4-1 moment tensor – applications 4-2 general remarks on applications to structures

4-1 Moment Tensor – Applications

4-2 General Remarks on Applications to Structures

4-3 Concrete – Fresh and Hardened

Damage MechanicsError EstimationVisualization

Kinematical information of crack motions is recovered by the SiGMA analysis.

Some applications are stated. The development of the SiGMA procedure is closely associated with error estimation.

In addition, the moment tensor is mathematically related with damage mechanics.

Trace component : Mkk = (3+2)lknkV.

A damage parameter in damage mechanics

[Kachanov 1980],

D = ∫ni(binj)nj dS = Vlknk = Mkk/(3+2)

Further the crack volume is obtained, V = Mkk/[(3+2)lknk].

Estimation of errors in the moment tensor analysis is fairly difficult. In the analysis of hydro-fracturing, the conditioning numbers were applied.

Then, error estimation was conducted, assuming the errors. As a result, it is found that the error estimation has been a really complicated task, because the errors are dependent on a spatial relation between the source location and the observation points.

Consequently, a post analysis is proposed [Ohtsu 2000], where AE waves to be detected at the observation points are synthesized from the source location and the moment tensor theoretically. The simulation analysis of theoretical AE waves is readily available.

102 AE events with detectable first arrivals were analyzed by the two-dimensional SiGMA analysis. Results are classified by the shear ratio X (%).

It is clearly found that almost 60 % events have the shear ratios over 60 %.

It was, however, realized that AE sources were distributed widely on the specimen, not concentrated around the slit.

Determination of both the amplitude of the first motion and the arrival time was actually not an easy task. This implies that some errors are unwillingly contained in results of the SiGMA analysis.

AE waveforms at sensor locations were synthesized in an infinite space, taking into the source location and the moment tensor components. The reflection coefficient was taken into consideration to simulate the waveforms. The SiGMA procedure was applied to synthetic waveform set as the post analysis.

Shear ratio

(%)

0 – 40

(%)

Tensile

40 – 60

(%)

Mixed-m

ode

60 – 100

(%)

Shear

SiGMA

analysis

20 20 62

Post analysis 12 10 79

Shear ratio

(%)

0 – 40

(%)

Tensile

40 – 60

(%)

Mixed-m

ode

60 – 100

(%)

Shear

No. of

events

10 3 33

Visualization procedure is developed by using VRML (Virtual Reality Modeling Language).

By applying the SiGMA code, AE events are displayed at their locations with symbols. In the previous results, a tensile crack is denoted by arrow symbol, of which direction is identical to that of crack opening. A shear crack is denoted by cross symbol, of which two directions correspond to two vectors l and n.

Although classification of cracks was readily made, crack orientation was not easily recognized. This was because two-dimensional projection was adopted for illustration but analyzed results are inherently suitable for three-dimensional visualization. In this respect, VRML is introduced.

Diagonal-shear failure at the shear span

Nucleation of cracks can be kinematically analyzed by the moment analysis. Applying the SiGMA code.

For the two-dimensional soltution, in-plane motions of AE waves are treated. The reliable solutions are selected by the post analysis.

Because visualization of results is desirable, three-dimensional visualization procedure is developed.

AE InspectionBasics of MeasurementElimination of NoisesSet-Up of AE System and

Measurement

AE techniques have been applied to a variety of structures and infrastructures in the field of civil engineering. Successful applications are concrete, rock, wood, superstructures of buildings and bridges, and substructures including railway structures.

A few codes have been established on concrete structures only in Japan [NDIS 2421 2000] and [JCMS-III 2003]. This is because concrete structures are known to be no longer maintenance-free.

When the techniques are going to be applied to existing structures or local members, inspection procedure shall be based on the codes and standards.

In other kinds of structures than concrete, in-situ inspection techniques of AE measurement or monitoring are under development for maintenance.

Some codes and manuals have been standardized in the governmental institutes and departments.

Due to damage evolution and deteriorations in the structures, AE events are observed, nucleating microcracks under in-service conditions.

An inspection method for active cracks or defects is a target for AE applications in engineering.

AE sensors shall be sensitive enough to detect AE signals generated in the target structure, taking acoustic coupling into consideration.

Sensitivity calibration of AE sensors shall be performed by employing the standard source or an equivalent piezoelectric sensor.

AE sensor also shall be robust enough against temperature changes, moisture conditions and mechanical vibrations in the environments.

Concerning coupling, several kinds of couplants are available.

So far there exist no regulated techniques as long-term inspection or durability of couplants are not confirmed yet.

This is because AE techniques are still immature for field applications.

=== Good enough strength and durability===

Amplifiers shall be set up as close as possible to AE sensors.

The frequency range, which are usually controlled by filters, and shall be determined prior to the measurement.

count, hit, event,

maximum amplitude,

energy,

rise time,

duration,

energy-moment,

RMS voltage,

frequency spectrum,

arrival-time difference.

Elimination of the noises is one of the most concerned aspects in the applications.

Usually, it is achieved by simply setting the threshold level over the noise level, or by a band-pass filtering and a post-analysis of the data.

Environmental Noise In advance to AE

measurement, the noise level shall be estimated on site.

Counteract against external noises, such as wind, rain, sunshine shall be made.

In the case that the noises have similar frequency contents and amplitudes to AE signals, or sources of the noises are unknown, characteristics of the noises shall be estimated prior to the measurement.

Then, separation of AE signals from the noises shall be made. In this respect, the use of filters is useful after determining the proper frequency range.

the measuring system was turned off in each case of traffic passing

AE events detected under traffic passing.

A measurement system consists of AE sensor, amplifier, and filter.

Total amplification by the pre-amplifier and the main amplifier is usually set from 60 dB to 90 dB.

To decrease the noises, a band-pass filter between several kHz and 1 MHz is mostly desirable.

The noises should be lower than 20 V as input voltage after detected by AE sensors.

Elimination of the noises is achieved by simply setting the threshold level over the noise level, or by a band-pass filtering and a post-analysis of the data.

In any cases, the averaged amplitude of the noise should be managed to be lower than 10 V as input.

Sensor array is determined from the attenuation properties of AE waves, setting the distance where attenuation during travel is less than 30 dB.

In most cases, the distance between the sensor and an AE source is set to be shorter than 1 m, in relation to the attenuation property,

The frequency range from 20 kHz to 100 kHz is recommended for in situ monitoring of concrete structures.

In advance of the test, attenuation properties of the target structure shall be estimated, by employing the standard source or the equivalent.

Sensor array shall be determined so as to keep the equivalent sensitivities in all the sensors.

The period of the measurement shall be prescribed, depending on the following conditions:

(1) Propagation property of AE signals in the target structure

(2) Stress distribution in the structure under inspection

Sensitivity of AE channels shall be conducted routinely by employing the standard source. Variation within the channels shall be less than 3%. Based on the spatial area to be covered, AE sensors of proper frequency characteristics shall be selected

AE signals shall be detected properly for the period of the measurement.

Concerning AE parameters detected, their trend, distribution, correlation, and locations are monitored and measured.

In principle, AE tests are conducted under loads which must not cause any damages on functions of the structure during detection and location of active cracks.

Under the following loads:Service load lower than the serviceable limit Incremental load lower than the serviceable

limit Variable and repeated load during service

Relations among AE parameters, at least AE hits, time, and loads shall be analyzed.

Applied fields and structures of AE measurement are still under developing and expansion.

This is because set-up conditions of AE measuring systems and target of analyses are different in each case and each site.

As a result, it is stated that AE techniques in practical fields are still immature. But, the applicability is already confirmed and successful applications have been extensively reported.

AE Techniques in Concrete Concrete of Early Age Hardened concrete

AE techniques have been extensively studied in concrete engineering for approximately five decades [Ruesch 1959].

They are applied to practical applications [Ohtsu 1987] and are standardized in the code [NDIS2421 2000].

This is because the increase of aging structures and disastrous damages due to recent earthquakes urgently demand for maintenance and retrofit of reinforced concrete structures in service.

It results in the need for the development of advanced and effective inspection techniques.

Wells studied the relationship between strain measurement and AE event [Wells 1970].

Studies on fundamentals of AE activity and the effects of mixture proportion were conducted [McCabe et al. 1976, Nielsen & Griffith 1977, Mlakar, Walker et al. 19845].

A frequency analysis and a source location analysis were also reported [Fetis 1976, Niwa, Kobayashi et al. 1978, Reymond, Raharinaivo et al. 1983, Berthelot & Robert 1987, Weiler, Xu et al. 1997].

Applications to reinforced concrete structures were investigated [Niwa, Kobayashi et al. 1977, Kobayashi, Hawkins et al. 1980].

These studies have resulted in practical applications to monitor micro-cracks in concrete structures and going to be made practical as diagnostic applications [Ohtsu 1988].

Nondestructive tests for fresh concrete at early ages have been summarized in the Rilem state of the art report [Reinhardt & Grosse 2005].

The AE applications to concrete of early ages are included in the report. Based on the article, noteworthy examples of the applications are briefly stated.

Roller-compacted concrete is developed and extensively applied to construct concrete dams in Japan.

In order to control the consistency of fresh concrete, the VC test is carried out.

Fresh concrete after mixing is placed in the mold on the vibration table.

During the vibration, time is measured by second, which is named “VC value”, until breeding water is observed at the surface of the mold.

The measurement is neither practical nor easy to estimate concrete properties of all mixes.

During mixing, where AE sensor was attached directly to the outer surface of a concrete mixer.

With the increase in the VC value, concrete becomes so sticky due to low water content that fretting aggregate with the mixer wall ceases earlier.

As a result, the transition points of AE energy move lower. This implies that the consistency of concrete is successfully estimated by just observing AE energy under mixing.

In normal concrete, the consistency is measured by a slump test.

AE activities during dynamic compaction were measured to simply estimate the degree of compaction [Kunisue, Yokoyama et al. 2002]. AE sensor was attached to the outer surface of mold.

Micro-cracking due to drying shrinkage was studied in plain cement paste and cement-based composite [Shiotani, Bisschop et al. 2002].

In the cement-based composite, 35% volume percentage of 6-mm glass sphere with smooth surface was mixed as aggregate.

Six AE sensors of 500 kHz resonance were mounted with wax onto the drying surface.

The specimens were dried in an environmental cabin ventilated with air of 25% RH and temperature of 31oC for 16 hours.

micro-cracking in the plain cement paste is due to self-restraining, while cracking in the composite results from both self-restraint and aggregate-restraint stress.

In hardened concrete, AE events are

normally detected by micro-cracking. The basic creep strain is proportional to

the total number of AE hits, which are associated with micro-cracking created in concrete.

For evaluation of fatigue, AE amplitude distributions observed in reinforced concrete beams under bending are useful.

Gutenberg-Richter Relation on earthquakes

Log N = a – bM M=LogA: Magnitude

Log N = a - bLogA

b-value Large:small amp.

Small:large amp.

From the arrival time differences, AE sources are located one-dimensionally [Heam & Shield 1997].

Water-Proof AE sensors of 60 kHz resonance

Short Course 101-1111-00L: Fundamentals and Applications of Acoustic Emission by Prof. Masayasu Ohtsu

October 25, 2007

4-1 In the relation: V = Mkk/[(3+2)lknk], taking into account the relations: = E/[(1+)(1-2)] and = E/[2(1+)],

Show that the crack volume is equal to zero when Poisson’s ratio = 0.5.

4-2 After reading “Draft1.pdf”, summarize the procedure for AE measurement with remarks.