Interfacial Crack Propagation Research Articles

Effects of conventional interweaving and secondary deposition on the microstructure and properties of Ti6Al4V/AA2024 alloy component fabricated by laser-directed energy deposition

The connection of dissimilar materials has always been a great challenge, especially titanium alloy and aluminum alloy, which have broad application prospects in the aerospace industry. In this work, the Ti6Al4V/AA2024 alloy component was prepared by laser-directed energy deposition (L-DED) technology based on a conventional interweaving structure, and the microstructure of four typical position interfaces was analyzed. Aiming at the problem of poor forming quality of titanium alloy in the second interweaving layer caused by high laser reflectivity of aluminum alloy, a secondary deposition process was proposed. The microstructure and mechanical properties of Ti6Al4V/AA2024 alloy component fabricated by two different processes were characterized and tested. The problems of discontinuity and penetrating cracks at the Ti6Al4V layer in the second interweaving layer are effectively tackled by secondary deposition. As a result, the mechanical property of the deposited structure can be effectively improved, of which the average compressive shear strength is 8% higher than that of the conventional interweaving Ti6Al4V/AA2024 alloy component. The stress at the Ti6Al4V/AA2024 interface presents a zig-zag distribution because of the interweaving structure, which avoids the generation of continuous stress bands and effectively inhibits the interface crack propagation.

Interfacial fatigue fracture of pressure sensitive adhesives

Pressure sensitive adhesives (PSAs) are viscoelastic polymers that can form fast and robust adhesion with various adherends under fingertip pressure. The rapidly expanding application domain of PSAs, such as healthcare, wearable electronics, and flexible displays, requires PSAs to sustain prolonged loads throughout their lifetime, calling for fundamental studies on their fatigue behaviors. However, fatigue of PSAs has remained poorly investigated. Here we study interfacial fatigue fracture of PSAs, focusing on the cyclic interfacial crack propagation due to the gradual rupture of noncovalent bonds between a PSA and an adherend. We fabricate a model PSA made of a hysteresis-free poly(butyl acrylate) bulk elastomer dip-coated with a viscoelastic poly(butyl acrylate-co-isobornyl acrylate) sticky surface, both crosslinked by poly(ethylene glycol) diacrylate. We adhere the fabricated PSA to a polyester strip to form a bilayer. The bilayer is covered by another polyester film as an inextensible backing layer. Using cyclic and monotonic peeling tests, we characterize the interfacial fatigue and fracture behaviors of the bilayer. From the experimental data, we obtain the interfacial fatigue threshold (4.6J/m2) under cyclic peeling, the slow crack threshold (33.9J/m2) under monotonic peeling, and the adhesion toughness (~ 400J/m2) at a finite peeling speed. We develop a modified Lake-Thomas model to describe the interfacial fatigue threshold due to noncovalent bond breaking. The theoretical prediction (2.6J/m2) agrees well with the experimental measurement (4.6J/m2). Finally, we discuss possible additional dissipation mechanisms involved in the larger slow crack threshold and much larger adhesion toughness. It is hoped that this study will provide new fundamental knowledge for fracture mechanics of PSAs, as well as guidance for future tough and durable PSAs.

Effect of intermetallic compounds on the mechanical properties of Cu/Al clad sheets with an SS304 interlayer

Annealing treatments were conducted on Cu/Al clad sheets incorporating a 304 stainless steel (SS304) interlayer to investigate the impact of intermetallic compounds (IMCs) on the mechanical properties of these clad sheets with an interfacial layer. Additionally, the mechanisms of interfacial strengthening and crack propagation behavior were examined. It was found that the best overall performance was achieved for samples annealed at 200 °C, showing tensile strength, fracture elongation, and peeling strength values of 219 MPa, 7.19 %, and 19.6 N/mm, respectively. For the rolled clad sheet, cracks typically initiated at the edges of the SS304 fragments, accompanied by minor residual holes and stress concentrations. Post-annealing, the Cu and Al layers underwent varying degrees of recovery and recrystallization, and element diffusion increased, leading to improvements in elongation and interfacial bonding strength of the clad sheets. As the thickness of the IMCs grew, the interfacial crack propagation path shifted to the Al2Cu/AlCu interface within the IMC layer, significantly reducing the bonding strength.

Thermally-induced fracture in the oxide scale of T91 ferritic/martensitic steel after exposure to oxygen-saturated liquid lead–bismuth eutectic

Ensuring the continuity and stability of the oxide scale to separate T91 steel from lead-bismuth eutectic (LBE) is an effective method for limiting corrosion in the lead-based fast reactor system. In this study, we specifically investigated the impact of cooling thermal stress on the stability and damage of the oxide scales of T91 steel when exposed to oxygen-saturated liquid lead–bismuth eutectic at high temperatures. A corrosion test was conducted on T91 steels exposed to stagnant oxygen-saturated LBE at 500 °C. Experimental results indicated that oxide damage exists without any external force, including interface cracks and through-scale cracks perpendicular to the interface. We developed a three-layer peridynamic model of T91-(Fe,Cr)3O4-Fe3O4 to simulate the deformation and failure of oxides under a cooling process from high temperature to room temperature. The simulation results show that a temperature drop of about 200 °C from 500 °C can cause through-oxide cracks in the oxide scale, and subsequent cooling can lead to the propagation of interfacial cracks. Additional modifications were introduced in the model to account for the porosity of the oxide scale. Parametric investigations illustrate the effects of oxide scale thickness and porosity on the resulting crack patterns.

Models analyzing the response of the adhesive joint between FRP and concrete: a comparative analysis

Fiber-reinforced polymers (FRP) for structural elements’ strengthening and rehabilitation are increasingly gaining popularity because of their proven efficiency and ease of application. The response of the adhesive joint formed between the FRP and concrete is a crucial factor influencing the overall behavior of FRP structures. The analysis of the interface crack propagation is a complex task involving the consideration of multiple interacting mechanisms. This work aims to provide a background to a recently proposed model that analyzes the behavior of the adhesive joint formed at the interface between the FRP and concrete, by creating its counterpart. The study presented herein discusses a model based on fracture mechanics to obtain a reference point needed to assess the output provided by an algorithm utilizing a damage-based model for concrete and an empirical-based procedure for the loss of bond action. Typically, model validation is performed via a comparison against experimental data. However, additional insights into the interacting phenomena can be provided by comparing the results of various applicable models.

Mechanical property analysis and optimization of nano-hydroxyapatite coated carbon fiber reinforced hydroxyapatite composites

Carbon fiber (CF) is one of ideal reinforcements to hydroxyapatite (HA) owing to its unique structure, excellent biocompatibility and high mechanical properties. However, the mechanical property of CF reinforced HA (CF/HA) composites cannot be effectively improved due to the mismatch in surface properties and thermal expansion coefficients between CF and HA. In order to solve the problems, nano-hydroxyapatite (nHA) coating with different thickness was fabricated on the CF surface. The effect of the coating thickness on mechanical property of nHA-coated CF/HA composites was studied by finite element analysis. The CF with proper nHA coating thickness was subsequently used to reinforce HA. The compressive strength of nHA-coated CF/HA composites was approximately 97.3 % and 164.6 % higher than those of the uncoated CF/HA composites and pure HA ceramics, respectively. The interface properties and crack propagation were analyzed by cohesive element in the finite element analysis. The toughening mechanism of the composites included interfacial debonding, crack deflection, crack branching, and crack bridging. The mechanical properties of nHA-coated CF/HA composites were enhanced due to the strong interfacial bonding strength and reducing cracking in HA matrix. The prepared nHA-coated CF/HA composites satisfy the requirement of the mechanical properties of human load-bearing bone. It can be used to prepare cages, plates, screws, pins and other compact bone substitute parts in bone grafting. The study provided a method for construction of nHA-coated CF/HA composites with the designed properties by adjusting the nHA coating on the fiber and can be used to design other fibers reinforced ceramic matrix composites.

Interfacial reaction and strengthening mechanism of thermo-compression bonding foam Ni reinforced SAC105 and SAC105–0.3Ti solder joints

The foam Ni holds great potential as a reinforcement phase, attributed to its distinctive three-dimensional (3D) continuous structure. In this study, high-strength Cu connections were produced by vacuum thermo-compression bonding using Ni foam reinforced SAC105 and SAC105–0.3Ti solders. The microstructure, interfacial morphology and shear strength of the connections with different Ni foam composite solders were systematically examined. The results demonstrate that the incorporation of Ni foam as a reinforcing phase substantially enhances the strength of SAC105 solder. The introduction of foam Ni resulted in the formation of a prismatic (Cu, Ni)6Sn5 phase in the matrix and altered the composition of the IMC layer. As the bonding time increased, (Cu, Ni)6Sn5 further grew in the matrix and formed a double interpenetration structure with the foam Ni. The IMC layer, composed of (Cu, Ni)6Sn5 and Cu3Sn, exhibited a random orientation. The Cu/SAC105–0.3Ti-Foam Ni/Cu joints bonded for 10 min exhibited the highest shear strength of 70.23 MPa. Shearing failure predominantly occurs within the soldering seam. The double interpenetration structure and the random orientation of the interfacial IMC impeded crack propagation, thereby significantly enhancing the strength of the Cu joints.

Peridynamic fracture analysis of film–substrate systems

When subjected to mechanical, thermal, or other loads, film–substrate systems may undergo complex cracking behaviors, which encompass film and substrate cracking, interfacial debonding, and their combinations, exhibiting rich fracture patterns, such as three-dimensional helical cracks. Identifying the mechanisms underlying these fracture phenomena may lead to more advanced strategies for technologically significant applications. In this paper, we develop an interfacial cohesive peridynamic method for fracture analysis of multiple-phase materials. Particularly, we focus on the modeling of coupled film cracking and interfacial debonding in film–substrate systems. By introducing cohesive interfacial bonds to describe the mechanical properties of the interfaces and adopting a displacement-based cohesive failure criterion, the model is able to predict the critical condition and path of interfacial crack propagation. The robustness of the interfacial cohesive peridynamic method is validated through a series of representative examples. We also demonstrate its efficacy in simulating three-dimensional cracks and identify the essential role of the interfacial energy release rate in controlling the cracking mode transition from a restricted pattern to a helical pattern. The numerical predictions of cracking paths and stress distributions agree well with the previous experimental results. This study provides a valuable tool for analyzing different cracking patterns in film–substrate systems and composite materials.

Enhanced Mechanical Properties of Ti/Mg Laminated Composites Using a Differential Temperature Rolling Process under a Protective Atmosphere.

Addressing the issue of low bonding strength in Ti/Mg laminated composites due to interfacial oxidation, this study employs a differential temperature rolling method using longitudinal induction heating to fabricate Ti/Mg composite plates. The entire process is conducted under an argon gas protective atmosphere, which prevents interfacial oxidation while achieving uniform deformation. The effects of reduction on the mechanical properties and microstructure of the composite plates are thoroughly investigated. Results indicate that as the reduction increases, the bonding strength gradually increases, mainly attributed to the increased mechanical interlocking area and a broader element diffusion layer. This corresponds to a transition from a brittle to a ductile fracture at the microscopic tensile-shear fracture surface. When the reduction reaches 47.5%, the Ti/Mg interfacial strength reaches 63 MPa, which is approximately a 20% improvement compared to the bonded strength with previous oxidation at the interface. Notably, at a low reduction of 17.5%, the bonding strength is significantly enhanced by about one time. Additionally, it was found that a strong bonded interface at a high reduction is beneficial in hindering the propagation of interfacial cracks during tensile testing, enhancing the ability of the Ti/Mg composite plates to resist interfacial delamination.

Preparation and mechanical properties of Cf/SiC composites via pressure-assisted gel impregnation and PIP

Non-uniform shrinkage of SiC precursors will lead to cracking and thus weaken the mechanical properties of the Cf/SiC composites. This study used the pressure-assisted gel impregnation method to introduce SiC particles (SiCp) into the carbon fiber preform and combined the PIP technique to fabricate Cf/SiC composites. The mechanism of enhancing the mechanical properties of composites was studied. Results showed that SiCp effectively eliminated matrix cracking, and the PyC interface layer inhibited interfacial crack propagation. The combined effect of the two resulted in a significant improvement in the mechanical properties of the composites. Specifically, the maximum flexural strength of the Cf/SiC composites was 360.63±26.02 MPa, and the maximum fracture toughness was 8.57±1.82 MPa·m1/2.

Alternating load failure analysis under the high-temperatures vibration of thermal barrier coatings

During thermal barrier coatings (TBCs) service, high-temperature and vibration are environmental factors that cannot be ignored. The vibration fatigue damage mode of TBCs under operating conditions that couple vibrations with high temperatures is unclear. In this paper, a high-temperature vibration coupled environment simulator was developed to investigate the evolution of high-temperature vibration fatigue damage modes of LaZrCeO/7-YSZ(LZC/YSZ) TBCs under alternating loads by employing acoustic emission detection. The results show that in the initial fatigue stage, the damage appears as discrete micro-cracks formed by the initial defects in the TBCs. With the increase in the number of cycles, the initiation of transverse microcracks began to occur inside the LZC layer and at the LZC/YSZ interface. When the transverse microcracks accumulate to the critical value, the applied alternating loads accelerate the propagation of opening interfacial cracks. Finally, the columnar crystal structures of the ceramic layers fracture at different heights under cyclic loading by high-temperature vibrations.

Interfacial stress and crack propagation experimental study in mini-LED chip debonding

The ultra-miniaturization trend of chips presents new challenges for non-destructive peeling-off techniques. This study models the mini-LED chip-UV film adhesive system as a micro laminated structure and conducts the interfacial stresses and crack propagation analysis of the structure using theoretical, finite element and experimental methods. The innovations of this paper are the illustration of the negligible effect of size effect on 125 μm x 75 μm x 60 μm chips, and the determination of the technology parameter tuning method to promote chip peeling-off. Firstly, applying the first-order shear deformation theory, and considering the UV-curable acrylic adhesive layer and the UV film as elastic materials, Lamé coefficients are introduced to explore the free vibration of the micro laminated structure. The analyses indicate that the scale effect has an extremely small impact, within 0.5%, on natural frequencies of the 100-micron scale laminated structure with a small length-to-thickness ratio. Thereafter, a three parameter elastic foundation (3-PEF) model of the chip-UV film adhesive structure is established and the interfacial stresses distributions are parametrically analyzed. Additionally, an experimental platform for needle-ejecting chip peeling-off is designed and constructed. Finite element simulation, experimental observations, and image recognition of crack generation and propagation during the peeling process were performed. Results show that cracks are generated at both free ends of the adhesive layer and progressively extended inwards. This is consistent with the 3-PEF analytical solution, in which the stress concentration also occurs at free ends of the adhesive layer. The agreement between theoretical, simulation and experimental results regarding interfacial stresses provides a solid foundation for further analysis of chip peeling-off mechanism.

A novel strength-energy criterion for bimaterial interface crack propagation

This paper aims to propose a novel fracture criterion to predict complex propagation behaviors of a bimaterial interface crack, either delaminating along the interface or kinking out of the interface into one of the adjoining materials. A strength-based criterion is used to predict the crack deflection, and an energy-based criterion is adopted to assess delamination along the interface. To determine the competition between these two cracking modes, a competing strategy is developed by comparing the strength-based and energy-based criteria. This coupled strength-energy fracture criterion eliminates the disadvantages arising from a criterion based solely on strength or energy, and is convenient to be calculated and embedded into numerical models. As practice, we embed the criterion into a homemade extended finite element model to simulate bimaterial interface crack propagation. Several numerical examples are performed to verify the effectiveness and accuracy of the developed model. Finally, this model is applied to simulate an interface crack propagation in a sandwich structure that has a specially designed peel-stopper. Parametric studies reveal that the material and interface toughness as well as geometrical dimensions of the structure have significant effects on the propagation mode of the interface crack. The results provide practical suggestions on the optimal design of peel-stopper in sandwich structures.

Dynamic enhancement induced by interface for additively manufactured continuous carbon fiber reinforced composites

Additive manufacturing (AM) has become one advanced manufacturing method for continuous carbon fiber reinforced polymer (CCFRP) composites, which is promising to work as aerospace materials. To promote their efficient design and extensive application, series of tensile and fiber-pullout tests were implemented to investigate the rate-dependent mechanical behavior and fracture mechanism. Both tensile strength and fracture strain significantly increase with the increase of strain rate in the studied range of 0.0001–1600 s−1. Fiber-pullout test also confirms that the interfacial shear strength of composites increases with the increasing loading rate. Dynamic enhancement is analyzed quantitatively, and it is mainly contributed by the rate-dependent behavior of fiber-matrix interface. Fracture analysis indicates the stronger interfacial bonding under dynamic loading, which is attributed to the impeded interfacial crack propagation by high strain rate. A constitutive model is built to predict the rate-dependent strength of CCFRP composites, and it fits well with the experimental results.

Hydrogen Bonds-Pinned Entanglement Blunting the Interfacial Crack of Hydrogel-Elastomer Hybrids.

Anchoring a layer of amorphous hydrogel on an antagonistic elastomer holds potential applications in surface aqueous lubrication. However, the interfacial crack propagation usually occurs under continuous loads for amorphous hydrogel, leading to the failure of hydrogel interface. This work presents a universal strategy to passivate the interfacial cracks by designing a hydrogen bonds-pinned entanglement (Hb-En) structure of amorphous hydrogel on engineering elastomers. The unique Hb-En structure is created by pinning well-tailored entanglements via covalent-like hydrogen bonds, which can amplify the delocalization of interfacial stress concentration and elevate the necessary fracture energy barrier within hydrogel interface. Therefore, the interfacial crack propagation can be suppressed under single and cyclic loads, resulting in a high interfacial toughness over 1650 J m-2 and an excellent interfacial fatigue threshold of 423 J m-2. Such a strategy universally works on blunting the interfacial crack between hydrogel coating and various elastomer materials with arbitrary shapes. The superb fatigue-crack insensitivity at the interface allows for durable aqueous lubrication of hydrogel coating with low friction.

Prediction on residual flexural fatigue life of rock-concrete interface based on a deformation-controlled fatigue crack propagation criterion

To assess the cracking resistance of rock-concrete interface, three-point bending tests were carried on composite rock-concrete specimens under the monotonic and high flexural fatigue loading. The crack opening displacements along the ligament of the beam were continuously recorded in the whole test process. According to the measured crack mouth opening displacements (CMODs), the correlation between the monotonic and fatigue deformations at different cracking statuses was discussed. It has been shown that the deformation development of composite specimens under flexural fatigue loading was mainly caused by the propagation of interfacial crack. The P-CMOD curve under monotonic loading was the envelope curve of those under the different maximum fatigue loads. There were three CMOD control points in the monotonic deformation response to indicate the fatigue crack propagation states, which corresponded to the initial cracking load, the ultimate load and the applied maximum fatigue load in the post-peak part of the monotonic response, respectively. Correspondingly, they manifested the fatigue crack initiation, the inflection point of the fatigue crack propagation rate from deceleration to acceleration and the fatigue failure of the rock-concrete interface. Based on the three CMOD control points, a deformation-controlled fatigue crack propagation criterion was proposed to assess the cracking state of the rock-concrete interface under fatigue loading. Accordingly, a theoretical prediction model, termed as fatigue deformation evolution law, was established to predict its residual flexural fatigue life. This method was verified by the reasonable agreements between experimental and predicted results in the residual fatigue life.

Prediction of Crack Propagation Directions in Dissimilar Metal‐Welded Joints Using Phase‐Field Models and Discussion of Its Mechanisms

Stress corrosion cracking (SCC) in dissimilar metal‐welded joints (DMWJs) poses a significant threat to the safe operation of nuclear power plants. This study employs the phase‐field method to analyze crack propagation paths at various positions in DMWJs of nuclear power safety ends. A user‐defined material (UMAT) subroutine was implemented to characterize the mechanical heterogeneity of the heat‐affected zone (HAZ) and fusion zone (FZ). The effects of Young’s modulus (E), critical energy release rate (GC), and mechanical heterogeneity on crack propagation paths were investigated. Results indicate that E has minimal impact on crack propagation paths, while GC significantly influences them. Mechanical heterogeneity in local regions, particularly in the HAZ and FZ, substantially affects crack propagation paths, with the HAZ having the most pronounced effect. Interface crack propagation is identified as the most hazardous. Notably, cracks in 316L/52Mw are less affected by mechanical heterogeneity compared to those in SA508/52Mb.

Research on Meso‐Damage Experiment and Simulation in the Anchorage Bonding Interface Between Rock Bolt and Rock Mass

Anchorage failure occurs mostly at the anchorage interface, and the mechanical mechanism of the anchorage interface is the key point and difficulty in the analysis and research of anchorage. In order to study the mechanics and failure mechanism of the anchorage interface, the shear failure characteristics of the anchorage interface and the deformation rule of the anchored rock under different rib spacings were analyzed through the indoor plane anchorage pull‐out test. Reasonable rib spacing can significantly improve the shear strength of the anchored solid. Finite element software ABAQUS was used to create an axisymmetric plane model of rock bolt pull‐out under different rib conditions to simulate the whole process of rock bolt pull‐out and anchorage interface failure. The results show that during the pull‐out process of anchorage rock mass, stress concentration zones occur at rib corners, and peak shear stress moves to the depths of the anchorage section. As the load increases, cracks emerge from the top of the rib and the edge of the rib angle of the rock bolt and propagate to the deep anchorage along the edge of the costal angle, resulting in damage at the rock mass interface. Load–displacement curves of different transverse rib shapes, heights, widths, and spacings indicate that rib changes affect the anchor‐bearing limit. The anchoring failure mode changes from rib wear and spreading slip, rib shear convex body propagation to interfacial crack propagation, and rib causes varying degrees of damage to the surrounding rock mass. Therefore, reasonable rib parameter improvements can enhance anchoring force performance.

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.

Meso-crack propagation process of concrete based on macro-fracture parameters: Numerical and experimental

Formation and propagation of cracks along mortar-aggregate interfaces or penetration of cracks into aggregates highly influences the mechanical performance of concrete composites. This paper presents a numerical model to calculate the meso-crack propagation process of concrete based on the macro-material and interfacial parameters that can be directly measured by basic mechanical experiments. In the model, the issue of aggregate crack penetration versus interface crack propagation has been addressed by employing the stress intensity factor (SIF)-based criteria for crack propagation. After the validity of the numerical model is verified by test results on three-point bending concrete beams with single embedded cylindrical aggregate, the applicability of the model to meso-crack propagation in concrete with various geometrical dimensions, interfacial properties, and aggregate gradations is discussed through a series of numerical experiments. The results show that the proposed numerical model is capable of predicting the meso-crack propagation process of concrete with reasonable accuracy.