mechanics meets informatics daad summer · pdf filenon destructive testing –the example...

MECHANICS MEETS INFORMATICSDAAD Summer School | Chania 4-14 July 2016

Dr. Evangelos V. LiarakosPostdoctoral Researcher

w: https://eliarakos.wordpress.com/

Technical University of Crete

School of Architectural Engineering

Applied Mechanical Lab.

Non Destructive Testing of

engineering structures based on

their vibration characteristicsApplications in concrete structural health monitoring

https://eliarakos.wordpress.com/

Technical University of Crete | Applied Mechanics LabLiarakos E.V.

MECHANICS MEETS INFORMATICS | DAAD Summer School – Chania 4-14 July, 2016

This presentation is distributed from the author Evangelos V.

Liarakos, under a Creative Common License and the following terms

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Technical University of Crete | Applied Mechanics Lab

Outline of presentation1. Structural integrity and Health Monitoring (SHM)

• Evaluating the mechanical integrity of constructions

2. Non Destructive Testing (NDT) based on construction’s

vibration characteristics

• Ultrasonic Methods (P-Wave Propagation)

• Impact-Response Method | Damage detection/Evaluation

3. Laser Scanning Vibrometry

• Non-contact and remote monitoring

• Laser Doppler Vibrometry and Interferometry

• Scanning of vibrating surfaces – Monitoring Mesh

• Finite Element Analysis (FEA) verification

4. Summary – Discussion Liarakos E.V.


Slide: 3/60


Abbreviations/Acronyms AMEL: Applied MEchanic Laboratory (Technical University of Crete) | web

DAQ: Data Acquisition systems

DVA: Displacement-Velocity-Acceleration (Time-Histories / Signals)

FEA/M: Finite Element Analysis / Method

IRM: Impact Response Method

LASER: Light Amplification by Stimulated Emission of Radiation

LDE: Laser Doppler Effect

LDV: Laser Doppler Vibrometry

LSV: Laser Scanning Vibrometry

NDT: Non Destructive Testing

PZT: Lead Zirconate Titanate (Piezoelectric Material)

SHM: Structural Health Monitoring

Liarakos E.V.


Slide: 4/60

http://users.isc.tuc.gr/~kprovidakis/Research_Lab.htm



1. Structural integrity and

health monitoring

Slide: 5/60


Structural integrity and mechanical behavior

Structural integrity. Ability of a construction to respond efficiently

and safely to mechanical loading both from internal (own weight /

gravity loads) and external forces (e.g. live loads, earthquake).

1. Structural integrity and health monitoring

Liarakos E.V.


Construction Safe

behavior. For a specific

level of mechanical loading

the induced stresses and

deformations are complying

with the acceptable values

that predicted from design

standards (e.g. Eurocode 1-

8 etc) [1].

Large deformations – Possible

mechanical damage

Safe behavior –

acceptable

deformation

Equivalent

strain

Slide: 6/60


Mech

an

ical

Str

ess

Strain / Deformation

Mechanical

strength

Young Modulus,

Stiffness of material

Destructive compression

testing of concrete

Structural properties

Structural properties govern the mechanical and dynamic behavior of a

construction.

Concerning Building materials

Mechanical Strengths: Tension, Compression, Shear etc.

Mechanical properties: Young Modulus, Poisson ratio, Velocity of

mechanical waves etc.

Physical properties: Density etc.

Liarakos E.V.



Slide: 7/60


Structural properties

Concerning structure’s global behavior

Marco-geometric characteristics. General framing of construction,

Outer shape.

Geometry of each individual constructional member. Sections of

beams and columns, Curvature of shells etc.

Connections between structural elements

Foundations of structure

Liarakos E.V.



Sharp change of macro-

geometry – Stress

concentration

Beam Section – Element

Stiffness

Horizontal shear loading –

Seismic ground motion

Connection joint

among structural

elements

Slide: 8/60


Mechanical Damages

Each irregular variation of structural properties that change

negatively the mechanical behavior of a structure.

Cracking. Mainly in brittle materials (Concrete, Rocks,

Ceramics). Alteration of constructional members internal

geometry. Formation of new surfaces.

Permanent plastic deformations. Ductile materials - Metallic

structures.

Decreasing of mechanical strength of materials. Age

hardening, fatigue, creep, ambient-related erosion.

Macroscopic fracture of structure. Collapse of connection

joints among constructional members.

Liarakos E.V.



Slide: 9/60


Mechanical Damages

Liarakos E.V.


1. Structural integrity and health monitoringB

ritt

le m

ate

rials

: C

on

cre

te

Concrete strain-stress curve

Necking: Large Plastic Deformation

Tensile fracture

Du

cti

le m

ate

rials

: S

teel

Neck

Steel strain-stress curve

Slide: 10/60


Max Compressive

Stress

Max Tensile Stress

Micro-Cracks. Regional Stress Concentration

Mechanical Damages – Consequences

Mechanical damages initially appear locally, especially in brittle

materials (Concrete, Masonry etc).

Damages like cracks increase stress intensity in nearby areas and

result the overloading of building material.

Liarakos E.V.


The existence of a topological

restricted damage, may does not

affect initially the global behavior of

construction, but …

… non-timely detection and

effective restoration deteriorates

the structural integrity and gradually

drives to total collapse incidents.


Slide: 11/60


Aims of structural health monitoring (SHM)

Inspection of structural integrity

Establishment of a monitoring grid. Sensing devices

installation in specific control points of construction's

space.

Combining inspection methods. Visual control, Material

sampling, Non-Destructive Testing (NDT).

Assessment of building materials strength.

Control of constructional members integrity.

Record of possible mechanical damages.

Post-inspection procedures. Comparative interpretation of

qualitative and quantitative inspection data.

Liarakos E.V.



Slide: 12/60


Aims of structural health monitoring (SHM)

Damage detection and evaluation

Detection of possible mechanical damages

Positioning of damages

Identifying damage causes

Assessment of damage severity – Estimation of influence in structure’s

general behavior.

Decision if a restoration is needed

Estimation of available time before restoration where structure is safe

Non-Destructive Testing (NDT) methods provide the option of

in-situ control of structural properties and because of this have

contribute significantly in efficiency improvement of SHM and

damage detection/evaluation procedures.

Liarakos E.V.



Slide: 13/60



2. Non Destructive Testing

(NDT) based on construction’s

vibration characteristics

Slide: 14/60


Non Destructive Testing: General Features and Principles

Assessment of structural properties without need to submit

material samples or constructional members in destructive

laboratorial test (E.g. Compression of concrete, Tension of structural steel).

Essential decrease of costs which are related to specimens

sampling, transportation, storage and conservation.

Non intervention to constructional members integrity

(materials cores sampling)

In-Situ evaluation of structural properties. An essential benefit

that is exploited in Structural Health Monitoring applications.

Liarakos E.V.


2. Non Destructive Testing (NDT) based on

construction’s vibration characteristics

Slide: 15/60


Non Destructive Testing: General Features and Principles

Basic philosophy: Indirect determination (estimation) of

structural properties by measuring directly an observating physical

quantity.

Observed physical quantities are correlated with structural

properties either via empirical models or by adapting exiting

theoretical model to acquired measurements.

Empirical models are formulated after an extensive statistical

analysis and identification of relation between measured quantities

and structural properties.

Disadvantages. Effects vary among NDT methods.

a) High cost of instrumentation

b) Variance in accuracy of measuring quantities

Liarakos E.V.




Slide: 16/60


Non Destructive Testing – The example of ultrasonic method

Aim: Estimation of materials’ Elasticity (Young) Modulus by measuring

the velocity of longitudinal (P-Waves) mechanical waves in a

constructional member.

Primary structural property: Elasticity dynamic modulus (Ed)

Direct measured quantity: P-Wave velocity (Vp)

Liarakos E.V.


Transmitting

Transducer

Receiving

Transducer



Slide: 17/60


Non Destructive Testing – The example of ultrasonic method

Theoretical Formula (Model):

Applied formula (cj: Correction factors) :

Model’s parameters that are not measured, are assumed based on

collateral measurements: Poisson ration (v), Materials Density (ρ).

Liarakos E.V.


v1

v12v1ρVE 2

pd

m:1j,cv,fρVE j

2

pd

Transmitting

Transducer

Receiving

Transducer



Slide: 18/60


Structures Dynamic Response and NDT

Several NDT methods that rely on different physical principles

have been proposed and implemented [2-3].

Some of the most widespread NDT methods are based in

observation of constructions vibration mode under an

artificial or ambient dynamic excitation [4].

Structural systems exhibit the characteristic of vibrating in a

unique way when they are excited from an external dynamic

load.

Vibrating modes are identified from structure’s dynamic features

and especially from resonant frequencies and corresponding

amplitudes [4].

Liarakos E.V.




Slide: 19/60


Structures Dynamic Response and NDT

Dynamic features are usually time invariant and strongly related

with macroscopic structural properties like modal masses

(inertia), stiffness of constructional members and internal

attenuation of mechanical energy (mechanical damping).

If a mechanical damage occur,

structural properties will change.

Damage related changes will imprinted directly as alteration of

structure’s dynamic features and hence will affect structure’s

dynamic behavior (vibrating mode).

Liarakos E.V.




Slide: 20/60


Structures Dynamic Response and NDT – DVA Signals

The dynamic response of a construction can be captured by recording

time-histories of Displacement, Velocity or/and Acceleration (DVA

signals), in specific control points.

Depending the purpose of monitoring, DVA-signals can be acquired:

a) In constant time intervals. Systematic Monitoring. e.g. A traffic excited bridge.

b) When an excitation incident occurs. Trigger Monitoring. e.g. Earthquake,

Impact etc.

Liarakos E.V.


Typical impact-

related,

acceleration

response signal.

Triggering time

Time (ms)



Slide: 21/60


Structures Dynamic Response and NDT – Control points

Defining control points on a construction.

The proper choice of control points is a vital issue for efficient monitoring.

Control points are ordinarily set in regions that expected high stress

concentration and the hazard to occur a mechanical damage is high.

The construction’s areas that expected to suffer from overloading and

stress concentration can be assessed by employing finite element

analysis (FEA).

Liarakos E.V.


Concentration of compression/tensile stresses

Possible position for control point

installation

FEA modelling



Slide: 22/60


Structures Dynamic Response and NDT – Sensors

Measuring of DVA monitoring data is performed utilizing appropriate

sensors such as accelerometers, geophones (velocity), strain gauges

etc [5].

Liarakos E.V.


Data logging is achieved by employing

Data Acquisition Cards (DAC).

Convert the analog signal of sensors to

digital.

Acquire measurements by applying a

user-defined sampling frequency (Fs)

which is depended from required

resolution of monitoring signals.

Provide the options of data filtering and

signals post-processing.

Accelerometer

installed on concrete member

Piezoelectric patch –

Mechanical Strain Sensor



Slide: 23/60


Structures Dynamic Response and NDT – Fourier Analysis

Dynamic features can be extracted from DVA signals by calculating

the Fourier Response Spectra.

DVA time-histories are discrete signals, as obtained via sampling,

and the calculation of Fourier spectra is performed by Discrete

Fourier Transform (DFT)[6].

Liarakos E.V.


F

j

jnjn txdtX1

i-exp

Resonant frequencies

peaks (pk)

Fourier Amplitude

Spectrum

Time-History

Χn:

ωn :

F:

xj:

dt :

:

Fourier spectrum’s value that correspond to …

… n-th DFT angular frequency, n=1:F/2

Number of signal’s measured values

Signal value in tj sampling time

Sampling time interval

nF

Fn

dtFFdtdtjt S

nsj1

2

1

2,/1,1

Time (ms)



Slide: 24/60


Structures Dynamic Response and NDT – Spectrogram

Determination both of resonant frequencies and of specific time

ranges where each frequency contributes significantly to signal’s form.

Liarakos E.V.


Essential tool in cases where is

required to identified the effect of

a specific excitation incident (e.g.

an impact) in general response.

Discrete Short-Time Fourier

Transform (DSTFT)[7].

F

j

jnmjjmn tttwxdtX1

, i-exp,

Time Window Function

FFdtFtttmt wwwwwm ,,5.01

Fw: Window width(Number of Signal Values)

twtm

x(tj)

x(tj)w(tj,tm)

2

2

2exp, mj

w

mj ttt

attw

Gaussian-type Time Window Function

aexptm: Window Center

tw: Window Length

Time (ms)



Slide: 25/60


Structures Dynamic Response and NDT – Spectrogram

Liarakos E.V.




Time window where high Fourier amplitudes

are appeared in most of examined frequenciesAcceleration

signal

DFT spectrumTime (ms)

Time (ms)Slide: 26/60


Non Destructive Testing and Structural Health Monitoring

Effective SHM demands a time efficient and accurate

estimation of specific structural properties, on certain

control points.

Assessment of materials strength and mechanical properties

Determination of constructional members dynamic features

(Resonant frequencies and Amplitudes).

Damage detection and evaluation

Comparative analysis

System Identification

Multi-point Evaluation - Scanning of Monitoring Area

Liarakos E.V.




Slide: 27/60


Impact Response Methods (IRM)

Tapping surficial a structural member with a hammer-type tool,

usually metallic, and trying to verify the present of flaws from

changes in impact sound, is one of oldest NDT techniques.

Impact Response Methods is a family of NDT methods that are

based on propagation of a hammer-generated (impact) stress

wave in constructional members interior [2,8].

Hammer-impact generates simultaneously shear (S-waves),

compression (P-waves) and surface (R-waves) waves.

Dynamic motion sensors, such as accelerometers, are employed

in recording of monitoring structure’s dynamic response, on

specific control points.

Liarakos E.V.




Slide: 28/60


Impact Response Methods (IRM)

Liarakos E.V.


Data Acquisition Unit (Fs, Filtering etc.)

Impact

Acceleration Response

S-Wave

P-Wave

R-Wave

Accelerometer

Hammer – Piezoelectric

Force Sensor

Concrete Structural

Member

P and S - waves

R-Waves

sr

ds

dp V

v

vV

v

EV

vv

vEV

1

12.187.0

12

1

211

1

Velocity

WavesR

Velocity

WavesS

Velocity

WavesP



Slide: 29/60


Impact Response Methods (IRM) – Impact Hammer

Impact hammers carry spherical heads (tips) which vary concerning

diameter, material elasticity and hardness (metallic, vinyl etc.)[9].

Generated wave frequency depends from radius and material of

hammer tips.

Appropriate tip is chosen for each monitoring case by investigating the

frequency ranges where impact results sharp and clear peaks in

Fourier spectrum of acquired DAV signal.

Liarakos E.V.


Frequency response curves for

different tips[9]

Range of Frequency that

stimulates each hammer tip

Hammer Tip

Impact Hammer - Force piezo-sensor



Slide: 30/60


Impact Response Methods (IRM) – Concrete P-Wave Velocity

Measuring P-Wave Velocity

Liarakos E.V.


Digital Oscilloscope –

DAQ

Hammer

AccelerometerImpact

Point

Accelerometer

Hammer

Accelerometer

Sensors output is voltage

Fs=500kHz

Acceleration and impact force are

calculated via electro-mechanic coefficients

Hammer Coeff: 2.5 mV/N

Accel. Coeff: 1.02 mv/m/s2



Slide: 31/60


Impact Response Methods (IRM) – Concrete P-Wave Velocity

Measuring P-Wave Velocity

Liarakos E.V.


Impact

time

P-wave

Arrival

Time

Dtp

sm

ms

mm

Dt

DV

p

p /4210)(076.0

320

Time (ms)

Time (ms)



Slide: 32/60


Impact Response Methods (IRM) – System Identification

Determination of concrete members dynamic features – Calculation of

DVA signals, Fourier spectrum or spectrogram.

Dynamic response of a structure can be simulated from the response

of an equivalent discrete dynamic system.

Equivalent discrete system consist of mass mj (inertia phenomena),

springs kj (stiffness of structural members) and dampers ηj

(mechanical energy attenuation).

Liarakos E.V.


Structure

Under

Monitoring

(SUM)

m1 k1 η1

mj kj ηj

mM kM ηM

Observed

dynamic

response

Dynamic System

Identification

Equivalent discrete

dynamic system

M-th Mass-Spring-

Damper sector



Slide: 33/60



Structures response simulation based on Impact and Acceleration

acquired signals.

Liarakos E.V.


AmplitudeFourierForceImpact

AmplitudeFourieronAccelerati :

ωFF

ωFAωHFunction Tranfer

Dynamic System

Identification :

FF(ω)=H-1(p,ω)FA(ω)

Output Acceleration FA(ω)

Input Impact Excitation force FF(ω)



Slide: 34/60



Theoretical transfer function derives from equivalent discrete system

response.

M: Number of mass-spring-damper sectors.

p-vector’s optimum values are calculated via minimization of sum of

squared differences (residuals) between experimentally measured and

model-calculated spectra (Least Squares Method - LSM)[11].

Liarakos E.V.


1

0tosubject

min,min

j

1

2

1

2

p

ppp

N

n

n

N

n

n

estmeas

n rHH

M:1jM:1jM:1jj0,jj1,

2

j0,j0,jjj0,

M

j 2

n

2

j1,

22

nj0,

2

nj

n

est

ηkmωηCωC/mkω

ωCωC

ωm1H

p

p

,,

/,

1



Slide: 35/60



System identification contributes to correlation of potential damage

existence with changes of specific set of parameters.

Efficient system identification means

effective damage detection and

interpretation.

Liarakos E.V.


j m(kg) kj(GN/m) ηj Freq0,j(kHz)

⁞ ⁞ ⁞ ⁞ ⁞

2 1.78 0.83 0.09 3.44

3 2.26 1.45 0.07 4.03

4 4.78 5.14 0.03 5.22

5 2.99 7.63 0.02 8.04

6 7.91 27.76 0.02 9.43

7 6.67 33.21 0.01 11.23

⁞ ⁞ ⁞ ⁞ ⁞



Slide: 36/60


Impact Response Methods (IRM) – Concrete Damage Detection

Damaged detection is achieved from comparative analysis between

undamaged and damaged structure frequency response.

PZT Teflon Based Sensors [10-12]

Liarakos E.V.




FFT spectrum of Sensors Response

Undamaged

Artificial Damages

Frequency (Hz)

Digital Oscilloscope – DAQ

Artificial Damages Undamaged concrete member

AMEL Teflon

Piezoelectric

sensor

Impact point

Slide: 37/60

http://www.sciencedirect.com/science/article/pii/S1877705811002700


Impact Response Methods (IRM) – Concrete Damage Detection

Statistical indices for damage existence verification[10-12].

Statistical quantities based on cumulative variance between the

undamaged (Und) and damaged (Dmg) cases related spectra.

Root Mean Square Deviation - RMSD

Liarakos E.V.


N

n

n

N

n

nn

Und

UndDmg

RMSD

1

2

1

2

100%

n=1:N, N : Length of spectra values vector.

Undn and Dmgn, value of spectrum that corresponding to n-th angular frequency ωn.

minmaxmin1

1

N

nn

Sequence of angular frequencies.



Slide: 38/60


Impact Response Methods (IRM) – Concrete Damage Scanning

Scanning dynamic response by applying impact excitation in different

points and keeping constant the acceleration control point.

Evaluation of effect of a shear crack in a concrete beam.

Liarakos E.V.


× × × ××

Scanning Impact Points

Hammer

Accelerometer. Steady response point

Digital Oscilloscope and DAQ

Shear Cracks



Slide: 39/60



Accelerometer FFT response spectra for each impact case.

Liarakos E.V.


Crack is between the

impact point and

accelerometer.

Essential decrease of

FFT amplitude that

derives from mechanical

energy attenuation and

beam stiffness weakness

due to crack existence.

Distance between Impact

point and sensor



Slide: 40/60


Shear Cracks

Freq

uen

cy (

kHz)

Accelerometer


2D interpolation of Fourier spectra. X-axis: Distance from Accelerometer. Y-

axis: Frequency (kHz). Color map: Fourier Amplitude

2D scanning spectrum appears strong decrease of amplitude in left side of

crack (x > 32 cm) depicting the damage of concrete member.

As bigger is amplitude’s reduction so serious is damage effect.

Liarakos E.V.




Slide: 41/60


Impact Response Methods (IRM) – Summary

Artificial dynamic excitation of structure under monitoring. Usually pulse

type waveforms.

Multi-point and single point excitation, depending the monitoring aims.

Acquiring response acceleration signal on specific control points.

Calculation of stress waves velocities. Estimation of mechanical

properties.

Determination of frequency content of DVA signals. Dynamic features

obtaining.

Simulation of structures response and parameterization of dynamic

behavior via transfer functions and system identification.

Scanning of structures dynamic response, detection of damages and

assessment of their severity.

Liarakos E.V.




Slide: 42/60




Slide: 43/60


Necessity of non-contact and remote SHM

In several monitoring cases there are serious restrictions

regarding the accessibility in construction's space.

Bridge pillars and decks

Dam faces

Hazardous and instable environments. Mining tunnels,

Excavations front, Semi-collapsed structures.

Cases where physical contact is forbidden or it is

dangerous.

Moving machine elements

High pressure or temperature industrial plants

Historic frescoes

Monuments structures

Historic marble or granite sculptures

Liarakos E.V.



Slide: 44/60


Monuments structures, Frescoes and Sculptures

Structural integrity control via NDT techniques in most cases.

Generally: Take no material samples and specimens.

Material sampling only from nearby existing fragments.

No contact with monitoring objects. Occasionally regional and limited

contact only for dynamic excitation (PZT actuators, Low voltage Shaker

actuators).

No Impact Excitation.

Point measurement of structures dynamic response is not always

adequate for damage evaluation. Large areas scanning.

Monuments structures are usually built from brittle materials.

Therefore suffer from crack type flaws.

Damage positioning is not enough. Extensive mapping of cracks is

necessary for reliable evaluation.

Liarakos E.V.



Slide: 45/60


Laser Doppler Vibrometry (LDV) – Features

Applications in SHM of heritage constructions and

monuments as vibrations based NDT technique.

Physical principle: Laser Doppler Effect (LDE)

Meets the specific standards that monuments structures and

heritage artifacts demand.

Real time measuring of structure’s surfaces vibration velocity in time

domain (vibration velocity signals).

Obtaining frequency response spectra via DFT (Structural dynamic

features).

Multi-Point scanning of vibrating faces of constructional members.

Mapping the vibrations modes.

Mapping the micro-topography of monitoring surface.

Liarakos E.V.



Slide: 46/60


Laser Doppler Vibrometry (LDV) - Doppler Effect

Doppler effect is the apparent change of frequency of a

mechanical (e.g. sound) or an electromagnetic (e.g. Laser) wave as

it is sensed by an observer…

… because of the relative motion between

observer and wave’s source.

Concerning the type of relative motion the following cases can be

termed (Acoustic waves cases – Observer: Human):

Steady wave source – Moving Observer (e.g. Alert siren of a building)

Moving wave source – Steady Observer (e.g. Police car, Formula 1)

Moving wave source – Moving Observer (e.g. How an observer on a

moving train sense-hear the sound of another moving train)

Liarakos E.V.



Slide: 47/60


Laser Doppler Vibrometry (LDV) - Doppler Effect

Observer

Human ear. Acoustic waves. Sensitivity: From 20 Hz to 20 kHz

(Depends from age and heath condition)

Photo-detector. Electromagnetic waves. In Laser Scanning

vibrometry, optic spectrum. Wavelength λ: 400 (750 THz) – 700 (429

THz) nm

Observer distinguish different waveforms by detecting the

changes in their frequencies.

Waveform frequencies fr are depended from wave velocity c

and wavelength λ.

Liarakos E.V.



cfreq

Slide: 48/60


Laser Doppler Vibrometry (LDV) – Implementation Principles

Laser beam hits and reflected from a vibrating point that can be simulated

as an excited oscillator (mass-spring).

Assume that vibrating point the time that wave is arriving and reflected is

moving away both from laser source and photo detector.

Liarakos E.V.



Vpt(t): Velocity of vibrating point

c: Speed of light, Velocity of Laser (≈3x105 km/s)

Received frequency f2

Reflected frequency f1

(Moving Source – Steady Observer)

Emitted frequency f0

Arriving frequency f1

(Steady Source – Moving Observer)

01 fc

Vcf

pt

12 fVc

cf

pt

Photo-detector

Laser source

Vib

rom

ete

r

pt

Vibrating Mass

Linear spring

tVpt

Vib

rati

ng

Po

int

Slide: 49/60


Laser Doppler Vibrometry (LDV) - Implementation Principles

Vibration velocity of a control point is determined as function

of laser frequency shift Δf between emitted and received

beams[13].

Liarakos E.V.



λ

2Vpt

0

pt

02

0

pt

0

ptVc

0

pt

pt

2

fc

2VffΔf

fc

2V1f

c

2Vcf

c

Vc

Vc

cf

pt

λ

2Vpt 0

pt

02 fc

2VffΔf

Vibrating point

moves away from source

Vibrating point

moves toward source

Slide: 50/60


Laser Doppler Vibrometry (LDV) – Interferometry

Frequency shift between emitted and received (after reflection) laser beam

can be measured using interferometry technique[13].

Interferometry is based on interference phenomenon of emitted and

received laser beams.

Liarakos E.V.



Slide: 51/60


Laser Doppler Vibrometry – Interferometry

Photo-detector is sensitive to laser intensity IL, which is varies

between a maximum value ILC (Constructive Interference) and a

minimum value ILD (Destructive interference).

Frequency shift Δf=g(IL) between emitted and received laser

beams is approximated by measuring the intensity of laser beams

that arrive to photo-detector.

Liarakos E.V.



20

2

1sL EcI

2

tΔfλtVpt

ε0: Vacuum electrical permittivity (8.854e-12 F/m)

Εs: Electrical amplitude of wave that derives after

interference of emitted and reflected laser beam

Slide: 52/60


Laser Scanning Vibrometer – PSV 500H[13]

TUC’s Applied MEchanics Laboratory (AMEL) equipment.

Structural dynamic applications in frequency domain.

Experimental modal analysis

Integrated vibrations measuring system:

Liarakos E.V.



Portable laser head and processing unit.

1D vibration’s velocity measuring.

Laser beam wavelength: 633 nm (red light) – 473

THz EM wave.

Embedded data acquisition system.

Bandwidth: 0 Hz-100 kHz.

Max sampling frequency: 250 kHz

Max FFT points: 12800.

Vibration amplitudes range: 1 mm/s – 10 m/s.

Slide: 53/60


Laser Scanning Vibrometer – Vibration Velocity Mapping

Scanning an area of monitoring structure. Acquiring of vibration velocity

time histories in multiple control points.

DFT transform of velocity signals. Vibration velocity amplitude mapping

for each frequency point of Fourier transform.

Experimental Modal Analysis.

Liarakos E.V.



Fourier response

spectrum on a control

point

Laser Scanning

Vibrometer

Scanning Area Structure under

monitoring Mapping of velocity

amplitude in a specific

frequency point

Slide: 54/60


Laser Scanning Vibrometer – Vibration Velocity Mapping

From velocity signal: a) displacement via integration and b)

acceleration via derivation.

Control points grid derive from the monitoring area discretization

using a FEM-type mesh.

Finite element analysis of monitoring surface and validation of

experimental modes.

Liarakos E.V.



Monitoring Area Discretization-

Monitoring Mesh

Vibrometer experimental analysis

FEM modal

analysis

Displacement mapping in

approximately 6 kHz

Slide: 55/60


NDT evaluation of a cracked concrete beam

A structural system of concrete beams is examined.

The upper beam carries a large shear crack

Dynamic excitation from a shaker device driven by a frequency

generator.

Sinus excitation frequency: 355 kHz. Output voltage: 1 Volt.

Liarakos E.V.



Cracks Area Discretization

Shaker

Slide: 56/60


NDT evaluation of a cracked concrete beam

Snapshots of displacement’s Fourier amplitudes distribution in 355

and 1760 Hz.

Displacement is calculated from time integration of velocity signals.

In cracked or discontinuity regions, displacement’s amplitudes

increase rapidly, reflecting the high mobility due to dynamic

stiffness reduction.

Liarakos E.V.



355 Hz1800 Hz

Vibration Modes MappingCracks zone

Slide: 57/60



4. Summary and discussion

Slide: 58/60


Structural health monitoring is an essential aspect in reliable

inspection of engineering structures.

Non destructive testing methods have crucially expand and

improve the efficiency of structural integrity control and monitoring.

Dynamic response and vibration based NDT methods have been

used in several structural health monitoring cases for damaged

detection and severity assessment of existing flaws.

Impact-Response methods can be employed in construction

integrity control via several different implementations that varies

from point evaluation to dynamic response scanning.

Laser Scanning vibrometry contributes significantly to non-contact

and remote NDT and SHM of constructions and indicated for

monitoring of heritage structures and monuments.

Liarakos E.V.


4. Summary and discussion

Slide: 59/60


References[1]. Bamforth P., Chisholm D., Gibbs J. and Harisson T., 2008. Properties of concrete for use in EuroCode 2. The

Concrete Center Report CCIP-029

[2]. International Atomic Energy Agency, 2002. Guidebook on non-destructive testing of concrete structures. Training

Courses Series No. 17, pages 26-32 Vienna.link

[3]. International Atomic Energy Agency, 2005. Non-destructive testing for plant life assessment. Training Courses Series

No. 26, Vienna link.

[4]. Doebling S.W., Farrar C.R., Prime M.B. and Shevitz D.W., 1996. Damage Identification and Health Monitoring of

Structural and Mechanical Systems from Changes in Their Vibration Characteristics: A Literature Review. Los

Alamos National Laboratory Report, LA-13070-MS link.

[5]. Harris C.M. and Piersol A.G., 2002. Shock and Vibration Handbook. McGraw-Hill, 5th Edition, USA.

[6]. Fourier Analysis Notes: http://ens.ewi.tudelft.nl/Education/courses/et2405/notes/champagne04.pdf

[7]. Short Time Fourier Transform Lectrure: http://research.cs.tamu.edu/prism/lectures/sp/l6.pdf

[8]. Carino N.J., 2004. The impact Echo Method: An Overview. Proceedings of the 2001. Structures Congress &

Exposition, May 21-23, 2001, Washington, D.C. link.

[9]. PCB Impact Hammer; http://www.pcb.com/products.aspx?m=086C03

[10]. Providakis CP, Liarakos EV and E. Kampianakis, 2013. Nondestructive Wireless Monitoring of Early-Age Concrete

Strength Gain Using an Innovative Electromechanical Impedance Sensing System. Smart Materials Research; vol.

2013, doi:10.1155/2013/932568. link

[11]. Liarakos E.V. and Providakis C.P., 2013. A miniaturized early age concrete strengthening and hydration monitoring

system based on Piezoelectric transducers. 10th HSTAM International Congress on Mechanics. May, 25-27 2013,

Crete, Greece. link.

[12]. Liarakos E.V., 2015. Damage detection in concrete structures using “smart” piezoelectric sensors as concrete’s

aggregates. PhD Dissertation, School of Architectural Engineering, Technical University of Crete. link

[13]. Polytec GmbH, 2014. PSV-500 Training Courses. www.polytec.com

Liarakos E.V.


Slide: 60/60

http://www-pub.iaea.org/mtcd/publications/pdf/tcs-17_web.pdf

http://www-pub.iaea.org/mtcd/publications/pdf/tcs-26_web.pdf

https://www.researchgate.net/publication/236398559_Damage_Identification_and_Health_Monitoring_of_Structural_and_Mechanical_Systems_From_Changes_in_their_Vibration_Characteristics_A_Literature_Review

http://ens.ewi.tudelft.nl/Education/courses/et2405/notes/champagne04.pdf

http://research.cs.tamu.edu/prism/lectures/sp/l6.pdf

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.588.1653&rep=rep1&type=pdf

http://www.pcb.com/products.aspx?m=086C03

http://www.hindawi.com/journals/smr/2013/932568/

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.588.1653&rep=rep1&type=pdf

https://eliarakos.files.wordpress.com/2015/09/liarakos_phd_defence_final_english_ver3.pdf

http://www.polytec.com/

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