Conference & Workshop Proceedings

2016

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.

Full paper: http://www.ndt.net/article/aero2016/papers/Pawe%C5%82HMalinowski_TomaszWandowski_Wies%C5%82awMOstachowicz.pdf

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 GICand 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: 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.

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: https://www.imp.gda.pl/fileadmin/doc/projects/ComBoNDT/101700B.pdf

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.

E. Moutsompegka (LTSM-UPAT), 2017. Experimental investigation of the effects of pre-bond fingerprint contamination and ageing on fracture toughness of composite bonded joints, Proceedings of the 7th EASN International Conference on Innovation in European Aeronautics Research, EASN Association, Warsaw, Poland, September 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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