·Table of Contents
·Methods and Instrumentation
Ultrasonic Chain Influence on the Materials Non Destructive Testing
Badidi Bouda K. Alem R. Matoug A. Benchaala
Laboratoire de Caractérisation et d'Instrumentation
Centre de Soudage et de Contrôle, Route de ély Brahim, B. P. 64 Chéraga (Algiers) ALGERIA
Tél. & Fax. : (213) (2) 36 18 50 ,Email: abadidi hotmail.com
One of the huge problem met during industrial facilities ultrasonic testing, is the equipment used influence, that, can affect the testing results.
This is very annoying, because the ultrasonic technique is preferred (from the others), to detect plan defects which are the most harmful.
In ultrasonic technique the defects are not shown, as in radiographic technique, but deduced from electrical signals.
An adequate analysis of these signals allows detection, localization and identification of defects as well as taking decision on the defect acceptability in comparison with the codes and standards in force.
In this work, we describe an experimental installation built up at the instrumentation and characterization laboratory and which allows to quantify the equipment chosen and theirs parameters influences on the ultrasonic testing quality.
This installation, piloted by a computer, allows to measure the different parameters influencing the testing such as generator, receiver, cables and transducers electrical parameters as well as the spatial distribution of the acoustical pressure generated by the immersion transducers used in automatic testing and the contact transducers used on plants when testing welded pipes.
To command measurement apparatus, for signal processing and for a data bank creation, a computational program has been developed.
These data allow, on the one hand, to detect any modifications in the parameters due to a deterioration or to an electronic drifts, and on the other, to realize a computational simulation program allowing to choose the best testing chain configuration for a given defect.
In ultrasonic technique the defects are not shown, as in radiographic technique, but deduced from electrical signal, which is called an echo signal. The testing quality (detection, precision in the positioning and sizing of the defects and possibility to identify the defects ) is therefore conditioned by that instrumentation used. The great number of characteristics entails an important dispersion in testing results. Therefore, the operator have to know in one hand , each element performances he uses, and in the other hand, the influence of each of its characteristics on results. This will allow him to find the best configuration of the testing chain so as to reply to a given problem.
We have therefore undertaken to develop techniques, assisted by computer, to measure temporal, frequential and acoustical characteristic of testing chain elements in order to reduce error risks of manipulation and to simplify the task to the operator. The measured characteristics are stocked in a data bank in order to be used for :
This will have to allow:
- to know if a transducer is blind or faulty
- the verification of transducer characteristics and their invariability
- to anticipate rectifications induced by the replacement of one element of the system
- to serve as data in the chain functioning simulation program
- to increase the probability of defects detection
- to increase the precision in the estimation of defects dimensions
Ultrasonic testing chain description
The ultrasonic testing chain (fig.1) is generally constituted by :
- a generator of excitation signals
- one or several ultrasonic transducers
- one or several coaxial cables
- one receiver amplifier
Fig 1: Testing chain scheme|
The generator delivers an electrical signal that is transferred to the transducer transmitter which converts the electrical energy in a mechanical energy (or ultrasonic wave). This wave by propagating in the sample to be tested, reflects and refracts on each defect.
The wave collected by the receiver transducer is converted in an electrical signal that will be amplified, visualized and interpreted.
The interpretation, accordingly to standards and specifications of testing will allow the operator to reject or accept the piece before or after an eventual reparation.
The generator transmits an excitation signal to the ultrasonic transducer. The characteristics that affect the testing are temporal and frequential characteristics of the excitation signals as well as electrical characteristics of the apparatus and this, for each adjustment.
- Temporal characteristics:
- amplitude of the pulse excitation
- time of climbing of the pulse excitation
- width of the pulse excitation to -6 dB
- Frequential characteristics
- spectrum bandwidth to -3 dB
- spectral energy density.
damping impedance (adjustable impedance in parallel with the burden cable - transducer ).
- Electrical characteristics :
The coaxial cable
Coaxial cables will be characterized by :
- their impedance.
- their electromagnetic wave attenuation coefficient
The amplifier receiver
The amplifier receiver is characterized by :
- its electrical impedance in admission
- its frequency bandwidth
The ultrasonic transducer
The ultrasonic transducer is generally characterized by:
- the echo signal and its frequency spectrum
- the resonance frequency
- the bandwidth to -3 dB
- the spatial distribution of the acoustic pressure in the propagation media
Testing chain characterization, 
We have developed techniques, assisted by computer, to measure temporal, frequential and acoustic characteristics (only for the transducers) of each element of the testing chain. We describe in the following some examples.
Electrical impedance measurements
An impedance-meter is used to measure the module and phasis or real and imaginary parts of the impedances for frequencies range from 0.2 MHZ to 13 MHZ. Its exit GPIB (IEEE 488) allows its order by the computer and the stocking of results. This installation allows to characterize generators as well as transducers (fig.2). The impedance meter allows us to have supplementary inquiry on characteristics transducers. Indeed the real part (fig. 3-a) allows to determine the resonance and the anti-resonance frequency of the transducer and the imaginary part, (fig. 3-b) allows to check that the transducer (ref. WB70-2) contain an adaptation circuit in order to limit losses to the surroundings of this frequency.
Fig 2: Experimental device of electrical characteristics |
Fig 3a: Transducer electrical impedance
Fig 3b: Transducer electrical impedance |
The experimental device (fig.2) permits the electrical characterization of generators, receivers and transducers. The generators characterization allows us to have the echo signal and its spectrum for different values of energy (graduated at 0 to 4) and damping (graduated at 0 to 10).
| Fig 4: generator electrical characteristics|
Indeed the energy (fig.4 & 6), contrarily to the damping (fig.5 & 6), increases the excitation pulse but narrowed the frequency range of the generator. It is necessary therefore to choice a compromise between these two parameters according to whether we desire to have a good sensitivity or a good resolution during the detection of defects in a piece.
| Fig 5: generator electrical characteristics
| Fig 6: generator electrical characteristics
The characterization of the transducers is made by pulse method. This allows us to have by an alone measuring, the behavior of the transducers in the range of frequency selected. The pulse excitation is delivered by a generator. The signal emitted by the transducers is reflected by the rear face of a standard sample (fig.2). A numerical oscilloscope permits to obtain digital signals of 512 points for temporal windows of 5 µsec. The elaborate program allows then the acquisition of this temporal signal by the computer. After the acquisition and the temporal characteristics computation , An algorithm of fast Fourier transform (FFT) computes the spectrum and gives the result with a step frequency of 0.2 MHZ.
| Fig 7: electrical characteristics of transducer (ref.WB70-2)
| Fig 8: Electrical characteristics of transducer ref.(WB70-2)
| Fig 9: electrical characteristics of transducer (ref. WB70-2)|
The echo signal measurements (fig.7 , 8 & 9) allows to determine the real value of the frequency of work of the transducers (2.03 MHz) while the constructor gives the value of 2 MHz for transducer WB70-2 . More we can study the influence of energy of the pulses and the damping impedance: Indeed, the energy, contrarily to the damping, increases the echo signal but narrowed the frequency range of the transducer . It is necessary therefore in compromises, like with generators, between these two parameters according to whether we want to have a good sensitivity or a good resolution during the detection of defects in a piece  and .
Spatial distribution measurement of the acoustical pressure delivered by the immersion transducer
The acoustic characterization consists in determining the spatial distribution of the acoustical pressure in the field radiated by the transducer. The spatial distribution is obtained by measuring the reflected echo amplitude on a marble situated in the propagation media (water). Marble dimensions are sufficiently small so as it should be regarded as a punctual reflector.
Acoustical pressure evaluation
For a planar and circular transducer the acoustical potential at an M point of the radiated field in a fluid (density ro) is given by Rayleigh integral  :
|F ( M ,t ) = v (t)Ä ÆiE ( M , t)
| R2 = (x - xo)2 + (y - yo)2 + (z - zo)2
|P ( M ,t ) = v (t)Ä PiE ( M , t)
The chain measurement (fig. 10) consist of the following elements:
- an immersion vat with a micrometric displacement system
- two step by step engines
- a punctual reflector (marble)
- an ultrasonic apparatus
- a transducer (element to characterize)
- a metallic ball displacement command and results measurements acquisition computer
Fig 10: synoptic of the chain acoustical
Experimental elaboration: To determine the geometrical shape of the acoustical beam, an horizontal scanning has been adopted
The manipulation aim is to represent the acoustical beam sections cut, perpendicularly to its ultrasonic waves theoretical propagation axis. Selected sections cut distances are taken so as near and far field are included. For all points the echo amplitude is stocked in the memory computer for further processing.
Spatial distribution measurement of the acoustical pressure delivered by the contact transducer
The contact transducer acoustic characterization is more requested than immersion transducer characterization by the manufacturers because these transducers are much more used when testing on site.
To find out this spatial distribution we have conceived and built up a special standard sample in which 2.5 mm holes diameter were drilled.
The technique consist on measuring the reflected holes echoes amplitude at different depth and with 0.5mm step displacement from one side of the hole to the other.
The measurement had been taken another time at different orientations of the transducer relatively to each hole.
This experimental work allows for an ultrasonic application in non destructive testing or in materials characterization to:
Transducers sustaining an invisible deterioration  are recognized thanks to fluctuations brought on values of electrical impedance among others. The knowledge of the spatial distribution of the acoustical pressure delivered by the transducer allows to improve the precision in the defects localization in the propagation media. This work shows that the knowledge of the transducer characteristics is indispensable to improve the ultrasonic testing quality. This survey finds its application in the ultrasonic nondestructive testing of welded joints in pipelines that CSC makes on behalf of the industry oil in Algeria.
- foretell the appropriate characteristics of each element of the testing chain and to choose the convenient elements (transducers, cables...).
- adjust the generator and receiver parameters, to obtain a compromise between the sensitivity and the resolution required.
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