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Ultrasonic Inspection technique using multi-element probes (phased array): application to tube inspectionB BISIAUX
Manager NDT/STAT Vallourec CEV
The material used to manufacture the active element is a piezo-composite. This is a material composed of ceramic elements organized in a polymer matrix which is piezo-electronically inert. This configuration means that the acoustic properties are modified in such a way as to enhance the performance of the ultrasonic inspection system.
With this technique focalization, deflection or electronic scanning are possible.
To obtain a focalization effect, the adjusted delays remain in phase with respect to the focalization point and are annihilated elsewhere. By precisely controlling the transmission delays, it is thus possible to focalize the wave on a point.
Electronic deflection consists of directing the ultrasonic beam in a specific direction, again using time-delay laws. This principle makes it possible to deflect and focalize the beam at the same time.
Electronic scanning is based on the following property: not all the elements of a multi-element probe are active at the same time. Only part of them are used for focalising and forming what is known as a "virtual probe". Furthermore, it is possible to change the delay law for each element in order to change the angle of incidence for example.
These electronics are characterised by the number of elements they can control, and the number of elements on which they can construct ultrasonic beams (128/16-128/32-128/2*16), the delay pitches on transmission and reception (1-5 or 10 ns)
This technique however, (in the case of encircling coils) requires a very precise tube guiding system (mechanical to 2.5 tenths, with tube sag taken into account). Calibration is also more complex. The tube is covered over 360° by several virtual probes, while conventional calibration only tests a number of virtual probes. Furthermore, the cost of the electronics and the probes remains high. The "electronic scanning" technique must also be well mastered as it defines the most efficient point density.
1. Application of the Phased Array technique to flaw inspection, longitudinal/transverse inspection in the case of tubes with linear advance.
The idea here is to use an encircling multi-element probe (in one or several parts) for longitudinal inspection. For transverse inspection, it may be necessary to fit a plexiglass shoe to provide the necessary mechanical deflection and focalization for detecting transverse flaws. The advantage of this system is that the same probe is used for detecting longitudinal and transverse flaws. The shoe may be adapted to tubes in the 10 to 90 mm range, with wall thicknesses varying from 0.5 mm to 15 mm. The inspection is conducted between the two critical angles using longitudinal and transverse waves.
As in order to inspect the entire tube using the electronic scanning technique, the probe has to encircle the tube, the machined shoe must also encircle the tube concentrically. Furthermore, as the inspection concerns transverse flaws, the ultrasonic beam is deflected to strike the steel at a 17° angle (this gives good results on reception). To improve the results, it is possible to focalize the beam at the same time as it is deflected.
We may therefore imagine a curved surface for the shoe at the plexiglass-water column interface.
Some echos. 32 element probe, 5 MHz, 0.6 mm pitch, on tube (37.7;3.64)
Transverse flaw detected directly
Transverse flaw detected by skip
Tube diameter (mm) 20 < OD < 90
Wall thickness (mm) 3 < WT < 171
Number of elements: 128
Inspection pitch 70 mm per rev
The axial resolution and efficient point density are calculated and represented in graph form (e.g. according to the axial resolutions) cf. graph below.
Application of the Phased Array to weld inspection
The phased array technique may be used to detect flaws located in a zone of ± 15°. The multi-element probe used is concentric to the tube to provide a constant water column. During the inspection the probe is fixed. An angle of incidence i is imposed (16° or 17°) from which is deduced the electronic angle of orientation a :
Application of the Phased Array technique to rotary inspection of tubes
This technique used with special electronics allows flaw detection at industrial rates regardless of the length and/or orientation of the tubes.
Research in this technique at the CEV to date gives us entire confidence in this method when used on our tubes (cold drawn or not). The following diagram shows a type inspection unit with the standard probe used in our plants (frequency 2 or 4 MHz, element width 0.6 to 1 mm, number of elements > 64).
|Flaw at 15°||Flaw at 0°|
Simulations and tests on pre-prototypes to extend this technique to both spiral and linear feed tube inspection are most encouraging as to the potential improvement in productivity and in the selectivity of our ultrasonic inspections.
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