course 29th and 30th may 2012 duratinetdurati.lnec.pt/pdf/duratinetcourse/9_laurent.pdfcourse...

COURSE Testing techniques for structures inspection 29th and 30th May 2012

Investing in our common future

PART 1 : GENERAL CONSIDERATIONS ON ULTRASOUNDS


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Definition :

Ultrasonic waves are acoustic or elastic waves which frequency is higher than 20 kHz

Waves consist of a disturbance in materials (media) that carry energy and propagate

Waveforms are often represented as sine waves

Ultrasonic waves

DURATINET COURSE - Testing Techniques for Structures Inspection LNEC • Lisbon • Portugal • 29-30 May 2012

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L. Braile, Purdue University

Wave propagation :Different propagation modes exist at different velocities and with different oscillation directions for these waves. The principal ones are :

Longitudinal or P-waves in which the oscillations occurring in the direction of the wave propagation

Ultrasonic waves




Shear waves or S-waves in which the oscillations occurring perpendicular to the direction of the wave propagation

Ultrasonic waves


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Rayleigh waves which are surface waves in semi-infinite solids with elliptic particle motion in planes normal to the surface and parallel to the direction of the wave propagation

Ultrasonic waves



Material

P-Waves (Cp)

km/sS-Waves (C

s)

km/sRayleigh (C

R)

km/sLamb (C

L)

km/s

Steel 5,9 3,2 3 5,1

Aluminum 6,3 3,1 2,9 5,1

Cast iron 5 3 2,7 4,7

Lead 2,2 0,7 0,7 1,2

Concrete 3.9-4.2 2.3-2.5 2-2.3 -

Waves speeds in solids :

The different waves speeds can be roughly estimated from the properties of the solids using the following relations :

Ultrasonic waves

Some values of speed for common material (from Muravin)

S

S


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Modifications during propagation :

Attenuation (energy spreading out and the absorption during propagation). The amplitude change of a acoustic wave during attenuation can be expressed as:

A = A0 exp - z

Where A0 is the amplitude of the wave at the initial location, z the distance from that initial location and the attenuation coefficient of the wave in the z-direction

Dispersion (material heterogeneity and frequency dependence of speed for waves)

Diffraction (spreading of waves interacting with objects of dimension of inclusions, porosity sites)

Scattering (dispersion, deflection of waves encountering a discontinuity in the material)

Adding phenomena : reflection of waves from boundaries and interactions with reflections

Ultrasonic waves



PART 2 : NON-DESTRUCTIVE TECHNIQUES BASED ON ULTRASONICS


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Application fields :

Crack detection in welding zones or fatigue stress concentrations

Thickness measurement of the metal, protection coating or corrosion products

Specific principle :

Emission of a ultrasonic wave (transducer as Emitter)

Propagation within the structure, reflection on defects and production of echoes

Typical defects : cracks, …

Time of propagation of the echo from the wall side of the metallic structure = thickness

PART 2 / Ultrasound methods for steel



Equipment and handlingVarious types of systems are commercially proposed :

Simple systems to measure thickness of metal part

Multichannel systems to detect and locate defects

Ultrasound system for thickness measurement (left) or defect detection (right)(Olympus products)


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Test procedure and results interpretation

For crack detection

Knowledge of damage type as preliminary works

Two main modes : transmission (attenuation) and pulse-echo (reflection)rent

Different types of representation : A-scan, B-San and C-Scan

Principle of flaw detection in pulse-echo mode (A-Scan)



For thickness measurement

Calibration procedure to convert time of flight of waves to thickness measurements

Example of US inspection and linked calibration curve


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Key aspects

For crack detection Advantages:

- Access from only one face of the element for pulse-echo mode

- Good depth of examination

- Very good sensibility of detection

Limitations:- Dependence of detection into the type and geometry of defects

- A care surface preparation and coupling between transducer and structure

- Requirement of a preliminary calibration

For thickness measurement

Advantages:- Access from only one face of the element

- Very good precision of thickness

Limitations: - Care coupling between transducer and structure

- Good preparation of metallic surface that must be without paint or coating

- Operators must be qualified in US technique and in metal structure inspection


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Definition :

Acoustic Emission is a phenomenon of elastic wave emission due to local

deformations in material

Main parameters :

Amplitude, count, rise time, energy, duration of an AE hit which reaches a fixed threshold

PART 2 / Acoustic emission technique


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Acoustic emission for steel

Application fields

Evaluation of damage in metallic elements and cables

Mechanical loading of the structure (lorry on bridge,…)

Equipment and handling

Various kind of commercial systems are proposed

1-2 channel systems using for specific detection purposes

4-128 channels systems used to monitor a full structure with location of defects capacities




Instrumentation by AE sensors of the inspected zone after a surface preparation

Calibration procedure based on standardized Hsu-Nielsen test

Loading of the metal element

Acoustic waves emission from the zone (cracking,…)

Collection of these waves via AE sensors located at several places on the structure

Results interpretation based on :

Correlations between physical damage and AE activity or/and energy

Correlation between other ND techniques, like strain gauges

Classic representation of the results as AE parameters (waves, energy, ) versus time.

Another analysis in frequency domain or by a statistic approach


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Key aspects

Advantages :

Access from only one face of the element

Real time detection or/and remote monitoring

Limitations :

Acoustic waves attenuation though material and mechanical perturbations from the near environment

The inspected damage must be active

Operators must be qualified in AE technique and in metal structure inspection



Acoustic emission for concrete

Application fields

Evaluation of concrete cracking and the detection of rebar's corrosion

Loading of the concrete element

Equipment and handlingSame as AE for steel


Same test procedure and result interpretation as for steel element

Specific analysis to classify active cracks based on a correlation between AE parameters

Values of RA (rise time / amplitude) and Fa (count /duration) as determined from AE parameters

Example of AE set-up for concrete cracking detection


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Key aspects

Same advantages and limitations as for steel

Applications under development

Current research concerns the detection of rebar’s corrosion. The source of AE can be

ferrous oxide cracking or concrete cracks at the rebar / concrete interface



Acoustic emission for monitoring

Application fields

Detection of wire rupture occurring in prestressed structures or others cable bridges

Permanent acoustic monitoring

Location and identification of wire failures in real time

Specific principle

Acoustic waves are emitted when a wire or a cable break

Occurrence and location of damage possible within cm range during the monitoring period

Equipment and handling

Several systems are commercially proposed (IFSTTAR, Advitam, Mistras group,...)

Two main methods : pre-filtering of the breaks detection or transmission to a remote data center for analysis


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IFSTTAR system to detect wire breaks on bridges based on a pre-processing of rupture data

Results of the acoustic monitoring of “pont neuf” prestressed bridge



Test procedure and results interpretation (for IFSTTAR system)

Locations of sensors roughly follow the plan of the prestressing strands position

Information from and to each sensor gathered by a central data unit which allows a remote control and data transmission

Detection thresholds for monitoring sensors adjusted with the background noise

Result interpretation based on :

Different parameters (location, rate of propagation,amplitude of waveform)

When an acoustic event reaches these parameters = a structural alarm generated

Automatic structural alarm

Example of acoustic monitoring sensors placed on lateral face of a prestressed beam


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Key aspects

Advantages :

Remote monitoring of wire breaks in real-time

wire break liberate an acoustic wave of great energy easy to detect

Limitations:

Accuracy of the rupture location depending on the sensitivity of the sensors, spacing of the sensors, the distance between the sensors and the event location

The wire ruptures monitoring system must be operational at any time

Signal analysis/interpretation is a specialist item

Applications under development :

Current research on wi-fi, GPS, energy independent possibilities for IFSTTAR system


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course 29th and 30th may 2012 duratinetdurati.lnec.pt/pdf/duratinetcourse/9_laurent.pdfcourse...

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