well as the flexible constants of IIITH
order as material-specific characteristic values and the three components of the
orthogonality pressure tensor and/or the three principal stresses as condition
parameters of the measuring body are received.
The running time of the ultrasonic waves, which spread within the measuring body, is
measured highly reolution with a TDC circuit.
The adaptation of the ultrasonic transducers into or to metallic bodies is easily
possible. The acousto-elastic effect can take place both via the measurement of the
longitudinal wave and via the measurement of the transversals wave or via
evaluation of the change of both waves. It is valid the reversibitity between expansion
and upsetting.
The Hook law is valid only for the elastic range.
s (tension) = E (elastic module) * e (stretch)
The ultrasonic waveguide of metal fulfill the Hook law. The relative change of the
wave velocity by the tension effect is very small. The change of speed of the
ultrasonic waves is an approximately linear function. The change of the speed of
sound depends apart from the
dependence on the influencing
mechanical stress also on the
temperature. In practice the
temperature equalizing places
itself between measuring bodies
and surrounding building
sufficiently fast.
Fig. 2: Acousto-thermal effect

Larger variations in temperature are concrete in the stationary installation in the
mountains or in tunnels, in the annular space between Tübbing and mountains not to
expect. With applications, where on a changing ambient temperature is to be
counted, temperature measurements are capable of being implemented for
compensation conceivably and easily in the measuring body. By the elastic behavior
of the measuring section between the ultrasonic sensors also the length of the
measuring section is changed.
It is well-known that those speed of sound changes by the effect of a mechanical
stress. [Split 2002] via the measurement of the speed of sound a sufficiently exact
determination of the tension can take place within the measuring body.
The change of the speed of sound is very small in relation to the absolute speed of
sound. The direct instrumentation evaluation by a usual measurement running time is
too inaccurate, since the dissolution is not sufficient here. A direct frequency counting
over microprocessors separates, there the cycle time (computing clock) around the
factor 1000 to 10000 is larger than the demanded usable dissolution. Metal plates of
few centimeters result in running times of the ultrasonic impulse smaller 10 μs. If
loads are to be measured by only some MPa, and/or Nmm-2, the dissolution must be
below 10 ns.
For the measurement of small changes (10 kPa) and smaller the increase of the
dissolution must take place via calculation of average values of many single
measured values.
TDC circuits can dissolve with a measurement better than 50 ps. By the short
measure-strain in the measuring body can problem-free by 10.000 measurements
per second be made. In one second so a resolution is very fast and easily possible
1 ps for better by calculation of average values.
In the alga meaning the stress measurement in the mountains or concrete is not a
time-critical task. The resolution of the running time under 1 ns requires from there
only sufficient measured values. Resolution-limiting the temperature influence affects
the running time. Modern TDC circuits possess special measuring entrances for
temperature measurement and permit a resolution of the temperature of 0.004 ° C.
The temperature is to be determined if possible with high resolution. The changes of
temperature in the rock and/or concrete take place in practice slowly and are not

time-critical in relation to the measurement of flying time. In principle nearly each
highly soluble temperature measurement is suitable.
A standard deviation of the temperature of 0,001 °K causes an additional deviation of
the tension from 1,31 kPa. Technically is executable with different electronic
construction units and by the principle different temperature sensors. Temperature
measurement principle:
• Pt-Resistors
Evaluation in the TDC circuit; (0,002°C)
• Digital temperature sensors
• 1-Wire-Interface
Dallas DS18S20, resolution: 12 Bit, (0,0625°C)
• 2-Wire-Interface
National Semiconductor LM76CHM, resolution: 14 Bit
• SPI-Interface
Analog Devices ADT7310, resolution: 16-bit; (0.0078 °C)
Advantage of the digital temperature sensors: Clear addressing already in the sensor
contain. The absolute accuracy can be brought by calibration in ice water on better
0,1°C. The resolution can be further increased by calculation of average values.
Since the temperature sensor is a firm component of the load sensor, the influence of
the absolute accuracy can be neglected. The zero-measurement without load and
the current measurement under load take place always also and the same
temperature sensor.
2. Sensitivity and factors of influence
The measurement of the running time took place with 2 different laboratory
superstructures with in each case a H8-Prozessor for the controlling of the TDC-GP2
with digital display and/or the TDC501 with serial interface. For the determination of
the thermal dependence of the speed of sound the running time with the TDC501
was determined and handed over the serial interface to a PC with the DATA
Aquisitions system DASYLab by national instruments.
Further the temperature of the measuring body with a semiconductor sensor was
determined. With a microprocessor determined were likewise serially handed over
and with a DASYLab module in °C scaled. From the pair of the running time and the

appropriate temperature result temperature-dependent correction for the running
time.
These factor to the correction are specific a material constant and for the respective
measuring body alloy. Thus the influence equal thermal coefficients of expansion with
is considered. The measurements
confirmed that for instance to 10
times influence of the thermal
dependence of the speed of sound
in relation to the influence the
thermal length variation measure-strain
on the result of a
computational determination of the
speed of sound.
Fig. 3: Determination of the thermal dependence of the running time with DASYLab
The speed of sound in solids, decreasing with rising temperature, is not linear. For
the interesting temperature range hardly concrete values are to be found in the
literature. The construction unit temperature changes the flexible behavior in linear
kind and run time change per 10 K temperature difference can confirmed
temperature coefficients be corrected due to one for many steel approximately 1.1 ‰
[Längler 2007].
Own measurements were accomplished by the author at inspection pieces from
aluminum with a thickness of 10 mm. Became in the temperature range of - 25°C to
+75°C the following dependence determines:
linear regression:
regression curve: Y = a + b*x ( 5 )
wih a = = 3079,314922
and b = = 0,886518
dimension X values = °C
dimension Y values = ns
number of measured values = 65
correlation coeffizient R = 0,998204
coefficient of determination R² = 0,996412
exponential regression:
regression curve: Y = a * exp (b*x) ( 6 )
with a = = 3079,341260

and b = = 0,000285
correlation coeffizient R = 0,998401
coefficient of determination R² = 0,996805
For practical application for the correction of the running time the use of the linear
involution is sufficiently exact.
Fig. 4: Run time change as function of the temperature
Following fig. 5 shows exemplary the run time increase in an aluminum body of 25
mm of thickness of approximately 15 ns during a rise in temperature of approx. 20°C
to 32°C. The curve down shows the result of the numeric run time correction. Even
during the dynamic change of temperature and the still taking place heat flow
amounted to the deviation of the corrected running time from the computational
reference running time (0°C) less than 100 ps.
18:22:30 18:27:30 18:32:30 18:37:30 18:42:30 18:47:30 18:52:30
h:min:s
7850
7840
7830
7820
7810
7800
45
40
35
30
25
20
15
1050
5,0
2,5
0,0
-2,5
-5,0
-7,5
-10,0
run time [ns]
temperature [°C]
deviation [ns]
Fig. 5: Compensation of the temperature dependence of the running time functional dependence of
the running time in a 25 mm of measuring bodies

Laufzeit = f (Spannung)
7780
7775
7770
7765
7760
7755
7750
7745
7740
7735
7730
0 10 20 30 40 50 60
Spannung MPa
Laufzeit ns
Fig. 6: running time as function of the stress
In the case of use of a measuring body with 25 mm measuring distance a change of
stress results in a change of the running time of 10 MPa of approx. 7800 ps. Each
individual measuring with the TDC circuit TDC-GP2 brings a resolution of ca.50 ns,
i.e. the resolution amounts to approx. 64 kPa without calculation of average values.
Fig. 7: Dependence of the
speed of the longitudinal
wave of the tension
3. Measurements of concrete bodies and reinforces elastomeric bearings
under the hydraulic press
For the static loading tests that the acousto-elastical sensors were centrically
concreted in concrete bodies with the dimensions with the dimensions of 300 mm of

length, 200 mm of depth
and 100 mm height. The
concrete bodies lay on bed
made of powder and leed
sheet.
Fig. 8: equipment Fig. 9: hydraulic press
The upper load introduction took place over elastomer camp (make Gumpa). Over
this camp to the distribution of the load a steel plate with a thickness was put of 30
mm. In order to achieve at the sensor a higher stress concentration, the elastomer
camp was made smaller on a surface von100 mm x 200 mm. With following
experimental setup became within the range of 0… 12.5 MPa load lines of aluminium
bodys with 10 mm up to 25 mm of edge length taken up to concrete.
.
For higher mechanical stresses the
surface was reduced for force
application. The surface of the reinforces
elastomeric bearing was made smaller
on 100 mm x 200 mm.
Fig.10: Stress concentration over the sensor

Fig. 11: experimental set-up Fig.12: Data Aqusition System DASYLab 9
Order for load took place with a hydraulic press upto max. 25 tons without load
control. The measurement the load took place with a ring torsion cell RTN C 47t from
gives with a 24-Bit AD-transducer ADS1232. The PC program TIADS123X
(LABVIEW) for it ran separately when running. The temperature measurement took
place with a digital temperature sensor. The stress in MPa, measured in Fig.12,
became after a calculation specification from the running times measured with the
TDC and with DASYLab as “sigma measuring “represented. The comparison load
measured with the load cell (resolution 10 g) as “sigma target” seized. The resolution
of the tension took place in each case in 1 kPa-walked.
Fig.13: DASYLab 9 computation “sigma is
For the computation of “sigma measuring “simplified according to the following
regulation one proceeded:
The stress s results from the temperature-compensated running time LT1, the
reference on time LT0 and that acousto-elastic factor of the measuring body material
Ks too

s = ( LT1 - LT0 ) / Ks ( 7 )
Hereunder applies for LT1 the measuring temperature T1 of the measuring body and
for LT0 the reference temperature T0 = 0 °C and the reference stress s = 0.
Whereby the temperature-compensated running time LT0 from the measured running
time LT and the correctur factor KT is determined after
LT0 = LT * KT ( 8 )
The thermal factor KT is for a large temperature range a nonlinear function
KT = f ( T ) ( 9 )
The thermal factor KT of the running time determines itself according to (5) with the
linear regression for the selected sensors too
KT = 0,94684 ns°C-1 ( 10 ) .
On the sensor test stand the acousto-elastical factor Ks, intended for the selected
metal alloy and sensor thickness, too
Ks = 4,4585 Mpa ns-1
and/or Ks = 4,4585 Nmm-2 ns-1 ( 11 )
4. Field measurements at a building of the federal motorway
4.1. Sensor
In order to permit the installation into boreholes under the elastomeric bearings, as
low an overall height of the sensors as possible was selected. The installation of the
sensors takes place into boreholes from
approx. 25 mm in diameter. The sensors
possess a 1-Wire-Interface DS18S20 von
Dallas with a resolution of 12 bit. Each
sensor is clearly identifiable with the sensor
coding in the ROM.
Fig. 14:stress sensor BBS_10_DS

4.2. Measurement with TDC
The measuring instrument with TDC is accommodated in a GFK box as well as the
processor for temperature measurement. The ultrasonic impulse is generated by an
ultrasonic thickness-measuring meter CL204
von Krautkrämer Branson. The
announcement of the thickness is not
evaluated and serves only for control of the
operating condition. The starting and stop
impulse for the measurement of running time
with the TDC are inferred from the CL204
and supplied to the TDC board. The control
TDC board takes place with a batch-program
Fig. 15: Measuring box with TDC and CL204
The running time, those with the TDC board is determined over a serial Interface with
a Windows program seizes. The tax and evaluation programs run multitasking on
Panasonic a Toughbook CF-M34.
4.3. Measurement of the running time with ultrasonic material testing set
With a further independent the running times of the sensors under the loaded
bearings additionally with the ultrasonic material testing set USP1 one seized. The
measurement of running time takes place with this measuring instrument only with a
dissolution of 1 ns. The run time data were seized with a further notebook. The
visualization of a-picture and the measurement of the echo amplitude make the
estimate for the operability possible of each sensor.
4.4. Stress and load measurement
The data at running time and the temperature, as well as the sensor number are
processed serially over a USB stroke in Panasonic the CF-M34. The representation
of the data takes place in a special program for the data evaluation under DasyLAB.

At present only approx. 3 measured values/second can be evaluated with the batch-program
for the selection of the TDC. The TDC is to be implemented able at
appropriate software 1000 to 10000 measurements/second. The measurement act of
the ultrasound measurements with the CL204 amounts to 1 ms, i.e. 1000
measurements/second are accomplished at present. The TDC queries however only
3 measurements/second, since it is limited over the serial interface by the data
transmission rate of 9600 Baud.
Fig. 16: (Screen of the Windows program for the
sigma determination)
4.5 Taking measurement
Fig. 17: BAB 9, Munich-Berlin Fig. 18: bored hole for sensor
After the hydraulic raising of the bridge and removing the elastomeric bearings the
mounting holes for the sensors were bored for bearing load measurement if possible
dare quite and centrically and/or close of the center of the surface of the elastomeric
bearings. The drillings were slit with diamond gumption sheets. The sensors must be
embedded actuated in the concrete under the elastomeric bearings. As mortar for
actuated imbedding construction mortar of Pagel served.

Fig. 19: Preparatory hole Fig. 20: Sensor inserted
Fig. 21: Before using the elastomeric bearing Fig. 22: Elastomeric bearing assigned
After using the sensors the zero-measurement without load influence took place. The
accomplished temperature measurements could not determine a rise in temperature
by exotherm tying the mortar. The concrete was heated altogether still of the day
before clearly. The air temperature on the day using the sensors was by a cooling
break-down approx. 12°C to 13°C clearly under the concrete temperature from 17°C
to 23°C. After sticking the elastomeric bearing together lowering the bridge took
place.

Fig. 23: Sensors after load measurement Fig. 24: Sensor row direction the west
After the bridge sinking take place the measurement of the load admission after 6
days. Apart from the measurement with the TDC board for each sensor the pertinent
temperature was determined. After a further week the sensors with the USP1 were
additionally examined.
5. Results of measurement
5.1. Static stress measurement
If one lays on the stress measured under the elastomeric bearings as bar chart
transverse to carriageway width, one receives the following representation: bearing
location 1 is west (motorway center), the bearing location 27 is east (standing tires).
With the TDC board no usable signal could be measured with bearing 4. The
examination with that USP1
resulted in, which is still
functional the sensor to
generate the signal amplitude
is too small over in the CL204
a stop signal for the running
time. With a changed
hardware this sensor is
further evaluable.
stress in concrete
100
80
60
40
20
0
1
3
5
7
9
11
13
15
17
19
21
23
25
27
bearing number
stress MPa
Fig. 25: stress and bearing number

5.2. Dynamic stress measurement
To measure despite the slow data transmission rate the attempt undertaken at a
camp the load entry dynamically.
The time axis in the continuous line
recorder Windows program was
adjusted to shorter time units.
Fig. 26: Dynamic load measurement
bearing 20
The break in fig. 26 possibly is on
work on the opposite counter bearing to lead back. At the same time work in the
hydraulics section. at the opposite bearings were accomplished.
The fluctuations of the running time are induced by traffic on the motorway.
The amplitude of the changes over 10 ns. That is 3 to 4 times more than the statistic
noise of the zero-measurement.
The next generation with improved
controlling of the TDC will make 1000
measurements per second.
13:36:00 13:36:10 13:36:20 13:36:30 13:36:40 13:36:50 13:37:00
h:min:s
Fig. 27: Sensor at the bearing 20 with a resolution time of 60 seconds.
6. View on applications
The advantage of the acousto-elastical stress measurement is recordable with the
following criteria:
• Low cost on-line measurement
• Practically indestructibly
• No measuring range delimitation upward
50
45
40
35
30
25
20
15
10
5
0
Schreiber 0

• Measurement in the heap of debris and fracturing zone
• Measurement in the water
• Inexpensive lost probe
From this concrete applications result how:
The monitoring of all possible effect and structural parameters during the building
phase and the enterprise of buildings is the basis for the condition and safety
analysis of the building. The data seized with different
geotechnical methods represent the basis for numeric
and mechanical concept. On-line measuring
procedures to stress measuring, do not have to be
replaced in its force of expression and topicality. With
on-line stress measurement the so far only modelful
parameters at small expenditure can be measured
and thus the verification of all past models be
substantially improved. Fig.28: fracturing in rock
By the use of expansive cements can be
manufactured an analogy to the hydraulic frac.
During a longer period such an equilibrium must
adjust itself to the minimum ground pressure.
Statements to the time performance of
expansive ones to be cement in fig. 29
described [Mehta and Monteiro, 1993].
Fig. 29:Compressive stress in expansive cement
From instrumentation view also the employment of RFI technology is conceivable. So
measuring bodies with planar antennas or induction pick-up coils could be attached
for the power supply of the ultrasonic units behind the Tübbings. Thus the installation
is made possible for on-line stress measurement in the tunnel tube. Special meaning
can attain the monitoring of buildings. So the in-situ stress sensors in the concrete
could measure the load changes and stress changes with a building damage after
earthquake immediately on-line. The alert with GPS item data is spread world-wide
over the Internet. The combination also for everyone accessible maps of the world

can facilitate the management after disasters substantially. Material costs of a
measuring system amount to less than 50 €. By networking of several measuring
systems with modern systems to the data communication, how Internet, mobile or
Satelite know thereby a kind of secondary low cost array Seismometer are
developed.
7. Results
Xiaojun Huang, Daniel R. Burns und M. Nafi Toksöz, ERL, MIT „The effekt of stress
on the sound velocity in Rocks.Theory of Acoustoelasticity and Experimental
Measurements“, Consortium Reports 2001, Earth Resources Laboratory, Cambrigde,
MA 02142.
Jäger, F.-M., Vorrichtung zur Ermittlung der Gebirgsspannung in einem Bohrloch -
DE102005047659B4, Verfahren und Vorrichtung zur Früherkennung von
Bauwerksschäden - DE102006053965A1, Vorrichtung und Verfahren zur
Lastmessung an Lagern von Bauwerken - DE102007014161B4, Verfahren und
Vorrichtung zur Bestimmung der Gebirgsspannung – DE102008037127.0, Verfahren
und Vorrichtung zur Überwachung und Bestimmung der Gebirgsspannung –
PTC/DE102009/001105
Längler,F., Wissensbasierte Automatisierung und kontinuumsmechanische
Erweiterung der Ultraschall-Eigenspannungsanalyse zur Beschreibung des
Spannungszustands im gesamten Bauteil, Dissertation 2007,Universität des
Saarlandes, Saarbrücken, S.10
Splitt, G., Schraubenspannungs-Messung mit Ultraschall - moderne Messtechnik für
sichere Schraubenverbindungen, Agfa NDT GmbH, DGZfP-JAHRESTAGUNG 2002
Mehta and Monteiro. (1993) Concrete Structure, Properties, and Materials, Prentice-
Hall, Inc., Englewood Cliffs, NJ
Author:
Dipl.-Ing.(FH), Dipl.-Ing.Ök. Frank-Michael Jäger
IBJ Technology
Ingenieurbüro Jäger GBR
Colkwitzer Weg 7
04416 Markkleeberg

The acousto elastical stress measurement - a new procedure for the geotechnical on-line monitoring

Recommandé

Recommandé

Contenu connexe

En vedette

En vedette (11)

Similaire à The acousto elastical stress measurement - a new procedure for the geotechnical on-line monitoring

Similaire à The acousto elastical stress measurement - a new procedure for the geotechnical on-line monitoring (20)

Plus de Frank-Michael Jäger

Plus de Frank-Michael Jäger (16)

Dernier

Dernier (20)

The acousto elastical stress measurement - a new procedure for the geotechnical on-line monitoring