Adhesive Fracture Toughness Research Articles

“Like binding a book”: Interfacial improvement of pre-melted ultrahigh molecular weight polyethylene by pulse vibration molding

Ultrahigh molecular weight polyethylene (UHMWPE) is widely used in industrial, military, and civilian applications owing to its excellent properties. However, the high entanglement degree of UHMWPE melt leads to great difficulties in diffusion and crystallization, which causes poor interfacial strength and mechanical properties of pre-melted UHMWPE, restricting its high-quality processing and high-value recycling. To this end, we propose a rapid interface stitching technology with a pulsed vibration force driving free volume homogenization and chain segment orientation. For the first time, pulse vibration molding (PVM) is applied for the hot-pressing of pre-melted UHMWPE, successfully realizing the interface stitching and product strengthening. Compared with those of the convention molding (CM) samples at 180 °C, the interfacial adhesive fracture toughness of the adhesive samples processed using PVM is increased by 105.9 % and the work of tension of the recycled samples processed using PVM is also increased by 49.09 %, respectively. The fracture strength of the PVM recycled samples at 180 °C reaches 91.77 % of that of the pure samples and the yield strength is also higher, which significantly enhances the recycling of UHMWPE. The characterization results of the interfacial molecular chain entanglement and crystallization show that PVM strengthens the molecular chain diffusion and induces crystallization by promoting the orientation of chain segments, especially at the interface. Consequently, we believe that the proposed technology can play a significant role in the UHMWPE processing methods that involve surface healing.

Influence of modelling hypotheses on strength assessment of CFRP stepped repairs

Composite stepped repairs can achieve high strength recovery without the addition of bolts or fasteners to the structure. They are therefore a major issue in the field of aerospace composite structure damage repair. However, there is no standardized method to design this type of repairs. Many analytical, semi-analytical and finite element models were proposed throughout the years to predict the strength of stepped repairs, using various hypotheses and simplifications. The aim of this paper is to investigate the influence of modelling hypotheses on stress distribution and strength prediction of composite stepped repairs. Five simplified stepped joint models using macro-element (ME) modelling and finite element method (FE) are compared to a full 3D FE model of a stepped repaired panel. The influence of step length and adhesive fracture toughness was investigated to determine the field of validity of each model. Among FE models, it was shown that modelling the equivalent joint under 2D generalized plain strain gives a very close strength prediction to the 3D stepped repair model while saving computation time. Simplified macro-element models under bar or beam hypotheses are fairly close to the results of FE modelling, but the deviation between those and FE is sensitive to step length and adhesive fracture toughness.

Tuning mechanical properties of TiAlSiN/TiAlN multilayers by interfacial structure

Multilayer structure as an effective way to tailor the structure and performance of hard coatings has attracted wide attentions. Here, we aim to elucidate the change of the interface, mechanical properties and oxidation resistance for TiAlSiN/TiAlN multilayers by varying modulation ratios from ML (tTiAlSiN/tTiAlN = 1:1) to HML (tTiAlSiN/tTiAlN = 1:2). Increasing TiAlN sublayer thickness stabilizes the cubic growth of TiAlSiN layers, where the HML exhibits coherent epitaxial growth in contrast to the cubic-wurtzite mixed structure of ML. This coherent strengthening effect is responsible for the noteworthy enhancement of the hardness values from ∼30.9 GPa of ML to ∼33.2 GPa of HML. Significantly, the presence of a coherent interface effectively withstands external pressure, thereby endowing HML with exceptional adhesive strength and fracture toughness. Nevertheless, the oxidation resistance of the coating is reliant on the chemical composition, with the multilayers displaying lower oxidation resistance compared to the TiAlSiN monolayer.

Ultra-tough architected adhesive joints for integrated composite processing and bonding

Deployment of advanced polymer-based composites in critical structures requires, among others, breakthroughs in adhesive bonding solutions. Indeed, available methods still suffer from limited fracture toughness of adhesives and from time-consuming bonding processes. Here, we demonstrate a novel concept of architected thermoplastic joints with exceptional fracture resistance up to 5000 J/m2, fully integrated with the composite resin transfer molding process, hence simultaneously targeting both limitations. This extreme toughness is activated through controlled 3D printed hollow pattern within a Nylon bondline. A synergetic combination of plastic dissipation, crack deflection, branching and arrest is tuned by changing the pattern characteristics. Three failure regimes are unraveled through fractographic analyses and finite element models. A stress-at-a-distance fracture criterion, identified for each constituent, quantitatively predicts the toughness variations along the crack path. This approach, amenable to dissimilar bonding between metals and composites, paves the road towards novel and higher performance structures and manufacturing approaches.

Testing method to determine the strength and fracture toughness of adhesives in a single continuous test

In this work, a new testing method for adhesive joints under tensile loading is presented. This method comprised three pull-off tests and one peel test in one single loading step, which enables the determination of the strength and fracture toughness of the adhesive. The properties determined can be of the adhesive, if cohesive failure occurs, or of the interface, in the event of adhesive failure. To validate the testing method, the adhesive used was a commercially available silicone, for which strength and fracture toughness had already been determined using standard methods. It was concluded that this method accurately enables the determination of strength and toughness, using one single joint and loading step.

Adaptive phase-field modeling of fracture in orthotropic composites

An adaptive phase-field method is developed and applied to model fracture propagation in orthotropic composites. The proposed approach extends the work of Muixí et al. (2020) to orthotropic composites through following modifications: (a) elastic tensor is evaluated in the global coordinate system from a given engineering stiffness matrix in the principal material coordinates, (b) strain energy density is additively decomposed into fiber and matrix-dominated modes, and (c) a penalized second-order structural tensor is introduced in the phase-field equation facilitating preferential damage growth along the fiber orientation. The developed approach is validated through several benchmark examples and available experimental results.Several numerical failure experiments are conducted varying fiber orientation, adhesive layer thickness, shear modulus, and fracture toughness. A large increase in ductility and peak-load carrying capacity is observed for laminate configurations where the fibers are oriented such that the principal material coordinates and the global coordinates are aligned. An increase in the adhesive layer’s fracture toughness increases the macroscopic ductility and prevents catastrophic failure of laminate. The results of this study provide valuable insights into the failure mechanisms in orthotropic composites.

Spontaneous formation of a nest-like frame structure in sprayed coatings for superhydrophobic applications

The accretion of ice, contaminants, oxidants, and organisms on exposed surfaces has been a perpetual concern in many fields of engineering and often presents a serious safety hazard. Green methods to make superhydrophobic (SH) coatings via spraying nanoparticles are attracting routes to resolve these issues, especially considering the low energy inputs and capital costs. However, these nanoparticle coatings do not yet meet the demanding standards of practical applications. Here, we show that spraying SiC whiskers (SiCw) with optimized dispersity in water spontaneously forms nest-like frame structures that readily host hydrophobic nanoparticles (NPs), yielding SH coatings with excellent durability and usability. The SiCw frame structure improves the wear resistance of the NPs coating by two orders of magnitude and withstands repeated bending and twisting (180°), making it more appealing than the state-of-the-art SH surfaces fabricated by sophisticated techniques. Exhibiting a low surface friction coefficient, superior hardness, and adequate adhesive strength and fracture toughness, the SiCw frame structure endows our novel SiCw/NPs composite coating with the necessary properties to meet the stringent demands for efficient SH coating fabrication.

Uncovering the toughening mechanisms of bonded joints through tailored CFRP layup

Aiming to increase damage tolerance of adhesively bonded joints, this work explores the influence of CFRP layup of the adherends on the crack onset and crack propagation of composite bonded joints under mode I loading. Quasi-static Double Cantilever Beam tests were performed using four different CFRP layups bonded with two adhesives. Parallel to the experimental program, finite element analyses were performed to aid in understanding and identifying the various damage mechanisms in each specimen type. The results show that the CFRP layup and adhesive fracture toughness significantly influence the joint fracture phenomena at crack onset and further crack propagation. An enhancement of the joint's mode I fracture toughness values at crack onset was observed in the specimens where a crack competition between the propagation within the bondline and the composite's layers was triggered. During crack propagation, the fracture toughness of the joint increases at crack deflections between the different plies of the CFRP layup until reaching the 0° ply, where sudden delamination occurs. It has been shown that CFRP layup tailoring is a promising toughening method that, when carefully designed, has the potential to increase the maximum effective fracture toughness up to 100% when compared to pure cohesive failure.

Effect of graphene nanoplatelets on mode I fracture Toughness of epoxy adhesives under water aging conditions

ABSTRACT In this work, graphene nanoplatelets (GNP) are used to reinforce epoxy adhesives, and the effects of GNP on mode I fracture toughness of adhesive under water aging condition are studied through double cantilever beam (DCB) test. Although the GNP reinforced adhesives deliver slightly higher GIC than neat epoxy before water aging, they exhibit an exceptional 21.93% higher GIC after immersing in water for 56 d, at a GNP content of 0.25 w.t.%. According to the experimental results, the effectively dispersed GNP could significantly alleviate the degradation rate in mode I fracture toughness due to the water aging. For example, compared to the non-aging condition, the mode I fracture toughness of neat epoxy decreases by 29.19% after 56 d of water aging. In a stark contrast, the degradation rate for 0.25 w.t.% GNP reinforced adhesive is only 15.76%. In addition, it is observed that GNP greatly improves the fracture surface roughness of DCB samples, which suggests that the incorporation of GNP is able to delay the negative effect of water aging on mode I fracture toughness of adhesive by lowering the diffusion of water into the adhesive, while increasing torturous of the crack propagation path.

A new technique to measure shear fracture toughness of adhesives using tensile load

The end notched flexure (ENF) test is the most common method for measuring the mode II fracture energy of adhesives which provides reliable results but has some inherent problems. The shear fracture energy calculated using ENF for brittle adhesives is often based on a single data point (corresponding to the fracture load) obtained from the test where the crack grows catastrophically and the results are significantly sensitive to pre-crack tip conditions. The aim of the current study is to introduce a new pure mode II fracture test suitable for measuring the shear fracture energy of adhesives. A new technique for measuring the mode II critical strain energy release rate (SERR) of brittle adhesives is proposed based on the central cut ply (CCP) configuration. Fracture tests were carried out on adhesively bonded steel CCP specimens. It is shown that the crack growth in CCP joints is more stable, and the failure develops in multiple steps that generates more experimental data point during the crack propagation. The mode II fracture energy of a brittle epoxy-based adhesive is extracted using CCP method and is compared with the shear fracture energy obtained from the ENF tests. The effect of adhesive thickness on the obtained fracture energies is also analyzed. The CCP technique can be used to find both the crack initiation and crack propagation toughness values whereas the ENF method only provides the crack initiation fracture energy.

Effect of rubber modification on fracture toughness of epoxy adhesives under mode I and mode II loading

In the present study, the effects of rubber modification on the joint strength and fracture toughness of epoxy adhesives under mode I and II loading were investigated using Arcan-type adhesively bonded joints. The results of the fracture tests under mode I loading revealed the occurrence of stress whitening owing to triaxial tensile stresses near the crack tip. The joint strength and fracture toughness were calculated using the J-integral value, where the adhesive layer was considered as an elastic–plastic material. The crack-growth energy was also calculated using the J-integral value, where the adhesive layer was considered as an elastic material. The results revealed that rubber modification under mode I loading induced an increase in the joint strength, fracture toughness, and crack-growth energy. Rubber modification under mode II loading did not induce stress whitening near the crack tip owing to low triaxial tensile stresses. However, macroscopic void growth and significant plastic deformation were observed in the adhesive layer. Rubber modification induced a slight decrease in the joint strength; however, it induced a significant increase in the fracture toughness owing to the growth of the plastic zone in the adhesive layer. The calculated crack-growth energy decreased slightly, in accordance with the joint strength. It was concluded that rubber modification contributed to an increase in the crack-growth resistance under mode I loading and not under mode II loading. Furthermore, a correlation was observed between the joint strength and crack-growth energy.

Ozone functionalized graphene nanoplatelets and triblock copolymer hybrids as nanoscale modifiers to enhance the mechanical performance of epoxy adhesives

In the present work, the mechanical and fracture performance of a newly developed high-performance nanocomposite adhesive was characterized experimentally. The nanocomposite adhesive was based on an epoxy resin, which was modified with 10 wt% of triblock copolymers (BCPs) including styrene-butadiene-methylmethacrylate (SBM) and methylmethacrylate-butylacrylate-methylmethacrylate (MAM), and ozone-functionalized graphene nanoplatelets (OZ-GNPs). The mechanical properties of the adhesive joints were systematically evaluated using lap shear, tensile-butt, and Mode-I fracture toughness tests. The lap shear strength and the ultimate shear strain of the 1.0 wt% OZ-GNP + SBM ternary nanocomposite epoxy adhesives obtained 91% and 97%, while 1.0 wt% OZ-GNP + MAM adhesives attained 64% and 63% improvement, respectively, compared to that of the pure epoxy. Consequently, the potential load capacity was improved by about 93% (1.0 wt% OZ-GNP + SBM) and 62% (1.0 wt% OZ-GNP + MAM) compared to that of pure epoxy joints. However, the tensile butt joint strength of the nanocomposite adhesives was slightly reduced due to the lower stiffness of the BCPs. The mode-I fracture toughness results for the 1.0 wt% OZ-GNP + SBM and 1.0 wt% OZ-GNP + MAM increased significantly by approximately 1000% and 200%, respectively, compared to the unmodified epoxy adhesive. Fractography using scanning electron microscopy showed the formation of BCP nanostructures, plastic deformation, crack deflection, debonding of the BCP, and subsequent plastic void growth, which all contributed to the increase in the fracture toughness of the nanocomposite adhesives. The extent of the plastic deformation zone was enhanced by the addition of OZ-GNPs.

Effect of slitting of carbon fiber-reinforced plastic strip on flexural properties of reinforced concrete

In this study, reinforced concrete was manufactured by attaching a carbon fiber reinforced plastic (CFRP) strips to the surface of the concrete. Strips with the same total area were slitted into one, two, three, four, and six pieces and attached to the upper, lower, and side parts of the concrete. The flexural strength and fracture toughness of the reinforced concrete were investigated. As the number of slits in the strip increased, for the upper reinforcement, the flexural strength gradually increased from 7.88 MPa to 11.21 MPa; for the lower reinforcement, the flexural strength increased significantly from 7.88 MPa to 26.48 MPa and then gradually increased to 33.90 MPa; and for the side reinforcement, the flexural strength increased from 7.12 MPa to 13.96 MPa and then gradually decreased. In the adhesive fracture toughness test, the fracture toughness energy significantly increased from 142.38 J m−2 to 516.63 J m−2 as the contact area between the adhesive and the strip increased. Therefore, in reinforcing concrete using a CFRP strip, it was confirmed that the reinforcement effect was enhanced when the strip was slit and then attached.

Numerical analysis of the mixed-mode fracture of bonded joints depending on the adhesive thickness

The mixed-mode strength of bonded joints can be predicted by techniques such as cohesive zone models (CZM), which requires the estimation of the adhesive strength and fracture toughness ( GC). Under the scope of fracture properties, the tensile and shear fracture toughness ( GIC and GIIC, respectively) and the corresponding mixed-mode behaviour are particularly relevant. Moreover, these parameters highly depend on the adhesive thickness ( tA), making it relevant to validate and propose CZM laws for bonded joint design. This work numerically addressed the tA influence on the mixed-mode fracture process of adhesive joints. For this purpose, single-leg bending (SLB) test experimental data was used, considering joints with composite adherends and a ductile adhesive, and tA from 0.1 to 2.0 mm. A numerical CZM analysis was performed, including the experimental and numerical load-displacement ( P- δ) curves’ comparison for validation, followed by the CZM law estimation and fracture envelope validation, for all tA. The effect of the different CZM parameters on the results was finally evaluated. Overall, it was possible to numerically ascertain the tA effect on the fracture behaviour of adhesive joints and to propose a numerical technique for mixed-mode bonded joint analysis.

Quasi-static and intermediate test speed validation of SHPB specimens for the determination of mode I, mode II fracture toughness of structural epoxy adhesives

The study of the behaviour of adhesive joints under impact loadings is a very active field of research, driven by significant industrial interest, such as the automotive industry, in order to improve mechanical behaviour, reduce weight and simplify manufacturing. Furthermore, due to high standards in terms of safety required in the automotive industry, namely the occupant’s safety during collisions, the behaviour of adhesives under impact loads is becoming a prominent object of study. This work proposes a novel and simplified method to experimentally obtain a custom set of TSLs (traction-separation laws) and estimate the fracture energy both for mode I and II that can be tested from quasi-static to impact conditions resorting to a SHPB (Split Hopkinson Pressure Bar) equipment with especially designed specimens. The method is validated by comparing those results to fracture energy values obtained using DCB (Double Cantilever Beam), ENF (End Notch Flexure) and prior validated mixed mode apparatus Finally, the SLJ (Single Lap Joint) behaviour is predicted by resorting to the custom TSLs and comparing model performance to experimental results. With the proposed method the obtained fracture energy was found to be equivalent to that obtained with more traditional fracture mechanical tests used in the field of adhesive bonding. The custom TSLs were able to predict the behaviour of SLJ fairly accurately.

Fracture energy assessment of adhesives – Part I: IsGICan adhesive property? A neural network analysis

Different joint types and geometries have been considered to measure the fracture toughness ( Gc) of adhesives. However, results show that, sometimes, the obtained Gcvalues for the same adhesives are not similar in different reports. Several factors including the joints geometry, material properties of the substrate, and also the test conditions influence the obtained results. This study is conducted to find a logical connection between these factors and to find their influences on the obtained mode I fracture toughness ( GIc) to understand which parameters are the most influential and which ones are less significant. To this aim, 115 values of different GIcalready reported in the literature were collected and examined regarding the geometrical and material parameters. To find the rational relationship between the effective parameters and the reported GIcvalues, a method based on the artificial neural network technique was employed. The results revealed that whilst the fracture energy of aluminum adhesive joints is more influenced by geometrical parameters including the joint type and substrate thickness, the steel adhesive joints are more sensitive to the adhesive properties and bondline thickness. With this study, it is possible to design an optimum test by minimizing the effects of variables that cause errors in obtaining fracture toughness and also to estimate GIcof adhesives by using the developed model. On the other hand, it is also possible to design real structures in which the fracture toughness of the adhesive reaches its maximum value.

Fracture energy assessment of adhesives Part II: Is GIIc an adhesive material property? (A neural network analysis)

Different joint configurations were considered to analyze the fracture toughness of adhesives. However, based on the literature, fracture toughness depends on various factors including geometrical parameters, material properties, and testing conditions. In Part I of this study, the tensile fracture toughness (GIc) of adhesives was assessed. The present Part II of this research deals with the influencing factors in shear fracture energy of adhesive materials. The main goal of this study is to find the role of the effective parameters on mode II fracture toughness (GIIc) of adhesives. The rational connection between these parameters and the shear fracture energy is also obtained. The interaction between the effective variables is also investigated. To attain this goal, the artificial neural network (ANN) technique was conducted on over 45 values of GIIc already obtained and reported by different authors using different joint geometries and adhesive materials. The influence of the variables was obtained, and the least important variables were detected and omitted from the model. The effect of different parameters and their interaction on the obtained GIIc were also analyzed. The results indicated that GIIc is remarkably sensitive to the alteration of the adherend Young's modulus and its thickness, the loading rate, and the crack length. It was found that GIIc is less influenced by the adhesive stiffness. In addition, the interaction results indicated that the loading rate has the highest interaction with the other studied variables. This study enables to estimate the mode II fracture toughness of adhesive joints and also to advise a configuration to improve the shear fracture energy of a specific adhesive. Designing a test procedure to minimize the effects of substrate parameters on the obtained GIIc is also possible using the obtained results.

Comparative Study of the Impact Wedge-Peel Performance of Epoxy Structural Adhesives Modified with Functionalized Silica Nanoparticles

Epoxy structural adhesives have strong adhesion, minimal shrinkage and high thermal and chemical resistance. However, despite these excellent properties, their high-energy impact resistance should be improved to satisfy the increasing demands of the automotive industry. For this reason, we used four types of silica nanoparticles with different surface groups, such as polydimethylsiloxane (PDMS), hydroxyl, epoxy and amine groups, as toughening agents and examined their effect on the glass transition temperature (Tg), crosslinking density and phase separation of epoxy structural adhesives. High-energy impact resistance, mode I fracture toughness and lap shear strength were also measured to explain the effect of surface functional groups. Silica nanoparticles with reactive functional groups increased the mode I fracture toughness of epoxy structural adhesives without sacrificing the crosslinking density. Although the mode I fracture toughness of epoxy structural adhesives could not clearly show the effect of surface functional groups, the dynamic resistance to cleavage obtained by impact wedge-peel tests showed quite different values. At a 0.3 vol% content, epoxy-functionalized silica nanoparticles induced the highest value (40.2 N/mm) compared to PDMS (34.1 N/m), hydroxyl (34.9 N/mm), and amine (36.1 N/m). All of these values were significantly higher than those of pristine epoxy structural adhesive (27.7 N/mm).

Evaluation of structural adhesive joints fracture toughness without crack measurement

The adhesive fracture resistance of structural adhesive joints in mode II, 𝒢IIC, is accessed through the application of two models that prescind of the measurement of the crack length during the subjection of ENF specimens to three-point bending monotonic tests. The load vs transversal displacement results are used in two different approaches: a damage model and a cohesive zone model. The first one was used in test where the crack propagation was unstable and the second one was used for stable crack propagations. These initial results show the viability of the models and show that both: initial crack length and transversal load rate; have great influence in 𝒢IIC evaluation.

Toughening Effect of Rodlike Cellulose Nanocrystals in Epoxy Adhesive

Epoxy structural adhesives have been used extensively in the automotive and aerospace industries to bond assembly parts. Much effort has been devoted to enhancing the mechanical properties of epoxy resin by incorporating fillers. Among a variety of fillers investigated for this purpose, nanocelluloses are regarded as a promising type of emerging green filler material because of their excellent mechanical and physicochemical properties. Indeed, nanocelluloses have been used as a filler for polymer nanocomposites. However, the toughening and reinforcing effects of nanocelluloses on rigid epoxy adhesives have not yet been fully revealed, particularly from the perspective of adhesive bonded joints. Here we report that epoxy adhesive containing cellulose nanocrystal (CNC) aggregates produced using a solvent-free ball milling method achieves drastically improved adhesive strength and fracture toughness compared with that of the reference adhesive without CNCs. The epoxy adhesive containing CNC aggregates exhibi...