Interfacial Fracture Behavior Research Articles

Understanding the mechanisms behind ultrasonic vibration in resistance spot welding of aluminum and steel

An ultrasonic-assisted resistance spot welding process, which can improve the quality of aluminum/steel spot-welded joints, has been developed. However, the mechanism of the thermal-electrical-mechanical behavior caused by the ultrasonic longitudinal vibration to welding process has not been clarified. In this study, the role and impact mechanisms of ultrasonic longitudinal vibration during the welding process to form joints were confirmed by combining the microstructural characterization of the formed joints with the signal analysis of the process, as well as the analysis of the independent finite element numerical calculation results. Additionally, the interface metallurgical reaction behavior, welding defects, fracture behavior, and joint strength characteristics of ultrasonic-assisted aluminum/steel resistance spot welding joints were comprehensively analyzed, and the results indicated that the sizes of the aluminum/steel nuggets decreased. It could be attributed to the competition among multiple ultrasonic effects. Specifically, ultrasonic vibration decreased the contact resistance between the aluminum/steel sheets, whereas the acoustic cavitation and acoustic streaming effects increased the electrical conductivity of molten steel and thermal conductivity of molten aluminum, respectively. Moreover, ultrasonic vibration promoted the radial creep of molten and near-molten aluminum, which expanded in the effective bonding area between the two workpieces. Furthermore, ultrasonic vibration effectively reduced the thickness of the intermetallic compound layer (<3.0 µm), concurrently inhibited the formation of interfacial welding defects. Finally, the peak tensile-shear load of aluminum/steel joints is significantly increased under multiple ultrasonic optimization mechanisms.

Characterization of the interfacial structure and fracture behavior of in situ synthesized ceramics to reinforce Ni-based composite coatings

This work used the in-situ synthesis of molten-state nitride ceramic phase-reinforced Ni-based alloy coatings, aiming to improve the phase-interface bonding through the interdependent co-solidification between molten droplets. The XRD was used to analyze the physical phases of the composite coatings. The microstructure and phase-interface structure were characterized in detail by combining SEM, TEM, HRTEM, FFT, and SAED techniques. Microhardness tester and microforce microhardness tester were employed to measure the surface hardness and elastic modulus of the composite coatings. The fracture behavior of the composite coatings was characterized by observing the fracture morphology of the coatings using SEM combined with the EDS technique. It was found that the formation mechanisms of interfacial misfit dislocation assistance, lattice distortion, aggregation of stacking faults, and specific growth orientation between the γ-Ni matrix phase and each ceramic phase in NiCrBSi-TiCrN composite coatings improved the lattice matching between the two-phase interface, which resulted in the formation of atomically corresponding coherent lattice relations and stepped interfacial semi-coherent lattice relations, and enhanced the degree of phase-interface bonding. On this basis, the composite coatings with high Cr content further inhibited the expansion of interphase penetration cracks due to the existence of Cr-rich zones at the phase interface, thus exhibiting high fracture toughness. This work provides new opinions on the improvement of phase-interface bonding and composition design of Ni-based composite coatings.

Revealing the grain boundary effect on interfacial adhesion in polycrystal Fe/Ni interfaces by using a multi-scale CZM-CPFEM approach

In layered structure, interface plays a significant role in property improvement, whereas its multiple factors at various length scales restricted further understanding. Previous research found a novel grain boundary (GB)-interface interaction: crack initiation locally occurred at the junctions between pre-existing GBs and interface. This leads to decreased interfacial strength. To clarify the mechanisms of such GB-interface interaction, the present study proposed a multi-scale cohesive zone model-crystal plasticity finite element method (CZM-CPFEM) model to investigate the effect of pre-existing GB on intrinsic bonding strength at atomic scale and interfacial stress distribution at micro scale, respectively. At atomic scale, interfacial fracture criteria were modelled according to Molecular Dynamics (MD)-based traction-separation relationships. The accuracy of CZMs was improved by considering both isothermal holding and tilting configuration effects. At micro scale, a practical interfacial microstructure was used to construct CPFEM models. Tensile simulations showed that the local stress concentration occurred at junctions, attributed to the anisotropic plastic deformation behaviours across the GB. On the other hand, weak bonding adhesion at the junction only affects the local crack initiation strength, showing insignificant impacts on the overall interfacial fracture behaviour.

High strength diffusion bonding of Ti2AlNb to GH4169 with (TiZrHfNb)95Al5 high entropy interlayer

A high entropy interlayer (TiZrHfNb)95Al5 was designed for vacuum diffusion bonding of Ti2AlNb to GH4169. The influence of bonding temperature on the interfacial microstructure morphology, mechanical properties and fracture behaviour of the resultant joints was investigated. The evolution of microstructure and fracture mechanism were revealed. The typical interface microstructure of Ti2AlNb/B2/Solid solution/(Ti, Zr, Hf)2(Ni, Nb)+(Ti, Zr, Hf)(Ni, Nb)+(Ti, Zr, Hf)(Ni, Nb)2+Solid solution/(Ti, Zr, Hf)(Ni, Nb)+(Cr, Ni, Fe)ss/(Cr, Ni, Fe)ss/Cr-rich(Cr, Ni, Fe)ss/Ni-rich(Cr, Ni, Fe)ss/GH4169 was formed. As the bonding temperature increased, the thickness of interfacial reaction layer gradually increased, and the shear strength gradually increased until 980 °C, then decreased sharply. The highest shear strength reached 376 MPa at 980 °C for 90 min. The highest microhardness region of 11.79 GPa existed on the GH4169 side, which was mainly composed of (Ti, Zr, Hf)(Ni, Nb) and (Cr, Ni, Fe)ss phases. Meanwhile, the elastic modulus dramatically changed interface formed near this region. The fracture surface morphology indicated typical brittle fracture characteristics such as cleavage steps, transgranular fracture and intergranular fracture. According to the fracture analyses, major cracks initiated in the (Ti, Zr, Hf)(Ni, Nb) phase+(Ti, Zr, Hf)2(Ni, Nb) phase and gradually extended to the (Cr, Ni, Fe)ss phase. Thus, the major crack emerged region was located at the elastic modulus dramatically changed interface.

Microstructure evolution and strengthening mechanism of CoCrFeMnNi HEA/Zr-3 brazed joints reinforced by fine-grained BCC HEA and HCP Zr

In the pursuit of manufacturing intricate components for the nuclear industry, we developed a novel Zr63.2Cu36.8 (wt.%) alloy via vacuum melting for brazing applications involving equiatomic high-entropy alloys (HEA) of CoCrFeMnNi and zircaloy (Zr-3). We systematically investigated the influence of various brazing parameters on microstructure evolution and shear properties. Furthermore, we established a comprehensive understanding of the relationship between the lattice structure of interfacial products, residual stress, and fracture behavior in HEA/Zr-Cu/Zr-3 joints. Our findings revealed that under specific conditions (1010 °C for 10 min), the reaction products in HEA/Zr-Cu/Zr-3 joints consisted of lamellar HEAP/lamellar Zr(Cr,Mn)2, granular (Zr,Cu)/Zr2(Cu,Ni,Co,Fe), bulk Zr(Cr,Mn)2, and Zrss. With increasing temperature and prolonged holding time, the layered HEAP and Zr(Cr,Mn)2 phases adjacent to the HEA substrates thickened, while the relative amounts of Zr2(Cu,Ni,Co,Fe) decreased, with a remarkable increase in ductile Zrss. Growth kinetics analysis of the reaction layer and EBSD analysis indicated that the HEAP phases exhibited a lower growth rate compared to the Zr(Cr,Mn)2 layer during brazing, and both phases exhibited random grain orientations. Particularly noteworthy was the precipitation of (Zr,Cu) within the layered Zr(Cr,Mn)2, which increased and coarsened with higher temperatures and extended durations. Finite element analysis and TEM analysis revealed higher residual stresses at the non-coherent Zr(Cr,Mn)2/HEAP interface with a lattice mismatch of 40.6%. The body-centered cubic (BCC) structural HEAP, composed of fine grains, effectively mitigated the concentrated residual stresses due to its superior plasticity. Moreover, micro-nanoscale close-packed hexagonal (HCP) precipitates (Zr,Cu) were distributed within the brittle Zr(Cr,Mn)2 phases, contributing to the overall strength improvement of the joints. Consequently, high-quality HEA/Zr-3 joints were achieved, featuring a maximum strength of 172.1 MPa, equivalent to approximately 62.6% of the yield strength of Zr-3. These results highlight the potential of Zr63.2Cu36.8 (wt.%) alloys in advanced brazing applications.

Effect of pin offset on interface evolution and fracture behavior of aluminum/copper dissimilar friction stir welded

In this study, triangular prismatic threaded structural pins at different offsets were used to study the forming and performance of Al/Cu dissimilar joints. In the experiments, the Al alloy was placed on the advancing side, and the stirring pin was offset 0.5 mm, 0.8 mm, 1.0 mm, and 1.2 mm towards the Al side, respectively. The results show that sound joints can be obtained under each offset. The number and size of Cu particles in the stirred zone of the joint with a pin offset of 0.5 mm was large. The Cu particles in the stirred zone gradually refine as the stirring pin is offset towards the Al side. According to the EDS analysis, the stirring zone and interfacial intermetallic compounds (IMCs) mainly consist of Al2Cu and Al4Cu9, and a lamellar structure with IMC layers was observed on the Cu side near the Al/Cu interface. The size and number of Cu particles as well as the thickness of the IMCs layer had a significant effect on the tensile strength of the joints, with a maximum tensile strength of 203.4 MPa (equivalent to 71.3% of the Cu base metal) obtained at a pin offset of 1 mm, which was fractured in the heat-affected zone on the Al side, and the fracture surfaces demonstrated ductile fracture.

Evolution of interfacial structure and fracture behavior in ZL702A/SUS304 compound castings with different surface treatments

Evolution of interfacial structure and fracture behavior in ZL702A/SUS304 compound castings with different surface treatments

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.

A multi-scale study on the effect of interfacial microstructure on interfacial strength and fracture behaviour in Fe/Ni bonded interfaces

Multilayered Steel (MLS) enables impressive strength-ductility combinations by controlling the interfaces. Previous studies indicated that the interface is affected by various factors at multiple length scales, whereas their mechanisms had not been fully clarified. In this study, we focused on the effects of interfacial microstructure on interfacial strength of Fe/Ni interface. At macro scale, the pre-exist grain boundaries (GBs) were controlled to fabricate bicrystal and polycrystal interfaces. Uniaxial tensile test results showed that the bicrystal interfaces exhibit relatively higher interfacial crack initiation strength compared to the polycrystal interfaces. In-situ observation revealed that the decreased strength was owing to the unique crack initiation mechanism: the pre-exist GB-interface junctions behaved as weak spots in tensile separation. At atomic scale, interfacial atomic behaviour at the junctions was investigated by the molecular dynamics (MD) simulation method. The result indicated that the disordered High-Angle GBs (HAGBs) blocked the growth of interfacial dislocations and formed local dislocation pile-ups, where subsequent crack initiation was observed. This is attributed to the relatively lower intrinsic bonding strength at the GB-interface junctions.

Multiscale experimental characterisation of mode-I interfacial fracture behaviour of vitrimer composites

Vitrimer composites with bond- exchange reactions offer significant advantages in terms of repair and recyclability. The mechanical properties of such composites can be designed by modifying the epoxy/anhydride ratio, thus allowing their interface properties to be customised. In this study, the mode-I interfacial fracture properties of vitrimer carbon fibre composites with different epoxy/anhydride ratios are investigated experimentally and numerically. The results show that composites with an epoxy/anhydride group ratio of 1:0.5 exhibit the highest fracture toughness, with a maximum value of 1.6 mJ/mm2. Meanwhile, the analysis of the composites’ microscopic fracture morphology indicates their good bonding performance with carbon fibres. Subsequently, a multiscale numerical approach is developed to simulate the mode-I fracture response of composite laminates and establish a trilinear cohesive model to simulate the macroscopic interfacial fracture behaviour. In this approach, a matrix elastic–plastic constitutive model and the J-integral method are applied to obtain the stress distribution in the fibre-bridging phase via a double cantilever beam fracture test. The macroscopic fracture behaviour obtained via simulation is consistent with the experimental results. Therefore, the proposed multiscale numerical simulation method, which correlates the matrix properties with macroscopic fracture parameters, allows the interlaminar properties of composites with novel matrices to be evaluated effectively.

Development of high thermal conductivity of Ag/diamond composite sintering paste and its thermal shock reliability evaluation in SiC power modules

Diamond has the highest thermal conductivity among naturally occurring materials and a relatively low coefficient of thermal expansion (CTE), making it a promising interconnected material for next-generation semiconductor power devices. In this study, Ag/diamond composite sintering paste was developed. The agglomerated diamond particles were uniformly dispersed in the sintered Ag porous structure, and the thermal conductivity was increased from 171.4 W/(m·k) for pure Ag sintering to 288 W/(m·k) for an Ag@2%diamond (2% of the total weight of Ag) specimen. The interfacial microstructure evolution and fracture behavior were investigated in detail for a SiC/Direct Bond Copper (DBC) joint structure during an extreme thermal shock test (TST) in the range of −50 °C–250 °C. Importantly, with a moderate amount of diamond additive, the microstructural degradation and severe deformation of the DBC substrate were effectively suppressed. This is mainly attributed to the diamond addition, which can block the migration pathways of Ag atoms toward long-term thermal shock and adjust the thermo-mechanical performance of the sintered layer. This detailed study about Ag/diamond composite paste can provide new guidance for strengthening sintered Ag interconnections for SiC power modules.

Elemental diffusion, atomic substitution mechanisms and interfacial fracture behavior in laser welded–brazed Al/Ti

Interfacial microstructure, fracture behavior, elemental diffusion, and Si atom substitution behavior were investigated in laser-welded–brazed Al/Ti joints. The results indicated that discontinuous intermetallic composite (IMC) and thick Ti(Al, Si) + Ti(Al, Si)2 + Ti(Al, Si)3 were generated under extremely low and high heat inputs, damaging joint strength. Interfaces with 1.02–3.20 μm Ti(Al, Si)3 was fractured at heat affected zone of Al and exhibited the highest strength of 252.0 MPa. Thermodynamic calculations indicated that elemental Mg and Si possessed lower chemical potentials at interface. This resulted in their aggregation at interface and formation of Mg2Si. Notably, the formation of Ti(Al, Si)3 acted as a barrier for further diffusion of Mg to IMCs. Density functional theory calculations were employed to clarify Si substitution behavior of Al. The calculated results indicated that smaller formation enthalpies and binding energies were obtained when Al atoms were substituted with Si atoms. Furthermore, the introduction of AlSi covalent bonds, lower total density of state (TDOS) values at EF level, and wider and negatively shifted pseudo-gaps contributed to a more stable structure and favored this substituted behavior. In addition, Si substitution would release charge accumulation level around the Ti atoms, resulting in a softening effect on the IMCs.

Plastic deformation of Ag/MgO interface system during tensile fracture: A molecular dynamics study

Fracture experiments on layered metal-ceramic composites show that the plastic dissipation in the metal near the crack tip accounts for most of the macroscopic fracture toughness. In this work we studied the microscopic plastic deformation mechanisms of Ag/MgO interface systems using molecular dynamics (MD) method. As tensile strain increases, lattice dislocations nucleate at interface misfit dislocations and propagate in metal layer. The formation of immobile Hirth dislocations in Ag causes hardening. As the metal layer increases, the yield strength decreases and the hardening range is shortened. Large interface area increases the degree of inhomogeneity of dislocation motion and interface fracture behavior, leading to decreased interface yield strength.

Mechanical properties degradation of Sn 37Pb solder joints caused by interfacial microstructure evolution under cryogenic temperature storage

The interfacial microstructure, shear property and fracture behaviors of Sn37Pb joints after cryogenic temperature storage for different durations were investigated in this study, storage test at room temperature (298 K) and thermal aging test at 423 K were also carried out as comparison. The morphology of interfacial Cu6Sn5 IMCs for the joints stored under 77 K and 173 K transformed from scallop-like to column-like, moreover, the IMC layer thickness slowly increased with prolonged storage time. The growth rate of Cu6Sn5 layer in Sn37Pb joint stored at 77 K was slightly faster as compared with that stored at 173 K. However, the morphology and thickness of Cu6Sn5 IMC layer still remained roughly unchanged after storage at 298 K for up to 40 days. The Cu6Sn5 IMC growth of Sn37Pb joint stored at 77 K and 173 K was expected to be caused by Cu atom diffusion from Cu pad towards Sn37Pb solder, which was induced by the stress gradient. Additionally, the shear force declined and fracture position changed from in Sn37Pb solder to primarily in Sn37Pb solder and partly along the interface with the prolonging of storage time at 77 K and 173 K, which were induced by IMC layer growth.

Interfacial properties and fracture behavior of β"/Mg interface in Mg–La alloy: A first-principles study

A deeper investigation into the characteristics of the β"/Mg interface represents a promising strategy for improving the ductility and strength of Mg–La alloys. This study systematically investigates the interface adhesion, stability, electronic structure, and tensile fracture behavior of three β"/Mg interfaces with different orientations through the first-principles method. The results indicate that the Mg1-T4-β"(1‾ 010) interface has the strongest binding capacity, and the atomic arrangement at the interface is consistent with the observation results from previous studies using HAADF-STEM. The PDOS and CDD analysis reveal differences in the atomic orbital hybridization, resulting in the formation of Mg–Mg homometallic bonds and Mg–La heterometallic bonds, contributing to the interface strength. The three interfaces exhibit varying mechanical properties with different strengths and strains, influenced by orientation and arrangement of atoms. The fractures occur near the La atom at the interface when the strain reaches a certain level due to insufficient bonding strength between Mg and La. The ductility of the β" precipitate is critical to the interface's ability to undergo plastic deformation. These findings offer new insights into the precipitate interface of Mg–La alloy and could guide the optimization of its properties for various applications.

The interfacial properties and fracture behavior of the graphite (0001)/Cu (111) interface from a first–principles investigation

Interface considerably influences the properties of copper (Cu)–graphite composites. Therefore, considerable understanding of the interface is required to improve the mechanical and tribological properties of these composites. Because the graphite (0001) and Cu (111) interfaces are a good match, this study focuses on calculating the related properties of the interface structure using first–principles calculations. The parameter setting of this work is reasonable when comparing the theoretical and experimental results of the bulk Cu and graphite. Slabs of the nine-layer graphite (0001) and seven-layer Cu (111) surfaces have excellent convergence. The order of interfacial configuration stability is [Formula: see text] and HCP possesses the largest work of adhesion and smallest interface energy among the studied interface structures. Furthermore, the electron density and Mulliken population method are used to examine the interfacial bonds.

Effect of silane coupling agent and concentration on fracture toughness and water sorption behaviour of fibre-reinforced dental composites.

This paper presents the effect of silane treatment of S-2 Glass fibres on the fracture toughness and water sorption/solubility behaviour of fibre-reinforced flowable dental composites. The effect of epoxy- and methacrylate-based silane coupling agents (SCAs) on the mechanical strength and hydrolytic properties were investigated. The concentration of the selected SCAs on the mechanical and physical properties were investigated. The influence of molecular structure and concentration in the interfacial adhesion at the fibre-matrix interfaces was also studied. Short S-2 Glass fibres of 250µm in length and 5µm in diameter were etched with acid to remove any impurities and roughen the surface. The acid-etched fibres were silane treated with 3MPS, 3GPS, and 8MOTS at different concentrations by weight (%). The silane-treated fibres were incorporated at 5 % into the dental resin mixture. Untreated fibres were added at 5 % to the dental resin mixture and served as the control group. The physical properties such as water sorption, solubility, and desorption along with mechanical properties such as fracture toughness and total fracture work of the fibre-reinforced dental composites grafted with the above-mentioned SCAs were evaluated. The surface morphology of the fractured surface was studied and analysed. The fracture toughness tests showed that the dental composites grafted with optimum weight per cent (wt. %) concentration of the SCA had a better stress intensity factor (KIC) when compared to the 2.0wt. % and 3.0wt. % concentration. The KIC value of dental composites grafted with untreated surface etched glass fibres was less than the KIC values of dental composites grafted with optimum concentrations of 3MPS, 3GPS, and 8MOTS by 81.6 %, 38.6 %, and 110.5 %, respectively. A similar trend was found while investigating the total work of fracture of the dental composites, between optimum concentration, 2.0wt. % and 3.0wt. % concentration of respective SCA. The increase in silane concentration also led to an increase in the water sorption/solubility characteristics. The absorption of water was most severe in the fibre-reinforced dental composites without silane treatment (32.9µg/mm3). The ANOVA results showed that the fibre-reinforced dental composites grafted with 8MOTS at optimum concentration showed an increase in fracture toughness when compared to optimum concentrations of 3GPS and 3MPS by 51.9 % and 15.9 %, respectively. The enhanced mechanical and physical characteristics are due to the increased adhesion between the fibre and silane achieved from the optimum wt. % concentration of 8MOTS. Similarly, dental composites grafted with 8MOTS at optimum concentration showed a decrease in water sorption characteristics when compared to optimum concentrations of 3GPS and 3MPS by 18.2 % and 0.6 %, respectively. The decreased water sorption characteristics at the optimum concentration of 8MOTS could be due to the reduced availability of reactive hydroxyl groups and the hydrophobic characteristics of 8MOTS. Silane coupling agents (SCAs) are important components of dental composites. The type and concentration of SCA have a significant effect on material properties. The current study focuses on understanding the effects of different SCAs and wt. % concentrations on the interfacial fracture behaviour and the influence of different SCAs on the water sorption and solubility behaviour of S-2 Glass fibre-reinforced flowable dental composites.

Pullout characteristics of functionally graded and degraded adhesive anchors

Three-dimensional axisymmetric elasticity solutions for pull-out stresses in bonded anchors with spatially stiffness-varying adhesive are presented. A stiff rod embedded in a semi-infinite rigid half-space through an adhesive bondlayer, representing a general anchor problem is analyzed. The adhesive layer is considered to have a smoothly varying stiffness over embedded length. Two cases of particular engineering relevance are considered: (i) stiffness grading of the bondlayer to enhance performance while retaining the critical length characteristics of bonded anchors, and (ii) modulus reduction of the bondlayer representing adhesive degradation proximal to the loaded-end. Theoretical solutions are developed adopting a stress function approach in conjunction with a variational method that compare well with 3D axisymmetric finite element (FE) results. Both theoretical and FE results indicate that the maximum shear stress in the adhesive decreases over 60% for a graded bondlayer for the parameters considered here without warranting a longer embedment length. In contrast, the degraded bondlayer reduces shear stress peaks significantly but warrants a larger embedment length to enable shear-dominated stress-transfer, disadvantageously loading the embedded-end in tension. A design map showing the critical embedment length required for degraded bondlines as a function of fractional embedment length over which bondline is regarded to have degraded is presented. In addition, interfacial fracture behaviors of the tailored and degraded adhesive anchors were examined through FE analyses finding that the tailoring reduces the energy release rate and has the potential for enhancing damage tolerance. The findings of the study indicate that the stiffness-tailored and -degraded bondlayers significantly redistribute the stress field with concomitant influence on stress-transfer and interfacial debonding characteristics of bonded anchors.

Interfacial fracture analysis for heterogeneous materials based on phase field model

Simulating crack initiation/propagation in heterogeneous materials has long been a challenge due to their complex topology and the complicated interactions between each phase. In order to address this task, an interfacial debonding phase field model is established for interfacial fracture behavior of highly heterogeneous solids in this work. Inspired by the phase field framework, a discrete interface is transformed into a smeared interface using interface phase field. The developed model has the following novelties: (1) interface properties are smeared such that singularity of material property can be eliminated; (2) due to the interaction between characteristic length scale parameters of the interface and the crack, critical energy release rate of interface is regularized; and (3) it can predict complex cracking phenomena such as multiple crack initiation, merging and branching. Particularly, the proposed model is validated experimentally and numerically. With the proposed model, influence of interface properties such as Young’s modulus and critical energy release rate on fracture behavior is studied in detail. The proposed model is potential in predicting interfacial debonding and the crack kinking path in complicated quasi-brittle heterogeneous materials.

Influence of calcium hexaluminate gradation on interfacial microstructure and fracture behavior of cement-bonded alumina castables

To reduce the brittleness of cement-bonded alumina castables for purging plug, different gradations (5-3, 3-1 and 1-0 mm) of calcium hexaluminate aggregates are introduced to replace the tabular alumina aggregates. The interfacial microstructure and fracture behavior with or without calcium hexaluminate aggregates are systematically analyzed. The results show that the large (5-3/3-1 mm) and small (1-0 mm) particle sizes of calcium hexaluminate contributed to the interfacial bonding and prolonged crack propagation path of cement-bonded alumina castables, respectively. Accordingly, the strength is promoted and the fracture mode is significantly affected under tensile stress. Specifically, the proportion of cracks propagating along the interface can be reduced by introducing 5-3-or 3-1 mm calcium hexaluminate particles. Besides, the introduction of 1-0 mm particles reduces the proportion of cracks propagating within the aggregate. Cement-bonded alumina castables containing three particle sizes (5-3/3-1/1-0 mm) of calcium hexaluminate have simultaneously better strength and toughness than castables containing a single particle size.