Bond Line Research Articles

Effect of interlayer adherend inclusions on strength of composite bonded joints

Foreign objects such as backing tapes may get embedded between the plies of a laminated polymer composite during its fabrication which can compromise the composite panel or the adjacent bond joint. This paper investigates the effect of such inter-layer inclusion defects in laminated carbon fiber composite adherends, on the strength of an adjacent single lap bonded joints, using both experimental and computational methods. Composite laminates composed of 0° unidirectional lamina were fabricated with Polytetrafluoroethylene (PTFE) strips embedded between the first two plies closest to the bond line. Three different inclusion positions were investigated and the location of the inclusion defect yielding the lowest strength was identified. The inclusion location in the most compromised joint was then used to study the effect of this type of defect in adherends having different ply orientations. Finite Element (FE) stress analysis was performed to understand the variation in the shear and peel stresses due to the presence of the inclusion. FE analysis revealed higher shear and peel stresses in bonded joints adjacent to inclusion defects.

Temperature dependent fatigue crack growth in electrical steel laminates with μm-thick epoxy layers

The effect of adhesive layer thickness on monotonic roll peel strength and fatigue crack-growth kinetics was investigated for electrical steel laminates with bond line thickness values ranging from 7 to 48 μm. Roll peel strength was increasing with epoxy layer thickness up to 20 μm. In contrast, roll peel strength was leveling at higher thickness values. The fatigue fracture mechanics approach in mode I delivered an equivalent ranking such as roll peel testing. However, energy-based fracture mechanics analysis was much more sensitive than monotonic roll peel testing, also at thicker bond lines. The different failure modes were correlated with the plastic deformation zone size developing at the propagating crack. In case of a constrained plastic zone, cohesive failure mainly within the epoxy layer was ascertained. In contrast, interfacial failure and interestingly also a better fatigue crack growth resistance was deduced for a weakly or non-constrained plastic zone. Regarding the temperature dependency, an enhanced fatigue crack resistance was obtained at 60 °C. At this temperature about 50 °C below the maximum of the loss factor of the epoxy adhesive, the effect of epoxy layer thickness was much smaller than at 23 °C. At testing temperatures slightly below or above the glass transition temperature, the fatigue crack propagation rate was a factor of 10 to 100 higher.

Graphene as reinforcing filler in polyvinyl acetate resin

Graphene gel and powder were added to polyvinyl acetate (PVA) resin to investigate its potential improving effects on shear strength of the resin. Graphene flakes were synthesized based on a simultaneous electrochemical exfoliation/deposition procedure. Flakes were decorated in face with magnetic nanocrystals in hetero-structure. They were mixed with PVA resin at 3% content, based on the wet weight of the resin. Beech wood specimens (Fagus orientalis L.) were prepared according to ASTM D143-14. Separate sets of specimens were prepared to be first heated at 165 °C and 185 °C for 4 h. Results demonstrated significant improving effect of graphene powder on shear strength of PVA resin. Graphene gel did not improve shear strength due to dilution of resin. Density functional theory simulations indicated very low adsorption energy of either pure graphene (−0.45 eV) or Ni-doped graphene (−0.91 eV) flakes on cellulose. Therefore, the improving effect was not attributed to formation of new bonds between graphene flakes and cell-wall polymers. It was concluded that graphene powder acted as a reinforcing filler in the PVA resin matrix, significantly improving its shear strength in the un-heated beech specimens. However, as a result of thermal degradation of cell-wall polymers and the consequent decrease in mechanical properties of the wood, as well as a considerable decrease in wood wettability, failures occurred within the wood structure of the heat-treated specimens rather than within the bond line. Therefore, graphene powder could not improve the overall shear strength in the heat-treated specimens at 185 °C.

An analytical approach to predict cohesion and interfacial failures of CFRP/steel composites under prolonged exposure to aqueous solutions

The long-term performance of Carbon Fibre Reinforced Polymer (CFRP) strengthened steel joint under prolonged exposure to moisture conditions is imperative in the applications of structures exposed to groundwater or seawater. Analytical models to determine the bond characteristics of CFRP/steel composites subjected to prolonged exposure in aqueous solutions are yet to be explored. This paper explores the cohesive and interfacial failure parameters of such CFRP/Steel composites theoretically. Thirty double-strap joints and twenty adhesive coupons were tested to determine the residual strength after exposure to seawater and distilled water for 6 and 12 months. An analytical model was developed to determine the degradation of adhesive material, bond and the interfaces of composite exposed to seawater and distilled water. The developed model is in good agreement with test results and capable of predicting initial bond degradation rate and stabilized degradation under-considered exposure conditions. Furthermore, the model could predict the contribution from the interfaces, bond line, and adhesive component on the degradation of composite in service.

Diffusion bonding of dissimilar titanium alloys via surface nanocrystallization treatment

A TC17 titanium (Ti) alloy, with and without surface nanocrystallization (SN) treatment was joined to another Ti alloy, TC4, via solid-state diffusion bonding. The interfacial void shrinkage of SN-TC17/TC4 bond was accelerated comparing with that of coarse-grained TC17/TC4 bond. Dynamic recovery and recrystallization were mainly responsible for the fiber texture in β phase and the random orientation in α phase on SN-TC17 side, respectively. The promoted grain growth effect on SN-TC17 side at 1093 K contributed to the disappearance of original bond line (BL). The shear strength of SN-TC17/TC4 bond increased as bonding temperature increased to 1013 K due to the enhanced void shrinkage process, which further increased up to 789 MPa at 1093 K as a result of the disappearance of original BL. Moreover, the shear strength of SN-TC17/TC4 bond at 1093 K was about 60 MPa higher than that of TC17/TC4 bond due to the disappearance of original BL and the Hall–Petch strengthening effect.

Simultaneous tailoring of pulsed thermography experimental and processing parameters for enhanced defect detection and sizing in adhesive bonds in carbon fibre composites

Thermographic inspection provides opportunity to tailor non-destructive evaluation to specific applications. The paper discusses the opportunities this presents through consideration of adhesive bonds between composites, such as those joining structural members and outer skins, where access is restricted to a single side. To date, literature focusses on the development of either an experimental procedure or data processing approach. This research aims to demonstrate the importance of tailoring both of these aspects to an application to obtain improved defect detection and robust quantification. Firstly, the heating stimulus is optimised to maximise the thermal contrast created between defect and non-defect regions using a development panel. Traditional flash heating is compared to longer square pulse heating, using a developed shutter system, compromising between experimental duration and heat input. A pulse duration of 4 seconds using two 130 W halogen bulbs was found double the detection depth from 1 mm to 2 mm, revealing all defects in the development panel. Temporal processing was maintained for all data using thermal signal reconstruction. Spatial defect detection routines were then implemented to provide robust defect/feature detection. Spatial defect detection encompassed a combination of image enhancement and edge detection algorithms. A two-stage kernel filter/binary enhancement method followed by the use of Canny edge detection was found most robust, providing a sizing error of 1.8% on the development panel data. This process was then implemented on adhesive bonds with simulated bond line defects. The simulated defects are based on target detection threshold of 10 mm diameter void found at 1–2 mm depth. All simulated void defects were detected in the representative bonded joint down to the minimum diameter tested of 5 mm. By considering the tailoring of multiple aspects of the inspection routine independently, an overall optimised approach for the application of interest has been defined.

Microstructure and mechanical properties of Ti2AlNb diffusion bonding using multi-phase refractory high-entropy alloy interlayer

This paper conducts diffusion bonding of Ti2AlNb based alloy, in which a novel refractory high-entropy alloy (Ti40Nb30Hf15Al15; RHEA) was used as the interlayer. The RHEA interlayer is designed to eliminate bond line and restrain the aggregation of orthorhombic (O) phase at bonding interface. The microstructures of joints were investigated by scanning and transmission electron microscopy (SEM and TEM, respectively). Defect-free joints were obtained when bonding was performed in the temperature range of 950–1000 °C at 30 MPa for 2 h. The joint microstructure was mainly composed of a disordered bcc-type solid solution and nanosized basketweave ordered O phase. According to the TEM results, the matrix was rich in Hf, which strengthened the matrix by solid solution strengthening. The tiny O phase retained a specific coherent relationship with the bcc matrix, (001)O// (011‾)bcc and [01‾1]O// [1‾11]bcc, which strengthened the bonding interface by precipitation strengthening. This multiphase coupling interface indicated reliable metallurgical bonding and guaranteed excellent joint performance. The mechanical properties of the joints were evaluated using nanoindentation and shear tests. The microhardness and Young's modulus were distributed evenly without any noticeable fluctuation and ranged from 5.01 to 5.52 GPa and 103.97–117.22 GPa, respectively, illustrating the suitable property matching between the high-entropy interlayer and Ti2AlNb. The maximum shear strength of the joint was 463 MPa with bonding at 970 °C and 30 MPa for 2 h. The main crack was significantly deflected into the parent materials, rather than propagating along the interface, which further demonstrated that the bonding face had higher strength than the base metals. The precipitation mechanism of the nanoscale O phase was revealed through transmission Kikuchi diffraction–electron backscatter diffraction (TKD-EBSD). The O phase variants formed with equal probability could lead to a basketweave morphology. The successful bonding of Ti2AlNb using RHEA as the interlayer provides a new interlayer system to bond Ti-based intermetallic compounds.

Micro-tensile testing of the bond line in hot isostatic pressed aluminum

Considerable effort is being devoted to development and regulatory qualification of low enriched fuels for research and test reactors by many agencies worldwide. One promising fuel configuration being examined for United States higher power research and test reactors (USHPRRs) are plate-type fuels composed of a metallic uranium-molybdenum foil clad in an aluminum alloy. The two pieces of aluminum alloy cladding are bonded using a hot isostatic pressing method. The mechanical properties of the resulting bond line in the aluminum alloy cladding will vary by the HIP'ing parameters, requiring a need to characterize the bond line. Small scale mechanical testing can provide a path for evaluating the mechanical properties and deformation behavior of the bond line both prior to and following irradiation. In this research, room temperature micro-tensile specimens of non-irradiated and irradiated samples containing an Al alloy (AA 6061) bond line were tested to evaluate its strength and deformation behavior. Observations indicated that the strain rate did not affect the deformation behavior or strength and most of the micro-tensile specimens failed in a ductile mode in grains around the bond line. There was no indication that the microstructural features from the bond line affected the mechanical properties of the micro-tensile specimens. An initial examination was performed on irradiated material but further systematic studies of the effects of irradiation can be performed in the future.

The Value of Transitioning from the Lap Shear to an Internal Bond for Testing Plywood*

Abstract The lap shear test has been the standard for bond strength testing in plywood for years. Its goal is to predict the long-term durability of the plywood panels. This test has also been used for root cause analysis by mill quality management teams to identify issues. There are several problems with the test, two significant problems being, (1) the only bond tested is the one that is next to the veneer tested, and (2) the test is highly subjective to the accuracy of the kerfing. This paper will address the first problem, which is the larger issue. During the long-term exposure of the panel, the bond lines most likely to fail are the exposed surface or, more likely, the weakest bond. The lap shear test does not test all the bonds simultaneously, so there is no way to ensure the weakest bond is tested on each sample. The data included in this article clearly showed that there was a difference between the bond lines that would be missed in the standard lap shear test. Lastly, the main bonds tested are in the center of the panel; therefore, the result would be biased and may not be an accurate representation of how the panel would perform in the field. These deficiencies are remedied by shifting to the standard internal bond testing common in other wood products.

In-Situ Ultrasonic Inspection of Thickness and Morphology of Thermal Interface Material in Multilayer Structure Using Modified Minimum Entropy Deconvolution

Power density of electronic devices inevitably increases as the feature size of integrated circuits (IC) shrinks, leading to the overheating of chips. The heat dispersion performance and reliability of electronic package devices are significantly affected by the bond line thickness (BLT) and morphology of the thermal interface material (TIM). The conventional ultrasonic inspection with pulse-echo mode fails to accurately quantify the BLT and morphology of TIMs due to the overlapping of echo waves in the multilayer structure. In this work, a third-order nonlinear minimum entropy deconvolution (TON-MED) algorithm is proposed to decouple the overlapped high frequency ultrasonic signals, and the best performance on sparsity of the theoretical and experimental signals is demonstrated by comparing the TON-MED algorithm with the commonly used deconvolution algorithms, Wiggins Minimum Entropy Deconvolution (WMED) and Optimal Minimum Entropy Deconvolution Adjusted (OMEDA). The TON-MED algorithm is consequently implemented to accurately determine the BLT of TIM and image the internal defects, in-situ acquired from the Si-TIM-Cu three-layer structure. The low measurement deviation (6%) and high-resolution image prove the reliability and capability of the proposed TON-MED algorithm for characterizing the BLT and morphology of the TIM simultaneously, which benefits to design and fabricate high performance TIMs for heat dispersion of chip devices.

Effect of Grade 5 titanium interlayer on microstructure and pitting corrosion behaviour of AA1100/A36 explosion welds

Explosion welding is a solid-state welding process that is used for the metallurgical joining of similar and dissimilar combinations of metals and alloys with varying physical and mechanical properties. In this process the metal surfaces are brought into sufficiently close contact, the valence electrons overcome the repulsive forces and result in sharing of their orbits. In recent years ocean acidification is the main problem due to a variety of pollutants. In naval applications A36 steel is used for making ship hulls that are very poor in corrosion. The present study is aimed to improve the corrosion resistance of the A36 steel by cladding aluminium alloy (AA110) using the explosion welding technique. Grade 5 titanium alloy has been used as an inter-layer between AA110 and A36 plates. The presence of Ti-6Al-4V inter-layer facilitated good bonding between aluminium and steel dissimilar weld joint with smooth bond line. The microhardness values were measured at various points start from the weld interface and towards both base metal regions. Higher hardness values were observed in the bond region and the Inter-layer regions, owing to the inherent higher hardness of Titanium and the formation of Martensitic structure near the bond region, as a result of higher and sudden impact energies. Potentiodynamic polarization tests were made in 3.5% NaCl solution. Corrosion resistance in AA1100 is higher than A36. All the observed results were correlated with microstructural features as evidenced by SEM studies.

Fabrication of Sn-plated Cu foam for high-efficiency transient-liquid-phase bonding

To solve the problems of long reflowing time and poor high-temperature resistance of traditional transient-liquid-phase (TLP) bonding, a low cost Sn-plated Cu foam (CF@Sn) preform was developed and fabricated for low-temperature die attachment of high-temperature electronic devices. The preform is composed of a strip-shaped Cu skeleton coated with a uniform Sn layer, displaying a low-bonding-temperature and high re-melting temperature. The microstructure evolutions of interconnection layer and the mechanical properties of joints bonded at 280 °C under 2 MPa for different bonding time were investigated. After bonding for 40 min, the preform demonstrated excellent metallurgic bonding to the substrates, and the bond line with only Cu and Cu3Sn phase was obtained. Additionally, the average shear strength of bonded joints was improved to 28.1 MPa, the failures were occurred along the interface between the Cu skeleton and IMCs. The bonded joints presented good thermal reliability without micro-crack interfaces after the high temperature aging tests.

An experimental and numerical investigation into the effects of bond line thickness variations on the strength of composite bonded joints

The structural integrity of adhesive joints used in the attachment of laminated composite panels has received considerable attention over the past few decades. Of particular concern and the topic of this paper is the effect that a nonuniform bond line thickness has on the load carrying capacity of a bonded joint. To this end, carbon fiber adherends were joined in a single lap joint configuration using a two-part epoxy adhesive. The joints were initially fabricated and tested with various uniform bond line thickness in a single lap joint configuration to establish a benchmark. Bonded single lap joints with linearly varying bond line thickness were then investigated. A finite element study was performed to predict bond strength of the uniform and linearly varying bond lines using the Critical Zone method. The failure strength of the uniform bond line thickness specimens was found to decrease with an increase in the bond line thickness. In the case of linearly varying bond line thickness specimens, the bond strength decreased as the bond thickness at one end of the bonded joint overlap deviated from its intended value. The finite element model using the Critical Zone method was able to predict the failure strength of the joints in both uniform and linearly varying bond thickness configurations. The reduction in failure strength was attributed to an increase in the stresses in the adherend ply adjacent to the bond line as determined through the finite element stress analysis.

Modelling adhesively-bonded T-joints by a meshless method

Bonding of non-parallel substrates has many applications in the transport industry. Adhesively-bonded T-joints are employed for that purpose. However, geometrical dimensions, substrates’ shape and material choices are broad. The effect that varying upper substrate thickness has on the joint strength (P max ) was investigated in this work through numerical analyses. The numerical analysis was performed using a meshless method, the natural neighbour radial point interpolation method (NNRPIM), which has been proven accurate and robust in another adhesive joint configuration. Materials were considered elastic-plastic. A yield criterion developed for rubber-like materials, the Exponent Drucker-Prager, was used for the adhesive layer, while the metallic substrates were analysed with the von Mises yield criterion. P max was determined numerically using a strain-based continuum mechanics failure criterion. Normalised peel and shear stress distributions along the bond-line are presented. Effective strains, both elastic and plastic, were also obtained. The estimated P max was compared with experimental data; a good agreement was found. The stress distribution along the bond line becomes asymmetric in joints with unbalanced substrate thicknesses. At P max , from 10 to 25% of the bond-line has entered into plastic regime. The results indicate that the proposed methodology is suitable to analyse adhesively-bonded joints under different load solicitations.

Prevention of thinned wafer deformation during thermocompression bonding and multi-die stacking supported by temporary bonding materials

Abstract Process flows for memory stacking or other heterogeneous integration schemes benefit from die bonding on a thinned silicon wafer 100 μm or less. In scenarios where a thinned device wafer contains features such as microbumps or Cu pillars, a carrier and temporary bonding material (TBM) facilitate the support of the fragile landing wafer during thermocompression bonding (TCB). The landing wafer in this case is vulnerable to deformations including loss of die planarity, Si bulging, Si or low k dielectric cracking, and damage to the underlying device wafer topography. In this paper, a dual layer system for temporary bonding is presented that maintains the integrity of a thinned device wafer during and after TCB. This is achieved with TBM materials which do not reflow at typical TCB conditions. The approach is to simulate TCB conditions which demonstrate the performance between different underlying TBM materials. A method which tracks the bond head z-axis over time during a TCB cycle is described which in turn yields information on the degree of temporary substrate deformation due to TCB force and temperature. The experiments include a worst-case scenario of multiple TCB cycles in the same position to mimic multi-die stacking. Finally, the impact of process conditions on Cu pillars with solder caps embedded in a thinned wafer bond line will be discussed.

Interplay of the Ring and Steric Strains in the Highly Substituted Cyclopropanes.

Using the concept of a complete set of homodesmotic reactions for the analysis of molecular energetics of polysubstituted methyl- and fluorocyclopropanes allows assessing the strain energy SE of cyclopropanes, free from interfering effects, in full accordance with the IUPAC definition ("relative to a reference ... hypothetical 'strainless' structure"). The correct SE calculation requires quantifying nonvalence interactions in the products of formal homodesmotic reactions (HDRs) using a routine multiregression analysis. The complete HDR set provides the information necessary for the analysis, namely, the heat effects of HDRs calculated by the G4 composite method and the wide set of reference compounds with various combinations of nonvalence effects. We have found that the SE value for methylcyclopropanes lies in the range from 117.0 (1.1-dimethylcyclopropane) to 146.1 kJ/mol (hexamethylcyclopropane). It is the sum of the ring strain energy RSE = 117.9 ± 0.3 kJ mol, which does not depend on the number of methyl substituents, and the Pitzer strain energy of 4.4±0.1 kJ/mol per one contact (the standard deviation is shown as an error of determination). In the series of fluorocyclopropanes, SE varies from 137.9 (monosubstituted cyclopropane) to 260.0 kJ/mol (hexafluorocyclopropane) and well correlates with the ∑DBCP parameter deduced from the QTAIM analysis of the electron density of the compound, representing the total deviation of bond critical points from geometrical C-C bond lines of CC bonds. The ∑DBCP parameter characterizes the curvature of banana-like bonds in cyclopropanes.

Microstructure and mechanical properties in TLP joint of FeCoNiTiAl alloy and IC10 superalloy using Mn-Ni-Cr filler

A filler metal of Mn-Ni-Cr was used to transient liquid phase (TLP) bonding high-entropy alloy (HEA) of FeCoNiTiAl and Ni3Al base IC10 superalloy for 10 min ~ 120 min at 1150 °C and a pressure of 2 MPa. There was no diffusion affected zone (DAZ) existed in the bonded zone as the filler material Mn-Ni-Cr contained no conventional melting-point depressants (MDPs) of Si/B. Hence, the TLP joint was composed of two isothermally solidified zones (ISZs) and an athermally solidified zone (ASZ). The ISZ beside IC10 consisted of strip Ni(Al, Mn) phases, resulted from the diffusion of Al atoms, and only γ-(Ni, Mn) phases formed in another ISZ adjacent to HEA base metal (BM). Moreover, sluggish diffusion of HEA can prevent the BM from dissolving and formed a straight bond line consisting of “(Hf, Ta)C + Al0.89Mn1.1” compound in the interface between HEA and bonded zone. Furthermore, increase of bonding time can lead to enrichment of Al in the bonded zone and induce composition segregation during solidification. A compound of “Ni(Al, Mn) + Ni3Ta” formed in ASZ of joint bonded for 30 min ~ 120 min. Hardness results revealed that formations of Ni(Al, Mn) phases and the compounds in ASZ can cause the alternant hardness distribution of valley and peak across the joint and lead to the generation of stress concentration. Therefore, the maximum shear strength, 269.9 MPa, was achieved in the joint dwelled for a period of 10 min.

Piezoresistive bond lines for timber construction monitoring\u2014experimental scale-up

Several laboratory studies and experiments have demonstrated the usability of polymer films filled with electrically conductive filler as piezoresistive material. Applied to adhesives, the glue lines of wood products can achieve multifunctional—thus bonding and piezoresistive/strain sensing—properties. Based on critical load areas in timber constructions, upscaled test setups for simplified load situations were designed, especially with regard to a stress-free electrical contact. In a second step, another upscaling was done to small glulam beams. Based on an experimental test sequence, the piezoresistive reactions as well as the behaviour until failure were analysed. The results show in all cases that a piezoresistive reaction of the multifunctionally bonded specimens was measurable, giving a difference in the extent of relative change. Additionally, measured phenomena like inverse piezoresistive reactions, electrical resistance drift and the absence of a piezoresistive reaction were discussed, based on additional strain analysis by digital image correlation. A model of macroscopic and microscopic strains influencing the piezoresistive reaction of the electrically conductive bond line in wood was used to explain all experimental results. Finally, a first scale-up of piezoresistive bond lines from laboratory samples to glulam beams was possible and successful.

Soft and Self‐Adhesive Thermal Interface Materials Based on Vertically Aligned, Covalently Bonded Graphene Nanowalls for Efficient Microelectronic Cooling

AbstractUrged by the increasing power and packing densities of integrated circuits and electronic devices, efficient dissipation of excess heat from hot spot to heat sink through thermal interface materials (TIMs) is a growing demand to maintain system reliability and performance. In recent years, graphene‐based TIMs received considerable interest due to the ultrahigh intrinsic thermal conductivity of graphene. However, the cooling efficiency of such TIMs is still limited by some technical difficulties, such as production‐induced defects of graphene, poor alignment of graphene in the matrix, and strong phonon scattering at graphene/graphene or graphene/matrix interfaces. In this study, a 120 µm‐thick freestanding film composed of vertically aligned, covalently bonded graphene nanowalls (GNWs) is grown by mesoplasma chemical vapor deposition. After filling GNWs with silicone, the fabricated adhesive TIMs exhibit a high through‐plane thermal conductivity of 20.4 W m−1 K−1 at a low graphene loading of 5.6 wt%. In the TIM performance test, the cooling efficiency of GNW‐based TIMs is ≈1.5 times higher than that of state‐of‐the‐art commercial TIMs. The TIMs achieve the desired balance between high through‐plane thermal conductivity and small bond line thickness, providing superior cooling performance for suppressing the degradation of luminous properties of high‐power light‐emitting diode chips.

Validation of a lightning protection system for a fusion-welded thermoplastic composite wind turbine blade tip

Fusion welding for thermoplastic composite wind turbine blades is being investigated as a replacement for adhesives in bond lines to reduce manufacturing time and cost and to promote ease of recyclability. Resistance welding uses an electrically conductive material, such as carbon fiber, embedded in the bond line to heat the joining surfaces and fuse them together under pressure. One main challenge with fusion joining for large wind blades is the ability to use this conductive heating element in the bond line while still protecting the blade from the catastrophic effects of a lightning strike. This paper outlines the design and manufacturing of a lightning protection system for a 5-m-long thermally welded blade tip. The outcomes of high voltage and high current lightning testing applied to the 5 m blade tip show that the proposed design was able to protect the fusion welded bond lines from significant lightning damage.