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S4: In situ monitoring of metallic aeronautic structures at Amphi Bézier (AB)
Chair: Mohamed El May and Marc Rebillat
10:00
20 mins
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Controlled localized corrosion damage generation
Cyril Nicard, Olivier Devos, Mohamed El May
Abstract: To precisely evaluate the efficiency and precision of a new ultrasonic based corrosion monitoring technique to detect and follow the growth of pits on metallic material it often needs to be tested and calibrated on damage which are known in size and location. Some ultrasonic technic such as Lamb Wave or Coda Wave interferometry technics need a measurement to be taken at “healthy” state that will be used as a reference for the signal treatment of following measurements. These technics are very sensitive to a lot of phenomenon and any manipulation of the sample can disturb the measures, thus, the ideal would be to directly generate damage on a sample instrumented with ultrasonic technic devices. Those ultrasonic technics can be preliminary tested with damage made mechanically by micro-drilling devices but corrosion is not just the loss of dissolved matter, it involves various oxides formation and selective dissolution that can sometimes lead to complex geometry, roughness, porosity or even undermining corrosion so it is preferable to generate realistic corrosion damage.
The generation of single localized corrosion damage on stainless steel with chloride ions has been attempted by others in the past in many different ways. The first approach investigated [1,2] was to simplify the problem by limiting the accessible surface for corrosion pit to initiate by using a steel wire or thin plate embedded in resin, letting only the smallest cross section accessible to corrosion. This configuration does not fit our needs as we need a large sample to fix the piezo electrical disc required to perform ultrasonic technics. Moreover, the obtained pit morphology is not realistic due to the non-corroding resin at damages edges. Another approach attempted in literature is to limit the area where the chloride ions will react with the surface either by generating them very locally with Ag/AgCl probe [3] or use micro-capillaries [4] sealed on the surface with silicon to prevent the NaCl solution to spread elsewhere on the surface. The use of Ag/AgCl probe is very effective to precisely initiate the pit but the size of pits generated this way is limited and the role of Ag cations needs to be investigated as they are generated alongside Cl anions. The use of sealed micro capillary seems a good option to rapidly obtain confined condition due que the small volume accessible in the capillary and damage generated this way can easily be grown or stopped by respectively imposing drastic anodic or cathodic potentials. The fact that the capillary is sealed on the surface is an issue for our application as its influence on ultrasonic wave propagation is unknown and it makes optical observation during the experiment impossible. A last option is to protect the surface with a varnish, coat, or paint [5] except for a little area for the pit to develop but the impact of the coating on the ultrasonic measure will have to be looked into. The generation of single pit using non-sealed micro capillaries has been performed by S. Heurtault [6] were he determined proper condition (concentrations and potential) to create a single pit but it was done on very small samples embedded in resin, in acidic (pH 0.25) conditions and despite the fact that there was corrosion at the edges of the sample, it seems to be the best starting point in term of methodology for our work.
Our challenge is to generate a single localized corrosion damage that can be controlled in size (grown or stopped at will) by electrochemistry on an industrial stainless steel while enabling the use of ultrasonic methods on the sample instrumented with piezo-electric discs. In order to reach this objective an electrochemical cell has been designed to fit our requirement. This electrochemical cell is a 3 electrodes cell with O-rings to maintain water-tightness with the Stainless 316L grade steel positioned horizontally. The top of the cell is let open to position the reference and counter electrodes and letting the possibility to install a long focal microscope in order to observe in-situ the pit size and morphology. In addition, a capillary holder and guide is included to the design to enable the positioning of a capillary in contact (but not sealed) with the surface with a fixed angle. The part that press the sample against the O-ring is designed in order to let enough space to install piezo electrical discs to implement ultrasonic technics and avoid any contact or stress on these. Preliminary tests had been conducted to identify the experimental parameters needed to control the reactivity of the steel. In order to create favourable conditions to have a single localized corrosion damage the pitting condition has to be reached on a very limited area. Two concentrations and two potentials have been identified. An aqueous Na2SO4 solution to fill the electrochemical cell in which the steel in passive state at both potential and the Na2SO4 + NaCl solution that will be injected to the capillary in which the steel is in passive condition at one potential and in pitting condition at the other potential. This way, by imposing the potential it is possible to control the generation and growth or the stabilisation of the localized corrosion damage. The size of the corrosion damage is estimated from the electric charge exchanged with the system measured by the potentiostat during the imposed pitting potential phases and optically observed thanks to the microscope. As there is a difference between the expected size calculated from the current and the observed size the corrosion damage is further characterized by other methods post experiment to investigate further this difference and attempt to explain it.
[1] E.D. Parsons, H.H. Cudd, H.L. Lochte, synthetic corrosion pits and the analysis of their contents, Journal of Physical Chemistry, 45 (1941) 1339 - 1345.
[2] P. Ernst, R.C. Newman, Pit growth studies in stainless steel foils. I. Introduction and pit growth kinetics, Corrosion Science, 44 (2002) 927 – 941.
[3] K. Fushimi, K. Azumi, M. Seo, Use of a liquid-phase ion gun for local breakdown of the passive film on iron, Journal of the Electrochemical Society, 147 (2000) 552 - 557.
[4] S.M. Ghahari, A.J. Davenport, T. Rayment, T. Suter, J.P. Tinnes, C. Padovani, J.A. Hammons, M. Stampanoni, F. Marone, R. Mokso, In situ synchrotron X-ray micro-tomography study of pitting corrosion in stainless steel, Corrosion Science, 53 (2011) 2684.
[5] R.C. Alkire, K.P. Wong, the corrosion of single pits on stainless steel in acidic chloride solution, Corrosion Science, 28 (1988) 411.
[6] S. Heurtault, R. Robin, F. Rouillard, V. Vivier, On the propagation of open and covered pit in 316l stainless steel, Electrochimica Acta, 203 (2016), 316-325.
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10:20
20 mins
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Hardware for ultrasonic inspection of aeronautical structures
Mathieu Thomachot, Frédéric Letellier, Marc Fournier, Timotéo Payre
Abstract: INTRODUCTION
Condition based maintenance of aeronautical structures requires monitoring systems that are non-destructive, easy to integrate, and deliver an easy-to-read information on the system’s condition. In the frame of the COQTEL project (ANR-20-CE42-0014), we developed a hardware used for monitoring corrosion damages by using Lamb waves.
DAMAGE DETECTION PRINCIPLE
Ultrasonic technologies relying on Lamb waves are used to monitor corrosion damage with piezo transducers (PZTs). Two different strategies are currently available: the passive and the active one. In a passive mode, PZTs only record corrosion damage acoustic activity generated by the mechanico-chemico-physical corrosion process going on within the structure. In the active mode, the same PZTs are sending Lamb waves to interrogate the structure and to listen to potential corrosion damage echoes. When encountering a corrosion damage, Lamb waves will face a local diminution of the plate thickness due to corrosion which will generate reflected Lamb waves indicative of damage position and size. Both methods are complementary as passive method targets corrosion premises and active method larger corrosion damage. The same PZTs can be used for both methods, which can thus rely on the same hardware.
LAMB WAVE DETECTION SYSTEM HARDWARE
The Lamb Wave Detection System (LWDS) contains up to 36 independent channels. For each channel, the LWDS has a power amplifier (±15V, 1A) to drive the PZT at more than 1MHz, a low noise programmable gain amplifier to detect the PZT echoes and the switching system to toggle between emitting (Pulse mode) and receiving (Echo mode). The LWDS includes an interface board to send the Pulse/Echo and to retrieve the measured signal. It can also be used to adjust the gain of each programmable amplifier. User can program a wave pattern with a user interface or with a Matlab or Python code through the USB port, which will be converted with a 12-bit resolution at 10MSPS (±5V).
SOFTWARE
In the passive mode, a MATLAB script configures all the PZTs in sensing mode (Echo mode). The script acquires and processes the electrical signals generated by the PZTs in real time. We apply a high-pass filter (of the order of 10 kHz) to the acquired signals to eliminate most of the interference. To determine the occurrence of an event, the MATLAB script compares the acquired signals with a threshold voltage value. All detected events are recorded.
In the active mode, a MATLAB script configures one PZT in actuator mode (Pulse mode) and all the others in sensing mode (Echo mode). The PZT in pulse mode will generate a Lamb wave at a frequency that can be varied (from 10 kHz to 200 kHz) and, once the transmission is complete, it will switch to sensing mode. The script MATLAB will acquire and record the signals generated by the PZTs after the Lamb wave has been transmitted. For each excitation frequency, the PZTs will be used in turn as actuators, while the others will be in detection mode.
MEASUREMENT EXAMPLES
Several hemispherical holes were generated mechanically on test plates, to simulate corrosion pits, with diameters varying from 0.15 mm to 1 mm, and a 1 mm. By comparing the Lamb wave signals from a healthy plate and damaged plates in active mode, the defects were clearly identified, whatever their size. By using the Time of Arrival method, the defects were localized with a millimetre range precision. The damages’ sizes were estimated by using an Image Post Processing algorithm and could fit the real sizes with less than a 0.2 mm error.
CONCLUSION
We confirmed the capability of the LWDS to be used for corrosion damage detection on test plates, including localization and size measurement. In the frame of the COQTEL project, 2 main targets still need to be reached. Firstly, the LWDS must be used in passive mode to monitor the early steps of damages creation by corrosion. Secondly, the LWDS must be used on a demonstrator using a real aircraft part, to demonstrate its operability in real conditions. Regarding the LWDS, development work is being made by Cedrat Technologies, to develop a more compact device that will integrate all the hardware functions seamlessly.
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10:40
20 mins
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In-situ monitoring of µm-sized electrochemically generated corrosion pits using Lamb Waves managed by a sparse array of piezoelectric transducers
Marc Rébillat, Cyril Nicard, Olivier Devos, Mohamed El May, Frédéric Letellier, Nazih Mechbal
Abstract: Corrosion within metallic structures can render an aircraft un-airworthy by weakening structural components, roughening the outer surface, loosening fasteners, hastening cracking, and facilitating the entry of water into electronic fixtures. In 2016, the combined commercial aircraft fleet operated by European airlines was around 7900 airplanes. The annual corrosion cost for this number of aircraft was estimated by the US National Association of Corrosion Engineers to 2.2 B$, which includes corrosion maintenance at 1.7 B$ and downtime due to corrosion to 0.3 B$. Anticipating corrosive conditions ahead of time can lead to significant cost savings and less aircraft downtime. It is estimated that savings between 15% and 35% of the cost of corrosion could be realized [1]. In order to limit corrosion issues, a typical aeronautic structure is made of qualified steel or aluminium alloys eventually protected from the environment by treating the surface with adequate coatings. It is generally expected that the protective surface is perfectly flawless over the whole structure. But on a large structure undergoing everyday service this cannot be the case. Impact damage during service life is a common occurrence of coating failure. Damage during maintenance (tools and split fluids) can also occur as well as paint cracking at high stress points around joints. From these initial premises, corrosion pits can start and threaten the structural integrity at a global level, thus motivating the need for in-situ pitting corrosion monitoring technologies.
Industrially speaking, an ideal corrosion monitoring technology should allow to monitor large-scale structures, to automate measurements, to detect corrosion pits from their premises, and to exhibit a high correlation between sensors measurements and the size and locations of corroded areas. Although various effective non-destructive testing (NDT) methods have been developed to monitor corrosion [2], they remain limited in their ability to detect and assess corrosion premises and to reliably size a given corrosion damage. The first reason for that is technological. Eddy currents allow local monitoring (one side), are slow, difficult to calibrate, and remain a small-scale approach. Standard ultrasonic methods (A-scan and C-scan) are again small scale, rely on coupling fluid & human intervention and are thus rather slow. Optical methods (visual inspection, liquid penetrant) are large scale but can cope only with surface features that can be visually inspected and remain difficult to interpret. Radiography (X-rays, tomography) is small scale and relies on extensive hardware that cannot be used on the field. The second reason is methodological. Actual monitoring procedures are referred as planned maintenance and imply regular parts inspection by means of visual inspection and non-destructive testing through human highly qualified operators. Many of these inspections are (fortunately) unnecessary and as they rely on human intervention, their reliability is then subject to confidence intervals that establish a certain probability of detection. Moving from planned maintenance to condition based maintenance (CBM), i.e. maintenance only when necessary, seems mandatory to solve these issues. Monitoring in real time and autonomously the health state of structures is thus of high interest in this context. Such a process is referred to as Structural Health Monitoring (SHM) [3, 4]. To achieve this goal, structures become “smart” in the sense that they are equipped with sensors, actuators, and algorithms that allow them to state autonomously regarding their own health. One can compare smart structures with the human body which, thanks to its various senses and nerves, is able to assess if it has been hurt, where it has been hurt, and to estimate how severe it is. Following this analogy, the SHM process is classically decomposed into four steps: damage detection, localization, classification, and quantification [5]. There is thus a real need for a large scale, automated, sensitive, low cost and embedded technology to develop corrosion monitoring SHM methodologies of aeronautic parts.
Ultrasonic technologies relying on Lamb waves have been shown to be extremely sensitive to corrosion damage [6, 7] and can be easily automated thanks to permanently embedded piezoelectric elements (PZT) networks [8]. In the present work, the focus is thus put on corrosion monitoring methods based on ultrasonics waves applied experimentally to plate-like structures. A literature on that topic survey has shown that ultrasonic inspection method based on spare PZT transducers arrays are the most suitable from a practical point of view but that they have never been validated on small corrosion pits realistic of actual corrosion (from 10 µm to 100 µm) but rather on large generalized corrosion areas (in cm). The reason explaining the fact that it was yet not possible to generate a single corrosion pit in a controlled manner and to simultaneously and in-situ record ultrasonic Lamb waves interacting with such a damage.
The objective of this contribution is thus first to introduce an experimental setup allowing to control electrochemically the growth of a µm-sized corrosion pit and to simultaneously monitor it in situ by means of ultrasonic Lamb Waves emitted and received using a sparse array of PZT transducers. It will secondly be demonstrated that such measurements can be post-processed to compute damage indexes (DIs) that correlate very well with actual corrosion pit radius as estimated electrochemically and that using a linear model relating DI values to corrosion pit radius, corrosion pit size from 10 µm to 150 µm can be reliably detected, located, and their upcoming size extrapolated.
[1] S. Benavides, Corrosion control in the aerospace industry, Elsevier, 2009.
[2] S. J. Harris, M. Mishon and M. Hebbron, "Corrosion sensors to reduce aircraft maintenance," in Workshop on enhanced aircraft platform availability through advanced maintenance concepts and technologies. , 2006.
[3] D. Balageas, C.-P. Fritzen et A. Güemes, Structural health monitoring, vol. 493, Wiley Online Library, 2006.
[4] C. R. Farrar et K. Worden, «An introduction to structural health monitoring,» Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences, vol. 365, pp. 303-315, 2007.
[5] K. Worden, C. R. Farrar, G. Manson et G. Park, «The fundamental axioms of structural health monitoring,» Proceedings of the Royal Society of London A: Mathematical, Physical and Engineering Sciences, vol. 463, pp. 1639-1664, 2007.
[6] C. Jirarungsatian and A. Prateepasen, "Pitting and uniform corrosion source recognition using acoustic emission parameters," Corrosion Science, vol. 52, pp. 187-197, 2010.
[7] P. Kudela, M. Radzienski, W. Ostachowicz and Z. Yang, "Structural Health Monitoring system based on a concept of Lamb wave focusing by the piezoelectric array," Mechanical Systems and Signal Processing, vol. 108, pp. 21-32, 2018.
[8] S. Grondel, C. Delebarre, J. Assaad, J.-P. Dupuis and L. Reithler, "Fatigue crack monitoring of riveted aluminium strap joints by Lamb wave analysis and acoustic emission measurement techniques," Ndt & E International, vol. 35, pp. 137-146, 2002.
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20 mins
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Development of an Experimental Approach to Study the Synergy and Damage Mechanisms of Corrosion Fatigue in Metallic Materials
Mohamed El May, Cyril Nicard, Marc Rébillat, Olivier Devos, Nazih Mechbal
Abstract: The study of the effect of the environment on the fatigue strength of metallic materials has been, and
still is, the subject of a great deal of research. Although the phenomena of crack initiation and
propagation in fatigue-corrosion appear to be well understood, they remain highly dependent on several
parameters linked to the environment studied and the material. Numerous dimensioning approaches
exist to consider the effect of corrosion on fatigue life. Some treat the two problems (fatigue and
corrosion) separately, while others attempt to model the complex effects of cyclic loading/corrosion
synergy.
A multi-physics and multi-scale experimental approach was developed to better understand crack
initiation and propagation mechanisms in fatigue/corrosion coupling situations. This approach is based
on the multi-instrumentation of fatigue-corrosion tests, combining electrochemical and optical
microscope measurements. More recently, lambs-wave monitoring has been introduced to track the
evolution of damage and to propose a self-health monitoring based on physical evidence.
Based on the results of in-situ electrochemical measurements and optical observations, the mechanisms
and laws of fatigue/corrosion coupling have been identified:
• Corrosion pitting initiation is controlled by an original experimental setup developed to generate
a controlled size of a single corrosion pits in the middle of the fatigue specimen equipped with
four piezoelectric sensors to track the corrosion fatigue damage,
• The size of the corrosion pit is estimated from the measured electrochemical current using the
faraday law,
• The first phase of crack propagation (short crack regime) is highly dependent on corrosion.
Material dissolution and applied stresses contribute to lowering the crack initiation threshold
and increase the kinetics of short fatigue crack propagation,
• In the Paris fatigue crack propagation regime, crack growth kinetics are higher than in air.
This detailed understanding of damage mechanisms has enabled us to:
• identify the parameters and laws governing fatigue-corrosion crack initiation and propagation,
• determine the pit-to-crack transition step using in-situ lambs-wave monitoring and optical
microscope observation,
• and propose an analytical model for estimating fatigue-corrosion life.
Microstructure/cyclic loading/corrosion interactions and the role of hydrogen, introduced during
fatigue-corrosion processes, in crack initiation and propagation mechanisms remain to be studied.
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11:20
20 mins
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Manual surface non destructive inspection of metallic parts using total focusing method on surface waves for Maintenance Repair and Overhaul
Mathieu Ducousso, Stéphane Amiel, Olivier Ghibaudo
Abstract: We present in this communication a new, efficient, method for non destructive inspection of surfaces of metallic parts from manual inspection. The method is based on ultrasonic testing. Currently, ultrasound is mainly used to inspect the bulk of components or to evaluate material properties. The main advantages of ultrasound are the sensitivity, the ability to inspect thick and opaque materials, the possible automation of testing, and data recording. Moreover, phase-array (PA) probes, made up of numerous small ultrasound transducers, revolutionized ultrasound inspection 20 years ago. [1]. Particularly, PA introduces the possibility to electronically deflect and/or focus the acoustic energy of bulk waves by applying time delays to the elements of the probe before summing the contribution of all the elements. The Total Focusing Method (TFM) is the realization of such electronic delays and the summed use of PA: it allows us to focus the ultrasound beam at every point of a region of interest (ROI). It is considered by some as the “gold standard” of ultrasound imaging. [2] It now can be used in real time, with various acquisition schemes, such as Full Matrix Capture (FMC) or Plane Wave Imaging (PWI). [3] The FMC involves firing each array element individually with all the elements used to record the response of the structure, while in PWI all array elements are fired with appropriate time delays to generate sequences of different plane incident waves. The calculated image using TFM is called a T-scan.
The TFM technique has been extended very recently to Rayleigh waves. [4] Such waves can be used in an NDT procedure to detect surface and sub-surface indications. They show high sensitivity and propagate over long distances since their energy is confined to the surface. By applying TFM to Rayleigh waves, complex surface cracks, extending over 18 mm, composed of several sub-cracks (from hundreds of microns to few millimeters), and with a crack opening of a few dozen microns on metallic parts have been characterized precisely. Such characterization has been done from real time manual inspection at 27 Hz.
Following this first demonstration, we demonstrate here that Rayleigh waves combined with TFM imaging is a versatile tool for NDT in Maintenance Repair and Overhaul (MRO). [5] First, we will focus in the image formation. In this part, we will analyze the image synthesis using different acquisition schemes and algorithms to calculate the images. Second, we will demonstrate the suitability of the method for different type of defects, such as cracks and corrosion pitting-like defects (hemispherical bottom hole, HFB), and freckle inclusion. Finally, we will focus on the complexity of the environment, demonstrating the performance of the method even if the indication is located under a few tens of µm coating, close (few tens of µm) to a sharp edge, in a circular edge, after a sharp edge, or at the surface of a tube. Based on all of this, this work demonstrates a new type of real-time surface and sub-surface inspection, applicable to large and complex surfaces or for areas with difficult access. Furthermore, the method is free of chemical preparation and cleaning, and is fully recorded, making this form of inspection compatible with Industry 4.0 or digital twinning.
[1] B. Drinkwater and P. Wilcox, Ultrasonic arrays for non-destructive evaluation: A review, NDT&E International, vol. 39, pp. 525-541, 2006.
[2] C. Holmes, B. Drinkwater and P. Wilcox, Post-processing of the full matrix of ultrasonic transmit–receive array data for non-destructive evaluation, NDT&E International, vol. 38, pp. 701-711, 2005.
[3] L. Le Jeune, S. Robert, E. Lopez Villaverde and C. Prada, Plane Wave Imaging for Ultrasonic Non-Destructive Testing: Generalization to Multimodal Imaging, Ultrasonics, vol. 64, pp. 128-138, 2016
[4] M. Ducousso and F. Reverdy, Real-time imaging of microcracks on metallic surface using total focusing method and plane wave imaging with Rayleigh waves, NDT & E International, vol. 116, p. 102311, 2020
[5] M. Ducousso, S. Amiel and O. Ghibaudo, Surface imaging using total focusing method on surface waves for non destructive testing, submitted to NDT & E International
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11:40
20 mins
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Monitoring the phase transitions by acoustic emission – an application note to monitor the melting ice in aircraft fuel tanks and fuselages
Helge Pfeiffer, Johan Reynaert, David Seveno, Pieter-Jan Jordaens, Bram Cloet, Martine Wevers
Abstract: Matter can produce characteristic acoustic emissions when it undergoes a phase transition. The origin of this sound is not yet fully understood, but it is clear that sudden volume changes, cracks, friction and other sources must be taken into account, but there is not yet any generally accepted theory of the “sound of phase transitions”. Fortunately, this phenomenon can be applied to monitor this transition behaviour of related substances in technical structures. This is important, for example, in the case of water and ice in aircraft tanks. Water caused by contamination and condensation is often found in fuel tanks and regular draining procedures are important to ensure the safety of systems and structures. While ice dispersed in kerosene will be able to seriously hinder combustion processes, block-shaped ice such as in valves and pipes can results in serious cracking.
A practical problem related to water draining is the optimum time to start the process, being the time when all ice has been melted, because starting too early would mean that the draining would be incomplete, or even impossible, and waiting too long created economic issues due to unnecessary downtimes. Using acoustic emission, the mechanical personnel will be able to know when all ice has melted. For this purpose, acoustic sensors are applied on the skin of the tank, and the acoustic emission signals are sufficiently intense to propagate through the aluminum sheets and coatings. In this way, the completion of the ice melt can be determined by the time at which the acoustic emission stops.
We present measurements on a laboratory scale, resulting from a realistic climate chamber on a tank replica and the outcome of a first campaign on an operational aircraft being an Airbus A330.
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