Application of linear frequency modulated excitation on ultrasonic testing for austenitic stainless steel welds Sen Cong, Tie Gang, Jiaying Zhang State Key Lab of Advanced Welding &Joining, Harbin Institute of Technology, China 1
Harbin Institute of Technology Founded in 1920, Harbin Institute of Technology (HIT), with science, engineering and research as its core, encompassing management and liberal arts, economy and law, is now developing into an open, multidisciplinary and world-class university. 2
Outline 1. Introduction 2. Ultrasonic LFM-TOFD testing 3. Experiments and results 4. Conclusions 3
1 Introduction Austenitic stainless steel welds Nuclear power, petrochemical industry Important structure Crack and corrosion damage 4
1 Introduction Anisotropic, heterogeneous, bulky columnar crystal, big thickness Problems of ultrasonic inspection Low SNR, sensitivity, detection resolution Serious attenuation Beam distortion
1 Introduction Ultrasonic Time of Flight Diffracted (TOFD) testing technique is suitable for the ultrasonic testing of metallic block with big thickness Advantages: Big thickness specimen High detection precision Positioning and sizing Crack sensitivity Disadvantages: Diffracted wave with low energy Low SNR Non-detection zone 1 d = ( c t ) + 4 c ts 2 1 2 2 6
1 Introduction Linear Frequency Modulated(LFM) pulse is a common waveform with large time-bandwidth product in radar system, which was later investigated in the field of ultrasonic system in 1970s Pulse Compression (Matched Filter ) Long pulse excitation High average power High transmitting energy Long detection distance Advantages Narrower compressed wave High SNR High time resolution High detection accuracy 7
1 Introduction Ultrasonic TOFD testing Ultrasonic LFM-TOFD testing Ultrasonic LFM excitation Improving the problems of ultrasonic testing for coarse grain materials with big thickness: noise interference, serious attenuation, beam distortion, detection SNR, time resolution, and accuracy 8
2 LFM excitation Linear frequency modulated (LFM) waveform is one of most common pulse compression waveforms in radar systems. Expression is: 2 s( t) = Arect( t T )cos( 2 π fct ± k π t ) t T 2 Slope of frequency modulation: B Bandwidth of LFM pulse; T Duration of LFM pulse; fc Centre frequency; k = B T Instantaneous frequency of LFM pulse: f ( t) B = fc ± t T 9
2 LFM excitation Pulse compression processing: Differences between conventional excitation and LFM excitation: 10
2 Ultrasonic LFM-TOFD testing Ultrasonic time-of-flight diffraction testing with linear frequency modulated excitation Comparison between conventional TOFD and LFM TOFD 11
3 Experiments Set up C= 5800m/s, the separation of transducers 2S=46mm, the refraction angle in austenitic stainless steel θ= 58.3 12
3 LFM excitation signal Parameters of the transducer Type Freq.(MH z) Centre Freq. (MHz) B -6dB (MHz) B -12dB (MHz) Size (mm) Straight 5 4.75 3.77 6.75 12 LFM excitation pulse B = 7MHz, T = 7µs, fc = 5MHz, V =63Vpp, Gain= 20dB; Conventional pulse Frequency =5MHz, V =300Vpp,Gain= 20dB; 13
3 Experiment Specimen 1 Specimen 1-Austenitic stainless steel base metal 14
3 Test results Specimen 1 D-scan images of base metal LFM-TOFD: Defect images are clearer, SNR is higher, and the backwall reflection wave is narrower, whereas the system gain is lower LFM-TOFD technique can enhance the quality of ultrasonic D-scan image and the positioning accuracy 15
3 Test results Specimen 1 D-scan images of base metal Extracting the A-scan signals from the same position of D-scan images 16
3 Test results Specimen 1 The comparison of A-scan signals (a) : the waves can be distinguished, but the lateral wave and the backwall wave are wide. (c) : the waves can also be clearly distinguished, the waveforms are narrower, and the amplitude of diffracted wave is higher. 17
3 Test results Specimen 1 Conventional TOFD and LFM-TOFD LFM-TOFD technique can enhance the capability and quality of ultrasonic testing for base metal Ultrasonic testing for austenitic stainless steel welds? 18
3 Experiments Specimen 2 Specimen 2- Austenitic stainless steel weld 19
3 Test results Specimen 2 D-scan images of austenitic stainless steel weld Defect images are clearer, and the backwall reflection wave is narrower LFM-TOFD technique can enhance the positioning accuracy 20
3 Test results Specimen 1 D-scan images of base metal Extracting the A-scan signals from the same position of D-scan images 21
3 Test results Specimen 2 The comparison of A-scan signals (a) The echo of conventional TOFD (b) The echo of LFM-TOFD (c)the compressed waveform of (b) (a): serious waveform distortion, and difficult to distinguish the diffracted wave of defect and the backwall reflection wave. (c): the waves can be clearly distinguished, the waveforms are narrower, and the SNR of the defect wave is higher. 22
3 Test results Specimen 2 Conventional TOFD and LFM-TOFD LFM-TOFD technique can improve effectively the capability and quality of ultrasonic testing for austenitic welds, avoid the beam distortion, and enhance the positioning accuracy. 23
3 Summary Measurement results of the two methods Testing method Specimen t ( µ s) Diffrated t ( µ s) Backwall Time error(µs) SNR (db) Conventional TOFD LFM-TOFD Specimen 1 1.06 2.92 0.12 10.3 Specimen 2 1.30 2.58 0.24 5.6 Specimen 1 1.16 2.76 0.04 20.9 Specimen 2 1.44 2.48 0.04 18.1 t Diffrated --the time interval of defect diffracted wave relative to the lateral wave t Backwall --the time interval of the backwall wave relative to the lateral wave 24
4 Conclusions (1) The qualities of the images and the waveforms can be improved, and the waveform distortion in austenitic welds effectively can be avoided (2) The time resolution and the SNR (about 20dB) can be enhanced, and the positioning precision can be reduced to 0.04µs by the LFM excitation. (3) Ultrasonic LFM testing technique is an effective improvement of ultrasonic inspection of austenitic stainless steel welds. 25
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