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

IBJ Technology / Structu ral Health Monitoring – Real Time Stress Measuremen t

The acousto-elastical stress measurement - a new pr ocedure 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 m easurement

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 ν.

• 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 α).

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) ν:

( 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.

σσσσ (tension) = E (elastic module) * εεεε (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


h:min:s

18:22:30 18:27:30 18:32:30 18:37:30 18:42:30 18:47:30 18:52:30

7850

7840

7830

7820

7810

7800

4540353025201510

50

5,02,50,0

-2,5-5,0

-7,5-10,0

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.

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)

7730

7735

7740

7745

7750

7755

7760

7765

7770

7775

7780

0 10 20 30 40 50 60

Spannung MPa

Lauf

zeit

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 σσσσ results from the temperature-compensated running time LT1, the

reference on time LT0 and that acousto-elastic factor of the measuring body material

Kσ too


σσσσ = ( LT1 - LT0 ) / Kσσσσ ( 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 σσσσ = 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 Kσ, intended for the selected

metal alloy and sensor thickness, too

Kσσσσ = 4,4585 Mpa ns -1

and/or Kσσσσ = 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 ultrasoni c 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.

Fig. 25: stress and bearing number

stress in concrete

0

20

40

60

80

100

1 3 5 7 9 11 13 15 17 19 21 23 25 27

bearing number

stre

ss M

Pa


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.

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

h:min:s13:36:00 13:36:10 13:36:20 13:36:30 13:36:40 13:36:50 13:37:00

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

Engineering

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