Scientific Presentations

2015

Abstract: In order to exploit the full potential of CFRPs for the manufacturing of lightweight structures in aeronautic industry there is a need to apply adhesive bonding as a joining technology to load-critical primary structures. Quality assurance processes for adhesively bonded CFRP primary structures that are not load-critical already exist. Up to now it is not possible to detect defects like kissing bonds or bondline weakening by in- and post-process NDT techniques. ComBoNDT project meets the European Aviation Safety Agency certification requirements for structural bonding. The extended non-destructive techniques used in ComBoNDT aim at establishing reproducible and reliable NDT tools to ensure the reliable strength of joint structures.

2016

Abstract: More and more carbon-fibre-reinforced plastics (CFRP) are being used in lightweight constructions. Before these components can be processed further by adhesive bonding or painting, laborious cleaning is necessary to remove residues of release agents or other contaminants, which otherwise would strongly decrease the stability of an adhesive bond. Often, surfaces are additionally treated (activated) after cleaning, in order to further increase the strength of an adhesive bond. Here we present a system which determines the wettability of a surface by an aerosol wetting test. Originally patented by Fraunhofer IFAM, we put a strong focus on automated industrial testing applications during industrialization and product development. At this stage our system provides a reliable process for fast surface inspection of large components during the production process. The surface condition is investigated through the nebulization of a very fine water mist. The contact angle of the water droplets with the surface depends on its bonding characteristics. The distribution of the droplets size along with other parameters is analyzed through image processing. The analysis is reference based and hence can be calibrated, with various “reference samples” in order to achieve a reliable “ready to bond” signal. This technique is not only limited to CFRP parts, however it can be applied to numerous materials and industrial applications – in consequence leading to an increase in product quality, process reliability and thus enhanced cost effectiveness. In summary our work including the latest studies show an enhanced capability to detect local contaminations as well as global pre-treatment steps. The latest results even indicate a capability to differentiate activation/contamination levels and concentrations.

Abstract: Numerous techniques of non-destructive testing (NDT) of structural parts of CFRP are investigated. In this research we focus on electromechanical impedance (EMI) technique. This is a technique often considered as NDT or Structural Health Monitoring (SHM) method. It is based on a piezoelectric sensor that is surface mounted to the inspected structure. The electrical quantities of the sensor are measured in wide frequency range. Due to direct and converse piezoelectric effects the electrical response of the sensor is related to mechanical response of the structure to which the sensors is bonded to. In the reported research adhesively bonded CFRP samples were investigated. Within the investigation properly bonded (referential) samples were consider together with samples with modified bond. The adhesive bonds were modified in order to simulate repair-related defects. Following modifications were considered: pre-bond thermal treatment, pre-bond contaminated with a de-icing fluid, and faulty curing of the adhesive. The electromechanical impedance spectra were investigated searching for anomalies and changes caused by modification of the adhesive bond. These spectra for different cases were compared with reference measurement results gathered from pristine samples. Numerical indexes for comparison of the EMI characteristics were proposed. The sensitivity of the EMI method to modified bonds was observed.

Abstract: This study deals with the Laser Shock Adhesion Test (LASAT) and its use for the weak bond detection. This paper presents a new configuration called symmetrical laser impact. It consists in impacting the material target by two laser beams on its two free surfaces. The results of the preliminary tests conducted on the carbon fiber reinforced plastic (CFRP) samples are presented. After a quick explanation of the different Non Destructive Technics (NDT) used to assess the structural health of a sample, the first results on the LASAT in this configuration are analyzed. By playing with laser parameters (energy and delay between the two laser), the mechanical loading can be adjusted inside the material to test selected interfaces. A general outlook of the technology will be given, as well as the foreseen improvements of the LASAT process for bonded assemblies integrity assessment.

Abstract: Damaged carbon fibre reinforced plastic (CFRP) aircraft parts may have been subjected during service to a range of hostile chemicals before being repaired by adhesively bonded patches. One chemical that could contaminate aircraft parts is the de-icing fluid used to maximize the runway friction during all plane movements at airports in winter. The scope of the present work is to experimentally investigate the effect of pre-bond contamination with de-icing fluid on integrity of CFRP bonded joints by conducting mode-I and mode-II fracture toughness tests on both reference and contaminated specimens. The de-icer used to contaminate the surface of one adherent was diluted with demineralized water to obtain solutions with the following concentrations in vol%: 2% (low level contamination, DI-1), 7% (medium level contamination, DI-2) and 10% de-icer (high level contamination, DI-3). Then, it was applied on the surfaces by dip coating and dried in the oven for 2h at 40°C. Afterwards, acclimatization at RT was allowed for at least 24h. The dip coating results were controlled by XPS measurements. Since the de-icer contains potassium formiate, the potassium content on the surface was taken as a measure for the degree of de-icer contamination. XPS-results showed mean values and standard deviations from two dip coated samples with three measuring positions each: DI-1: 6.4 (±1.8) at% K, DI-2: 10.9 (±2.3) at% K, DI-2: 12.0 (±1.4) at% K. The experimental results revealed a detrimental effect of de-icing fluid on the integrity of the joints. Specifically, for DI-1 a reduction of 30% and 56% is observed for GIC and GIIC, respectively, with regard to the reference values (non-contaminated joints), for DI-2 the corresponding values are 37% and 62% and for DI-3 it is 56% and 83%. Regarding the fracture surfaces, it was observed that with increasing the contamination level the presence of light-fiber-tear failure mode increases, which indicates that the de-icing fluid has a deleterious impact on the composite material.

Abstract: Quality assurance concepts for adhesive bonding of aircraft composite structures are crucial in order to strengthen production and material testing processes in all stages of the life cycle of aircrafts. The development of innovative extended non-destructive testing (ENDT) can strongly contribute to enable an overall quality assurance concept for adhesive bonding like the detection of multiple contaminants, kissing bonds and bonds weakening. Fast and precise detection of surface contaminations and defects like kissing bonds in bondline can help to save time (up to 70% time savings by using ENDT) during production, maintenance, overhaul, repair, and retrofit. Within the on-going Horizon 2020 funded project Quality Assurance Concepts for Adhesive Bonding of Aircraft Composite Structures by Advanced NDT (ComBoNDT) activities regarding the development of extended non-destructive testing (ENDT) are performed.ENDT techniques such as Optically Stimulated Electron Emission (OSEE) and Laser Induced Breakdown Spectroscopy (LIBS) are applied for the investigations of surface and bondline quality assurance. Carbon Fibre Reinforced Plastic (CFRP) test coupon samples were successfully produced considering a series of single contaminations characterizing typical manufacturing and repair scenarios. The ENDT techniques were explored, matured and optimized showing great capacity of detecting the presence of the single contaminants in both considered scenarios of manufacturing and repair.

Abstract: The permanent incorporation of sensors in composite structures is becoming a new trend in aeronautics, to facilitate strain monitoring and damage detection, often in conjunction to existing NDT methods. Except from classical electrical strain gauges and OFBG sensors, new magnetostrictive (i.e. coupled magnetic-mechanical) elements have been developed, to be attached on the external surface of composite structures, in order to monitor through-thickness strain/damage development. One of the main advantages of this methodology is the absence of connection or even contact of the sensing elements (magnetostrictive wires) to the measuring device. Using a single magnetic transducer, strain measurements are retrieved at any point of the concerned structure in a contact-free manner, thus facilitating inspection, especially in cases of monitoring of large areas. The proposed method could serve for fast – low cost components’ inspection at regular maintenance intervals, as well as for the improvement of aircraft certification procedures, providing part’s “strain signature” in a continuous manner (mapping), instead of spot measurements achieved today using strain gauges or OFBG sensors. Retrieved strain signature is subsequently compared against either previous readings or theoretical models, in order to trace potential differences and / or anomalies, indicating damage. In this paper, the latest developments of this novel strain sensing method are presented, together with results from its application on actual aeronautical structures.

Abstract: Safety, reliability, and economical prices are required for the diversified low-weight materials with high performance and for the respective fabrication and assembly processes in the aviation sector. In the H2020 joint research project “ComBoNDT – Quality assurance concepts for adhesive bonding of aircraft composite structures by extended NDT”, a quality assurance concept is developed which can be applied within the whole life cycle of the aircraft. Extended Non-Destructive Testing (ENDT) permits to steadily monitor adherent surfaces prior to bonding and to detect adhesion properties of bonded components.

A detailed approach regarding in-line ENDT for Carbon-Fibre Reinforced Polymers (CFRP) materials with increased technological readiness level (TRL), in-process and post-process, will be established. We present approaches for improving the detection capabilities, reliability and robustness of innovative NDT techniques like Optically Stimulated Electron Emission (OSEE), Aerosol-Wetting-Test (AWT) and Laser Induced Breakdown Spectroscopy (LIBS) for CFRP adherent surfaces with non-planar geometries or with multiple contaminations.

Abstract: Numerous techniques of non-destructive testing (NDT) of structural parts of CFRP are investigated. In this paper electromechanical impedance (EMI) technique is studied as an NDT tool for assessment of adhesive bonds. In order to perform the assessment a surface mounted piezoelectric sensor is used. Due to the piezoelectric effects the electrical response of the sensor is related to mechanical response of the inspected object. The electrical quantities of the sensor are tracked in order to find a relation with the mechanical state of the object. In the reported research adhesively bonded CFRP samples were investigated. The adhesive bonds were modified simulating the conditions in manufacturing and repair stages. Sample surface was contaminated before bonding with release agent in order to simulate a manufacturing stage threat to the quality of the bond. Pre-bond thermal treatment, pre-bond contamination with de-icing fluid, and faulty curing of the adhesive were considered as repair stage threats to the quality of the bond. The electromechanical impedance spectra were investigated searching for anomalies and changes caused by modification of the adhesive bond. These spectra for different cases were compared with reference measurement results gathered from pristine samples. Numerical indexes for comparison of the EMI characteristics were proposed. The sensitivity of the EMI method to modified bonds was observed.

2017

Abstract: Composite materials are commonly used in many branches of industry. One method to join or repair CFRP parts is by the use adhesive bonding. There is a search of effective methods for pre-bond assessment of bonded parts and post-bond inspection. Research reported here focuses on post-bond inspection of bonded CFRP plates. In this paper we reported results of two methods. We used noncontact ultrasonic testing (UT) technique as reference method. Ultrasonic testing was made in an immersion tank using phased-array probes. The second method was the electromechanical impedance (EMI). A piezoelectric sensors were surface mounted on each of the samples. Due to piezoelectric effect the electrical response of the sensor is related to mechanical response of the structure to which the sensors is bonded to. Measurements were conducted using HIOKI Impedance Analyzer IM3570. In order to perform a detailed study three samples of each kind were tested. There were three reference samples. The samples with modified adhesive bonds had three levels of severity, so there were three samples with each level of modification. The ultrasonic testing was focused on C-scan analysis taking into consideration the amplitude and time of flight (TOF). Two probes were used, one with 5 MHz frequency, second with 10 MHz. The EMI spectra were gathered up to 5 MHz and they were processed with signal processing algorithms in order to extract differences between reference samples and samples with modified bonds. The UT results provided relevant information about the investigated samples, while the EMI showed sensitivity to the level of adhesive bond modification.

Full paper: http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=2618006

Abstract: This paper presents the LAser Shock Adhesion Test (LASAT) usingsymmetrical laser shocks. The study is part of ComBoNDT European project that develops new Non-Destructive Tests (NDT) to assess adherence properties of bonded composite structures. This NDT technique relies on the creation of a plasma on both side of the sample using two lasers. The plasma expands andgenerates shockwaves insidethe material. When combined, the shockwaves create a local tensile strength. Properly set, this stress can be used to test interfaces adherence. Numerous experiments have shown that this adaptive technique can discriminate a good bond from a weak one, without damaging the composite structure. Weak bondsare usually created by contaminated surfaces (residues of release agent, finger prints, ...) and were artificially recreated for ComBoNDTtest samples.Numerical simulations are being developedas well,to improve the comprehension of the physical phenomenon.And ultimately, using these numerical results, one should be able to find the correct laser parameters (intensity, laser spot diameter) to generate the right tensile strength at the desired location.

Link: http://meetings.aps.org/Meeting/SHOCK17/Session/T4.6

Abstract: The paper deals with adhesive bonding of CFRP plates. Adhesive bonding of composites is considered as good alternative to riveting, provided that good tools are available to assess the bond quality after manufacturing and during the lifetime of structures. The performance of an adhesive bond depends on the physico-chemical properties of the adherent surfaces. A weak bond can be caused by improper surface preparation before bonding. In the research reported herein, modifications of one of the surfaces to bebonded were considered. These modifications are realized using chemical contamination or thermal treatment. The influence of surface modification on bond performance was investigated by electromechanical impedance (EMI) method, ultrasounds and mechanical tests. In the EMI method, a piezoelectric transducer was mounted at the middle of each sample surface. Using an impedance analyzer the electrical parameters were measured for wide frequency band. Due to piezoelectric effect the electrical response of a piezoelectric transducer is related to mechanical response of the sample to which the transducers is attached. The ultrasonic testing (UT) was focused on C-scan analysis taking into consideration the amplitude and time of flight of the signals. Aprobewith 10MHz central frequencywas used, to highlight different effects on the responses. Mode-I and Mode-II fracture toughness tests were conducted aiming to gain knowledge on the influence of surface modification on the bond strength. The performed investigation showed that bond quality is influenced by the bond modifications.

Link: https://openconf.org/iwshm2017/modules/request.php?module=oc_program&action=summary.php&id=163

Abstract: There is a continuous search of non-destructive testing (NDT) techniques dedicated to assessment of adhesive bonds. In this research we focus on adhesive bonds of composite parts made of carbon fibre reinforced polymers (CFRP). The electromechanical impedance (EMI) technique was proposed. The technique uses a piezoelectric sensor that is surface mounted onthe inspected structure. The electrical quantities of the sensor are measured as a function of frequency. Due to direct and converse piezoelectric effectstheelectrical response of the sensor is related to mechanical response of the structure. Analysing changes in conductance, resistance and other quantities one draws conclusion about the mechanical state of the structure. In the reported research adhesively bonded CFRP samples were investigated. Each sample comprised of two CFRP plates joint by the adhesive. Pre-bonds modification of the plates were investigated. The modification comprised of sample treatment (for example: thermal treatment) or contamination (for example with release agent).The paper shows results of EMI technique for single pre-bond modifications as well as mixed modifications of two types. Differences between the cases are discussed.

Abstract: There is a continuous search of non-destructive testing (NDT) techniques dedicated to assessment of adhesive bonds. In this research we focus on adhesive bonds of composite parts made of carbon fibre reinforced polymers (CFRP). The electromechanical impedance (EMI) technique was proposed. The technique uses a piezoelectric sensor that is surface mounted onthe inspected structure. The electrical quantities of the sensor are measured as a function of frequency. Due to direct and converse piezoelectric effectstheelectrical response of the sensor is related to mechanical response of the structure. Analysing changes in conductance, resistance and other quantities one draws conclusion about the mechanical state of the structure. In the reported research adhesively bonded CFRP samples were investigated. Each sample comprised of two CFRP plates joint by the adhesive. Pre-bonds modification of the plates were investigated. The modification comprised of sample treatment (for example: thermal treatment) or contamination (for example with release agent).The paper shows results of EMI technique for single pre-bond modifications as well as mixed modifications of two types. Differences between the cases are discussed.

W. Ostachowicz (IMP PAN), 2017. Highlights and Challenges in damage assessment of composite structures, VIII ECCOMAS Thematic Conference on Smart Structures and Materials SMART 2017, Madrid, Spain, June 2017.

M. Sagnard (CNRS), L. Berthe (CNRS), R. Ecault (AIRBUS GROUP SAS), F. Touchard (CNRS), M. Boustie (CNRS), 2017. Weak Bond Detection Through Laser Induced Shock, Journée Nationale des Composites Champs-sur-Marne, France, June 2017.

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 636494.

This project is endorsed by the European Aeronautics Science Network - EASN.

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