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10th International Symposium on NDT in Aerospace
1 License: https://creativecommons.org/licenses/by/4.0/
The Use of Pulse Compression Technique in
Non-Destructive Testing: A Review
Muhammad Khalid RIZWAN 2, Hamed MALEKMOHAMMADI 1, Stefano LAURETI 1,
Pietro BURRASCANO 2, David A. HUTCHINS 3, Gui Yun TIAN 4, Junzhen ZHU 4,
Qiuji YI 4, Marco RICCI 1 1 University of Perugia, Perugia, Italy
2 Universita degli Studi di Perugia, Terni, Italy 3 University of Warwick, Warwick Manufacturing Group (WMG), Coventry, UK
4 Newcastle University, Newcastle upon Tyne, UK
Contact e-mail: muhammadkhalid.rizwan@unipg.it
Abstract
Non-Destructive Testing (NDT) refers to an extended group of techniques used both in
research and industry, which are exploited for inspecting, characterizing and evaluating
various kinds of materials and goods. Eddy Current Testing (ECT), Ultrasonic Testing (UT)
and InfraRed Thermography (IRT) are among the most employed NDT methods. In the said
methods, a short duration delta-like signal provided by different types of transducers is
typically employed to excite the sample under test (SUT) within an extended bandwidth.
Features of interest, e.g. presence of flaws and inclusions inside the SUT, can be extracted
by analyzing the system’s impulse response both in time and frequency domains. However,
the maximum achievable Signal-to-Noise Ratio (SNR) is directly related to the source power.
This limit can be overcome by using coded excitations together with Pulse Compression
(PuC) technique. In fact, in PuC the frequency spectrum of the coded excitation can be
tailored to suit the investigation of a given sample, while the SNR can be increased almost
arbitrarily by enhancing the signal time duration. Finally, the system’s impulse response is
retrieved by applying the PuC algorithm on the acquired data. In this paper, the use of PuC
technique in IRT, UT, ECT will be reviewed and discussed.
THE USE OF PULSE COMPRESSION TECHNIQUE IN NON-DESTRUCTIVE TESTING: A REVIEW
Muhammad Khalid Rizwan1, Hamed Malekmohammadi1, Stefano Laureti1, Pietro Burrascano1, David A. Hutchins2, Gui Yun Tian3, Junzhen Zhu3, Qiuji Yi3, Marco Ricci4
1Department of Engineering
University of Perugia, Italy
2School of Engineering
University of Warwick, UK
3School of Engineering
Newcastle University, UK
4DIMES Department
University of Calabria, Italy
THE USE OF PULSE COMPRESSION TECHNIQUE IN NON-DESTRUCTIVE TESTING: A REVIEW
Outline→ Background
→ Limits of PuE approach
→ Pulse Compression (PuC)
• Basic principle
• Signals for PuC
• Choice of the Signals
→ Application in NDT
• PuC Ultrasonic
• PuC Thermography
• PuC Eddy Current Testing
→ Conclusions
→ References
THE USE OF PULSE COMPRESSION TECHNIQUE IN NON-DESTRUCTIVE TESTING: A REVIEW
BackgroundGeneral procedures in Ultrasonic, Eddy Current, Thermography NDT inspections are based on
measuring the response ℎ(𝑡) of the sample under test (SUT) to an impulsive excitation which
approximates a delta 𝛿(𝑡) like function (e.g. Pulse Echo)
ℎ(𝑡)𝛿(𝑡)
THE USE OF PULSE COMPRESSION TECHNIQUE IN NON-DESTRUCTIVE TESTING: A REVIEW
Background
• In circuit theory, the response to a delta 𝛿(𝑡) excitation is called impulse response h(t) and it describes the input-output relation of the system as
𝑦 𝑡 = 𝑥(𝑡)⨂ℎ(𝑡)
Measuring ℎ 𝑡 allows the inspection of the sample as well as the calculation of the response of the SUT to any other possible excitation
THE USE OF PULSE COMPRESSION TECHNIQUE IN NON-DESTRUCTIVE TESTING: A REVIEW
Limits of the Pulse-echo approach
→ Maximum achievable SNR is not sufficient for the defect detection and characterization in the presence of noise or high attenuation
→ Aim is to measure the impulse response even in the presence of noise, to enhance sensitivity of the system
How?
→ Increase energy of the excitation pulse Amplitude of the pulse; constrained by the system and the transducer’s limitations
→ Averages Long time of measurements
THE USE OF PULSE COMPRESSION TECHNIQUE IN NON-DESTRUCTIVE TESTING: A REVIEW
Enhance SNR HOW?
Proposed Solution
Pulse Compression
An alternative technique to increase the SNR of the measurement setup
increase the time bandwidth product increase the time duration of
the excitation pulse rather than the amplitude
❖ Flexible both in time and frequency shaping
❖Widely applicable
❖ Various techniques to implement
THE USE OF PULSE COMPRESSION TECHNIQUE IN NON-DESTRUCTIVE TESTING: A REVIEW
➢ Pair of signals whose correlation is a delta 𝛿(𝑡) like function, i.e.,
𝑔 𝑡 ∗ Ψ 𝑡 = መ𝛿(𝑡)
➢ Excite the Linear (and in some cases nonlinear) system with one of the signals, 𝑔 𝑡
➢ Impulse response of the unknown system can be estimated when the output is
convolved to the second signal Ψ 𝑡 in the pair, i.e.,
𝑦 𝑡 ∗ Ψ 𝑡 = 𝑔 ∗ ℎ 𝑡 ∗ Ψ 𝑡 = ℎ 𝑡 ∗ መ𝛿 𝑡 ≃ 𝐶ℎ(𝑡)
➢ SNR is maximized when Ψ 𝑡 = 𝑔 −𝑡 matched filter theory
Pulse Compression (PuC)- basic principle
THE USE OF PULSE COMPRESSION TECHNIQUE IN NON-DESTRUCTIVE TESTING: A REVIEW
Pulse Compression- basic principle
𝑔(𝑡)
𝑦 𝑡 = [ℎ ∗ 𝑔] 𝑡 + 𝑛(𝑡) ℎ 𝑡 ≃𝐶 × ℎ 𝑡 + 𝑛(𝑡)LTI
system
Matched
Filter
ℎ(𝑡) 𝛹 𝑡 = 𝑘𝑔(−𝑡 + 𝑇)𝑛 𝑡noise
Excitati
on
signal
𝑔(𝑡)Matche
d
filter
𝛹 𝑡
Estimat
ed
A-scan
ℎ 𝑡
☺ High SNR Required SNR Gain can be achieved by increasing the time duration of the excitation pulse without effecting the bandwidth
Bandwidth is independent of TPuC
☺ Flexibility in time and frequency domains shaping
☺ No hardware complexity for implementation
Advantages
THE USE OF PULSE COMPRESSION TECHNIQUE IN NON-DESTRUCTIVE TESTING: A REVIEW
Signals for PuC
➢ Ideally, the signal should prevail the property of autocorrelation as;
𝑅𝑔𝑔 = 𝛿(𝑡)
➢ In Real cases, the closer the correlation 𝑅𝑔𝑔 = መ𝛿(𝑡)~𝛿(𝑡), the
more accurate is the estimation of the impulse response, ℎ(𝑡)~ℎ(𝑡)
➢ Practically, frequency spectrum of the selected signal should be wide
enough to cover the bandwidth of the sample under test
THE USE OF PULSE COMPRESSION TECHNIQUE IN NON-DESTRUCTIVE TESTING: A REVIEW
Applications of PuC in NDT
→ Wave Phenomenon Ultrasonic NDT
▪ SNRGain is significantly improved widely applicable
▪ h(t) consists of several reflections/paths
▪ Due to propagation, duration of impulse response, Th is much longer than the required time
resolution δT, i.e., Th >> δT ThB >>1 SNRGain is high
→ Diffusion Phenomenon EDDY Current, Thermography
▪ SNRGain is reasonable, but other advantages exist
▪ h(t) is smooth and exponentially decaying
▪ Time duration of the impulse response Th is greater than time resolution δT, but reasonable
THE USE OF PULSE COMPRESSION TECHNIQUE IN NON-DESTRUCTIVE TESTING: A REVIEW
Choice of the Signals
Swept frequency Signals:
Chirps (both linear and nonlinear)
Chirps are the widely used signals in PuC
NDT Easy control over the bandwidth and almost a flat spectrum
Coded Signals:
Binary codes, mapped into continuous waveforms (bits in the form of square wavelets), e.g. Multi-Length Sequences (MLS), Golay Complementary Sequences, Barker Codes, etc.
THE USE OF PULSE COMPRESSION TECHNIQUE IN NON-DESTRUCTIVE TESTING: A REVIEW
PuC Ultrasonic NDT: Schematic of the Setup
Signal for PuC
Signal Generation: Arbitrary waveform Generator
Transmission Transducer
Sample Under Test
Reception Transducer
Low Noise Amplifier
Data Acquisition:Digital Oscilloscope
Post Processing and Display Unit
Back wall EchoDefect
Acquired Signal Impulse Response 𝒉(𝒕) of the SUT
THE USE OF PULSE COMPRESSION TECHNIQUE IN NON-DESTRUCTIVE TESTING: A REVIEW
PuC Ultrasonic NDT: Results comparison to PuE
▪ PuC (using a Linear chirp as anexcitation signal) and PuE UTinspections of a steel forging,1.5m long and diameter of0.6m
▪ Using the same transducers,V108-VEDIOSCAN fromOlympus
▪ AWG Vp-p;❖ in case of PuE 200V
❖ in case of PuC 24V
SNRGain at Defect
SNRGain is enhanced significantly in case of PuC
THE USE OF PULSE COMPRESSION TECHNIQUE IN NON-DESTRUCTIVE TESTING: A REVIEW
PuC Ultrasonic NDT: Results
▪ Echograms of PuC UTinspection of forgings
▪ SNRGain is significantlyhigh, because theduration of the signal(4m) is much higherthan the resolution (inmm)
▪ Exploit the advantage ofbroad bandwidth of aLinear Chirp
▪ Obtain the optimumfrequency of Inspection
THE USE OF PULSE COMPRESSION TECHNIQUE IN NON-DESTRUCTIVE TESTING: A REVIEW
PuC Thermography, Setup
Schematic of the PuC based thermographic Experiment [1]
The Setup at our Lab (the NDT Laboratory, Terni) [2]
[1],[2] Diagram and the setup from presentation of Hamed Malekmohammadi,
Comparison of optimization strategies for the improvement of depth detection capability of Pulse-Compression Thermography, QIRT-2018
o Heating Source: 8 x 50W LED chips
o Thermal Camera: Xenics Onca-MWIR-InSb IR
o Signal Generation/Acquisition: LabviewTM
o Camera in Reflection mode, 40 FPS
THE USE OF PULSE COMPRESSION TECHNIQUE IN NON-DESTRUCTIVE TESTING: A REVIEW
PuC Thermography, Results
▪ CFRP sample with defectsat different depths
▪ Impulse Response ℎ 𝑡 isslow, SNRGain is not veryhigh
▪ Binary Sequences, BarkerCode is used for PuC
THE USE OF PULSE COMPRESSION TECHNIQUE IN NON-DESTRUCTIVE TESTING: A REVIEW
PuC Eddy Current Inspections, Setup
Time Amplitude and Phase response
THE USE OF PULSE COMPRESSION TECHNIQUE IN NON-DESTRUCTIVE TESTING: A REVIEW
➢ Comparison of Time and Frequency domain analysis for the defect detection, using Linear chirps (varying duration of the signal)
➢ A surface defect of (0.1x0.4x3 mm3), covered with another sound plate of thickness = 2 mm
PuC Eddy Current Inspections, Results
➢ SNRGain is better because the time resolution of impulse response is in between the UT and thermographic NDT
➢ In this case, time domain analysis outperforms the frequency domain analysis in terms of defect detection
➢ One can obtain more robustness of the system to the lift off variations, while applying time or frequency phase analysis
THE USE OF PULSE COMPRESSION TECHNIQUE IN NON-DESTRUCTIVE TESTING: A REVIEW
• PuC procedures enhances the sensitivity of NDT techniques,
• As discussed, the PuC methods,➢ Applied in UT inspection of large structures, improved SNR has resulted in better spatial and temporal
resolution➢ Applied in Eddy currents testing, lift off compensation can be achieved via time and frequency phase
analysis➢ Applied in thermographic, thermograms with improved SNR and fidelity can be achieved
• The PuC method can easily be incorporated to the NDT systems with simple hardware
• Flexibility over the time and frequency domain shaping of the signal, spectral density of the excitation pulse can developed well matched with the bandwidth of the system
• One can exploit the broadband of the PuC procedure to obtain a multifrequency defect evaluation system, to gain the optimum frequency of inspection for a specified SUT
• An NDT system with improved SNR gain opens further application of the method for the inspection of highly attenuating materials and structures, like CFRP and GFRP
• Performance of the PuC NDT system can further be improved combing other advanced signal and image processing techniques
Conclusions
THE USE OF PULSE COMPRESSION TECHNIQUE IN NON-DESTRUCTIVE TESTING: A REVIEW
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References
THE USE OF PULSE COMPRESSION TECHNIQUE IN NON-DESTRUCTIVE TESTING: A REVIEW
Acknowledgements
This research work has been supported from the European
Union’s Horizon 2020 research and innovation programm under
the Marie Skłodowska-Curie grant agreement No 722134 –
NDTonAIR
www.ndtonair.eu
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