Interfacial Crack Initiation Research Articles

Multi-crack spatial propagation evolution analysis of 3D-TSV under thermal-electric-mechanical coupling field

As an interconnected microstructure, Through-Silicon Via (TSV) play a vital role in three-dimension (3D) chip. With the improvement of interconnection density, the reliability problems origin from interface crack initiation and propagation become increasingly prominent. In this study, the effects of the crack type, crack propagation direction, current magnitude and direction on the spatial characteristic of crack propagation under thermal-electric-mechanical coupling field is deeply investigated based on 3D J-integral-based fracture mechanics method. Results shows that: 1) Crack J-integral is consistent with the variation of ambient temperature and positively correlated with the current magnitude; 2) When the current direction is same as crack propagation direction, electron holes will gradually accumulate at crack tip, which can accelerate the crack propagation rate; 3) Different cracks will present different morphological characteristics, the shell pattern cracks can be found at RDL-SiO2 and Si-SiO2 cracks, and the internal cracks TSV-Cu present irregular trapezoidal shape. Relevant result is hope to provide certain references for the reliability analysis and optimal design of TSV.

Interaction of propagating crack with microstructure in CGI: In situ tensile test and numerical simulation

Compacted graphite iron (CGI) is a double-phase material, in which complex graphite morphology greatly influences crack initiation and propagation. Despite its wide use and considerable past research on the fracture behaviour of CGI, the understanding of the interaction of propagating cracks with CGI's microstructure is still limited. In this study, a novel method is employed that integrates scanning electron microscopy (SEM) and an optical high-speed camera. This approach allows the clear visualisation of microstructures before and after fracture, as well as the calculation of crack-propagation rate during tensile tests. It is also revealed for the first time that in the case of graphite particles situated at a specific distance from the primary crack path, the initiation of secondary micro-cracks, near particles does not occur. A critical distance from the main crack path for crack initiation is analysed. Further, finite-element simulations are developed to study the effect of microstructure on the fracture behaviour of the tested microstructures. A Johnson-Cook (JC) damage model is calibrated and used in all simulations. The crack path and velocity of simulation results are in good agreement with the experimental data, demonstrating that the JC damage model can be used to predict the crack initiation and propagation of CGI. It is found that interface debonding and crack initiation always tend to appear near the tip of vermicular graphite. Thus, large vermicular graphite particles can affect negatively the toughness of CGI. Graphite particles situated farther from the primary crack significantly reduce the likelihood of crack initiation in that specific location. Furthermore, large nodular graphite particles absorb energy and generate secondary cracks, while small graphite particles have little effect on the crack-path direction. The newly discovered fracture mechanisms and simulation results provide new insights for the design and manufacture of metal-matrix composites (e.g., Al/SiC) with optimal mechanical properties.

Bonding characteristics between existing concrete substrate and high ductility geopolymer overlays under direct shear loading

High ductility geopolymer (HDG) looks promising as an exceptional repair material due to the faultless mechanical properties and environmental benefits. The interfacial bonding characteristics between existing normal concrete (NC) substrate and HDG overlay are the vital impact on the overall performance of repaired structures. This study presented the NC-HDG bonding behavior under the direct shear tests, and totally 45 specimens were designed with various concrete strength grades, interface roughness and elongations of HDG. To capture the fracture characteristics, the digital image correlation (DIC) technique was employed. Test results showed that the direct shear bonding strength (τu) had an increasing trend as the NC strength dropped or the interface roughness and elongations of HDG were increased, among the interface roughness affected the τu mostly, with the improvement amount of 185.07 %∼343.28 %. The direct shear strength ratio (defined as the ratio of τu to the inherent shear strength of NC) was ranged as 0.110∼0.489, i.e., at least half of shear strength of NC was lost during repairing. Based on the DIC measured results, the improvement of NC strength, interface roughness and elongations of HDG all delayed the interfacial crack initiation under direct shear loading. Moreover, the upper-bound limit analysis was adopted, a simplified method for predicting the direct shear bonding strength of NC-HDG interface was proposed and showed a sufficient precision.

Thermal-mechanical evolution behaviors of metal matrix diamond composite by powder bed fusion-laser beam

The thermal damage of diamond and the residual stress have always been the critical issues to be resolved in the development of metal matrix diamond composites. In the presented study, FeCoCrNi high entropy alloys (HEAs)-diamond composites were fabricated by powder bed fusion-laser beam of metals (PBF-LB/M), using un-coated diamond and Ti–Ni coated diamond, respectively. Based on the numerical and experimental analysis, the effect of thermal-mechanical behavior on the microstructures and mechanical properties of the composites was systematically investigated. By establishing the temperature field of the PBF-LB molten pool, the defects, diamond graphitization and in-situ TiC formation were studied. The start-temperature for diamond graphitization in coated samples was 365 °C higher than that of un-coated samples, due to the diffusion barrier effect of TiC. Subsequently, by thermo-mechanical coupling, the property and value of the residual stress exhibited sudden change at the diamond/matrix interface. The TiC intermediate layer significantly relieved the stress concentration at interface, and effectively avoided cleavage fracture of the diamond and the initiation of interfacial cracks. At the moderate molten-pool temperature, the coated diamond composites exhibited optimal performance, as bending strength of 913 MPa, friction coefficient of 0.25 and worn mass loss of 0.67 mg. Finally, the quantitative relationship between PBF-LB molten-pool temperature and the relative density, graphitization degree, residual stress, retention(bending strength) and wear performance was established, to demonstrate the thermal-mechanical evolution of the HEAs-diamond composites by PBF-LB.

The toughness of interlocking metasurfaces

Interlocking metasurfaces (ILMs) are arrays of autogenous latching unit cells patterned across a surface. These create structural joints similar to bio‐inspired suture joints but patterned over a 2D surface rather than a 1D seam. This enables ILMs to be an alternative to conventional joining technologies such as bolts, welds, and adhesives. However, compared to conventional joining methods, relatively little is known of the engineering considerations for designing structural ILMs. In this study, the interfacial toughness of an archetypal ILM was examined for the first time. Under the conditions studied here, the ILM was substantially tougher than the material from which it was made; in this case exhibiting up to a 50% increase in interfacial crack initiation energy over the solid base material, a photocured 3D printed polymer. Through experimental tests using in‐situ digital image correlation along with complementary computational analyses, the mechanism of toughening in the ILM structure and the origins of toughness anisotropy were revealed. The increase in toughness is associated with cross‐cell interactions, i.e. load‐sharing across unit cells, which gives rise to a finite process zone length with different effective material properties. In this way, ILM toughening is analogous to crack blunting in ductile materials or fiber bridging in composites; yet here, the ILM was composed of a single‐phase base material and so the architected toughening is geometric in nature and hence amenable to future topological optimization.This article is protected by copyright. All rights reserved.

Initiation and Propagation of Interface Cracks in Layered Composite Rocks: An Investigation Using True Triaxial Experiments

Initiation and Propagation of Interface Cracks in Layered Composite Rocks: An Investigation Using True Triaxial Experiments

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.

High-temperature mechanical performance of Ru doped cermets

The mechanical properties degradation of cermets under high temperatures greatly limits their applications. In this paper, the compressive yield strength of WC-Co based cermets at high temperatures was significantly improved by the addition of Ru. Based on the detailed microstructure analyses, the contribution of Ru to the plastic deformation of the cermets was disclosed. The addition of Ru would stabilize the fcc-Co structure, leading to an increase in the proportion of the coherent WC/Co phase boundaries. The coherent phase boundaries are beneficial to avoid stress concentration across the interface and premature crack initiation, inducing higher dislocation densities and larger plastic deformation in both Co and WC phases. The solid solution strengthening effect of Ru and the Ru cluster in the Co phase can inhibit the softening of the binder phase at high temperatures. The synergy of solid solution strengthening and dislocation strengthening is responsible for the higher yield strength of WC-Co cermets with Ru at high temperatures.

Enhanced interfacial joining strength of laser wobble joined 6061-T6 Al alloy/CFRTP joint via interfacial bionic textures pre-construction

The joining of metals and carbon fiber reinforced thermoplastics composite (CFRTP) is recently considered to be an effective and promising way to meet the lightweight requirements of manufacturing in the aviation field. Herein, novel bionic textures (fishbone-like and nacre-like), constructed by laser pretreatment, are proposed to enhance the hybrid joints of 6061 Al alloy and CFRTP by the laser wobble joining method. The effects of the proposed bionic textures on the hybrid joints of 6061/CFRTP were uncovered through a systematic experimental method and finite element analysis (FEA). The results indicated that the strength and toughness of the hybrid joints with novel bionic textures were both distinctly improved compared with that of conventional textures (groove-like and circular-like). This strengthening effect can be essentially attributed to the buffering of interfacial stress concentration and the deflection behavior of cracks. The observed interfacial gaps and fractured resin appearance both distributed near the sides of the textures demonstrate that the cracks propagate inside the resin on one side of the texture and propagate along the interface on the other side. A new reinforcement strategy for hybrid metal/CFRTP joints has been proposed: regulating interfacial bionic textures to inhibit the initiation and propagation of interfacial cracks.

Automatic simulation of the initiation and propagation of interface cracks and matrix cracks with VCFEM

Based on the transformation mechanism of interfacial cracks propagating into a matrix, this paper proposes a model containing the particle phase, interphase, matrix phase, interface cracks, and matrix cracks. The model derives the functional, and we develop a new Voronoi cell finite element method (VCFEM) containing the interphase to describe interface crack and matrix crack initiation and propagation. A new mesh-subdivision method reflecting crack propagation from the interface to the matrix is constructed, which can automatically simulate the initiation and propagation of interface cracks and the transformation and penetration of interface cracks into matrix cracks. With crack propagation, various combined elements models are proposed. For these combined elements, their shared external nodes change as the cracks propagate through the original element, while the external node numbers of other elements remain unchanged. The effectiveness of the proposed method is verified by comparing the results of the new VCFEM with those obtained with Abaqus and analytical solution through several numerical examples. Since the proposed method assumes a high-order stress field within an element and can capture the stress concentration with fewer elements, it is suitable for large-scale calculation.

Analysis of interfacial crack initiation mechanism of thermal barrier coatings in isothermal oxidation process based on interfacial stress state

In this paper, the interfacial stress state is used to analyze the interfacial crack initiation mechanism of the thermal barrier coatings (TBCs) during isothermal oxidation. The influence of thermal growth stress, initial residual stress, and creep behavior on the stress distribution is considered to have an accurate simulation result. A parameter that integrates the effects of interfacial normal and tangential stress is modified for evaluating interfacial crack initiation. It is found that, in the cooling stage, the interfacial cracks sprout at the top coat (TC)/thermally grown oxide (TGO) interface valley region and the TGO/bond coat (BC) interface peak region, which agrees with the experimental results. Furthermore, the influence of interfacial roughness on crack initiation is investigated. The result shows that different interfacial roughness affects the sprouting region of interfacial cracks and cracks within the TC layer.

Dynamic fracture behavior and fracture toughness analysis of rock-concrete bi-material with interface crack at different impact angles

The crack initiation and propagation behavior of interface crack under impact are of great significance for evaluating the stability of rock-concrete system and other layered structures. Dynamic test using split-Hopkinson pressure bar (SHPB) system was performed on bi-material cracked straight-through Brazilian disc (CSTBD) specimens, and the fracture processes of specimens were monitored by means of a high-speed camera. The effects of the impact angle, concrete strength and strain rate on the dynamic fracture behavior were investigated. The results show that three typical fracture types can be identified: interface fracture, combined fracture (interface/shear and tension fracture) and tensile fracture. The interface fracture appears under small impact angle, while the combined fracture occurs with the further increase of impact angle. The crack initiation time of wing cracks and the number of secondary cracks are mainly affected by the strength of concrete in the bi-material specimen. Crack initiates earlier and more secondary cracks are generated in concrete material (weak material), and the number of secondary cracks increases with increasing strain rate. The dissipated energy increases gradually as the impact angle increases from 0° to 75°, while it decreases when the impact angle exceeds 75°. As the concrete strength increases, the difference in dissipated energy for different impact angles decreases. Furthermore, the effects of impact angle, concrete strength and loading rate on the dynamic fracture toughness for different loading modes are also discussed. The results indicate that the normalized stress intensity factors (SIFs) at the two tips of the interface crack are distinct and the difference increases with increasing crack length-to-diameter ratio. The impact angle, concrete strength and loading rate influence the dynamic fracture toughness significantly.

Study on Interfacial Crack Initiation of Jointed Rock Mass Based on Interface Fracture Mechanics

The fracture of interfacial crack is the main failure type of jointed rock mass. Therefore, it is very important to study the interfacial fracture of jointed rock mass. For the similarity of jointed rock mass and composites (all are composed by two parts, intact materials and their contact interfaces), the interface fracture mechanics widely used for analysis the interface crack of the composites (bimaterials) can be applied to study the interfacial fracture of jointed rock mass. Therefore, based on the basic theories of interface fracture mechanics, the interfacial fracture of jointed rock mass was analyzed, and one new criterion of interfacial crack initiation for jointed rock mass is proposed. Moreover, based on the proposed interfacial crack initiation criterion, the effect of main influence factors on the interfacial crack initiation of jointed rock mass was analyzed comprehensively. At last, by using the triaxial compression numerical tests on a jointed rock mass specimen with interfacial crack, the theoretical studies were verified.

Study on the Initiation of Interface Crack in Rock Joints.

The interfacial fracture of rock joints is an important although easily ignored issue in jointed rock engineering. To conduct this study, an interface crack model of rock joints was proposed. By analyzing the ratio of stress intensity factor to fracture toughness, the fracture mode of the interface crack was studied. Based on the Mohr-Coulomb criterion, an interface fracture criterion considering T-stress was established. To verify the proposed fracture criterion, laboratory and numerical tests were conducted. Finally, the effect of relative critical size α, internal friction angle φ and cohesion c on the initiation of an interface crack was comprehensively discussed. It is concluded that the proposed fracture criterion can predit the initiation of the interface cracks properly. With an increase in cohesion c, mode II fracture toughness KIIC also clearly increases. When the absolute value of KI is small, the effect of α is much larger than that of φ. In addition, with an increase in the absolute value of the mode I stress intensity factor, the φ of the joint plays a more important role in the initiation of the interface crack.

Concurrent multiscale modeling of in-plane micro-damage evaluation in Z-pinned composite laminates

Most of the unit cell models for Z-pinned composite laminates (ZCL) are mesoscopic in scale, where the fiber reinforced zone is modeled as homogeneous but without detailed characterization and failure analysis of the fibers and matrix. This paper proposes a new concurrent multiscale unit cell model to investigate the in-plane micro-damage evolution of ZCL. The novelty worth mentioning is the proposed element-based method to generate random microbuckling spatial fibers in the fiber reinforced zone, where the microbuckling is caused by fiber waviness and fiber crimp, and then the mesh of fiber, Z-pin, matrix, and the interface between Z-pin and matrix is regenerated. In-plane tensile tests (ASTM D3039) are conducted for verification. There is good agreement between experiment and simulation, only with deviations of −0.97% and 2.69% for tensile modulus and fracture initiation stress, respectively. The research indicates that in-plane micro-damage initiates from the interface between Z-pin and matrix. The stress concentration and higher local fiber volume fraction, caused by the microbuckling spatial fibers clustered near the Z-pin, play a major role in the evolution of interface damage, crack initiation and growth.

Rationally Regulating the Mechanical Performance of Porous PDMS Coatings for the Enhanced Icephobicity toward Large-Scale Ice.

Ice accumulation on various surfaces in low-temperature and high-humidity environments is still a major challenge for several engineering applications. Herein, we fabricated a kind of PDMS coating with the introduction of porous structures under the surface by a two-step curing and phase separation method. The coatings with no further surface modification showed good hydrophobicity and icephobicity, and the typical ice adhesion strength was down to 40 kPa with a water contact angle of 116.5°. More than that, the porous PDMS coatings showed extraordinary icephobicity, especially toward large-scale ice (>10 cm2). In this case, the large-scale ice layer can be rapidly removed under a small external deicing force in a form of interface crack propagation rather than whole direct fracture. It was confirmed that by regulating the pore size and porosity of PDMS coatings properly, the stiffness mismatch between coatings and ice can be controlled to induce the initiation of interfacial cracks. On this basis, under the condition of a large-scale icing area, a small external deicing force can cause an increased surface stress concentration, and the formed interface cracks can propagate quickly, resulting in the ice layer falling off easily. In addition, under the influence of the size effect, ice can be removed without an additional force, and the minimum external force (per unit width) can be only 60 N/cm. This paper proposes that prefabricating a large number of microcracks at the interface can significantly weaken the bonding between ice and coatings, that is, reduce the fracture toughness. The new coatings have a remarkable effect toward large-scale icing.

On the crack propagation and fracture properties of Cr-coated Zr-4 alloys for accident-tolerant fuel cladding: In situ three-point bending test and cohesive zone modeling

The cracking behavior of a Cr-coated Zr-4 alloy for accident-tolerant fuel cladding at room temperature was investigated using an in situ three-point bending test and finite element (FE) analysis. The initiation and propagation of vertical and interfacial cracks were captured in real time. Moreover, the fracture properties of the Cr-coated Zr-4 alloy were evaluated by in situ observations and FE simulations based on the cohesive zone model. The results indicated that vertical cracks were generally initiated from the interface and promptly penetrated through the coating thickness at a small deflection. Under continuous external loading, the vertical crack density increased rapidly and finally reached a plateau. Additionally, interfacial cracks were initiated from the vertical crack tips and propagated gradually along the interface, which was driven by large local interfacial peeling and shear stresses. Through FE analyses, the fracture toughness of the Cr coating was estimated to be 65 J/m2, and the interfacial fracture strength and toughness were evaluated as 150 MPa and 200 J/m2, respectively.

A semi‐analytical approach for the efficient predicition of thermally induced interface crack initiation at bi‐material free edges

AbstractIn the present study, thermally induced interlaminar crack initiation emanating from the free edge of a bi‐material junction is investigated by the example of a uniformly cooled epoxy‐glass bi‐material specimen taken from literature. For this purpose, the coupled stress and energy criterion within the framework of finite fracture mechanics is applied. Interlaminar stresses and energy dissipation due to crack onset are determined efficiently using the semi‐analytical scaled boundary finite element method (SBFEM). Moreover, the effect of the layer thicknesses on crack initiation is studied. For this purpose, dimensional analyses yield simple scaling laws for interlaminar stresses and energy dissipation with respect to the layer thickness such that only few model evaluations are required. The obtained results are compared to a finite element reference solution. For validation purposes, a cohesive zone model is applied additionally. Finally, the obtained predictions are compared to experimental findings from literature.

Low-Cycle Fatigue Crack Initiation Simulation and Life Prediction of Powder Superalloy Considering Inclusion-Matrix Interface Debonding.

From the perspective of damage mechanics, the damage parameters were introduced as the characterizing quantity of the decrease in the mechanical properties of powder superalloy material FGH96 under fatigue loading. By deriving a damage evolution equation, a fatigue life prediction model of powder superalloy containing inclusions was constructed based on damage mechanics. The specimens containing elliptical subsurface inclusions and semielliptical surface inclusions were considered. The CONTA172 and TARGE169 elements of finite element software (ANSYS) were used to simulate the interfacial debonding between the inclusions and matrix, and the interface crack initiation life was calculated. Through finite element modeling, the stress field evolution during the interface debonding was traced by simulation. Finally, the effect of the position and shape size of inclusions on interface debonding was explored.

Dynamic deformation and fracturing properties of concrete under biaxial confinements

The study of deformation and fracturing properties of concrete is essential to understand failure mechanisms of concrete material, especially under extreme and complex loading conditions, e.g. high strain rates and multiple confinements. In this study, dynamic deformation and fracturing behaviour of ordinary concrete under biaxial confinements are investigated by using a triaxial Hopkinson bar system, three-dimensional digital image correlation, and synchrotron X-ray computed tomography techniques. Results show that compressive strain localisation areas appear around aggregates due to the elastic difference between aggregate and matrix, where accompanied with initiation of interfacial cracks and propagation of main cracks. Transgranular cracks can also be observed in the central of specimen in CT slices due to the effect of strain concentration. In addition, CT slices with distinct properties in various directions indicate the anisotropic fracturing behaviour of concrete due to the effect of biaxial confinements.