7 th Asia-Pacific Workshop on Structural Health Monitoring November 12-15, 2018 Hong Kong SAR, P.R. China Research on a Transmit-Receive Method of Ultrasonic Array for Planar Defects Zhenggan Zhou 1,2,3 *, Wentao i 1, Yang i 1 More info about this article: http://www.ndt.net/?id=24038 1 Beihang University, Beijing 100083, China Email: zzhenggan@buaa.edu.cn; liwentao@buaa.edu.cn; lyxml19890327@126.com 2 State Key aboratory of Acoustics, Institute of Acoustics, Chinese Academy of Sciences, Beijing 100190, China 3 The Collaborative Innovation Center for Advanced Aero-Engine (CICAAE), Beijing 100083, China KEYWORDS: Ultrasonic Array; Transmit-Receive Method; Planar Defects; Finite Difference Time Domain Method ABSTRACT The detection results have a strong dependence on the relative position of the transducer and planar defects. Therefore, detection of planar defects for complex structures has always been a difficult point in ultrasonic testing. In this paper, a transmit-receive method based on dual ultrasonic array transducers is put forward and described in detail. The influence of different positions of transmitting and receiving transducers on the planar defect detection method is analysed. Then, according to the structural characteristics of the specimen, a full coverage inspection scheme for butt weld of complex structure is established through the design of acoustic beam transmitting and receiving method. Combined with the flexible beam control method, the transmit-receive method of ultrasonic array can cover a larger detection range without moving the transducer, and the propagation distance of acoustic wave is shorter which can reduce the attenuation of acoustic energy. Additionally, a response simulation model of the ultrasonic array is established based on the finite-difference time-domain (FDTD) method. The echo signals of different wave modes are compared to optimize the focusing law. Finally, for the application on actual components, a set of transmitting and receiving transducers is designed for the ultrasonic array board that had 128 individual transceiver channels. The butt weld specimen with planar defects was fabricated and the practical experiments were conducted as well. The results show that the transmitreceive method of ultrasonic array can provide more accurate detection results in comparison with the conventional ultrasonic testing method using single transducer and greatly enhance the efficiency in planar defects detection. 1. Introduction Advanced welding technologies such as diffusion welding and friction welding are more and more widely used in aerospace and other important industrial fields [1]. Due to the influence of welding process and surface condition, the weld interface is prone to planar type defects [2]. Different from the volumetric defects such as hole and slag inclusions, the reflected acoustic waves of the planar type defect have a strong directivity. The defect reflection wave may not be received by the transducer when the incident beam axis is not perpendicular to the planar defect. For the detection of such defects, an ultrasonic transducer can be arranged perpendicular to the welding interface for testing [3]. However, the actual welding structure is usually complex, and the ultrasonic transducers are difficult to maintain perpendicular to the welding interface or within an effective detection range. * Corresponding author. Creative Commons CC-BY-NC licence https://creativecommons.org/licenses/by/4.0/
Recent years, ultrasonic array technology is widely used in the detection of complex welding structures due to its high detection resolution and flexibility [4]. Computer technology is used to control the delay time of transmitting or receiving ultrasonic waves of array elements, and then the imaging detection is realized by deflecting and focusing of the synthetic acoustic beam. Using a single ultrasonic array transducer, two or three-dimensional imaging in a certain area can be achieved without moving the transducer [5]. In order to achieve high accuracy and high efficient detection of planar defects in complex structures, A transmit-receive method based on the dual ultrasonic array transducers is proposed to achieve the overall detection of the turbine rotor diffusion weld. At the same time, simulation and experimental results verify the feasibility and correctness of the proposed method. Through the design of the beam transmitting-receiving scheme, this method has a good application prospect in the detection of planar type defects in complex structures. 2. Transmit-Receive Method of Ultrasonic Array 2.1 Theoretical analysis The relative location of the planar defects and acoustic source have great influence on the detection results. The amplitude of the defect echo received by the transducer is the largest when the axis of the acoustic beam is perpendicular to the planar defect. When the defect deviates from the centre axis of the acoustic source, the amplitude of the defect echo will decrease as the incident angle increases, and the degree of the decrease is related to the size of the defect and the directivity of the echo. Fig. 1 shows the reflection of the acoustic waves when the defect deviates from the central axis of the acoustic source. For planar defects, the amplitude of tip diffraction signal is far lower than the amplitude of reflected signal. However, the reflection signal is difficult to be received when a single transducer is used for detection. Therefore, a new detection method is needed to avoid defects missing in detection. As shown in Fig. 1, the angle of deviation between the defect and the line axis of the acoustic source is, and is the incline angle of the planar defect. It can be obtained that the incident angle at the centre of the planar defect is.the echo acoustic pressure of the defect can be expressed as [6] : sin[ kcsin( )] 2 Pt = r p0d ( )cos( ) kcsin( ) Here, r is the acoustic pressure reflectance of longitudinal waves at the interface between air and workpiece, p 0 is the acoustic sound pressure at normal incidence, D is the directivity coefficient of the acoustic source beam, k = 2 /, is the wave vector of the acoustic wave and the is wavelength. Therefore, the orientation of planar defect and the transducer position should be fully considered before designing the detection scheme. (1)
Acoustic source θ Tip diffraction signal Reflection signal γ-θ γ Planar defect Figure 1. Schematic diagram of acoustic reflection of planar type defects deviating from the central axis of acoustic beam 2.2 Design of detection scheme As shown in Fig. 2, The turbine rotor is a typical complex structure in which the turbine disk and the integral blade ring are connected by diffusion welding. In order to ensure the full coverage detection of the welding area, a plurality of conventional ultrasonic transducers with different angles is usually required for combined detection. On the other hand, the detection results of single channel transducer have poor signal to noise ratio and low detection efficiency. To improve the detection accuracy and reliability of planar defect, a new detection scheme based on ultrasonic array is established. Diffusion welding interface Figure 2. Turbine rotor sample and the diffusion welding interface. It can be seen from the Formulae (1), the acoustic pressure of the defect echo is the largest when. Therefore, it has strong reflection signals in a certain depth range when the transducer deviates from the smaller angle to detect the vertical planar defect. The reflection signal of the planar defect can be better received if the dual ultrasonic array transducer is used. In addition, the ultrasonic beam control technique can be used to improve the detecting efficiency and resolution. The schematic diagram of the detection scheme for the weld of the turbine rotor is shown in Fig. 3. An ultrasonic array transducer is placed on the upper and lower surfaces of the turbine disk respectively. The full coverage detection of the whole diffusion welding seam is realized through the exchange transmitting and receiving transducer.
Transmitting(Receiving) transducer Planar defect Welding interface Receiving(Transmitting) transducer Figure 3. The transmit-receive detection scheme of ultrasonic array for planar defect of turbine rotor. The transmit-receive method of ultrasonic array has the advantages of short propagation path and energy concentration, and it has high sensitivity to planar defects. However, the incident wave will produce waveform conversion at the defect, which will affect the evaluation of the detection result. Fig. 4 depicts two possibilities of mode conversion of longitudinal waves and shear waves [7]. Part (a) shows the possible reflected waves produced from a longitudinal incident wave at angle. When the incident angle increases, both longitudinal and shear waves are always reflected. Part(b) illustrates an incident shear wave at angle S. In this case, the longitudinal wave will disappear with the increase of the incident angle. The incident angle of the acoustic wave is large enough that only the reflected shear wave at the planar defect interface. Therefore, in this scheme, the shear wave is focused and received for detection. S S S θ θs (a) Figure 4. Mode conversion of reflected waves: (a) general input; (b) general S input. 3. Simulation and Experiment 3.1 Simulation validation A two-dimensional FDTD model has a mesh size of 0.02mm was developed to verify the detection scheme. The simulation time interval is set to be 1 ns and the total simulation time is 25 s. A 3-cycle, pressure-loaded pulse with a centre frequency of 5 MHz is used as the excitation signal for each array element. Full details of the transducer parameters are shown in Table 1. A long rectangle of 1 mm in length and 0.2 mm in width was prefabricated at the centre of the weld to simulate planar defect. The material of the turbine disk is a nickel-based powder superalloy with a longitudinal wave velocity of 5980 m/s and a shear wave velocity of 3138 m/s. (b) Table 1. Parameters of the ultrasonic array transducers Parameters value Frequency 5MHz Elements number 32 Elements pitch Elements width 0.4mm 0.3mm
According to the shear wave focusing method of the detection scheme, the propagation process of the synthetic beam and its effect result on defects are shown in Fig. 5. At the initial time Fig. 5(a), the elements are stimulated by a specific delay law, and the transmitting transducer excite longitudinal() and shear(s) waves. Part (b) shows that the reflected shear wave S d is generated when the focusing shear wave is incident on the defect surface, the remaining shear wave is S r, and r is the longitudinal wave reflected by the side wall. Finally, the shear waves of defect reflected S d and the sidewall reflected S r are received at different times by the receiving transducer. S Defect S d S r r S d S r (a) (b) (c) Figure 5. The wave propagation process of the focusing shear wave:(a) The initial moment of acoustic excitation; (b) The effects of the defect and the focusing shear waves; (c) The defect reflected shear wave propagates to the receiving transducer. The acoustic signals of the intact model and defect prefabricated model in the whole simulation time are extracted respectively. The corresponding phase delay is processed and the A-type signal diagram is drawn as shown in Fig. 6. Figure 6. Simulation result of the planar defect detection with transmit-receive method of ultrasonic array. 3.2 Experiment and analysis The transmit-receive experiment of ultrasonic array was conducted on the turbine rotor specimen that has an artificial prefabrication defects of a length of 3mm and a width of 1mm, as shown in Fig. 7(a). The parameters of the transducers are identical with that of the Table 1, and the excitation hardware is an ultrasonic board with 128 independent channels produced by AOS company of the United States. The beam emission scheme for detecting the middle and lower areas of the weld is shown in Fig. 7(b). The reflected signals of the prefabricated defects in this specimen will be received by the first few elements of the receiving transducer. Since the specific beam path of each channel has been determined, it is possible to push back to the position where any defect is located on the weld area. By switching the transmitting and receiving transducers, the upper half and the lower part of the weld can be detected respectively.
Transmitting(Receiving) transducer Transmitting transducer Prefabricated defect Prefabricated defect 1 32 3mm Receiving(Transmitting) transducer 1 32 Receiving transducer (a) (b) Figure 7. Experiment for the prefabricated defect of the turbine rotor: (a) Photograph of the specimen and the dual transducers; (b) Schematic diagram of acoustic beam emission scheme. As shown in Fig. 8, it could be seen that the defect can be identified in B-scan image clearly and the sizes measured by 6dB method are consistent with the actual sizes. Nevertheless, this defect cannot be detected by the conventional single channel transducer. In addition, the right side of the image is directly transmitted shear wave signal, that is, the lowest blue beam line in Fig. 7(b). Therefore, Designing the beam transmitting-receiving scheme after determining the transducers position is necessary to recognize the defect signal when using the transmit-receive method of ultrasonic array. Transmission shear wave signal Defect signal Figure 8. B-image for prefabricated defects using transmit-receive method of ultrasonic array. 4. Conclusions A transmit-receive method of ultrasonic array transducers is proposed for the planar defects in the welding area of the turbine rotor of aero-engine. The validity of the detection scheme is verified by the FDTD method, and the planar defect with the length of 1mm can be clearly detected from the simulation model. Then, a high precision ultrasonic array testing system is constructed, and the experiments are carried out on the turbine rotor sample with the artificial embedded defect. The experimental results in this paper indicated that the transmit-receive method of ultrasonic array is effective for detection the planar defects of complex components.
Acknowledgement The authors acknowledge the financial support from the National Natural Science Foundation of China (NSFC) No. 51775026 and 51375027. References and Footnotes References should be written in the order in which they appear in the text in the following format: [1] M katoh, K Nishio, T Tamaguchi, Materials evaluation of diffusion bonded steel bar and its impact characteristics, NDT & E International 35(4), pp 263-271, 2002. [2] Z Cao, H Chen, J Xue, Evaluation of mechanical quality of field-assisted diffusion bonding by ultrasonic non-destructive method, Sensors and Actuators A: Physical 118(1), pp 44-48, 2005. [3] P N Marty, N Desaï and J Andersson, NDT of kissing bond in aeronautical structures, 16 th World Conference on NDT,2004. [4] J Zhang, B W Drinkwater, P D Wilcox, Efficient Computation of Delay aw for Imaging Structure with a Complex Surface. Joint UFFC, EFTF and PFM Symposium, pp 139-142, 2014. [5] C Fan, M Caleap, M Pan, A comparison between ultrasonic array beamforming and super resolution imaging algorithms for non-destructive evaluation, Ultrasonics (54), pp 1842-1850, 2014. [6] J Rose. Ultrasonic waves in solid media, Cambridge University Press, pp 48-50, 1999. [7] W Schmerr Jr, Fundamentals of ultrasonic phased arrays, Iowa State University: Springer International Press, pp 38-42, 2015.