Interfacial Toughening Research Articles

Mechanism of interface performance enhancement of nano-SiO2 modified polyvinyl alcohol fiber reinforced geopolymer concrete: Experiments, microscopic characterization, and molecular simulation

This study proposes a method utilizing nano-SiO2 modified polyvinyl alcohol fibers to enhance the interface properties of geopolymer concrete. The surface properties of modified polyvinyl alcohol fibers were analyzed through microscopic characterization. The influence of different dosages of modified polyvinyl alcohol fibers on the mechanical properties of geopolymer concrete was investigated. Molecular dynamics simulations revealed the interfacial toughening mechanism between modified polyvinyl alcohol fibers and geopolymer concrete. Results showed that nano-SiO2 modified polyvinyl alcohol fibers increased surface roughness, thereby enhancing interfacial adhesion and mechanical interlocking between fibers and geopolymer concrete, significantly improving compressive, splitting tensile, and flexural strengths of geopolymer concrete with an optimal fiber dosage of 0.2 %. Relative to unmodified PVA fibers, the modified fibers enhance the compressive, splitting tensile, flexural strength, and elastic modulus by 20.19 %, 15.72 %, 22.34 %, and 30.58 % respectively. KH560 serves as an intermediate medium for nano-SiO2 modified polyvinyl alcohol fibers, generating numerous hydrogen bonds at the interface and tightly adhering nano-SiO2 to the surface of polyvinyl alcohol fibers. Additionally, nano-SiO2 can form ionic bonds and stable Si-O-Si bonds with the hydrated sodium aluminosilicate gel.

The effect of short Kevlar fibers interfacial toughening on impact properties of carbon fiber/aluminum honeycomb sandwich panels

Due to the susceptibility of carbon fiber/aluminum honeycomb sandwich structures to interface damage under impact loading, their overall mechanical performance can be compromised, potentially leading to structural failure. This poses a severe risk to the safety of such structures in engineering applications. This study initially conducted low-velocity impact tests and post-impact compression tests on carbon fiber/aluminum honeycomb sandwich panels with both untoughened and short Kevlar fibers toughened interfaces, using four different impact energy levels (10 J, 20 J, 30 J, and 50 J). The experimental results indicated that the toughening with short Kevlar fibers significantly enhanced the impact resistance and residual compression performance of the sandwich panels. Subsequently, the impact test on the toughened specimen was simulated numerically and the experimental results were in good agreement with the numerical simulations. Finally, based on the numerical model, the study explored the influence of varying face sheet parameters on the impact resistance of short Kevlar fibers toughened carbon fiber/aluminum honeycomb sandwich panels. Layup quantity and layup angle have significant influence on the impact resistance of sandwich panels. With the increase of layup quantity, the impact resistance improves. In several layup angles, [0/45/90/-45/0/45] layering method shows the best impact resistance.

Enhancing the flexural toughness of UHPC through flexible layer-modified aggregates: A novel interfacial toughening strategy

Enhancing the interfacial deformability of UHPC positively impacts its toughness and durability. In this work, a novel interfacial toughening strategy was proposed and employed for UHPC, in which the aggregates were treated with polyacrylate emulsion (PL) or PL modified by silica fume or carbon nanotubes to form an interfacial flexible layer (FL). The flexural characteristics of the prepared UHPC were comprehensively investigated, with attention to the damage evolution based on acoustic emission. Meanwhile, the corresponding toughening mechanism was discussed. The results showed that the FL modified by carbon nanotubes effectively enhanced the flexural deformation capacity, energy absorption capacity, and toughness of UHPC, while maintaining flexural strength. Introducing FL reduced ringing count and acoustic emission energy and mitigated damage rate of UHPC. The FL altered the flexural damage mode of UHPC by alleviating stress concentration to prevent sudden matrix cracking and fiber debonding. During the elastic stage, FL and the UHPC matrix jointly sustained tensile cracks, enhancing the matrix's energy absorption capacity, which correlated positively with the percentage of tensile cracks. In the softening stage, this capacity correlated positively with the percentage of shear cracks. Moreover, FL reduced the probability of microcracks at the interface. Although the FL reduced the average microhardness at the interface, it stabilized the performance of hydration products and increased their maximum microhardness. The FL promoted interfacial energy dissipation and synergistically bridged microcracks with steel fibers, ultimately enhancing the flexural toughness of UHPC.

Effect of plastic deformability and fracture behaviour on interfacial toughening mechanism at Fe/Ni interfaces

Fracture at interface causes plastic deformation in the vicinity region. Conventional plastic energy dissipation theory indicates that ductile vicinity toughens the interface by absorbing plastic deformation energy. However, the microstructure in the vicinity directly affects local plastic deformability and fracture behaviour, implying a more complicated toughening mechanism. In this study, the effect of microstructure and hardness on fracture behaviour of Fe/Ni interface was investigated. By experimental approach, interfaces with and without dynamic recrystallization (DRX) were fabricated by controlling the bonding conditions. It showed that compression-induced plastic deformation is the main source of the hardening behaviour in the vicinity. Moreover, the interfaces with hardened DRX vicinities exhibited improved fracture toughness, which is inconsistent with the plastic energy dissipation theory. To clarify this observation, the crystal plasticity finite element method (CPFEM) approach was employed to distinguish the effects of plastic deformation and interfacial microstructure. The result showed that although higher plastic deformability in the vicinity absorbs more dissipated plastic energy, severe stress concentration at the interface leads to early fracture and poor toughness. On the other hand, the interfacial hardened DRXed grains disperse interfacial stress distribution and provide potential sub-crack sites. A combined result of uniform plastic deformation and fracture energy dissipation is responsible for the improved toughness at interfaces with DRXed grains.

Molecular investigation on interfacial toughening between silane coupling agent treated glass fiber and cement

Silane coupling agents (SCAs) are widely used as adhesion promoters to tightly bond glass fiber (GF) and cement matrix (CM) for developing sustainable and high-performance glass fiber-reinforced cementitious (GFRC) composites. This study uses molecular dynamics (MD) simulation to examine the effect of 3-aminopropyltriethoxysilane (APS) on CM/GF interfacial properties. Pure cement and nine cement models bonded to APS-modified GF with APS concentrations of 0.00%, 12.50%, 25.00%, 37.50%, 50.00%, 62.50%, 75.00%, 87.50%, and 100.00% are constructed. It is shown that pure CSH system experiences interfacial delamination. With increasing APS from 0.00% to 37.50%, the failure mode transits from interfacial delamination to CSH delamination, and with increasing APS to 100.00%, it transits to interfacial delamination. Meanwhile, it is measured that with increasing APS from 0.00% to 37.50%, the tensile strength increases from 0.48 to 0.58 GPa and the adhesion energy increases from 0.71 to 0.90 J⋅m−2. Comparatively, with increasing APS to 100.00%, the tensile strength decreases to 0.26 GPa, and the adhesion energy decreases to 0.23 J⋅m−2. Moreover, the interfacial structure and chemical bond analysis demonstrate that with increasing APS from 0.00% to 37.50%, the peak of H2O molecules decreases, which denotes stronger interfacial bonding. With increasing APS from 37.50% to 100.00%, the peak of the H2O molecules increases, which induces low interfacial bonding. These findings provide a basis for optimizing APS concentrations for specific performance enhancements of GFRC materials.

Enhancing bond performance of CFRP-steel epoxy-bonded interface by electrospun nanofiber veils

The epoxy-bonded interfaces between carbon fiber reinforced polymer (CFRP) and steel usually have insufficient strength and toughness, and the toughening of bonded interface is a key problem for the usage of CFRP in steel structures. In this study, electrospun nanofiber veils were first proposed to enhance the bond performance of CFRP-steel epoxy-bonded interfaces. Firstly, shear tests were conducted on neat epoxy and nano-modified single-lap aluminum-aluminum joints to determine the optimal areal density and number of layers of nanofiber veils, as well as the optimal curing processes. Then, a series of neat epoxy and nano-modified CFRP-steel double-lap joints with different bond lengths were tested to investigate the size effect of the bond behavior. The displacement and strain field evolution of the joints were captured by the digital image correlation (DIC) technique, allowing for visualization of the detailed failure process. The failure modes, load-displacement curves, CFRP strain distributions, and bond-slip relationships of the CFRP-steel joints were obtained. Both the tests on aluminum-aluminum and CFRP-steel joints show that the optimal modification strategy is incorporating 3 layers of nanofiber wels with an areal density of 4.5 g/m2, with 5 h room-temperature and 2 h 80 °C high-temperature curing. The primary failure mode of CFRP-steel joints is CFRP delamination accompanied by CFRP-adhesive interface or steel-adhesive interface debonding. The bond strengths of the modified joints with 3 layers of 1.5 g/m2 and 4.5 g/m2 nanofiber veils are increased by 7 % and 25 % compared to those of un-modified joints, respectively. The 4.5 g/m2 nanofiber veils modified bonded interface has an effective bond length of about 152 mm, with a corresponding ultimate bearing capacity of 117 kN. Different from the triangular (brittle) shape of most neat epoxy interfaces, the nano-modified interfaces have trapezoidal (ductile) bond-slip relationships, providing superior cracking resistance. Moreover, a comparison with the bond strength of SiO2 nano-particles and carbon nanotubes (CNTs) modified joints revealed that nanofiber veil modification comes to higher bond strength in most cases. The proposed electrospun nanofiber veil modification technique provides a great insight into the interfacial toughening of CFRP-steel composite structures.

Entropically Toughened Robust Biodegradable Polymer Blends and Composites for Bone Tissue Engineering.

Biodegradable polymers and composites are promising candidates for biomedical implants in tissue engineering. However, state-of-the-art composite scaffolds suffer from a strength-toughness dilemma due to poor interfacial adhesion and filler dispersion. In this work, we propose a facile and scalable strategy to fabricate strong and tough biocomposite scaffolds through interfacial toughening. The immiscible biopolymer matrix is compatible by the direct incorporation of a third polymer. Densely entangled polymer chains lead to massive crazes and global shear yields under tension. Weak chemical interaction and high-shear melt processing create nanoscale dispersion of nanofillers within the matrix. The resultant ternary blends and composites exhibit an 11-fold increase in toughness without compromising stiffness and strength. At 70% porosity, three-dimensional (3D)-printed composite scaffolds demonstrate high compressive properties comparable to those of cancellous bones. In vitro cell culture on the scaffolds demonstrates not only good cell viability but also effective osteogenic differentiation of human mesenchymal stem cells. Our findings present a widely applicable strategy to develop high-performance biocomposite materials for tissue regeneration.

Controllable toughness enhancement in graphene hybrid materials via planar twist-angle of carbon nanotubes

A controllable toughness enhancement in graphene hybrid materials via twist-angle of CNTs was first investigated. The mechanical properties, such as out-of-plane deformation, stress–strain curves, strain energy and interlayer loading transfer of Gr Hybrid model coupled by twist-angle CNTs were investigated by molecular dynamics simulations and nonlinear fracture mechanics theory. We find that these mechanical properties are sensitive to the existence of twist-angle CNTs in the Gr hybrid model. The crosslinking sites can be considered as structural defects. In addition, the CNTs exert a strengthening influence on the out-of-plane deformation, interlayer loading transfer as well as bond strain energies, thereby deflecting the crack paths of Gr Hybrid model under axial tension. Nevertheless, the existence of CNTs also triggers a reduction in the in-plane stress and the associated strain due to sp2 + sp3 hybrid state. We find that this abnormal improvement in toughness derives from the localization of the stress fields arising from their out-of-plane displacements at the interface, resulting in the stress fields being localized only at the crosslinking sites. These interfacial toughening strategies can redistribute crack tip stress in a wider range, avoid stress concentration, and at the same time realize a lot of energy dissipation by promoting crack deflection, thus achieving overall high toughness.

Simultaneous Enhancement of Efficiency and Operational-Stability of Mesoscopic Perovskite Solar Cells via Interfacial Toughening.

The combined effects of compact TiO2 (c-TiO2 ) electron-transport layer (ETL) are investigated without and with mesoscopic TiO2 (m-TiO2 ) on top, and without and with an iodine-terminated silane self-assembled monolayer (SAM), on the mechanical behavior, opto-electronic properties, photovoltaic (PV) performance, and operational-stability of solar cells based on metal-halide perovskites (MHPs). The interfacial toughness increases almost threefold in going from c-TiO2 without SAM to m-TiO2 with SAM. This is attributed to the synergistic effect of the m-TiO2 /MHP nanocomposite at the interface and the enhanced adhesion afforded by the iodine-terminated silane SAM. The combination of m-TiO2 and SAM also offers a significant beneficial effect on the photocarriers extraction at the ETL/MHP interface, resulting in perovskite solar cells (PSCs) with power-conversion efficiency (PCE) of over 24% and 20% for 0.1 and 1cm2 active areas, respectively. These PSCs also have exceptionally long operational-stability lives: extrapolated T80 (duration at 80% initial PCE retained) is ≈18000 and 10000h for 0.1 and 1cm2 active areas, respectively. Postmortem characterization and analyses of the operational-stability-tested PSCs are performed to elucidate the possible mechanisms responsible for the long operational-stability.

Interfacial toughening and bending performance of the CFRP/aluminum-honeycomb sandwich

CFRP / honeycomb sandwiches often fail prematurely due to interface debonding when subjected to bending loads. In this paper, three kinds of toughening interfaces were designed, and their effects on the sandwich’s three-point bending performances (including peak force and energy absorption) were studied. The results showed that the peak force and energy absorption of the sandwich were increased by 43.6% and 141.3%, respectively by adding a multi-scale toughening film (short aramid fiber (AF) film grafted with polydopamine (PDA) and graphene oxide (GO)) at the interface. The toughening interface affected the deformation process and failure mode of the sandwich by changing the competition mechanism between the facesheet, interface, and honeycomb strength. The short fibers of toughening film formed bridges at the toughened interface, and the crack propagation mechanism changed from interface debonding to a combination of interface debonding and matrix fracture. In addition, the test results revealed that the wettability, tensile strength, surface roughness, and specific surface area of the fiber could be significantly improved by treating with PDA and GO, which lead to crack deflection at the interface. Thus, the interface debonding is effectively delayed or prevented.

Superior interfacial toughening of hybrid metal-composite structural joints using 3D printed pins

An experimental investigation is presented into the enhancement of interfacial toughness properties of hybrid titanium-to-composite structural joints using 3D printed metal pins created using selective laser melting (SLM). The joints were formed by printing an orthogonal array of thin (1.0 mm) diameter titanium pins over the titanium substrate using SLM, which were then embedded into a carbon-epoxy composite substrate to create a high toughness interface. SLM Ti-pinned hybrid joints were evaluated for the first time with and without film adhesive, which is co-bonded between the titanium and composite substrates. The SLM Ti-pins increased the modes I and II interfacial toughness of the hybrid joints by up to about 19- and 11-fold, respectively. The study reveals that pins created using SLM are highly effective at the strengthening and toughening of hybrid metal-composite joints, with the need for adhesive bonding dependent on the loading condition.

Toughening of hydrogel adhering interface based on soft/hard heterogeneous structures

Emerging technologies like hydrogel based stretchable devices call for new interfacial toughening strategies for the achievement of reliable hydrogel adhering interface. Here, we propose a soft/hard composite design to enhance the average peel force for stretchable hydrogel adhesion. We fabricate a hydrogel/elastomer composite with alternate structures where the hydrogel acts as the soft component and the elastomer acts as the hard component. The elastomer is embedded in the hydrogel, with a thin hydrogel layer coated on its surfaces to work as the interfacial hydrogel layers for adhesion. When two pieces of the adhering composites are separated in a 180°peeling test, the peeling front will get stuck when it extends from a soft phase to an adjacent hard phase. The interfacial hydrogel layers between the entire symmetric elastomer units near the peeling front are stretched and the elastomer bends steadily until a sudden debonding occurs, which releases the stored energy and improves the overall adhesion properties. We adjust the structural parameters and obtain an average peak peel force of 330 N/m, which is almost 5 times the peel force of 70 N/m required to peel homogeneous hydrogels. We establish a beam model to study the peeling behaviors of the hydrogel/elastomer composite in the 180°peeling test. The numerical results agree with the experimental results quantitatively. We also show that the peeling performance of the hydrogel/elastomer composite can be tuned by controlling the patterning of the composite structures.

An interfacial toughening strategy for high stability 2D/3D perovskite X-ray detectors with a carbon nanotube thin film electrode.

Single-crystalline metal halide perovskite materials hold great promise for developing next-generation low-dose X-ray detection. To bring this new technology into reality, it is important to improve the durability of perovksite detectors by suppressing the well-known corrosion and ion diffusion problems at the perovskite/electrode interface. For imaging application, it is also imperative to develop new assembling approaches to realise non-planar interconnection between thick perovskite crystals and thin-film transistor (TFT) backplanes. Herein, a flexible and mechanically robust carbon nanotube (CNT) film was proposed to replace noble metal electrodes. The proposed CNT film, whose binder contains a carboxyl group, can form solid contact with a phenethylamine-based two-dimensional (2D) perovskite via amide coupling, thus toughening the perovskite-electrode interface. The resulting CNT/2D-3D perovskite detector shows an applaudable low dark current, high sensitivity, a low dose detection limit and excellent stability, retaining 98% of its initial sensitivity after storage for three months. Moreover, the flexible CNT films are also beneficial for making non-planar interconnection between thick perovskite crystals and TFT backplanes. The proposed flexible CNT thin film electrode thus provides a facile route towards realising a low-dose, high-resolution and highly stable perovskite X-ray detector.

Restructuring the Interface of Silk-Polycaprolactone Biocomposites Using Rigid-Flexible Agents.

Natural fiber-reinforced biocomposites with excellent mechanical and biological properties have attractive prospects for internal medical devices. However, poor interfacial adhesion between natural silk fiber and the polymer matrix has been a disturbing issue for such applications. Herein, rigid-flexible agents, such as polydopamine (PDA) and epoxy soybean oil (ESO), were introduced to enhance the interfacial adhesion between Antheraea pernyi (Ap) silk and a common medical polymer, polycaprolactone (PCL). We compared two strategies of depositing PDA first (Ap-PDA-ESO) and grafting ESO first (Ap-ESO-PDA). The rigid-flexible interfacial agents introduced multiple molecular interactions at the silk-PCL interface. The "Ap-PDA-ESO" strategy exhibited a greater enhancement in interfacial adhesion, and interfacial toughening mechanisms were proposed. This work sheds light on engineering strong and tough silk fiber-based biocomposites for biomedical applications.

Evaluation of the Failure Mechanism in Polyamide Nanofibre Veil Toughened Hybrid Carbon/Glass Fibre Composites.

The interface of hybrid carbon/E-glass fibres composite is interlayered with Xantu.layr® polyamide 6,6 nanofibre veil to localise cracking to promote a gradual failure. The pseudo-ductile response of these novel stacking sequences examined under quasi-static three-point bending show a change to the failure mechanism. The change in failure mechanism due to the interfacial toughening is examined via SEM micrographs. The incorporation of veil toughening led to a change in the dominant failure mechanism, resulting in fibre yielding by localised kinking and reduced instances of buckling failure. In alternated carbon and glass fibre samples with glass fibre undertaking compression, a pseudo-ductile response with veil interlayering was observed. The localisation of the fibre failure, due to the inclusion of the veil, resulted in kink band formations which were found to be predictable in previous micro buckling models. The localisation of failure by the veil interlayer resulted in a pseudo-ductile response increasing the strain before failure by 24% compared with control samples.

Modulating impact resistance of flax epoxy composites with thermoplastic interfacial toughening

The application of natural flax fibre/epoxy composites is growing in the automotive sector due to their good stiffness and damping properties. However, the impact damage resistance of flax/epoxy composites is limited due to the brittle nature of both epoxy and flax fibres and strong fibre/matrix adhesion. Here, biobased thermoplastic cellulose acetate (CA) is deployed as a fibre treatment to alter the damage development of flax/epoxy composites subjected to low-velocity impact. The perforation threshold energy and the perforation energy of unmodified cross-ply composites increased respectively by 66% and 42% with CA-treated flax fibres. The CA-modification modestly decreased the transverse tensile strength and in-plane tensile shear strength of the composites. However, it altered the brittle nature of flax/epoxy laminates in quasi-static tests into ductile failure with clearly increased fibre–matrix debonding.

Interfacial toughening with self-assembled monolayers enhances perovskite solar cell reliability.

Iodine-terminated self-assembled monolayer (I-SAM) was used in perovskite solar cells (PSCs) to achieve a 50% increase of adhesion toughness at the interface between the electron transport layer (ETL) and the halide perovskite thin film to enhance mechanical reliability. Treatment with I-SAM also increased the power conversion efficiency from 20.2% to 21.4%, reduced hysteresis, and improved operational stability with a projected T80 (time to 80% initial efficiency retained) increasing from ~700 hours to 4000 hours under 1-sun illumination and with continuous maximum power point tracking. Operational stability-tested PSC without SAMs revealed extensive irreversible morphological degradation at the ETL/perovskite interface, including voids formation and delamination, whereas PSCs with I-SAM exhibited minimal damage accumulation. This difference was attributed to a combination of a decrease in hydroxyl groups at the interface and the higher interfacial toughness.

Three-point bending properties of carbon fiber/honeycomb sandwich panels with short-fiber tissue and carbon-fiber belt interfacial toughening at different loading rate

The bending performances at different loading rate are studied for carbon fiber/honeycomb sandwich panels toughened by short aramid fiber tissues and carbon fiber belts. The carbon fiber belts are stitched cross the pores of honeycomb core for interfacial improvements. Three-point bending tests at different loading rates are carried out to investigate the effect of short aramid fiber tissue and carbon fiber belt on mechanical properties of carbon fiber/honeycomb sandwich specimens. Experimental results firstly indicate that the short-aramid-fiber interfacial toughening and carbon fiber belts toughening could both enhance the energy absorption and peak load of sandwich specimens at loading rates ranging from 2 mm/min to 500 mm/min. The failure modes and microstructures of toughened specimens are observed to analyse and explain the underlying mechanism of enhancing effect. It is illustrated that crack isolation phenomenon is found to be the main mechanism for avoiding interfacial damage of the sandwich specimen.

Oxidation resistance, residual strength, and microstructural evolution in Al2O3-MgO–C refractory composites with YAG nanopowder

The decisive role of nanostructured yttrium aluminium garnet (YAG;Y3Al5O12) powder addition on oxidation resistance, residual strength and microstructural evolution were studied in Al2O3-MgO–C refractory composites. Oxidation index and rate constant calculations indicated that the oxidation resistance was almost 70 % improved for the nano-YAG containing refractories oxidized in air at 1600 °C. Residual compressive strength (Rc) estimations showed that there was nearly 75 % strength retained in these oxidized refractories fortified with nano-YAG. Residual bending strength (Rb) estimations based on cyclic thermal shock, exhibited that there was almost 70 % thermal shock resistance enhancement in refractories reinforced with nano-YAG, showed a good agreement between Rb and Rc values. These beneficial properties were attributed to the formation of a well-sintered framework of YAG/Spinel bonding grains throughout the dense oxidized layer microstructure of these new class of refractories. The concept of interfacial toughening and implications of these results to practical applications are discussed.

Molding and Encoding Carbon Nitride-Containing Edible Oil Liquid Objects via Interfacial Toughening in Waterborne Systems.

Charge interaction-driven jamming of nanoparticle monolayers at the oil–water interface can be employed as a method to mold liquids into tailored stable 3D liquid objects. Here, 3D liquid objects are fabricated via a combination of biocompatible aqueous poly(vinyl sulfonic acid, sodium salt) solution and a colloidal dispersion of highly fluorescent organo-modified graphitic carbon nitride (g-C3N4) in edible sunflower oil. The as-formed liquid object shows stability in a broad pH range, as well as flexible pathways for efficient exchange of molecules at the liquid–liquid interphase, which allows for photodegradation of rhodamine B at the interface via visible light irradiation that also enables an encoding concept. The g-C3N4-based liquid objects point toward various applications, for example, all-liquid biphasic photocatalysis, artificial compartmentalized systems, liquid–liquid printing, or bioprinting.