Bond Layer Research Articles

Curing monitoring of adhesive layers between metal adherends by ultrasonic resonance technique

Abstract Layer resonance induced by ultrasonic wave incidence is applied to monitor the viscoelastic properties of a curing adhesive layer between metal plates. The theoretical analysis shows that when the adhesive layer is modeled as a linear viscoelastic material, the ultrasonic reflection spectrum takes local minima at the layer resonance frequencies depending on the wave velocity of the adhesive. On the other hand, the local minima of the reflection spectrum can be expressed as a function of the loss factor of the adhesive. Based on these results, a characterization technique for the wave velocity and the loss factor of a curing adhesive layer is proposed. This technique enables the evaluation of the viscoelastic properties even if the reflected waves from both faces of a bond layer cannot be separated. The proposed method is employed to investigate the curing behavior of bi-component epoxy adhesives. As a result, it is shown that the bonding condition affects the variation of the wave velocity and loss factor. The estimated wave velocity increases as the curing proceeds, while the loss factor sometimes takes a local maximum depending on the bonding condition and the frequency range. When the mixing ratio of the main and curing agents is imbalanced, the variation of the wave velocity becomes gradual. Furthermore, the increase in the curing temperature leads to fast changes in the wave velocity and loss factor. The proposed technique has the potential to provide insight into the curing behavior of the adhesive layer by incorporating it into other measurement methods and theories.

Effects of oxidation‐induced aggregation structure transformation of aramid fiber on interfacial adhesion of epoxy resin

AbstractThe large‐scale pretreatment and efficient surface activation of aramid fibers (AFs) before composite fabrication remains a major challenge. In this study, we developed a heat treatment–induced surface modification method to change the reactive groups on AFs. Results indicated that the O/C ratio and the number of ester groups on the AFs as well as the surface morphology of the AFs could be flexibly controlled using the heat treatment–induced surface modification method. Furthermore, the surface oxidation mechanism of the AFs changed from point oxidation to surface oxidation during heat treatment. As the heat‐treatment time increased, the crystallinity and tensile strength of the AF monofilament considerably increased. An optimal heat‐treatment time of 30 min was provided considering that long‐time heating (>30 min) would destroy the surface molecular chains. Under this optimal heat‐treatment time, the number of ester groups reached the maximum, which enhanced the reactivity of the AFs in the epoxy resin matrix. The interfacial shear strength between the AFs treated for 30 min and epoxy resin microdroplet increased by 12.64%, reaching as high as 18.17 MPa. Moreover, the contact angle between the AF monofilament and epoxy resin droplet reached a minimum value of 54.7°. Furthermore, the thickness of the interfacial bond layer between the AFs and epoxy resin reached 50–60 nm. These results provide effective theoretical guidance for the large‐scale application of AFs in composite materials.

Assessing thermal resistance in fusion bond layers of 3D heterogeneous electronics packaging

We investigated thermal management challenges in semiconductor devices that use fusion bonding, with a focus on measuring the thermal resistance of the bonded interface. Starting with two fusion-bonded silicon wafers, we thinned the top wafer to below 100 μm and used frequency domain thermoreflectance to measure the effective thermal boundary resistance of the fusion bond. We established the measurement uncertainty with a multilayer thermal diffusion model, considering the effects of the top Si layer thickness and laser spot size. The thermal boundary resistance was measured to be 0.2 and 0.5 m2K/MW for two types of bonded wafers, with total SiO2 film thicknesses of 200 and 470 nm at the fusion bond layer, respectively. We also created defects at the interface and imaged these defects with the thermal phase response at specific frequencies chosen according to a thermal model. The method described can be applied to other types of wafer-to-wafer or die-to-die bonding including direct, polymeric, and metallic bonds.

Effects of irradiation on interfacial strength and microstructure of double-layer mullite and alumina coating on SiC

Silicon carbide (SiC) ceramics hold great potential for use in nuclear-reactor components due to characteristics such as high-temperature strength and low activation. Despite their resistance against corrosion in harsh environments, they suffer elevated corrosion rates under particle irradiation. Thin anti-corrosion coatings, such as mullite bond layers and alumina top layers, are essential to enhance the irradiation stability of SiC. The objective of this study was to evaluate the irradiation stability of a double-layer coating developed to enhance the corrosion resistance of SiC in nuclear applications. The coating, which comprised a mullite bond layer and an alumina top layer, was applied to a SiC substrate using laser chemical vapor deposition. Irradiation experiments were conducted at 300 °C with 5.1-MeV Si ions up to 10 displacements per atom. To assess the interfacial strength, a novel testing method, called the double-notch shear compression testing method, was developed based on ASTM standards and was implemented using a nanoindenter. This approach enabled precise measurements of the mechanical integrity at the interfaces of the coating under irradiation. The results showed an increase in the interfacial strength at the SiC/mullite and alumina/mullite interfaces with irradiation. Microstructural analysis of the fracture surface through scanning electron microscopy–energy-dispersive X-ray spectroscopy revealed that cracks propagated within the mullite layer, indicating the presence of mullite on the fracture surface. Transmission electron microscopy (TEM) images indicated that a 2-µm-thick transition layer existed at the SiC/mullite interface but not at the alumina/mullite interface. The TEM–electron energy loss spectroscopy suggested that the Al-O bonding structures in the transition layer were changed from tetrahedral (AlO4) to octahedral (AlO6) through irradiation, and this structural transition may directly affect the strength.

Oxidation mechanism of the Si-X (X=HfO2, Yb2O3 or HfO2+Yb2O3) composite systems in air and water vapor at 1300 ℃

The formation rate of SiO2 during oxidation of Si bond layer is a key issue to determine the lifetime of environmental barrier coatings. The oxidation kinetics and mechanism of Si and Si-X (X=HfO2, Yb2O3 or HfO2+Yb2O3) composite systems were investigated systematically. The thickness of the SiO2 layer was greatly reduced by the dynamic formation of silicates (HfSiO4, Yb2SiO5 and Yb2Si2O7) in air and water vapor, benefiting for long-term servicing. However, the addition of HfO2 promoted the oxidation of Si with an increased total SiO2 thickness, while that of Yb2O3 not. These results provide a guidance for the design of next generation bond layers.

Enhanced electromagnetic wave absorption of bacterial cellulose/ reduced graphene oxide aerogel by eco-friendly in-situ construction

Electromagnetic wave absorption materials (EWAMs) have become an effective means to address electromagnetic (EM) radiation and enhance stealth technology, among which aerogels are valued for their lightweight nature and excellent designability. This study utilized environmentally friendly preparation and in-situ reduction techniques to fabricate bacterial cellulose (BC) / reduced graphene oxide (RGO) aerogels, achieving tailored EM wave loss capabilities by controlling the reduction time of ascorbic acid. Benefitting from the effects of freeze-casting, BC winding, hydrogen bond, and RGO layers coupling, the aerogel maintains their original structure after reduction and exhibits satisfactory EM wave absorption. The minimum reflection loss (RLmin) is −38.52 dB, with an effective absorption bandwidth (EAB) of 6.68 GHz and a maximum radar cross section (RCS) reduction of 44.69 dBsm. Additionally, the aerogel’s lightweight (a low density of 9.03 mg/cm3) and outstanding thermal insulation properties enable it to adapt to complex conditions. Thus, the study provides a novel approach for the construction of industrialized and sustainable RGO-based EWAMs.

Anti-oxidation properties and phase composition evolution of the MoSi2-HfO2 duplex bond coat in air and steam/oxygen atmosphere at 1450 °C

Environmental Barrier Coatings (EBCs) can prevent SiC-based ceramic matrix composites from corrosion at high temperatures. However, the low melting point of conventional Si bond layers limits the operating temperature of EBCs to 1350 °C. Herein, a MoSi2-HfO2 duplex bond coat was designed and showed robust adhesion on SiC substrate after corrosion at 1450 °C for 100 h in steam/oxygen atmosphere. A mixed oxide layer was observed to in-situ form at the interface between the MoSi2 and HfO2 layers, containing SiO2, Hf-Si-O glass and HfSiO4. This oxide layer was proven to effectively inhibit further oxidation/corrosion of the coating and the substrate. The phase composition evolution and anti-oxidation mechanism were systematically analyzed.

Modelling of energy harvesting with bendable concrete and surface-mounted PVDF

Polyvinyl alcohol fiber reinforced engineered cementitious composite (ECC) using piezoelectric polymer film has attracted significant interest due to its energy harvesting potential. This work provides a theoretical model for evaluating the energy harvesting of bendable ECC using surface-mounted polyvinylidene fluoride (PVDF). In the mechanical part, concrete damage plasticity model based on the explicit dynamic analysis was utilized to simulate the dynamic flexural behavior of ECC beam under different dynamic loading rates. The mechanism of force transfer through the bond layer between the PVDF film and ECC specimen was simulated by a surface-surface sliding friction model wherein the PVDF film was simplified as shell element to reduce computational cost. Then, the electromechanical behavior of the piezoelectric film was simulated by a piezoelectric finite element model. A simplified model was also given for a quick calculation. The theoretical model was verified with the experimentally measured mechanical and electrical results from the literature. Finally, a parametric analysis of the effects of electromechanical parameters on the efficiency of energy harvesting was performed. The verified theoretical model can provide a useful tool for design and optimization of cementitious composite systems for energy harvesting application.

Biomedical Device Surface Treatment by Laser-Driven Hydroxyapatite Penetration-Synthesis Technique for Gapless PEEK-to-Bone Integration.

Polyetheretherketone (PEEK), a bioinert polymer known for its mechanical properties similar to bone, is capable of averting stress shielding. Due to these attributes, it finds applications in diverse fields like orthopedics, encompassing cervical disc replacement for the neck and spine, along with dentistry and plastic surgery. However, due to insufficient bonding with bone, various methods such as hydroxyapatite (HA) coating on the surface are attempted. Nonetheless, the interface between the polymer and ceramic, two different materials, tended to delaminate after transplantation, posing challenges in preventing implant escape or dislodgement. This research delves into the laser-driven hydroxyapatite penetration-synthesis technique. Differing from conventional coating methods that bond layers of dissimilar materials like HA and PEEK, this technology focuses on synthesizing and infiltrating ionized HA within the PEEK substrate resulting in an interface-free HA-PEEK surface. Conversely, HA-PEEK with this technology applied achieves complete, gap-free direct bone-implant integration. Our research involved the analysis of various aspects. By means of these, we quantitatively assesed the enhanced bone bonding characteristics of HA-PEEK surfaces treated with this approach and offered and explanation for the mechanism responsible for direct bone integration.

Water vapor corrosion behavior of SiC/Si/mixed Yb2Si2O7-Yb2SiO5 environmental barrier coating for C/C composites

Tri-layer environmental barrier coatings (EBCs) were developed to provide complete encapsulation for carbon/carbon (C/C) composites, consisting of SiC inner layer, Si bond layer, and mixed Yb2Si2O7-Yb2SiO5 topcoats with varying mass ratios. The protective properties of the EBCs were assessed by exposing the coated samples to a corrosive gas mixture of 50 % O2 and 50 % H2O at 1500 °C, the mass change, phase composition, and microstructure evolution were investigated. The results showed that a small amount of Yb2SiO5 doping could improve water vapor corrosion resistance, but the excessive content of Yb2SiO5 led to a mismatch of the coefficient of thermal expansion (CTE) with inner coating, resulting in coating cracking. The mixed topcoat with 60 wt% Yb2Si2O7 and 40 wt% Yb2SiO5 showed superior water vapor corrosion resistance. After corrosion for 20 h at 1500 °C, no penetrating cracks of large size appeared, the thickness of thermal growth oxide (TGO, 4.6 ± 0.4 μm) was relatively lower, and the C/C substrate was effectively protected from damage.

Understanding the wear mechanisms of diamond circular saw blades during machining hard rocks

Hard rock, prized for its exceptional resistance to corrosion and pollution, is extensively utilized in construction and transportation. However, its high hardness poses a significant challenge to the limited lifespan of diamond tools. This study delves into the wear mechanisms affecting diamond circular saw blades during hard rock machining. Initially, the dynamic behavior of saw blades was analyzed to uncover the wear and breakage characteristics of diamond segments. Through systematic characterization, the wear mechanisms and chemical element deposition were elucidated. The findings reveal that the front segment gradually loses sharpness due to primary rock impact, primarily experiencing bond wear, abrasive wear, and pull-out, while the rear segment encounters mainly abrasive and bond wear. Fresh and whole diamonds are approximately 45% with around 25% exhibiting macro-fractures and pull-out at the front segment, whereas the rear segment comprises over 50% fresh diamonds, with less than 20% showing macro-fractures and pull-out. Side edges predominantly undergo abrasive wear, bond wear, and pull-out or breakage, while medium edges experience abrasive wear, bond layer wear, and wear equalization. The rock is removed in a brittle and partly elastic-plastic manner. Through a comprehensive investigation, the wear process of diamond segments was clarified, attributing diamond wear to vibration shock, fatigue wear, and grinding heat, while bond wear factors include debris hardening point, fatigue wear, and chemical erosion. These systematic findings shed light on the wear characteristics of diamond segments, serving as a crucial theoretical guide for the material design of diamond segments for machining hard rocks.

INVESTIGATION OF THE BENDING BEHAVIOR OF INC625/SUS316L LASER-CLADDING LAYERS APPLIED TO GGG40

Laser cladding is a multi-purpose thermal coating technology whose application area has increased rapidly in recent years. It can be used for the purposes of obtaining a thick (mm to cm) layer on surfaces by the interaction of different materials in powder form (especially metallic and metal-matrix composite) with the laser beam, thus repairing, restoring, improving and protecting the surface resistance of various industrial metallic parts. It is normally used to refurbishment, create and repair metallic components, hydraulic mills, gears, shafts, turbine blades, drilling tools, and other static or dynamically loaded mechanical parts. The benefits of laser cladding over alternative technologies include better metallurgy (bonding, hardness or lower porosity) as well as reduced part deformation and stress due to lower overall heat input. The melting of substrate material ensures a good metallurgical bond between the cladding layer and the substrate material. However, the melting of the substrate material causes dilution. Therefore, the melting of the substrate material should be controlled and kept to a minimum because high substrate melting causes an increase in the dilution, which may degrade the mechanical and corrosion properties of the clad layer. The laser beam over the substrate materials creates temperature gradients in the thickness direction that can lead to mechanical deformation (such as bending) and changes in the microstructural and interface properties. Depending on the substrate type and production history, as well as the laser-clad powder properties and process parameters (power, feed rate, clad speed etc.) used in the application, the deformation properties of the cladded part and its bending behavior can change. In this study, INC625 and SUS 316L+INC625 clad layers were applied on a GGG40 substrate with optimized parameters and then subjected to bending tests. Bending-test results were examined comparatively and their microstructural properties were examined. Cladding powder feedstock material properties, substrate type and its thermo-physical properties, clad process parameters and surface preparation conditions, number and thickness of layers are the most important factors affecting the bending behaviour in laser-cladding applications and require optimization studies. The INC625 clad layer on the SUS316L bond layer has reduced the diffusional effects, and the hardness distribution has a more homogeneous profile. In this way, an increase in bending angles was observed. The highest bending angle of 32° was measured with dublex-clad layered samples.

Temperature-dependent debonding behavior of adhesively bonded CFRP-UHPC interface

The adhesively bonded structure, comprising carbon fiber-reinforced polymer (CFRP) and ultra-high-performance concrete (UHPC), enhances structural strength while reducing brittleness and strain softening behavior. However, the adhesively bonded structure is inevitably influenced by temperature variations throughout its service life. When the adhesive layer's temperature surpasses the glass transition temperature (Tg), significant damage occurs to the bond layer joint. To investigate the debonding behavior of the CFRP-UHPC interface at various temperatures, a series of three-point-bending tests were conducted at room temperature, 120 %Tg, 150 %Tg, and 160 %Tg. This study introduced a temperature degradation factor, employing a proposed maximum load capacity prediction formula to adjust both the test results and data from other researchers. By adjusting the cohesion parameters with the temperature degradation factor, a temperature-dependent mixed-mode cohesive zone model (CZM) was developed. The model's validity was confirmed through finite element (FE) analysis, considering two distinct experimental scenarios. The paper concludes that the proposed model effectively simulates interface debonding behavior across varying temperatures.

Structural evolution and enhanced thermal cycling life of HVAF-sprayed NiCrAlY bond coat modified by YSZ

Thermal barrier coatings with varied bond coat structures were deposited using high-velocity air fuel spraying. Highly hard YSZ particles are introduced into the NiCrAlY bond layer by accelerating to supersonic speeds at low temperatures to change the porous structure of the bond coat. We systematically increased the YSZ content in NiCrAlY bond coats and extensively investigated microstructure, oxidation behavior, thermal cycling life, and residual stress variation of the bond coats suing a combination of scanning electron microscopy, optical microscope, confocal Raman microprobe. Introduction of YSZ particles reduced the porosity of NiCrAlY bond coat from 2.69% to 0.45%, and altered the oxidation behavior of the bond layer. The NiCrAlY mixed YSZ composite structure induced the formation of a complex and thin thermal growth oxide layer on the bond coat during oxidation process, greatly alleviating and reducing the thermal stress generated of the coating and during the thermal cycling test. And the thermal cycling life of the NiCrAlY mixed YSZ composite structure coating is 2.8 times longer than that of the ordinary NiCrAlY coating. This finding highlights the significance of the bond coat structure in enhancing the service life of thermal barrier coatings.

Oxidation resistance and thermal insulation performance of Al–Cr–O/Zr–O + NiCoCrAlSiY and Al–Cr–O + NiCoCrAlSiY coatings prepared by arc ion plating

Al–Cr–O/Zr–O multilayer coating + NiCoCrAlSiY bond layer and Al–Cr–O single-layer coating + NiCoCrAlSiY bond layer were deposited on superalloy by arc ion plating, and subsequently they were heat treated at 1000–1200 °C. Scanning electron microscope and X-ray diffractometer were used to analyze the microstructure and phase of the coatings. The results showed that both the Al–Cr–O/Zr–O multilayer coating and Al–Cr–O single-layer coating exhibited a compact spherical structure. After heat treatment, cracks occurred on the surface of Al–Cr–O/Zr–O coating. Cracks increased and widened with rising the temperature. However, the surface cell clusters of Al–Cr–O coating after heat treatment were changed to tightly connected granular structures, and the granular structures grew up significantly with the increase of annealing temperature. Owing to the oxygen ion conductor and lower densification of t-ZrO2 than α-Al2O3, t-ZrO2 in the Al–Cr–O/Zr–O coating provided an access for the inward diffusion of oxygen. Therefore, the high temperature oxidation resistance of the Al–Cr–O coating + NiCoCrAlSiY layer was superior to that of the Al–Cr–O/Zr–O multilayer coating + NiCoCrAlSiY layer. The thermal insulation performance of Al–Cr–O/Zr–O multilayer coating + NiCoCrAlSiY layer was better than that of Al–Cr–O coating + NiCoCrAlSiY layer due to the large thickness and low thermal conductivity of t-ZrO2 phase and the heat reflection of interlayer interface.

Multi-layer solid-state ultrasonic additive manufacturing of aluminum/copper: local properties and texture

Ultrasonic additive manufacturing (UAM) is an advanced joining technique that utilizes ultrasonic vibrations to bond layers of metal foil together. UAM offers several benefits over traditional manufacturing methods, including enhanced design flexibility and reduced material waste, and its potential applications in various industries such as aerospace, automotive, and biomedical engineering are being actively explored. The study employs a nanoindentation apparatus to investigate the effect of the UAM process on the local mechanical properties of the bonded interface, along with changes in microstructure, which were investigated using scanning electron microscopy and electron back-scattered diffraction. The results revealed a significant correlation between material hardness and local plasticity. EBSD has revealed that the grain size distribution of Al far from the interface contains 57% of the grains less than 3 µm in size, while at the interface this number rises to approximately 78%, indicating that the average grain size decreases as it approaches the interface. This result is consistent with nanoindentation results that demonstrated a gradual change in the hardness of Al foil far from the interface to close to the interface (the maximum penetration depth near the interface was 500 nm less than far from the interface). Both EBSD and nanoindentation disclose the effect of work hardening close to the interface, which is related to dislocation accumulation with a density of 8.6×10-10cm-2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$8.6\ imes {10}^{-10} {{\ ext{cm}}}^{-2}$$\\end{document} beneath the interface. The consistency of hardness and Young’s modulus with the pole figures and microscopic images demonstrated that plasticity flow and fine grain distribution would only occur in the vicinity of the interface in the softer metal region. Although the harder metal did not exhibit plasticity or recrystallization, the hardness, and Young’s modulus map indicated the formation of a layer of small grains close to the interface on the aluminum side owing to strain hardening and dynamic recrystallization.

Enhanced osteochondral regeneration with a 3D-Printed biomimetic scaffold featuring a calcified interfacial layer

The integrative regeneration of both articular cartilage and subchondral bone remains an unmet clinical need due to the difficulties of mimicking spatial complexity in native osteochondral tissues for artificial implants. Layer-by-layer fabrication strategies, such as 3D printing, have emerged as a promising technology replicating the stratified zonal architecture and varying microstructures and mechanical properties. However, the dynamic and circulating physiological environments, such as mass transportation or cell migration, usually distort the pre-confined biological properties in the layered implants, leading to undistinguished spatial variations and subsequently inefficient regenerations. This study introduced a biomimetic calcified interfacial layer into the scaffold as a compact barrier between a cartilage layer and a subchondral bone layer to facilitate osteogenic–chondrogenic repair. The calcified interfacial layer consisting of compact polycaprolactone (PCL), nano-hydroxyapatite, and tasquinimod (TA) can physically and biologically separate the cartilage layer (TA-mixed, chondrocytes-load gelatin methacrylate) from the subchondral bond layer (porous PCL). This introduction preserved the as-designed independent biological environment in each layer for both cartilage and bone regeneration, successfully inhibiting vascular invasion into the cartilage layer and preventing hyaluronic cartilage calcification owing to devascularization of TA. The improved integrative regeneration of cartilage and subchondral bone was validated through gross examination, micro-computed tomography (micro-CT), and histological and immunohistochemical analyses based on an in vivo rat model. Moreover, gene and protein expression studies identified a key role of Caveolin (CAV-1) in promoting angiogenesis through the Wnt/β-catenin pathway and indicated that TA in the calcified layer blocked angiogenesis by inhibiting CAV-1.

A study on multi-layered surgical masks performance: permeability, filtration efficiency and breathability

Surgical masks have been widely used in the past for protection purposes against infection as a hygiene product. Masks, which are often used by healthcare professionals, are also widely used by the general public during pandemic periods. Surgical masks were designed in various ways according to colour, nose strip, earloop, grams per square meter and several layers. Some performance features are expected from these masks according to certain standards. In this study, it has been studied on multi-layered surgical masks. Thickness, air permeability, bacterial filtration efficiency and differential pressure properties of these masks consisting of polypropylene spun bond and melt-blown layers were investigated. The effects of an increase in grams per square meter, the number of layers and the melt-blown layer on the mentioned properties were determined. As a result; it has been observed that the optimal mask is one of the 4-ply masks. Also, it has been concluded that increasing the number of layers does not always provide an increase in performance; instead, an increase in grams per square meter can meet the expectation. Therefore, it can be said that it is necessary to determine the appropriate weight in grams and the appropriate number of layers with suitable raw materials to provide the expected features from the surgical masks.

Study on the processing parameter window for laser cleaning thermal barrier coating

In this paper, laser cleaning of thermal barrier coatings is investigated by finite element simulation and experiments. The effect of different parameters on the maximum temperature of the substrate surface during the cleaning process is studied. The results show that the repetition frequency and pulse width have little effect on the maximum temperature of the substrate surface, and the maximum temperature of the substrate surface has an approximately linear relationship with the laser power and an exponential decay relationship with the scanning speed. From the perspective of cleaning efficiency and damage to the substrate, the parameter windows for laser cleaning of thermal barrier coatings were obtained. The highest cleaning efficiency for the top ceramic layer is about 3.02 mm3/s. The highest cleaning efficiency for the bond layer is about 2.17 mm3/s.

Mixed harmonic generation of surface guided waves by collinear wave mixing for debond identification in FRP-strengthened beams

The high incidence of debonding as a failure mode in FRP-strengthened structures highlights the importance of regular inspections of the bond layer for any discontinuity. In contrary to most previous studies, which focus on the linear features of waves like amplitude and velocity, the present study utilises a novel collinear wave mixing technique for the generation of mixed harmonics of surface guided waves for debond characterisation. Experimental study is conducted in which a particular surface guided mode, sensitive to bond layer, is excited using wedge transducers. A three-dimensional finite element model is developed to understand the nonlinear interaction of surface guided waves with debonding, and the results obtained are compared with those obtained from the experimental results in both time and frequency domains. The numerical results are found to be in good agreement with the experimental ones. Later, extensive parametric studies are conducted on the validated model by changing the debond size. A damage index is proposed that rely on the band frequency amplitude of fundamental and higher order mixed harmonics paving the way for baseline-free debond quantification. The damage index (DI) exhibited a consistently increasing trend with debond size. Finally, a sensitivity analysis is conducted by increasing the debond size and comparing the relative parameter values of linear features (scattered amplitude and change in time of arrival) with the proposed damage index. The analysis results revealed that the proposed DI is more sensitive to the debond size than the conventional linear parameters that also requires pristine condition data. Thus, the study offers valuable insights into the collinear wave mixing phenomenon in FRP strengthened RC structures and proposes an efficient and practical technique for debond detection in the field.