Artificial Debonding Research Articles

A hierarchical multistage holistic model for acoustic emission source monitoring in composites

Abstract This paper introduces a multistage smart structural health monitoring (SHM) model for carbon-fiber composites, with a focus on multiple types of acoustic emission (AE) source localization and classification. The SHM model uses time-frequency data from various AE events (such as tool drops, impact, and artificial debonding) across different zones of a composite structure. The SHM strategy demonstrates a robust smart monitoring of composites with high accuracy. Further, a hypothesis testing has been carried out that supports the superiority of a 2-stage identification process, revealing statistically significant higher accuracy and confidence intervals across all zones and AE source types. This research establishes a novel framework for solving a hierarchical multistage holistic damage source identification problem, offering robustness in identifying various damage scenarios and quantifying associated prediction uncertainties.

Automatic high depth-resolvability inspection of TBC debonding defects using nonlinear frequency modulation photothermal radar

In this article, a novel nonlinear frequency modulation (NLFM) photothermal radar whose instantaneous frequency curve is a concave quadratic function is proposed and further combines K-means clustering image segmentation algorithm to achieve automatic high depth-resolvability inspection of debonding defects in thermal barrier coating (TBC) structure. Firstly, a 3-dimensional (3D) photothermal finite element (FE) model of TBC structure is established, where the extrapolated Beer-Lambert model is introduced to describe the spatially volumetric heat source within the ceramic coating. Afterwards, the superior depth resolvability of the proposed NLFM photothermal radar is demonstrated compared to conventional waveforms using the established FE model. Meanwhile, its depth resolvability dependence on the thickness and thermal diffusivity of the ceramic coating is also investigated. Finally, the photothermal experiments of the TBC structure containing artificial debonding defects are conducted, and the influence of the coating thickness and metal substrate on the non-destructive testing (NDT) detection of TBC structure by NLFM photothermal radar combining K-means clustering is also experimentally studied. The results reveal that NLFM photothermal radar combining K-means clustering can achieve automatic high depth-resolvability inspection of debonding defects in TBC structure and the coating thickness and metal substrate have also a great influence on the photothermal NDT of the debonding defects of TBC structure.

Visualized evaluation of mechanical bonding properties of cementitious material interfaces based on volumetric strain analysis

The interphase transition zone (ITZ) of cementitious materials interfaces is widely considered a weak location, therefore, the mechanical properties of the ITZ must be properly assessed. However, no currently available methods can accurately distinguish between good bonding or debonding conditions. This study proposes a method to visualize the debonding areas of new-to-old mortars based on a combination of in-situ X-ray computed tomography (XCT) and digital volume correlation (DVC). Under uniaxial compression and slant shear testing, samples with different artificial debonding areas were deformed by applying a relatively small load. Then, the in-situ loading XCT testing setup was used to track deformations, and DVC was used to calculate the volumetric strain. The results showed that under uniaxial compression tests and slant shear testing, the debonding area had larger deformation and higher strain values, and the different debonding ratios areas of the new-to-old mortars were successfully visualized.

Damage assessment of a titanium skin adhesively bonded to carbon fiber–reinforced plastic omega stringers using acoustic emission

This study is devoted to the use of acoustic emission technique for a comprehensive damage assessment, that is, damage detection, localization, and classification, of an aeronautical metal-to-composite bonded panel. The structure comprised a titanium panel adhesively bonded to carbon fiber–reinforced plastic omega stringers. The panel contained a small initial artificial debonding between the titanium panel and one of the carbon fiber–reinforced plastic stringers. The panel was subjected to a cyclic increasing in-plane compression load, including loading, unloading, and then reloading to a higher load level, until the final fracture. The generated acoustic emission signals were captured by the acoustic emission sensors, and digital image correlation was also used to obtain the strain field on the surface of the panel during the test. The results showed that acoustic emission can accurately detect the damage onset, localize it, and also trace its evolution. The acoustic emission results not only were consistent with the digital image correlation results, but also managed to detect the damage initiation earlier than digital image correlation. Finally, the acoustic emission signals were clustered using particle swarm optimization method to identify the different damage mechanisms. The results of this study demonstrate the capability of acoustic emission for the comprehensive damage characterization of aeronautical bi-material adhesively bonded structures.

Evaluation of Bonding Quality with Advanced Nondestructive Testing (NDT) and Data Fusion.

This work aims to compare quantitatively different nondestructive testing (NDT) techniques and data fusion features for the evaluation of adhesive bonding quality. Adhesively bonded composite-epoxy single-lap joints have been investigated with advanced ultrasonic nondestructive testing and induction thermography. Bonded structures with artificial debonding defects in three different case studies have been investigated: debonding with release film inclusion, debonding with brass film-large, debonding with brass film-small. After completing preprocessing of the data for data fusion, the feature matrices, depending on the interface reflection peak-to-peak amplitude and the principal component analysis, have been extracted from ultrasonic and thermography inspection results, respectively. The obtained feature matrices have been used as the source in basic (average, difference, weighted average, Hadamard product) and statistical (Dempster–Shafer rule of combination) data fusion algorithms. The defect detection performances of advanced nondestructive testing techniques, in addition to data fusion algorithms have been evaluated quantitatively by receiver operating characteristics. In conclusion, it is shown that data fusion can increase the detectability of artificial debonding in single-lap joints.

Experimental and numerical study of the effect of using polyurethane instead of Teflon strip to simulate debonding defect in composite patch repairs aluminum plate under thermography inspection

In order to evaluate the capabilities of a non-destructive testing (NDT) technique for detecting the potential defects, the simulation conditions of manufactured defects must be correspond to real defects from shape and material point of view. Debonding defects are one of the most dangerous types of damage in composite patches for repairing cracks and defects in aircraft structures. In the case of real debonding defect, a thin air layer is embedded between the aluminum and the epoxy resin adhesive. In general, an artificial debonding is simulated by inserting Teflon (Polytetrafluoroethylene) tapes within the joint. This paper proposes the application of polyurethane (PU) instead of polytetrafluoroethylene (PTFE) strip to simulate debonding in composite when the specimen is inspected by thermography technique. The effect of the simulation of debonding defect at the edge of composite patch using polyurethane, Teflon tapes, and air gaps under thermography inspection is studied. Finite element modeling (FEM) and experimental verification of debonding at the edge of composite patch show a different efficiency of thermography inspection due to different thermal properties of Teflon, polyurethane, and air gap (real debonding). Comparing the performance of thermography inspection of composite patch with debonding simulated by PU, PTFE and air reveal the importance of simulating defects with realistic industrial configurations. Finite element analysis and experimental results show that using the proposed polyurethane sheet instead of Teflon sheet can provide much more realistic simulation of debonding defect in composite patch subjected to thermography inspection.

The effect of interface debonding on flexural behaviour of composite sandwich beams

In the current article, the behaviour of sandwich beams with and without initial core–skin debonding is studied under flexural loads through numerical and experimental procedures. Sandwich beams with three different lengths of 100, 180 and 280 mm and two types of composite skin layups of [0/90]2 and [45/–45]2 are fabricated. An initial artificial debonding is created between core and face sheets during manufacturing the flawed sandwich beams. Numerical simulations and experiments of the short- and medium-sized intact beams revealed that the dominant failure mode is foam yielding and crushing. Thus, the composite skins layup sequence has almost no effect on the failure initiation and growth of those beams. However, in the long-sized sandwich beams, the layup sequence changed the load–displacement response of the beams. Moreover, ignoring the nonlinear behaviour of the composite skins caused a remarkable deviation from the experiment. It is shown that sandwich beams with initial debonding placed in tension side had a negligible effect on the loading capacity of the beams, while those on the compression side had remarkable effects. For instance, the ultimate load of the long-sized beam decreased by 56% compared to the intact sandwich beam. Similarly, in the medium-sized beam, the core–skin debonding in the compressive side caused near 20% reduction in the loading capacity compared to the corresponding intact beam. The cohesive zone model and the extended finite element method were utilized successfully to capture crack initiation and propagation between the core–skin interfaces as well as inside the foam core. Acceptable agreement was observed between the experiment and numerical results.

Research on Debonding Defects in Thermal Barrier Coatings Structure by Thermal-Wave Radar Imaging (TWRI)

In this paper, thermal-wave radar imaging (TWRI) is introduced to detect debonding defects in SiC-coated Ni-based superalloy plates. Linear frequency modulation signal (chirp) is used as the excitation signal which has a large time–bandwidth product. Artificial debonding defects in SiC coating are excited by the laser beam with the light intensity modulated by a chirp signal. Cross-correlation algorithm and chirp lock-in algorithm are introduced to extract the thermal-wave signal characteristic. The comparative experiment between TWRI reflection mode and transmission mode was carried out. Experiments are conducted to investigate the influence of laser power density, chirp period, and excitation frequency. Experimental results illustrate that chirp lock-in phase has a better detection capability than other characteristic parameters. TWRI can effectively detect simulated debonding defects of SiC-coated Ni-based superalloy plates.

Eddy Current Thermography with Adaptive Carrier Algorithm for Non-destructive Testing of Debonding Defects in Thermal Barrier Coatings

Interfacial defects can cause the premature failure of thermal barrier coatings (TBCs). An Eddy current thermography (ECT) method under the transmission mode in the heating phase is developed in this study to detect the artificial debonding defects in TBCs samples. When ECT is used, the temperature distribution of specimen surface is uneven on account of geometric heating effect, skin effect, edge effect and abnormal emissivity. Among them, the influence of the surface emissivity is the smallest, because the background noise is subtracted before the thermal images are processed. The uneven temperature distribution shields the weak thermal response characteristics of the defects and interferes with the identification of the defects. Adaptive carrier algorithms are established as post-processing algorithms to resolve this problem. The feasibility and validity of the developed methods are verified by simulation and validation tests using TBCs samples with a 2 mm artificial debonding defect and a 0.5 mm blind-hole defect.

A numerical procedure for the simulation of skin–stringer debonding growth in stiffened composite panels

In this paper, a numerical study on the skin–stringer debonding growth in composite panels under compressive load is presented. A novel numerical procedure, for the selection of proper material parameters governing the traction–separation law in Cohesive Zone Model (CZM) based elements, is introduced and demonstrated. Indeed, the proposed procedure uses Virtual Crack Closure Technique (VCCT) based FEM analyses on Double Cantilever Beam (DCB) and End Notched Flexure (ENF) specimen to characterize the traction–separation law, respectively, for fracture mode I and mode II. The established traction–separation laws are then applied to composite structures containing inter-laminar damages modeled by cohesive elements. To validate the proposed approach, a single stringer panel under compression with an artificial debonding between skin and stringer, has been considered. The numerical results, in terms of displacements and debonding size as a function of applied compressive load, have been compared to experimental data available in literature providing a good numerical–experimental correlation.

Stainless Steel Brackets: In Vitro Corrosion Behaviour and Mechanical Debonding Test

The purpose of the study was to investigate the microchemical, morphological, mechanical and anticorrosive characteristics of the metallic brackets by using energy-dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM), electrochemical and shear tests. The changes resulted from the exposure of the metallic brackets to artificial saliva and debonding forces were investigated under orthodontic appliances. The results have shown that metallic brackets studied in this paper present a good design, great electrochemical behaviour in artificial saliva for different pH values (2, 5 and 7), and also great mechanical properties. In the case of shear stress the obtained values were higher than the clinical accepted value. The metallic brackets studied in this paper present a good behaviour in artificial saliva, and good mechanical characteristics. The ARI investigation revealed that most of the adhesive was left on the bracket.

Experimental analyses of metal-composite bonded joints: damage identification

Abstract The advent of composite co-cured and co-bonded integrated construction in aircraft structures has lead to the replacement of fastened joints with bonded joints between the skins and the stiffeners. Skin-stiffener debondings could occur due to impact or other operational reasons and it is usually internal failure. Damage identification of bonded components, which are often vital elements in many structures, is crucial for the prevention of failure of the entire structure. Thus, different researchers have investigated vibration-based methods as an alternative technique to be used in the structural health monitoring (SHM) systems. Hence, this work consists of investigating experimentally through the vibration-based method, the dynamic behavior changes in a bonded metal-composite structure by using piezoelectric transducer and accelerometers in order to monitory the damage. The damage is an artificial debonding in the joint, which was simulated by inserting Teflon™ tapes within the joint. In-situ inspection as ensured by accelerometer and piezoelectric transducers (PZT) bonded to the structure. Indeed, with a simple comparison of the frequency response functions is difficult to conclude if there is damage in the structure, unless a large damage is presented. However, by using damage metrics, it is possible to identify the damage with more accuracy. Thus, the experimental results obtained by the accelerometers were compared to the data provided by the smart composite sensors (PZT). Finally, it was discussed the advantages and limitations of the experimental analyses and the identification technique proposal.

Skin Stringer Debonding Evolution in Stiffened Composite Panels under Compressive Load: A Novel Numerical Approach

In this paper, a numerical study, on the compressive behaviour of stiffened composite panels with skin-stringer debonding has been carried out. The analysis has been performed by adopting a novel robust (mesh and time step independent) finite elements based numerical model on a single stiffener panel with an artificial debonding. In order to prove the effectiveness of the proposed numerical tool, the results in terms of debonded area growth and compressive load versus applied displacement, have been compared with experimental data available in literature.

Active Debonding Detection for Large Rectangular CFSTs Based on Wavelet Packet Energy Spectrum with Piezoceramics

AbstractThe debonding between the steel tube and the confined concrete core of concrete-filled steel tubes (CFSTs) can weaken the confinement effect of the steel tube on the concrete core and may induce a decrease in the load-carrying capacity and in the ductility. The debonding detection technique for CFSTs is still a challenging problem resulting from the inaccessibility and invisibility of the interface. In this paper, by embedding piezoelectric lead zirconate titanate (PZT) based functional smart aggregates (SAs) in concrete core as actuators and pasting PZT patches on the predetermined locations of the outer surfaces of the specimen as sensors, a novel active interface condition monitoring approach for rectangular CFST members is proposed and is experimentally validated with a scale rectangular CFST specimen with artificial debonding defects. Analysis of the wavelet packet energy spectrum (WPES) of the measurements of PZT patches is carried out, and a damage index, called the weighted variation of WP...