Interfacial Fracture Toughness Research Articles

Spider silk-inspired heterogeneous interphase featuring hybrid interaction for simultaneously improving the interfacial strength and fracture toughness between carbon fiber and epoxy by regulating hydrogen bond density

Designing advanced fiber-reinforced polymer composites through biological inspiration proved to be a crucial strategy for overcoming limitations in simultaneously enhancing the strength and toughness of composites. To achieve simultaneous improvement in the interfacial strength and toughness between carbon fiber (CF) and epoxy, a spider silk-inspired interphase featuring hybrid interaction was constructed by introducing hyperbranched polyamide-amine (HPAA) and graphene oxide (GO) onto the surface of CF. The results suggested that manipulating the feed ratio to adjust the branching degree of HPAA allowed for the attainment of various hydrogen bond densities. The fiber surface with high hydrogen bond density provided more hydrogen bond interaction sites to promote the deposition of GO. Benefitting from the ameliorative interfacial adhesion force, surface energy and interface thickness, impressive improvements of 94.5 % and 110.0 % in respective interfacial strength and fracture toughness over those of untreated CF/EP composites were achieved for functionalized CF/EP composites. The enhancement mechanism of interfacial performance was attributed to the formation of a “nano-fishnet” structure, which improved the stress transformation efficiency and consumption of external energy absorbed by hydrogen bonds. The method of regulating the branching degree of hyperbranched polymers and hydrogen bond density has opened an advanced way for surface modification of high-performance fibers.

Mixed experimental-numerical method for quantitative measurement of mode II fibre/matrix interface debonding and comparison of fibre sizings in single short fibre composites

In short fibre composites, debonding cracks usually initiate from fibre tips and their growth is governed by the properties of the fibre/matrix interface. These properties are ideally acquired from a model test similar to the real composite, while excluding the effects of fibre fractures and the induced debonding interactions. Therefore, in this study, debonding growth was examined in detail, using novel single short fibre specimens, which were fabricated using a new methodology. These experiments were further complemented with a continuous fibre case study, where the established single fibre fragmentation specimens were evaluated. The progression of debonding and pull-out damages versus remote stress was studied for two fibre sizings utilizing real-time in-situ polarized light microscopy. A finite element model was built to obtain the fibre/matrix interfacial fracture toughness considering thermal residual stresses, friction and matrix plasticity, and a very good correlation was obtained between the model and the experiments.

Investigation on the interface bonding and reinforcement mechanism of nano Ti/AZ31 magnesium matrix composites

In this study, nano Ti/AZ31 composites were successfully prepared by powder metallurgy method, and the strength and plasticity of the composites were improved in both directions. The SEM shows that the Ti particles were evenly distributed at the grain boundary in the sintered composite, and the Ti particles pin the grain boundary to make the grain of the composite significantly refined. EBSD shows that the addition of nano Ti particles significantly reduces the texture strength of the composite and increases the starting probability of the slip system. TEM shows that nano Ti and AZ31 matrix formed stable Al3Ti and MgO phases at the interface, and shows a strong coherent relationship. The nanoindentation test shows that the Ti/AZ31 interface of the composite was stable, the interface bonding energy and interface fracture toughness were gradiently distributed, and the bonding energy and fracture toughness of the interface bonding area were better than those of the matrix. Tensile test shows that the 1.5 wt%Ti/AZ31 composite obtained the best comprehensive mechanical properties with yield strength, ultimate tensile strength and elongation of 143 MPa, 243 MPa and 11.5%, respectively, obviously higher than those of AZ31 alloys. The increased strength was mainly due to the grain refinement and strengthening at grain boundaries. The improved ductility was the result of weakened texture, increase of slip system start-up probability and strong interfacial bonding between Ti particles and AZ31 matrix.

“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.

Capacity fading and inter/intra-granular fracture of polycrystalline cathode particle induced by charge/discharge cycles: A numerical study

The capacity fading of Lithium-ion batteries (LIBs) containing polycrystalline cathode materials can be attributed to the degradation of secondary particles caused by inter- and intra-granular fractures occurring during charging and discharging cycles. In this work, a diffusion-induced stress (DIS) analysis method based on the finite-discrete element method (FDEM) is developed for studying the fracture behaviors of polycrystalline cathodes. The analysis of lithiation and delithiation processes shows that C-rate and interfacial fracture toughness have significant effects on inter-granular crack morphologies. Our method enables precise analysis of the competition between intra- and inter-granular cracks and the effect of crack-impeded diffusion on crack morphologies. We compared numerical simulation results for scenarios where fracture toughness changes with the number of cycles, and demonstrated that the decreasing fracture toughness with cycle number is an important factor contributing to the slow crack growth. Additionally, we established the relationship between crack length and capacity loss, which agreed well with experimental results.

Antibacterial and dynamical behaviour of silicon nanoparticles influenced sustainable waste flax fibre-reinforced epoxy composite for biomedical application

Abstract This article explores the impact of nano-silica on the properties of woven flax fibre/epoxy composites. Using compression moulding, epoxy/flax/silica hybrid nanocomposites were produced. The nano-silica was dispersed in the epoxy matrix via ultrasonication at various weight ratios. A series of tests, including crack durability, dynamic mechanical analysis, and scanning electron microscopy, were conducted to evaluate the modified materials. Notably, a 3% nano-silica filler load resulted in a 54% and 57% improvement in initiation and transmission interfacial fracture toughness, respectively. Scanning electron microscope imaging confirmed that fibres pull out at the crack tip during initial debonding, accounting for the increased toughness. Dynamic mechanical analysis further revealed enhancements in mechanical properties. Moreover, the 3% nano-silica content led to less fibre pull-out, suggesting higher heat resistance than standard flax/epoxy composites. The material also demonstrated promising antimicrobial efficacy against gram-positive and gram-negative bacteria, offering a potential alternative to conventional antibiotics.

A four-point bending technique for characterizing the interface fracture toughness between soft thin films and stiff substrates

Interface fracture toughness, also called work of adhesion or adhesion energy, is a quantity characterizing the resistance of an interface between two adhered solids against interfacial delamination. Determining the interface fracture toughness should be of great value to the studies on the related adhesion and delamination problems. Four-point bending test is an experimental approach that was originally used to measure the flexural stiffness and fracture toughness of monolithic materials. It was also used to characterize the fracture toughness of the interface when a bi-layered notched composite beam specimen is adopted (Int. J. Frac. 1989; 40: 235). However, this method does not work very well when a thin and soft material is encountered because the interface delamination cannot be triggered easily. To address this problem, in this paper, we revise the configuration of the four-point bending specimen from bi-layer to tri-layer by imposing an additional stiffer layer on the top of the thin and soft layer. The analytical solution to the energy release rate of the preexisting interfacial crack is revisited. Finite element analysis is carried out to assess the applicability and limitations of the analytical solution. With the modified four-point bending specimen, the interface fracture toughness between an epoxy coating and a steel substrate is successfully measured from the critical load that leads to the delamination of the preexisting interfacial crack.

Effects of adhesive layer thickness on the fracture properties of a concrete-epoxy resin interface

Adhesive layer thickness is a crucial factor that affects the performance of interfaces in cement-based composite structures. However, the influence of adhesive layer thickness on the fracture properties of concrete–epoxy resin remains unclear. In this study, a two-parameter fracture model of a concrete-epoxy resin interface from the perspective of fracture mechanics. Digital image correlation (DIC) technique was used to investigate the evolution of the fracture process zone (FPZ) of nine specimens with adhesive layer thicknesses of 2 mm, 5 mm, and 8 mm, respectively. The interfacial fracture toughness parameters of the concrete-epoxy resin interface were determined based on the four-point bending (FPB) tests. The test results indicated that increasing the adhesive layer thickness changed the failure mode of the interface from approximate-brittle to quasi-brittle. The load-carrying capacity and interfacial stiffness of the concrete-epoxy resin interface decreased by approximately 13.8 % and 61.1 % with an adhesive layer thickness from 2 mm to 8 mm, respectively. The deflection across the specimen increased by approximately 37.2 % with an increase in adhesive layer thickness due to the lower elastic modulus of epoxy resin compare to the concrete. Furthermore, the FPZ for the adhesive layer thickness of 5 mm was the longest; it was approximately 18.7 % longer than that for the test specimen with an adhesive layer thickness of 2 mm. In terms of structural load-bearing capacity and fracture toughness, the concrete-epoxy resin interface exhibited the best working performance when the thickness of the adhesive layer was 5 mm.

Modelling the anisotropic fracture behaviour of cold spray additive manufactured 6061 aluminium alloy deposits based on the phase field method

Cold spraying has been widely accepted as a promising additive manufacturing technique to produce large metallic structures. The fracture behaviour of cold spray additive manufactured deposits is complex because of their inherent anisotropy. In order to study the anisotropic fracture, a multiple-particle impacting model was established in a Lagrangian frame. Then, an image recognition program was developed to convert the simulated deposit image into a finite element discretisation scheme. In finite element framework, a phase field model was proposed to simulate the fracture process of deposits under tensile loading along (0°) and perpendicular (90°) to the particle impacting direction. The deposit anisotropic fracture behaviour for two impact velocities, at 600 m/s and 800 m/s, was investigated. It is shown that the interfacial fracture toughness is the most important parameter that affects the tensile properties of deposits. The tensile ultimate load of the deposit increases thrice when the interfacial fracture toughness increases by an order of magnitude. For the 800 m/s deposit, the ultimate load and fatigue life in 90° loading direction are about 11.3% and 39.2% higher than those in 0° loading direction, respectively. The fracture anisotropy relates to the degree of flattening of the deformed particles. The higher interfacial fracture toughness of mode II can lead to more transparticular cracks, shorter crack length and smoother crack path. The comparison between simulations and published experimental results suggests that the proposed phase field model can capture the complex anisotropic fracture behaviour of cold sprayed deposits.

A combined interface phase field (CIPF) model for interfacial crack propagation

The phase field method has been very popular in the past decade because it can handle complex crack propagation problems without ad hoc criteria. In this paper, a combined interface phase field (CIPF) model is proposed to better investigate the fracture behaviours of composite systems with a finite-thickness interface. To build the CIPF model, a new numerical method named the modified crack surface displacement extrapolation (MCSDE) method is proposed to determine the real interfacial fracture toughness. Then, an effective interfacial fracture toughness is given based on the coupling relationship between the length scale parameter and the interface thickness. We investigate crack propagation behaviours in bi-material structures with varied initial notch positions and sandwich specimens using the CIPF model. The results show that the MCSDE method can effectively and accurately determine the interface fracture toughness, and the difference between the calculated results and the results obtained by the sing-leg bending (SLB) experiment is 6%. The CIPF model can not only simulate the propagation of cracks in bulk, but also simulate the propagation of cracks between interfaces. For different positions of the initial notch, the propagation patterns of cracks and the mechanical response of the structure are very different.

Interfacial fracture toughness of different surface treatments on zirconia-reinforced lithium silicate glass-ceramics.

This study investigated the influence of different surface treatments on unfiring or firing zirconia-reinforced lithium silicate (ZLS) glass-ceramics. Celtra Duo and IPS e.max CAD blocks were cut and process following manufacturer protocols. The specimen surface was treated with seven different protocols. Two ceramic blocks with the same surface treatment were bonded with luting agent and prepared for mini-interfacial fracture toughness tests (mini-iFT). The specimens were tested after 1-week storage. The data was statistically analyzed using two-way ANOVA and Dunnett's T3 comparison (α=0.05). The highest mini-iFT of both Celtra Duo unfired and fired was shown in the HF+S group, which was not significantly different from HF+S+UA. For IPS e.max CAD, the mini-iFT was higher in the groups treated with hydrofluoric acid. Additional adhesive after silane application did not significantly improve bonding effectiveness. Therefore, surface treatment with hydrofluoric acid and silane is recommended for both unfiring and firing ZLS glass ceramics.

Different effects of interfacial properties on the tensile and compressive damage mechanisms of 3D woven composites: Multiscale damage model and numerical comparative study

A multiscale damage model is proposed based on a micro-mesoscale bridging model and a multiscale kinking model, which can predict the damage development processes of 3D woven composites (3DWCs) under different loading conditions using only few material parameters. At the microscale, the various mesoscopic damage modes of yarns, including tensile fiber breakage, compressive fiber kinking and transverse inter-fiber crack, can be simply divided into matrix- and fiber-dominated damage. Using this multiscale damage model, a numerical comparative study is conducted to investigate the effect of interfacial properties on the mechanical performances and damage mechanism of 3DWCs. The numerical results indicate that the interfacial properties exhibit different effects on the tensile and compressive strengths and damage mechanisms of 3DWCs. The interaction of different damage modes controls the overall trend of the tensile strength of 3DWCs. There is an optimum point for the tensile strength of 3DWCs at different interfacial fracture toughness. Beyond the optimum point, the tensile strength of 3DWCs is negatively correlated with interfacial properties. Unlike the tensile loading case, the better the interfacial properties, the later fiber kinking occurs, resulting in higher compressive strength of 3DWCs.

Stress distribution variations during nanoindentation failure of hard coatings on silicon substrates

Regarding quality inspection of technologically important nanocomposite hard coatings based on Ti, B, Si, C, and N and bioceramics such as hydroxyapatite that are used in small-scale high-precision devices and bio-implants, it is essential to study the failure mechanisms associated with nanoindentation, such as fracture, delamination, and chipping. The stress imposed by the indenter can affect the fracture morphology and the interfacial fracture energy, depending on indenter shape, substrate type, crystallographic properties, pre-existing flaws, internal micro-cracks, and pre-strain. Reported here are finite-element-based fracture studies that provide insights into the different cracking mechanisms related to the aforementioned failure process, showing that the fracture morphology is affected by the interaction of different cracking events. The interfacial fracture energy, toughness, and residual stress are calculated using existing models with minor adjustments, and it is found that increasing the indenter sharpness improves the shear stress distribution, making the coating more prone to separation. Depending on the prevailing type of stress, the stress distribution beneath the depression results in either crack formation or a dislocation pile-up leading to strain hardening. Different forms of resistances resulting from the indentation process are found to affect the tip–sample conduction, and because of its stronger induced plasticity than that of a Berkovich indenter tip, a sharper cube-corner tip produces more resistance.

Research on crack propagation behaviour of EB-PVD TBCs based on TGO evolution

Thermal Barrier Coatings (TBCs) are functional coatings used to protect high-temperature components that are prone to early damage and premature failure under the influence of complex working conditions. This paper examines the crack propagation behaviour of 8% yttria-stabilized zirconia (8YSZ) EB-PVD TBCs under different oxidation conditions at 1100 °C. The morphology of interfacial cracks after oxidation was summarized and the evolution of thermally grown oxide (TGO) was quantified. Based on the evolution of TGO, the causes of crack propagation were analyzed. For the specimens after oxidation experiment, the interfacial crack propagation behaviour was observed and analyzed by SEM, and the reason of lateral crack propagation was explained from the perspective of interfacial fracture toughness. The reason for crack deflection is analyzed from the perspective of energy release rate. The equivalent thickness, normalized rumpling index and two-dimensional roughness index were calculated, then the TGO growth behaviour was comprehensively analyzed and related to the crack propagation.

Ice Adhesion Characterization Using Mode-I and Mode-II Fracture Configurations

Abstract The ice buildup on airborne structures operating in cold weather conditions has detrimental impacts on their safety and performance. Due to practical applications, there has been a significant interest in ice removal strategies. However, the current body of literature lacks comprehensive insights into the mechanistic aspects of the ice adhesion/breakage process, resulting in a wide range of reported adhesion strengths that differ by two orders of magnitude. To address this gap, we employed a fracture mechanics-based approach to investigate the fracture behavior of a typical ice/aluminum interface in terms of mode-I and mode-II fractures. We examine a range of surface roughness values spanning from 0.05 to 5 micrometers. An experimental framework employing a single cantilever beam and direct shear tests were developed. The near mode-I and mode-II interfacial fracture toughness and strength values were extracted from the experimentally measured force and displacement by both analytical and numerical models employing cohesive surfaces. The combined experimental and numerical results show that ice adhesion is primarily driven by cohesive interfacial failure, which exhibits almost mode-independent fracture behavior. Mode-I fracture shows directional instability of crack propagation, which is attributed to thermally induced residual tensile stress at the ice layer-substrate interface. The fractographic inspection reveals similar ice-grain size over the examined range of substrate roughness values. For the examined range of surface roughness and temperature, which induces the Wenzel state with full surface wetting at the interface, the ice adhesion is insensitive to the interfacial roughness in both mode-I and mode-II fracture.

Interfacial fracture behavior and adhesive strength in tensile and shear loading of SiC-PyC-SiC composites by micro-scale specimens

Micromechanical tests were performed to understand the local fracture behavior and adhesive strength of a fiber-reinforced ceramic matrix composite (silicon carbide fiber reinforced silicon carbide matrix with the pyrolytic interphase in between, SiCfiber/PyC/SiCmatrix) and to determine the mode-dependent interfacial fracture toughness. A combined approach of pico-second laser ablation and focused ion beam milling was used to fabricate notched and unnotched micro-cantilever and in-plane micro-shear specimens for mode I and II testing. Due to the complexity of the sandwich system, finite element simulations were implemented, which took into account the elastic heterogeneity, the influence of PyC thickness, Young's modulus and beam thickness. This allowed us to obtain mode-specific geometry functions and to increase the accuracy of the obtained stress intensity factor. In all cases, a failure at the SiCfiber/PyC interface was observed. While straight through notched specimens exhibit a systematic overestimation of fracture toughness due to the less accurate alignment of notch root and PyC layer, curved notched specimens show a very low interfacial fracture toughness of 0.24 ± 0.02 MPa√m and 0.17 ± 0.06 MPa√m in mode I and mode II, respectively. Combined with tests on unnotched specimens, a critical flaw size for the present microstructure of the PyC phase of ca. 21 nm was identified. The data underlines the important role of the PyC interlayer in damage resistant ceramic composites and the developed methodology, which can be used for systematic studies of their properties as a function of process, geometry, and boundary conditions.

Bond strength of ceramic veneered CAD-milled alloy upon prolonged sintering.

Ceramic-sintering affects bond strength and longevity of metal-ceramic. This study investigated the effect of sintering temperatures and times on metal-ceramic bond strength vis-a-vis interfacial fracture toughness. One hundred eighty rectangular-shaped (25 × 8 × 1mm) casting (Auriloy® (CA)) and CAD-milling (Ceramill Sintron® (MA)) alloys were prepared and randomly veneered with ceramic at normal (930°C; (TN)), increased (940°C; (TI)), and extremely increased (950°C; (TE)) sintering temperatures and normal (1min; (HN)), increased (2min; (HI)), and extremely increased (3min; (HE)) sintering time (n = 10/group). Pre-cracked was subjected to four loading-unloading cycles at 0.05mm/min speed to determine interfacial fracture toughness from strain energy release rate (G). Microstructures were examined with a scanning electron microscope (SEM), energy-dispersive X-ray (EDX), and atomic force microscopy (AFM). ANOVA and Tukey comparisons were determined for significant differences (α = 0.05). Significant differences in G due to the effect of alloy, sintering temperature, and time (p < 0.05) were indicated. MA revealed higher G than CA. Raising temperatures enabled increasing G for CA, not for MA. Extended sintering permitted increasing G for both alloys. Rougher surface of MA than CA was observed. Interfacial ion exchange was differently indicated between CA and MA. Bond strength was influenced by alloy, sintering temperature, and time. Ceramic has better adhesion to MA than CA. Enhancing bond for CA was succeeded through increasing sintering temperature and time, whereas through extended sintering for MA. MA offers stronger bond than CA. Enhancing bond is suggested by extended sintering. Raising temperature can enhance bond for CA, not for MA.

Enhanced mechanical and tribological properties of polymer nanocomposites by improving interfacial properties by hexagonal boron nitride nanosheets: Molecular dynamics simulations

AbstractTo comparatively investigate the mechanical, tribological, and interfacial properties of polytetrafluoroethylene (PTFE) strengthened by graphene (Gr) and hexagonal boron nitride (h‐BN) nanosheets, molecular models of PTFE nanocomposites containing a similar weight fraction of Gr and h‐BN nanosheets were established. The constant‐strain approach, the three‐layer friction structures, and the pull‐out simulations were respectively employed to calculate the mechanical, tribological, and interfacial properties of the nanocomposites. Results indicate that the Young's, shear, and bulk moduli of the nanocomposites are increased by 17.22%, 20.72%, and 4.78%, respectively, by introducing h‐BN nanosheets than those by introducing Gr nanosheets. Meanwhile, a decrease of 8.49% in the friction coefficient of the nanocomposites is obtained by incorporating h‐BN nanosheets than that by incorporating Gr nanosheets. In addition, 84.71% and 5.18 times higher in the interfacial cohesive strength and interfacial shear strength, and 91.58% and 128.7% higher in the interfacial fracture toughness of PTFE nanocomposites in the normal separation and shear separation, respectively, are achieved by incorporating h‐BN nanosheets than those by incorporating Gr nanosheets. To provide a deeper understanding of the enhancement mechanisms of h‐BN nanosheets, the interfacial interaction energies, radial distribution functions, and von Mises stress distributions of the two PTFE nanocomposites were evaluated and interpreted accordingly.

Mixed mode fracture toughness of epoxy molding compound/printed circuit board interface of semiconductor packages with respect to temperature and moisture

Interfacial delamination is a critical factor for dissimilar bi-material interfaces in terms of structural mechanical behavior and reliability. In this study, the mixed mode interfacial fracture toughness of a thin epoxy molding compound/printed circuit board bilayer with different mixities was measured via the asymmetric double cantilever beam test with a reinforced metal jig. Additionally, mode II fracture toughness properties were measured through the end notched flexure test. All tests were performed under various temperature and moisture absorption conditions below 100℃/85 %RH. Then, a finite element simulation was performed with the cohesive zone model technique to validate the experimentally measured fracture toughness values by comparing the load–displacement curves from the experiment and simulation. Additionally, the single leg beam test and its finite element simulation were conducted and compared for validation of the measured mixed mode fracture toughness values. It was demonstrated that the developed mixed mode fracture toughness measurement method could provide accurate fracture toughness, which could effectively predict the overall load–displacement curve and damage propagation behavior.

Investigation on the effect of interface properties on compressive failure behavior of 3D woven composites through micromechanics-based multiscale damage model

In this paper, the effect of interface properties on the compressive failure behavior of 3D woven composites (3DWC) is investigated by incorporating a micromechanics-based multiscale damage model (MMDM). The correlation between the mesoscopic stress of yarns and microscopic stress of constituents is established by defining a stress amplification factor. With the microscopic stresses, the fiber breakage and matrix failure can be separately evaluated at the microscale, without assuming the yarns as transversely isotropic homogeneous materials. Especially, the interfacial debonding between yarns and matrix is also a dominant damage mode within 3DWC. Given that there is still a lack of studies on the influence of interfacial properties on the compressive failure behavior of 3DWC, it is meaningful to perform numerical parametric studies to reveal how the interface properties contribute to the damage mechanisms of 3DWC under compressions. The predicted results indicate that with the increase of interface strengths and fracture toughness, the compressive resistance of 3DWC can be significantly improved, resulting in higher strength and failure strain. Additionally, the studied 3DWCs with weak, medium and strong interfaces exhibit different damage development processes.