INSPECTION THROUGH AN OVERLAY REPAIR WITH A SMART FLEXIBLE ARRAY PROBE. Ph. Brédif (*), G. Selby (**), S. Mahaut (*), O. Casula (*) (*) CEA/DRT, Saclay, France (**) EPRI, USA 1. ABSTRACT Contact inspection of welds on steel pipe can be drastically disturbed by the irregular surface. These specific conditions can come from an overlay repair covering entirely the weld. This overlay presents an irregular state of surface on the outside diameter of the pipe that strongly limits the use of conventional ultrasonic probes in a contact configuration. Further to the works carried out by CEA on a new concept of contact flexible probe (TCI), EPRI asked CEA to estimate the feasibility of using such a technique to perform the inspection through a representative overlay surface. The principle of this method is based on the use of a smart flexible linear phased array integrating a specific instrumentation. Such a probe is able to fit the inspection surface, optimizing the coupling conditions and preserving the orientation of the ultrasonic beam through the irregular surface. This paper deals with the design of such a transducer. This study has involved the definition of a number of mechanical constraints to combine with acoustical ones. Acoustical properties of the selected transducer have then been simulated and appreciated by means of the CIVA software developed by the CEA. Moreover, an experimental validation has allowed detecting a defect through a realistic irregular profile by using this kind of transducer. Experimental and theoretical studies are presented, that shows the feasibility of such a control. 2. INTRODUCTION A number of circumferential welds on steel pipe of US nuclear reactor cannot be completely inspected with conventional Non Destructive Testing methods. The special conditions of this inspection come from an overlay repair covering entirely the weld. This overlay exhibits an irregular state of surface on the outside diameter of the pipe that strongly limits the use of classical ultrasonic probes in a contact configuration. Further to works carried out by CEA on a new concept of contact flexible probe, EPRI asked CEA to estimate the feasibility of using such a technique to perform the inspection through the overlay surface. The inspection must cover the overlay thickness and the first 25% of the base metal around the weld. This article presents some results obtained in the framework of this feasibility study carried out with NDT simulation tools.
First, after having detailed the context and the objective of this study, the smart flexible phased array concept is presented. Second, configurations of inspection are detailed for each kind of flaws that have to be detected into the overlaid steel pipe. Then, some results of ultrasonic field simulations obtained with CIVA during the design stage of a new flexible phased array are presented. Finally, experimental result obtained with an existing smart flexible phased array for inside surface breaking notch detection is presented. 3. CONTEXT AND OBJECTIVE Some components of nuclear reactor are covered by an irregular overlay repair. Most of UT inspection for these components are performed using conventional contact probes. Unfortunately, the irregular overlay profile lead to bad coupling conditions. As a result, the ultrasonic beam transmitted through such an irregular surface is drastically degraded. Following image illustrates such a kind of component repaired by an irregular overlay. Irregular overlay repair Steel pipe Figure 1. Example of steel pipe covered by an irregular overlay repair
In the context of that study, a representative mockup has been provided by EPRI to CEA. As we can see on the following image, this steel pipe contains a circumferential weld covered by an irregular overlay repair. 600 mm Ø 273 mm 210 mm Steel pipe representative of a nuclear reactor Figure 2. Mockup provided by EPRI Irregular overlay repair In order to illustrate typical problems encountered during inspection, simulations have been performed using CIVA software tools. The first simulation shows an ideal coupling condition through a flat specimen. In that case, the ultrasonic beam is optimally transmitted. Figure 3. Simulation of ultrasonic field radiated through a flat surface The second simulation presents the same conventional transducer used to control the mockup. In this example, the profile has been measured along one axis of the pipe and digitized before to be used into CIVA. In that case, we observe that the coupling is variable, and it becomes impossible to predict the ultrasonic beam behavior.
Overlaid component variable coupling ANGLE? -5dB Ultrasonic beam drastically degraded Figure 4. Simulation of ultrasonic field radiated through an irregular surface 4. THE SMART FLEXIBLE PHASED ARRAY CONCEPT To solve this problem, CEA has been developing a new method able to control such an irregular geometry since few years. A new instrumentation has been developed based on three main key points: optimal coupling conditions, phased array techniques and real time measurement of array elements positions. Optimal coupling conditions are ensured by a flexible transducer able to fit the irregular overlay profile and to transmit a maximum of energy trough the surface of inspection. Nevertheless, this condition is not enough to ensure the control of the beam orientation. For this reason, phased array techniques are used to control the produced beam by adjusting delay laws. This technique involves knowing, for each transducer position, the distortion of the phase array. This can be achieved using a system able to measure in real time the displacement of each element. Following picture illustrates an example of ultrasonic beam transmitted through an irregular profile. This simulation has been made using Civa.
Optimal delay law Smart flexible phased array time Beam oriented at 45 10mm Figure 5. Illustration of the smart flexible array principle In this example, the transducer produces 45 longitudinal-waves focused at 10 mm under the surface of inspection. We observe that the generated beam is correctly produced through the irregular surface of inspection. Sensitivity and beam angle are similar to results obtained on the flat specimen. 5. DESIGN OF A DEDICATED SMART FLEXIBLE PHASED ARRAY. In the framework of this study, a new dedicated phased array has been designed. This instrumentation has to be able to detect three different kinds of flaw. For each kind of flaw, a specific mode of inspection has been defined. Following part illustrate expected types of flaw into the mockup and corresponding mode of inspection. 5.1 Inspection configurations 5.1.1 Inspection of flaws parallel to the surface Figure 6 below shows the configuration of inspection for defects embedded into the overlay and parallel to the surface. In that case, the transducer is used in pulse-echo mode to generate 0 Lwaves. The inspection of defects located very close to the surface could be performed using a reduced number of elements.
O L waves 25 Flexible Array Overlay 0 L Base metal Flaw Figure 6. Configuration for inspection of flaws parallel to the surface of inspection 5.1.2 Inspection of the outer 25% of the base metal Following figure 7 displays the configuration of inspection for defects embedded into the base metal. The transducer is used as pulse-echo transducer to generate longitudinal waves. Considering these defects orientations, refracted angles have to vary from 45 to 60. Steering beam 25 Flexible Array L or S waves Overlay Base metal Flaw Figure 7. Configuration for inspection of flaws parallel to the fusion line
5.1.3 Inspection of flaws perpendicular to the surface Defects embedded into the overlay and perpendicular to the surface of inspection can be detected by a TOFD (Time Of Flight Diffraction) configuration. Several elements of the transducer are used as transmitters and others as receivers. The transmitter part generates 45 L-waves and receiver part detects diffraction echoes from edges of defect. Here, the phased array is just symbolized by its elements. Blue ones represent transmitters (Tx), red ones represent receivers (Rx). TOFD Flexible Array Tx Rx Overlay Flaw Base metal Figure 8. Configuration for inspection of flaws perpendicular to the surface of inspection 5.2 Determination of the new dedicated transducer parameters The design of this new flexible phased array has been performed using Civa software tools. Thanks to simulation tools, it is possible to predict the ultrasonic field radiated by a given transducer. For the design of a phased array, it is thus possible to know the effect of each parameter on the field. Main parameters are: nominal frequency, pitch of the array, element size, number of active elements. In each configuration, it is possible to compute the ultrasonic beam by applying different delay and/or amplitude laws. Several simulations have been performed through a plate specimen as well as a complex profile extracted from the mockup. Following images show examples of ultrasonic field radiated by the same phased array. In that case, the nominal frequency is 4 MHz and the maximal number of active elements is 24.
Appropriated delay laws have been defined in order to generate 0, 45 and 60 L-waves through the surface of inspection. In the case of the complex profile, these delay laws take into account the mechanical distortions of the phased array on the specimen. 0 Longitudinal wave simulation Flat surface Realistic profile 45 Longitudinal wave simulation Flat surface Realistic profile 60 Longitudinal wave simulation Flat surface Realistic profile Figure 9. Simulation of ultrasonic fields radiated by the new flexible phased array. Left: simulation through a flat surface; Right: simulation through a realistic profile. We first observe that the designed phased array transducer is able to correctly generate modes required for the inspection on a flat specimen (0, 45 and 60 longitudinal waves). Moreover, simulations carried out on the realistic profile show that this transducer is also able to produce same required ultrasonic fields through the overlay repair. Thus, using NDT simulations tools of CIVA, it has been possible to design all parameters of the new smart flexible phased array according to required configurations of inspection. It has also been possible to evaluate its acoustical performances through a realistic irregular surface.
5.3 Experimental feasibility study The second part of this study consists in the evaluation of the mechanical behavior of an existing smart flexible phased array through the mockup provided by EPRI. This mockup contains several surface breaking flaws embedded into the base metal, parallel to the circumferential fusion line. A 2 MHz smart flexible phased array has been used to detect these flaws. 24 elements 20 mm 47.3 mm Figure 10. Illustration of the new flexible smart phased array Following picture shows the transducer during an acquisition on the mockup. The probe is moving along the axis of the pipe. 2 MHz smart flexible phased array Figure 11. Inspection of the EPRI s mockup by the existing smart flexible phased array Figure bellow shows an example of inspection result. A white line symbolizes the inner profile. The transducer is configured to generate 60 Shear waves focused on the backwall. On this true Bscan, we can observe the corner echo of a surface breaking notch. In that case, the signal to noise ratio is about 4 db.
32 mm Depth (mm) 15 mm Inner profile 60 Shear Waves Crack depth = 22.5 mm 0 Scanning (mm) 145 mm Figure 12. Example of true Bscan acquired with the existing smart flexible phased array These experimental tests have shown the possibility to use a smart flexible transducer for the EPRI s mockup inspection. Coupling conditions are always fulfilled and delay laws are correctly calculated taking into account surface inspection irregularities. 6. CONCLUSION The study exposed in this article concerns the development of instrumentation for the inspection of a steel pipe with an irregular overlay repair. It has been demonstrated that a conventional contact probe is not efficient to inspect such a varying profile due to bad coupling conditions. Further to works carried out by CEA, a solution to inspect such specimen can be given by a smart flexible phased array. In that way, a new 4 MHz TCI has been designed using CIVA software tools. Acoustical field simulations have shown the possibilities for this probe to radiate required modes of inspection: 0, 45 and 60 longitudinal waves. Furthermore, it has been checked that acoustical characteristics of the ultrasonic beam produced by the smart flexible phase array are preserved through the irregular profile. First results have been obtained with an existing 2 MHz smart flexible phased array. It has been experimentally used to inspect a representative mockup provided by EPRI. It has been possible to detect inside surface breaking notches. These results have then demonstrated that such instrumentation is useful for UT inspection through an irregular overlay repair. The designed smart flexible phased array is now under realization. It will be used on a new realistic mockup including flaws into the overlay and the base metal.
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