Disbonded Area Research Articles

Bonding quality monitoring of Carbon Fiber Reinforced Plastics bonded structures by fiber Bragg gratings

This paper deals with a new method for the continuous and remote monitoring of the bonding quality of Carbon Fiber Reinforced Plastics (CFRP) bonded structures. The proposed approach is based on the use of Fiber Bragg Gratings (FBGs) sensors and relies on the dependence of their temperature-induced strain sensitivity, generated by local inductions of (even small) thermal changes, on the quality of the bonding. Bonding composite laminates is a structural solution often adopted in aeronautics. The presence of disbonded areas during production is normally investigated with nondestructive tests (NDT), generally based on ultrasonic techniques (UT). However, these techniques do not always allow to detect areas with poor bonding quality (insufficient adhesion forces). To simulate such situations, some specimens were used for the experimental tests consisting of CFRP flat skins partially overlapped in a bonded area. To reproduce various bonding conditions, the specimens were bonded with different adhesive laying techniques. Experimental results here reported demonstrate that the FBGs thermal sensitivity can change significantly depending on the quality of the bonding in proximity of their position and can increase up to ∼ 50 % if the FBG is positioned in proximity of areas of the composite laminates presenting a disbonding or defects. The proposed monitoring approach is useful for in-service structural health monitoring of aircraft, supporting the NDT methods usually included in maintenance manuals.

The corrosion behavior and cracking susceptibility of the disbonded coating in the X80 steel pipeline by theoretical simulations and experimental measurements

Abstract The corrosion behavior and cracking susceptibility of the disbonded coating in the X80 steel pipeline were investigated by different methods. The oxygen content in the trapped solution decreased rapidly with the formation of an airtight disbonded area. The airtight system affected the electrode reaction process, resulting in the inhibition of corrosion in the center of the disbonded area and the more refined surface finish of the sample. The bottom of the disbonded area underwent a relatively intense and accelerated corrosion reaction controlled by the diffusion process. The cracking susceptibility of the X80 steel firstly decreased and then increased in the pointing direction.

Fabrication of MIL-88A sandwiched in graphene oxide nanocomposites using a green approach to induce active/barrier protective functioning in epoxy coatings

The MIL-88A as a Fe+3/fumarate-based metal-organic framework (MOF) was used for the modification of graphene oxide (GO). This MOF was grown successfully on GO sheets to achieve active corrosion inhibition and to improve barrier effect against corrosive agents. The electronic density functional theory with dispersion correction functional (DFT-D) computations revealed the interactions between the GO sheets and the complexes. The electrochemical impedance spectroscopy (EIS) and polarization analysis revealed an increase in the total resistance (79%) and a reduction in corrosion current density (83%) after 5 h immersion in the saline solution containing optimally synthesized MIL-88A/GO hybrid nanoparticles. This anti-corrosion pigment released 3.6 ppm Fe+3 and 225 ppm total carbon according to inductively coupled plasma-optical emission spectrometry and total organic carbon experiments. The scratched epoxy coatings with 0.15% of this nanocomposite showed active inhibition in EIS and salt spray study. Higher wet/dry adhesion strength values were detected in the coating incorporating synthesized particles. Also, the cathodic delamination results indicated a 50% and 42% decrement in the disbonded area compare to neat epoxy and epoxy/GO coatings, respectively. Finally, the EIS measurement confirmed modified GO sheets' excellent barrier effect. The coating damage percentage was calculated by the broad frequency range analysis method. The results expressed that the epoxy and epoxy-GO had 100% and 79% damage index after 105 days, while the damage index for the MIL-88A/GO epoxy coating was 4%.

Cathodic disbonding of self-healing composite coatings: effect of ethyl cellulose micro/nanocapsules

ABSTRACT The effect of ethyl cellulose (EC) micro/nanocapsules containing linseed oil (LO) on the cathodic disbonding of self-healing composite coatings is investigated by cathodic disbonding test and electrochemical impedance spectroscopy (EIS). Results reveal that the presence of EC nanocapsules significantly improves the cathodic disbonding resistance of the coatings so that the rates of disbonded area under the simulated condition of cathodic protection decrease from 3.882 mm2 h for the blank coating to 0.025 mm2 h for the nanocapsules containing coating. It may be ascribed to the releasing LO and filling the defects at the coating/substrate interface by oxidative polymerisation mechanism. EIS test results indicate that the corrosion resistance of the coatings is improved in the presence of EC nanocapsules. In addition, the coating resistance to cathodic disbonding and corrosion increase by decreasing capsule sizes. Finally, a proposed mechanism of ion diffusion through the self-healing coating’s defect is schematically illustrated.

Design new epoxy nanocomposite coatings based on metal vanadium oxy-phosphate M0.5VOPO4 for anti-corrosion applications

Epoxy nanocomposite coatings are an essential way to protect petroleum storage tanks from corrosion. For this purpose, the new nanocomposite epoxy coatings (P-M/epoxy composites) have been successfully designed. The P-M/epoxy composites are based on the metal vanadium oxy-phosphate M0.5VOPO4 (where M = Mg, Ni, and Zn). The function of P-M/epoxy composites as anti-corrosion coatings was explored using electrochemical and mechanical tests. Using electrochemical impedance spectroscopy (EIS), it has been noticed that the pore resistance and polarization resistance of the P-M/epoxy composites remain higher as compared to the neat epoxy. The P-M/epoxy composites have the greatest impact on the cathodic dis-bonded area and water absorption. Besides, P-M/epoxy composites exhibit a very high order of mechanical properties. Further, Mg0.5VOPO4 has the greatest effect on the anti-corrosion properties of epoxy coating followed by Zn0.5VOPO4 and Ni0.5VOPO4. All these properties lead to developing effective anti-corrosion coatings. Thus, the net result from this research work is highly promising and provides a potential for future works on the anti-corrosion coating.

Synergistic effect of imidazole dicarboxylic acid and Zn2+ simultaneously doped in halloysite nanotubes to improve protection of epoxy ester coating

Halloysite nanotubes as inhibitor container were introduced in an epoxy ester coating. To obtain synergistic impact of both inorganic and organic inhibitors, 4,5-imidazole dicarboxylic acid and Zn2+ were simultaneously doped in the halloysite nanotubes modified with 3-aminopropyltriethoxysilane (MHNTs). The electrochemical and surface analysis tests in NaCl solution showed that a mixture of inhibitors is able to provide higher corrosion inhibition for steel compared with the single inhibitors. With the mixture of the organic and inorganic moieties, an inhibition efficiency of 90% was achieved at the end of the 24 h test. Introducing of doped MHNTs in epoxy ester enhanced the barrier effects and active corrosion protection. EIS and EDS studies on the scratched coatings confirmed the effective active protection resulted from incorporation of double-doped MHNTs into the polymeric coating. As a result of salt spray test, lower disbonded area was found for the scratched epoxy ester film bearing the double-doped MHNTs. The precipitation of zinc hydroxide and a complex of Zn2+/Fe2+-IDC might be the main reason for the superior function.

X-ray microprobe characterization of corrosion at the buried polymer-steel interface

The simultaneous collection of x-ray fluorescence (XRF) intensity maps for several elements was combined with x-ray absorption near-edge spectroscopy (XANES) to perform a non-destructive study of polymer-steel interfaces exposed to high pH potash brine. Analysis of iron, manganese, potassium and chlorine XRF maps allowed the identification of anodic pits, disbonded areas containing precipitated corrosion products and several cathodic regions. Iron XANES was used to determine the phases of Fe corrosion products present. Our results confirm previous models of corrosion under polymer films where osmotic blisters are causally related to the spread of corrosion at the polymer-metal interface.

SHM of single lap adhesive joints using subharmonic frequencies

The increased usage of adhesive bonding as a joining method in modern aerospace components has led to developing reliable ultrasonic health monitoring systems for detection of regions of poor adhesion. Nonlinear acousto-ultrasonic techniques based on higher harmonics and subharmonic frequencies have shown to be sensitive to the detection of micro-voids and disbonds. Nonlinear resonance properties of disbonds generate various nonlinear phenomena such as self-modulation, subharmonics, hysteresis and so on. By exploiting the local natures of these phenomena, this paper demonstrates the use of subharmonics for detection and imaging of flaws in bonded structures. To optimise the experimental testing a two-dimensional analytical model and a three-dimensional finite element analysis simulation were developed for understanding the generation of nonlinear elastic effects with emphasis on subharmonic frequency components. The proposed analytical model qualitatively described the generation of subharmonics but also higher harmonics due to the nonlinear intermodulation of the driving and resonance frequencies associated with the disbonded region. The numerical model was developed by modifying the user defined cohesive element formulation with a bi-linear traction-displacement relationship in order to simulate the interaction of elastic waves with the structural disbond. Whilst the analytical model supported the selection of the driving frequency, the numerical one successfully predicted the generation of subharmonic frequencies originating in the disbonded area. Experimental tests were conducted on a disbonded single lap joint structure using surface-bonded piezoelectric transducers and a laser-Doppler vibrometer, and allowed to validate the analytical and numerical results. It was clearly demonstrated that the nonlinear resonance effects in the form of subharmonics could be used to discriminate reliably regions of poor adhesion in bonded structures. This work can lead to new in situ nonlinear acoustic based health monitoring system for locating and imaging defects in critical aerospace components.

Monitoring Cathodic Shielding and Corrosion under Disbonded Coatings

This work presents a novel corrosion monitoring probe designed for simulating the conditions developed under disbonded coatings and for measuring current densities and their distribution over a simulated pipeline surface. The probe’s concept was experimentally evaluated via immersion tests under Cathodic Protection (CP) in high resistivity aqueous solution. Under the disbonded area, anodic currents as well as cathodic currents were both measured. Anodic current densities were used to calculate metal losses by means of Faraday’s law. Calculated corrosion patterns were compared with corrosion damage observed at the probe’s surface after a period of test. The probe’s working principles are explained in terms of simple electrochemistry.

Experimental and numerical disbond localization analyses of a notched plate repaired with a CFRP patch

Through the use of finite element analysis and acoustic emission techniques we have evaluated the interfacial failure of a carbon fiber reinforced polymer (CFRP) repair patch on a notched aluminum substrate. The repair of cracks is a very common and widely used practice in the aeronautics field to extend the life of cracked sheet metal panels. The process consists of adhesively bonding a patch that encompasses the notched site to provide additional strength, thereby increasing life and avoiding costly replacements. The mechanical strength of the bonded joint relies mainly on the bonding of the adhesive to the plate and patch stiffness. Stress concentrations at crack tips promote disbonding of the composite patch from the substrate, consequently reducing the bonded area, which makes this a critical aspect of repair effectiveness. In this paper we examine patch disbonding by calculating the influence of notch tip stress on disbond area and verify computational results with acoustic emission (AE) measurements obtained from specimens subjected to uniaxial tension. The FE results showed that disbonding first occurs between the patch and the substrate close to free edge of the patch followed by failure around the tip of the notch, both highest stress regions. Experimental results revealed that cement adhesion at the aluminum interface was the limiting factor in patch performance. The patch did not appear to strengthen the aluminum substrate when measured by stress-strain due to early stage disbonding. Analysis of the AE signals provided insight to the disbond locations and progression at the metaladhesive interface. Crack growth from the notch in the aluminum was not observed until the stress reached a critical level, an instant before final fracture, which was unaffected by the patch due to early stage disbonding. The FE model was further utilized to study the effects of patch fiber orientation and increased adhesive strength. The model revealed that the effectiveness of patch repairs is strongly dependent upon the combined interactions of adhesive bond strength and fiber orientation.

A monitoring technique for disbond area in carbon fiber–reinforced polymer bonded joints using embedded fiber Bragg grating sensors: Development and experimental validation

This study evaluated fatigue-induced disbond areas in carbon fiber–reinforced polymer double-lap joints using embedded fiber Bragg grating sensors. When the disbond grew by cyclic loading, the embedded fiber Bragg grating sensors yielded reflection spectra having two peaks representing a step-like strain distribution generated by the disbond; the peak at the shorter wavelength corresponded to the unloaded disbond region. The ratio of the peak intensity at the shorter wavelength to that at the longer wavelength increased gradually with increasing disbond length. The relationship between the peak intensity ratio and the disbond length was analyzed by coupled structural–optical analysis and was validated by comparing analytical peak intensity ratio with the experiment results. The disbond length was then estimated from the measured spectra based on this analytical calibration relationship, but the estimated disbond area exceeded that observed using the ultrasonic C-scan technique. Additional experiments including destructive observation of the adhesive suggested that an embedded fiber Bragg grating sensor could detect a moving disbond tip earlier than conventional nondestructive techniques.

The Effect Rust and Over-Protection Voltage of Impressed Current Cathodic Protection towards LR Grade a Steel Disbondment

Corrosion at the bilge due to direct contact with the seawater is an issue which is necessary to be taken into account. The problem can be solved by giving combined protection such as coating and cathodic protection impressed current cathodic protection (ICCP). For broader range, there is occasionally a possibility of over-protection at certain area especially that is close to anodes. It can trigger cathodic disbondment to occur. Cathodic disbondment is a situation when the coating loses its adhesion to the steel due to the voltage originated from cathodic protection. This trial was conducted by using several samples of LR Grade A plate coated by primer coat epoxy and top coat polyurethane. The plate was then given artificial defect with the size of R2, R3 and R4 accoring to ISO 4628-3 or is equal to 0,5% ; 1% and 8% of the plate surface width. Futhermore, impressed current cathodic protection was conducted with the steel anode, Ag/AgCl reference electrode and over-protection voltage were-1,5 ; -2,0 ; -2,5 ; -3,0 ; -3,5 and-4,0 volt. Electrolyte media employed was seawater. Furthermore, the disbondment taken place was measured after 6 hours and analyzed the corrosion product produced. In the research, it could be seen that the broader the defect was, the smaller the disbondment area was. The width of disbondment area was depending on current density. The smaller the voltage (volt) was, the broader the disbondment in accordance with the increase of disbondment width taking place on the plate surface. The corrosion product produced was Fe3O4 (Magnetite).

Understanding the effectiveness of cathodic protection under disbonded coatings

Factors affecting the effectiveness of cathodic protection under disbonded coatings were studied using a partially covered two dimensional electrode array that simulates a crevice under a disbonded coating and allows for the mapping of electrochemical currents under the influence of cathodic protection (CP). This technique enabled the study of the effects of major factors, including crevice gap size, solution conductivity and applied CP potential on the distribution and evolution of CP currents over the electrode array surface. The effect of each of these factors on the overall current distribution profile has been explained using a new electrochemical model. This model suggests that, despite the detrimental effect of cathodic shielding, the steel under disbonded areas could still be protected, independent of the crevice geometry and solution resistivity, by means of concentration polarization instead of direct electrochemical polarization. A set of conditions for maintaining CP potentials more negative than −850mVCSE along disbonded coating areas was deduced from the proposed model and validated against potential profiles available in the literature. The possible practical implications of the current mapping technique used in this study for developing in-situ CP monitoring sensors has also been discussed.

Electrochemical impedance spectroscopy as a tool to measure cathodic disbondment on coated steel surfaces: Capabilities and limitations

The disbondment of protective organic coatings under excessive cathodic protection potentials is a widely reported coating failure mechanism. Traditional methods of evaluating cathodic disbondment are based on ex situ visual inspection of coated metal surfaces after being exposed to standard cathodic disbondment testing conditions for a long period of time. Although electrochemical impedance spectroscopy (EIS) has been employed as an effective means of evaluating various anti-corrosion properties of organic coatings; its application for assessing the cathodic disbondment resistance of coatings has not been sufficiently exploited. This paper reports an experimental study aimed at developing EIS into a tool for in situ measurement and monitoring of cathodic disbondment of coatings. A clear correlation between EIS parameters and the disbonded coating areas has been confirmed upon short term exposure of epoxy-coated steel electrodes to cathodic disbondment conditions; however the degree of this correlation was found to decrease with the extension of exposure duration. This observation suggests that EIS loses its sensitivity with the propagation of coating disbondment, and that in order to achieve quantitative determination of the coating cathodic disbondment localized EIS measurements are required to measure the parameters related to local disbonded areas.

An Electrochemical Measurement for Evaluating the Cathodic Disbondment of Buried Pipeline Coatings under Cathodic Protection

Steel pipelines are susceptible to corrosion by the action of corrosive substance in the environment and one of the most common failure modes in buried pipeline coatingis cathodic disbondment. Consequently, Electrochemical Impedance Spectroscopy (EIS) was used to assess the effect of three important parameters, the thickness of coating, the artificial defect and electrolyte type, on cathodic disbondment, according to ASTM G8. Experiments were done in 3.5 % wt NaCl, KCl and CaCl2 solutions at room temperature with different coating thicknesses (354, 483 and 1014μm) and several artificial defects diameters (3, 6 and 9 mm). Immersion time was 28 days and during this period, a -1.5 V cathodic potential (vs. SCE) was applied. Further, the EIS measurements were done in Open Circuit Potential (OCP). Investigations showed that a thick polyurethane coating was highly resistant to cathodic disbondment in intact areas and that coating thickness increases caused decreases in the rate of cathodic disbondment in areas with defects. On the other hand, the evaluation on artificial defects showed disputable results, though with no logical correlations between artificial defects’ diameters and any disbonded area individually. In addition, assessment on electrolyte type showed the rate of cathodic disbonding might be classified after two parameters: solubility and mobility of ions. So, the disbonded area was dependent on the cation type in the electrolyte, and the anion diffusion had less effect in the cathodic disbondment process. Furthermore, the comparison between the cations and the anions in solution showed that the most important coating disbondments occurred when cations / anions are in a 1:1 ratio.

Evaluation of the effects of surface treatments on the cathodic delamination and anticorrosion performance of an epoxy-nanocomposite on steel substrate

The St-37 type steel substrates were pretreated with Cr(VI) and Cr(III) conversion coatings where the latter was then post-treated with Co(II) and Ni(II) chemical treatments. The epoxy coatings containing 3.5 wt% nano-sized ZnO particles were applied over the chemically treated steel samples. The corrosion resistance of the samples was studied by a DC polarization technique. A scanning electron microscope (SEM) was utilized to investigate the morphology of the pretreated and post-treated samples. Electrochemical impedance spectroscopy (EIS) was utilized to investigate the corrosion resistance of the epoxy nanocomposites for different immersion times in 3.5 wt% NaCl solution. The adhesion strengths of the coatings were measured before and after 120 days of immersion in the corrosive electrolyte using a pull-off test. The cathodic delamination (CD) of the painted samples was also investigated. Results showed that conversion coatings can significantly increase the corrosion resistance and adhesion strength of the epoxy coating on the steel, and also reduce the rate of CD in comparison with an untreated sample. The adhesion strength and corrosion resistance of the epoxy coating on the Cr(III) pretreated samples were significantly greater than on the Cr(VI) sample. The increase in adhesion strength and corrosion resistance was more pronounced on the samples that were post-treated with Co(II) and Ni(II) chemical treatments. The cathodic disbonded areas of the Cr(III)–Co(II) and Cr(III)–Ni(II) post-treated samples were significantly lower than the Cr(III) and Cr(VI) pretreated samples. Results showed that Cr(III)-based conversion coatings can improve the anticorrosion performance and reduce CD compared with those with Cr(VI).

Evaluation of the corrosion resistance of an epoxy-polyamide coating containing different ratios of micaceous iron oxide/Al pigments

The corrosion resistance of an epoxy coating reinforced with different ratios of MIO/Al pigments was studied. The coatings properties were investigated by an electrochemical impedance spectroscopy (EIS), salt spray test, cathodic disbonding and a scanning electron microscope (SEM). The corrosion resistance of the epoxy coating was improved using MIO (micaceous iron oxide) and Al pigments. The corrosion resistance of the purely Al pigmented coating was considerably greater than the purely MIO pigmented coating. The cathodic disbonded area of coating was decreased using MIO and Al pigments. The decrease in disbonded area was more pronounced in the presence of Al particles.

Effective spring stiffness for a planar periodic array of collinear cracks at an interface between two dissimilar isotropic materials

Explicit analytical expressions are obtained for the longitudinal and transverse effective spring stiffnesses of a planar periodic array of collinear cracks at the interface between two dissimilar isotropic materials; they are shown to be identical in a general case of elastic dissimilarity (the well-known open interface crack model is employed for the solution). Since the interfacial spring stiffness can be experimentally determined from ultrasound reflection and transmission analysis, the proposed expressions can be useful in estimating the percentage of disbond area between two dissimilar materials, which is directly related to the residual strength of the interface. The effects of elastic dissimilarity, crack density and crack interaction on the effective spring stiffness are clearly represented in the solution. It is shown that in general the crack interaction weakly depends on material dissimilarity and, for most practical cases, the crack interaction is nearly the same as that for crack arrays between identical solids. This allows approximate factorization of the effective spring stiffness for an array of cracks between dissimilar materials in terms of an elastic dissimilarity factor and two factors obtained for cracks in a homogeneous material: the effective spring stiffness for non-interacting (independent) cracks and the crack interaction factor. In order to avoid the effect of the crack surface interpenetration zones on the effective spring stiffness, the range of the tensile to transverse load ratios is obtained under the assumption of small-scale contact conditions. Since real cracks are often slightly open (due to prior loading history and plastic deformation), it is demonstrated that for ultrasound applications the results obtained are valid for most practical cases of small interfacial cracks as long as the mid-crack opening normalized by the crack length is at least in the order of 10 −5.

A modelling technique for calculating stress intensity factors for structures reinforced by bonded straps. Part II: Validation

In this second part of the two-part paper validation of the 2D FE modelling technique described in the first part is presented for a range of test configurations. Each mechanism that influences crack growth behaviour of strap reinforced structures is modelled for different substrate geometries, strap materials and dimensions in order to test the accuracy and robustness of the methodology. First, calculated through-thickness strain energy release rate distribution is compared with the result of a 3D FE model to validate this 2D model. Second, calculated disbond areas, thermal residual stresses and their redistribution with crack propagation are validated against experimental measurements. Third, influence of geometric nonlinearity and the need to use the alternate analysis method described in part I are demonstrated by examples, and errors generated by not following this analysis rule are given. Finally, using the 2D model calculated stress intensity factors, fatigue crack growth rates and lives are predicted for different specimens, strap materials and applied stress levels and are compared with the experimental tests. Good or acceptable agreement has been achieved for each case.

Full-width Disbonding Effect on Repaired Cracks in Structural Panels with Bonded Composite Patches

The process of repair of structures by using bonded composite patch is an effective and economical method to increase the service life of damaged structures. In this study, the finite element method is used to analyze the behavior of a crack repaired by a circular patch by computing the stress intensity factor (SIF) at the crack tip with and without the disbond. The obtained results show that the boron/epoxy 0° fiber perpendicular to the crack, considerably influences the reduction of the SIF with regard to the boron/epoxy 0° fiber oriented in the parallel direction to the crack. Moreover, the increase of the disbond area amplifies the SIF of the repaired crack and the rate of this increase strongly depends on the disbond area. When the disbond is significant between the patch and the plate, it reduces the repair effectiveness and the crack growth under loading becomes important. The disbond growth results in patch separation.