Bond Line Research Articles

Two-Factor Rule for Distinguishing the Covalent and Tetrel Bonds.

Understanding and exploring the existence of a recognizable boundary between the noncovalent tetrel bond (TtB) and the coordination or weakened covalent bond are important for the bonding characterization. We have developed a simple methodology for analysing the type of bonds based on comparison of the electrostatic and total static potentials along the bond line. For the typical σ-hole noncovalent bond formed by a Tt atom in a tetrahedral molecule, we have found that the space gap between positions of the maxima of the total static potential and the negative quantity of electrostatic potential is much wider than that for the coordination bonds in a trigonal bipyramid molecular system for the Cl-Tt/Cl…Tt and N-Tt/N…Tt (Tt = C, Si, Ge) bonds in molecules and molecular complexes. The distinction between the weakened covalent and strengthened noncovalent bonds is well reflected in behaviour of the Fermi hole along the bond line. Two-factor empirical rule based on the superposition of the electrostatic and total static potentials is suggested.

Mode I cohesive law of birch wood-biobased adhesive systems

ABSTRACT The development and assessment of bio-based wood adhesives face challenges due to the limitations of conventional test procedures, particularly in predicting adhesive performance in real-world applications. This study investigated the mode I cohesive law of two birch wood-biobased adhesive systems, comparing them with conventional fossil-fuel-based systems. The experimental approach, developed during the ‘90s at Lund University, involves direct measurement of the traction-separation relation and allows evaluation of post-peak behaviour and fracture mechanical property characterisation. SEM analysis confirmed fracture development within the bond line for all tests. The birch–fish-adhesive bonds demonstrated peak stress of approx. 4 MPa, with serrated fracture surfaces and favourable fracture properties (specific fracture energy approx. 370 Nm/m2, brittleness approx. 40 GPa/m). Birch wood bonded with lignin-based adhesive showed lower, yet reasonable, levels of peak stress (approx. 3 MPa) but less favourable fracture properties (specific fracture energy approx. 110 Nm/m2, brittleness approx. 90 GPa/m), suggesting room for further optimisation.

An experimental-cohesive zone model approach to predict fatigue life of adhesive joints with varying modes of loading and joint configurations for automotive applications

ABSTRACT Predictive fatigue life models of adhesive joints are necessary to enable the assessment of automotive-bonded structures while reducing costly experimental testing. However, contemporary models have typically been calibrated for specific joint configurations and modes of loading, limiting their applicability to large-scale structures. Additionally, available models are based on simulation of cumulative fatigue cycling, making them computationally prohibitive. In the current study, cross-tension (CT) (load angles of 0°, 45°, and 90°) and single-lap shear joint (SLJ) configurations were bonded using an epoxy adhesive (BetaGuard CI6125R; PPG, France) used in automotive production (one part) and tested under fatigue cyclic loading. A total of nine joint configurations, having symmetrical (same material and thickness) and asymmetrical (dissimilar material or unequal thickness) joints, were tested. Fatigue tests at load levels between 25% and 75% of the static peak load were performed until joint failure or to runout (two million load cycles). The static tests of the joints were simulated to failure using finite element (FE) models with the cohesive zone method (CZM). The maximum strain energy release rates ( G max ) were calculated within the adhesive bond line at static loads corresponding to the peak loads of the fatigue tests. The G max values, computed from single cycle, specimen-specific FE simulations, were correlated with the measured fatigue life ( N f ) of the adhesive joints with varying modes of loading and joint configurations. The fatigue life prediction model, based on G max − N f correlation and following a crack propagation approach, predicted the cycles to failure for 85% of the fatigue tests, and 81% of the independent validation tests. The proposed fatigue life prediction approach provides computational efficiency and large-scale compatibility.

Influence of incorporating beta-cyclodextrin/essential oil-based wood preservative on the bonding strength of wood composite products

The introduction of additives during the production of wood composites may disrupt the development of bonding strength between wood and adhesive, potentially compromising the structural performance of wood composites. This study investigates the impact of incorporating a natural wood preservative into 3-ply southern yellow pine (SYP) plywood on its bonding strength. The formation of β-cyclodextrin and trans-cinnamaldehyde (βCD-tCN) inclusion complex as a natural wood preservative was confirmed by Fourier transform infrared and X-ray diffraction analyses, with an inclusion yield ranging 78 % to 120 %. The differential scanning calorimetry analysis found the curing of pMDI with wood was not significantly affected by the βCD-based additives. The influence of βCD-tCN addition on the bonding strength of a wood composite was assessed by a shear strength test of the 3-ply plywood, following ASTM D906-20. The average shear strength of all panels ranged 1.53 N/mm2 to 2.07 N/mm2, exceeding the minimum requirement of 1.00 N/mm2 for cross-layer bond lines according to EN 16351-2015. Statistical analysis indicated no significant differences between the bonding strength of pristine pMDI and pMDI containing βCD-tCN complex. The combined results suggest that βCD-tCN could be effectively implemented in the manufacturing of wood composite products.

Thermal interface materials: From fundamental research to applications

AbstractThe miniaturization, integration, and high data throughput of electronic chips present challenging demands on thermal management, especially concerning heat dissipation at interfaces, which is a fundamental scientific question as well as an engineering problem—a heat death problem called in semiconductor industry. A comprehensive examination of interfacial thermal resistance has been given from physics perspective in 2022 in Review of Modern Physics. Here, we provide a detailed overview from a materials perspective, focusing on the optimization of structure and compositions of thermal interface materials (TIMs) and the interact/contact with heat source and heat sink. First, we discuss the impact of thermal conductivity, bond line thickness, and contact resistance on the thermal resistance of TIMs. Second, it is pointed out that there are two major routes to improve heat transfer through the interface. One is to reduce the TIM's thermal resistance (RTIM) of the TIMs through strategies like incorporating thermal conductive fillers, enhancing interfacial structure and treatment techniques. The other is to reduce the contact thermal resistance (Rc) by improving effective interface contact, strengthening bonding, and utilizing mass gradient TIMs to alleviate vibrational mismatch between TIM and heat source/sink. Finally, such challenges as the fundamental theories, potential developments in sustainable TIMs, and the application of AI in TIMs design are also explored.

Application of microscale methods to study the heat-induced delamination in engineered wood products bonded with one-component polyurethane adhesives

One-component polyurethanes (1-c-PUR) are commonly used adhesives for the manufacture of cross-laminated timber (CLT). Typically, these adhesives do not govern the mechanical response of CLT under normal service temperatures. However, when subjected to heating from a fire, failures may transition from within the timber to the bond line interphase zones. CLT bonded with 1-c-PUR has been shown to be prone to heat induced delamination (HID), which may compromise its structural performance at elevated temperatures. Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and dynamic thermo-mechanical analysis (DTMA) were used to study the thermal and thermo-mechanical responses of engineered wood products and components (i.e. adhesives and timber) and their interphase (i.e. bond line). Two commercially available 1-c-PUR adhesive films from the same manufacturer, two timber species (Norway Spruce and Radiata Pine, as sawdust or as veneers), and four different veneer shear lap combinations, were studied. Shear lap specimens bonded with a conventional 1-c-PUR adhesive consistently experienced bond line mechanical failure in DTMA at about 220–240 °C. The same adhesive films tested via DSC and TGA softened at 240–260 °C. Conversely, a different 1-c-PUR adhesive, formulated specifically for enhanced performance at elevated temperature, displayed no detectable softening in DSC and TGA, and shear lap specimens in DTMA consistently failed within the timber; even at elevated temperatures. The presented thermo-mechanical methods allow identification of failure modes and temperatures, whilst the thermal microscale methods (i.e. TGA, DSC, DTMA) assist in understanding the various factors contributing to failures.

Interfacial microstructure evolution and mechanical response of TC19/Ti150 dissimilar joints obtained by diffusion bonding

The structure made by dissimilar titanium alloys can meet the differentiated requirements of performance for different parts, fully exerting their performance advantages during operation respectively, which holds significant technological and economic value. Here, we used diffusion bonding (DB) method to connect TC19 and Ti150 dissimilar titanium alloys in the temperature range of 810 °C–900 °C, and investigated the morphology, forming mechanism, deformation behavior and mechanical properties of the joints. The study showed that the interfacial voids disappeared with the increase of temperature, and the interfacial characteristics gradually evolved from the initial straight shape to a curved shape. The original bond line transformed into βTC19/αTi150 phase boundaries (PBs) and high-angle grain boundaries (HAGBs) formed by αTC19 and αTi150. No harmful intermetallic compounds were formed on the interface. The joint formation can be considered as a combination of three behaviors: plastic deformation, elemental diffusion and interfacial grain boundary migration (IGBM). In addition, it was observed that dislocation slip was the main deformation behavior of the joints. The dislocations generated in the both matrices were obviously accumulated at the interface during tensile deformation of the specimen, and the bonding interface hindered the dislocation transfer between two substrates. The room temperature uniaxial tensile results exhibited that the joints bonded at 810 °C and 840 °C were broken at the interface, showing poor mechanical properties caused by holes and straight interface. And yet the joints bonded at 870 °C and 900 °C occurred fracture on the Ti150 matrix. The strength of joints was comparable to that of the Ti150 matrix, indicating that the two dissimilar titanium alloys achieved a reliable bonding. This work provides valuable inspiration for the diffusion bonding process of titanium alloys with different microstructures.

Bond line shear strength of structural wood adhesives at high temperatures up to wood carbonization – A classification approach

The European timber fire design code EN 1995-1-2 presently specifies that adhesives for timber construction products shall provide integrity throughout the intended fire exposure time, yet no test or requirement specifications are given. Contrary, in North America elevated temperature tests are mandatory and product standards for glued and cross laminated timber demand sufficient bond line shear strength up to 220 °C. This paper deals with block shear compression tests at ambient and elevated temperatures for development of a high temperature resistance classification system for structural wood adhesives. The investigations comprised 1200 specimens made from Norway spruce bonded with twelve brands from five adhesive families. Polycondensation (pcon) resins encompassed phenolic resorcinol formaldehyde, melamine urea formaldehyde and melamine formaldehyde adhesives. Polyaddition (padd) adhesives comprised five one-component polyurethanes and an emulsion polymer isocyanate. The focus was on the strength-temperature (fv-T) relationship of bonded and solid wood reference specimens at 180–270 °C. The pcon adhesives showed almost throughout a fv-T relationship close to solid wood up to 270 °C. Contrary, the padd adhesives revealed massive strength degradations vs. solid wood from 180 °C onwards. An adhesive test, evaluation and classification procedure with four classes T270 to T200 was derived. It is based on a comparison of the temperature-dependent shear strength decline in bonded samples vs. solid wood reference samples beyond 180 °C. Class T270 adhesives are proposed being suited for the linear charring model of EN 1995-1-2 as PRF adhesives without additional fire resistance testing of components deemed necessary today.

Monitoring the curing, degradation and moisture ingress into alkyl 2-cyanoacrylate adhesives using electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) was employed in an attempt to gain insight into the mechanisms of ethyl 2-cyanoacrylate (ECA) curing (polymerisation) and bonding on aluminium alloy 2024 metal. EIS can detect ionic movement, adsorption processes, charge transfer and storage occurring at an adhesive/substrate interface and/or in a bulk bond line during curing. Low-frequency capacitance measurements demonstrated sensitivity to surface polymerisation reactions and were modelled using an equivalent circuit model with two time constants in series. At a frequency of 1 kHz, changes in the dielectric polymer could be readily followed with time, confirmed by employing a crown ether to accelerate the polymerisation process. Hydrolytic degradation of poly-ECA bonds at a stainless steel interface was also investigated. An equivalent circuit model containing a number of circuit components comprising pore, charge transfer and diffusional impedances, along with polymer film, double layer and diffusional capacitances (represented by constant phase elements), was developed. Three regions were identified in the frequency domain and ascribed to processes taking place at the polymer/electrolyte and polymer/metal oxide interfaces. In short, EIS can be employed to follow the rate of polymerisation of ethyl-2-cyanoacrylate and also the degradation of the resulting polymer in saline solution.

Residual strength and time to failure of bonded wooden lap joints at high temperatures and superimposed constant loading

ABSTRACT The study focused on the effect of high temperatures on wooden bond lines of single lap tensile shear specimens according to EN 302-1 made from beech wood at superimposed sustained mechanical loading. The quasi-duration of load (DOL) tests encompassed temperature levels of 160–232°C with specimens loaded at 30% of their mean shear strength at ambient temperature for 40 min. The investigations comprised one phenol resorcinol formaldehyde (PRF), two melamine urea formaldehyde (MUF) and three one-component polyurethane (1C-PUR) adhesives as well as solid wood reference specimens. Complementary ramp load tests were performed with specimens heated in unloaded state up to 250°C. The investigations revealed three highly differentiated adhesive DOL behaviours although showing a well comparable pure temperature-bound strength reduction effect at 200°C. PRF and one MUF showed little to no additional influence of applied load, while for two 1C-PUR brands a drastic impact on survival times and residual strength was revealed. One MUF and a special 1C-PUR forwarded results between both extremes. The investigations provide evidence that the assessment of the temperature behaviour of structural adhesives can be biased when relying exclusively on ramp load tests with specimens heated in unloaded state, and thus necessitates a superimposed load as present in realistic fire scenarios.

Predicting bond line thickness of polymeric thermal interface materials based on the rheological properties

Bond line thickness (BLT) is an essential parameter of thermal conductive composite gels as the thermal interface material (TIM). Extensive research has been performed on designing next-generation TIMs with high thermal conductivity; however, it remains elusive how to link laboratory measurements and the theoretically predicted BLT. Here, we propose a new model to estimate BLT based on a rheological property, in which the TIM is assumed to be a simple power law fluid. To avoid the unrealistic situation of the BLT tending to zero, we introduce a decaying exponential function to describe the influence from fillers during the lid attach process and rebuild the force balance equation. Compared with previously reported models, the theoretical prediction BLT based on the proposed model is in good agreement with the experimental data. Our model has a guiding significance in predicting the BLT, which may help to optimize the thermal performance of TIMs.

Supporting Manufacturing Process Changes with an Innovative Microscopy Method

Abstract Bakelite Chemicals LLC has developed an innovative microscopy technique to view adhesives “in use.” The cornerstone of the method is based on sample preparation. Samples of a composite wood product are sanded to a highly polished surface using a progression of sandpaper grits from lowest to highest. This preparation technique differs significantly from historical preparation techniques, which rely on soaking or embedding samples followed by thin-section creation using a microtome. The wet preparation technique causes wood cells compressed during production of the composite to swell and change how the adhesive is seen within those cells. The addition of water to the sample can also move uncured adhesive around the bond line. These changes can significantly alter the interpretation of how the adhesive performs during the manufacturing process. The new preparation technique combined with standard fluorescence microscopy has provided a groundbreaking and accessible ability to understand how an adhesive responds to process changes in manufacturing. Improving the understanding of the dynamic relationship between the adhesive and manufacturing processes will be crucial in helping the wood products industry take advantage of the rapid improvements in processing technology.

Analytical solution for the interfacial stress and energy release rate at failure initiation of the three-point bending test (ISO 14679:1997)

Determining adhesive failure in adhesively bonded joints is still challenging in the field. While the three-point bending test (3PBT) (ISO 14679:1997) has been helpful in identifying critical forces and displacements related to bond strength, it only allows for a qualitative assessment of the bond line. Recently, a new quantitative methodology has been developed to determine critical stress and fracture toughness using the coupled stress–energy criterion and the 3PBT. However, these assessments require a significant effort using semi-analytical or finite element (FE) analysis. Therefore, the present study proposes a reliable set of analytical equations to determine peel and shear stress distributions using a weak interface formulation and 1D Euler–Bernoulli approach. More precisely, a particular numerical method computes the integration constants from these equations. A new method has been proposed for calculating the interfacial stiffness in peel and shear mode, based on the material and geometrical parameters of the geometry. This method differs from the previous approach, where the interfacial stiffness was calibrated from the experimental behavior of the test. The whole approach has been validated through experimental and numerical analysis, including the costly 3D FE analysis. An analytical expression for interfacial energy release is suggested, developed following the works of Fraisse and Schmit on J-integral assessment of sandwich-type overlaps and depending on the applied force and a rotation, which could be experimentally measured. Therefore, this work is significant progress in determining bond strength using a simple mechanical test and equations applicable to various industries.

STRAIN MEASUREMENT AND ADHESIVE SELECTION FOR STRAIN GAUGES ON CARBON ROVING IN TEXTILE-REINFORCED CONCRETE STRUCTURES

Textile-reinforced concrete (TRC) is a promising building material that meets modern environmental protection and sustainability requirements. For the widespread use of this innovative building material, empirical models based on material properties determined by laboratory tests are needed. This study investigated the ability to measure the strain of carbon rovings with small strain gauges during standard tensile tests. The measured values were compared and validated using the digital image correlation (DIC) method. Another component of this study is the investigation of a suitable adhesive for the application of the sensors to SBR-coated (styrene-butadiene rubber) carbon roving. Three test series with three different adhesives, cyanoacrylate adhesive, epoxy resin, and polychloroprene, were produced and tested for tensile strength. The validation tests showed that a strain loss occurs in the adhesive joint of the strain gauges. It was found that cyanoacrylate adhesives, such as M-Bond 200, suffered the lowest losses and are recommended for further investigations. The epoxy resin influences the stiffness of the carbon fibers at the contact surface too strongly, while the polychloroprene adhesive could not transfer the actual roving strain due to insufficient stiffness of the bond line.

STRAIN MEASUREMENT AND ADHESIVE SELECTION FOR STRAIN GAUGES ON CARBON ROVING IN TEXTILE-REINFORCED CONCRETE STRUCTURES

Textile-reinforced concrete (TRC) is a promising building material that meets modern environmental protection and sustainability requirements. For the widespread use of this innovative building material, empirical models based on material properties determined by laboratory tests are needed. This study investigated the ability to measure the strain of carbon rovings with small strain gauges during standard tensile tests. The measured values were compared and validated using the digital image correlation (DIC) method. Another component of this study is the investigation of a suitable adhesive for the application of the sensors to SBR-coated (styrene-butadiene rubber) carbon roving. Three test series with three different adhesives, cyanoacrylate adhesive, epoxy resin, and polychloroprene, were produced and tested for tensile strength. The validation tests showed that a strain loss occurs in the adhesive joint of the strain gauges. It was found that cyanoacrylate adhesives, such as M-Bond 200, suffered the lowest losses and are recommended for further investigations. The epoxy resin influences the stiffness of the carbon fibers at the contact surface too strongly, while the polychloroprene adhesive could not transfer the actual roving strain due to insufficient stiffness of the bond line.

Repair Analysis of Overlay Woven Fabric CFRP Laminates

The increase in aerospace composites usage for structural components demands advanced repair analysis. Overlay repairs of carbon fiber-reinforced polymer laminates offer an alternative that is easier to perform and less time-consuming to produce than the widely used tapered scarf repair and stepped lap. Composite specimen manufacturing was based on both twill carbon/epoxy prepreg and wet lay-up. The repair was performed with both prepreg and wet extra plies to the parent prepreg structure. However, the design of overlay joints must be carefully investigated to avoid generating stress concentration regions at free edges. This study examined specific extra ply terminations' impact on peak stresses in the adhesive bond line. Linear finite element analysis was performed to conduct a maximum principal stress study with a focus on three joint design parameters: ply material, overply effect, and stacking sequence. FEA accurately predicted experimentally observed responses and provided further insight into the failure behavior of the structure. Results showed that overlay joints have a strong sensitivity to ply material type, the number of overply, and stacking sequence. The introduction of overplies provided protection and stiffness at joint tips, and an overply material behavior was identified. The location of 0̊ plies in the composite laminates was highlighted as an important factor. The analysis was then extended to three-dimensional FE models for verification. In conclusion, results showed that high-stress concentration in overlay joints can be mitigated with the introduction of overplies and appropriate changes in joint design parameters to reduce stress peaks at joint tips and corners.

Experimental determination of the charring rate of cross-laminated timber panels

Recent design guides include charring models for cross-laminated plane timber (CLT) members. When the bond line integrity is not maintained, charring is illustrated as a combination of sequenced phases. Significant research has been conducted on the charring behaviour of CLT panels, and the criterion of 300 °C has been an accepted definition for the char front line. The charring rate of a solid wood-based panel is determined by the char depth divided by the time to reach the char depth. In the CLT structure, this method can be applied to the first lamella layer. However, due to the char fall-off, it cannot be directly applied to the lamella layers behind the first layer. In this research, eight fire tests were conducted to investigate how charring rates of different lamella layers can be determined from measured temperatures. An assessment method based on the mean char depth development determined from experimental temperature distributions of CLT panels is introduced. Also, the effects of thermocouple positioning and specimen orientation on the assessment results were analysed. The charring rates determined for both horizontal and vertical specimens align well with the results calculated using the European Charring Model and a post-protection factor k3 of 2.

Effect of Short-Fiber Orientation on the Tensile Strength of Epoxy Adhesives

Short-glass-fiber-reinforced adhesives are state-of-the-art materials used in the bond lines of wind turbine blades. Various alignments of the short fibers are emerging, which depend on the adhesive flow during the application and joining processes. This induces a spectrum of direction-dependent mechanical properties. The tensile strength, for instance, can vary by about 20% depending on the load direction. Therefore, the adhesive performance, which is normally determined under controlled laboratory conditions and implemented in bond line design routines, can deviate from the in situ application. This work investigates the effect of the short-fiber alignment on the tensile strength distribution of an industry-standard adhesive used in wind turbine rotor blades. The smeared short-fiber orientation of the tensile specimens tested is estimated locally near the damaged surface using a material model that is fed with thermomechanical measurements and calibrated via micro-computed-tomography analysis. A linear correlation between tensile strength and short-fiber alignment has been identified. This augments the identification of the material’s upper and lower strength limits for bond line design purposes.

Experimental study and numerical simulation of adhesively bonded timber-concrete composite panels: bending behavior, adhesive shear and peel stress distributions

This study investigated the structural response of timber-plain concrete panels bonded with epoxy and PUR adhesives under a four-point bending load, both experimentally and numerically. Tests evaluating epoxy- and PUR-bonded glulam-plain concrete panels measured ultimate load (Fult), effective bending stiffness (EIeff), and mid-span deflection (∆ult). Shear stress within the adhesives was monitored using fiber optic sensors. Numerical simulations estimated Fult and EIeff with errors of 5% and 14%, respectively. These discrepancies may stem from assuming defect-free wood (e.g., no knots), idealized boundary conditions, uniform adhesive thickness assumptions, and unmeasured mechanical and damage properties of wood and concrete (Detailed error source analysis is available in Section 4). Qualitative validation involved comparing sensor-recorded shear stress with simulation results. A parametric study evaluated the effects of adhesive thickness (0.5, 1, and 3 mm), concrete-wood depth ratio (50/85, 85/85, and 150/85 mm/mm), wood type (spruce (Picea), beech (Fagus), and Azobe (Lophira)), concrete strength class (C12/15 and C30/37), and span length (2, 4, and 8 m) on the bending behavior of the panel, and the shear and peel stress distribution within the adhesive bond line. Shear stress prevailed over peel stress in the adhesives, with peel stress approaching shear stress as span length increased (max. τa / max. σa ∼ 3.5–7). The ultimate load typically resulted in concrete compression damage and wood fiber damage, influenced by the concrete strength class and wood type. Higher depth ratios led to tension damage in concrete, while adhesive thickness had a minimal impact on stress distribution and failure modes.

Diffusion Bonding 321-Grade Stainless Steel: Failure and Multimodal Characterization.

Vacuum diffusion-bonded printed circuit heat exchangers are an attractive choice for the high-temperature, high-pressure demands of next-generation energy applications. However, early reports show that the high-temperature materials desired for these applications suffer from poor bond strengths due to precipitation at the bond line, preventing grain boundary migration. In this study, a diffusion bond of the high-temperature stainless steel grade 321H is investigated, and poor mechanical properties are found to be caused by Ti(C, N) precipitation at the bond line. Through in situ studies, it is found that Ti diffuses from the bulk to the mating surfaces at high temperatures. The Ti subsequently precipitates and, for the first time, an interaction between Ti(C, N) and Al/Mg-oxide precipitates at the bond line is observed, where Ti(C, N) nucleates on the oxides forming a core-shell structure. The results indicate that small amounts of particular alloying elements can greatly impact diffusion bond quality, prompting further research into the microstructural evolution that occurs during bonding conditions.