Interfacial Fracture Behavior Research Articles

Effects of temperature and voids on the interfacial fracture of Si/a‐Si3N4 bilayer systems

This work studies the effects of temperature, voids at or near the interface on the interfacial fracture behavior, and mechanisms of the Si/a‐Si3N4 bilayer systems via molecular dynamics simulations. Under mode I loading, at 300 K, the interfacial strength of the bilayer system without voids is ∼22.5 GPa and it undergoes brittle fracture at the interface. However, at 600 K, failure occurs in the a‐Si3N4 layer in a ductile mode. With the existence of void in the Si layer, crack initiates at the void and propagates toward the interface when the temperature is 300 K. However, as temperature increases to 600 K, the defected bilayer system undergoes brittle fracture at the interface at a significantly lowered interfacial strength. With the presence of void at the interface, the bilayer system fractures at the interface with a deteriorated strength regardless of temperature. Under mode II loading, the Si/a‐Si3N4 system undergoes three deformation processes: elastic deformation, followed by plastic deformation in the a‐Si3N4 layer, and subsequently, interfacial sliding. The presence of voids at different locations and the increase in temperature lower the stresses required for interfacial sliding but do not have significant effect on the shear deformation processes. This study provides an insight to fracture behavior of Si/a‐Si3N4 system at certain operating conditions.

Interaction between the doubly periodic interfacial cracks in a layered periodic composite: Simulation by the method of singular integral equation

In this paper, a doubly periodic interfacial cracking problem in a layered periodic composite is analyzed under antiplane shear loads. To further understand the interaction effect among the periodic interfacial cracks, four crack configurations are considered, namely a bilayered composite containing a single crack and a periodic array of collinear cracks, a layered periodic composite with a periodic array of parallel cracks and a doubly periodic rectangular array of cracks. In view of the periodic symmetry, the three periodic problems are transformed to a mixed boundary value problem for a single crack problem in an appropriate cell with the suitable periodic boundary conditions on its boundaries. The treatment skills for the periodic array of collinear cracks and the disposal techniques for the periodic array of parallel cracks are used together to solve the doubly periodic rectangular array of cracking problem. To demonstrate the computational accuracy of the present method, detailed comparative analyses are carried out. Numerical results are presented to show the interaction effect among the periodic interfacial cracks. For the doubly periodic rectangular array of cracking problem, parametric studies on the stress intensity factors leads out the following rule: the shielding effect of multiple parallel cracks and the amplifying effect of multiple collinear cracks exist simultaneously, and a coupled effect between geometrical and physical parameters on the interfacial fracture behavior exists clearly.

The Yoffe-type moving tubular interface crack in a hollow composite cylinder with finite length

In this paper, the problem of a composite hollow cylinder with a constant-velocity Yoffe-type moving tubular interface crack is considered. For four frequently encountered constraint edges, i.e. free-free, clamped-clamped, free-clamped, or clamped-free edges, the mixed boundary value problem associated with a mode-III crack is reduced to a Cauchy kernel singular integral equation by applying the Fourier transform. Then, the numerical results of stress intensity factors (SIFs) are obtained by the Lobatto–Chebyshev quadrature technique from the singular integral equation. Numerical results of SIFs show that there exists a coupled effect of geometrical and physical parameters on the interfacial fracture behavior, which clearly relates to the selection of constraint edges. It is also observed that SIF increases as the crack moving velocity increases for the free–free, free–clamped, or clamped–free edges, and SIF is a decreasing function of the crack moving velocity for the clamped–clamped edge.

Interfacial effects on fracture nucleation and propagation in crystalline–amorphous energetic material systems

Local failure modes, such as the nucleation and propagation of a pre-existing crack, have been investigated for energetic materials with a viscoelastic binder and crystalline grains subjected to dynamic thermal and mechanical loading conditions. A crystalline plasticity with dislocation density, finite viscoelasticity, dynamic fracture nucleation and propagation methods, and finite element methods were used to study crack nucleation and propagation due to dynamic thermal and mechanical loading conditions. The interrelated effects of dislocation densities, grain boundary (GB) misorientations, polymer binder relaxation, and interactions between crystal and binder were coupled to material thermal decomposition, adiabatic inelastic heating, viscous dissipation heating, and thermal conduction to analyze interfacial fracture behavior in RDX–estane systems. The predictions indicate that cracks propagated toward the binder and were arrested due to the viscous nature of the polymer binder and plasticity buildup. For low angle misorientations, the pre-existing crack propagated toward the binder with increasing crack tip speed until it reaches the binder, at which point the crack was arrested. For high angle misorientations, the crack propagated toward the binder and was arrested, adjacent to the binder, due to plastic deformation and lattice rotations. A secondary crack eventually nucleated and propagated to the interface, where it was arrested.

Microstructure and mechanical properties of overcast aluminum joints

The aluminum joints were prepared by overcasting liquid aluminum A356 onto 6101 aluminum extrusion bars. The microstructure, element distribution, hardness and tensile strength of the joint interface area were investigated, the mechanism of interface formation and fracture behavior were analyzed. The results show that good metallurgical bonding was formed in the joints by electro-plating the solid 6101 aluminum alloy with a layer of zinc coating and carefully controlling the overcasting process. There is a transition zone between the two bonded aluminum alloys, and the fine equiaxed grained structure in the transition zone is due to the high undercooling during solidification. The tensile strength of the joint interface is higher than that of the as-cast A356 aluminum alloy (about 145 MPa) and the final fracture is always located in the as-cast A356 material.

Interfacial Creep Fracture Behavior of Foam Core Sandwich Composites with Different Resin

Interfacial creep fracture behavior of sandwich composites consisting of E-glass/ unsaturated polyester (UPE) resin and E-glass/vinyl ester (VE) resin facings over PMI foam core has been experimentally investigated. Digital image correlation (DIC) method was employed in the double cantilever beam (DCB) test to measure the creep displacement field surrounding the crack tip. Crack opening displacements (COD) at various creep time were extracted from the creep displacement field and mode I stress intensity factors were determined using the small region COD-based linear extrapolation method. Significant increments of COD and nominal stress intensity factor are found in specimen with UPE resin and specimen with VE resin after 24 hours creep test. It is also found that specimen with UPE resin has a better interfacial creep fracture resistance than that of specimen with VE resin.

Effect of interfaces of grain boundary Al2CuLi plates on fracture behavior of Al–3Cu–2Li

Transmission electron microscopy (TEM) and density functional theory (DFT) calculations were employed to investigate the interfacial characteristics and fracture behavior of Al–3Cu–2Li containing plate-like Al2CuLi (known as the T1 phase) precipitates at grain boundaries. TEM studies showed that T1 plates form at grain boundaries, with a coherent interface on one side parallel to the {111} planes of the matrix and a non-coherent interface with no preferred orientation relative to the grain on the other side. The low energy of the coherent interface leads to a serration of the grain boundaries due to the growth of the grain boundary T1 phase. Under tensile loading, the intergranular T1 phase leads to the formation of nanopores at the non-coherent side, and fracture mostly through the non-coherent grain boundary T1/matrix interface for under-, peak- and overaged conditions. DFT simulations showed that, under tensile loading, fracture is most likely to take place at the T1/Al interface, and the non-coherent side of the grain boundary is weakest as the decohesion energy is 25% lower than that of the coherent interface.

Finite Element Modeling of Mode I Failure of the Single Contoured Cantilever CFRP-Reinforced Concrete Beam

The single contour cantilever beam (SCCB) test method has been developed with the intent to capture Mode I opening failures of CFRP-reinforced concrete beams. Recent development in the method explores possible shifting damage into the concrete substrate by using the International Concrete Repair Institute (ICRI) Surface Profile Level Three (SP3) as the desired CFRP bonded interface to concrete. To validate and explain the interface fracture behavior, finite element analysis using special cohesive elements has been performed. The cohesive element allows separation of the concrete substrate from the CFRP. This paper presents the simulation of laboratory test results, where failure in the substrates has been successfully reproduced. The simulation results indicate that finite element method using cohesive elements can successfully replicate Mode I critical strain energy release rate and the peak capacity of the laboratory tests and may have the potential to simulate actual applications.

Interfacial structure and fracture behavior of 6061 Al and MAO-6061 Al direct active soldered with Sn–Ag–Ti active solder

Micro-arc oxidation (MAO), a novel coating method capable of depositing compact, ultra-hard ceramic composite coatings on Al and its alloys, is applied to heat sink surfaces. A micro-porous Al2O3 layer was synthesized on 6061 Al-Alloy (MAO–Al) using the MAO technique. The microstructure, shear strength and fracture of Al/Al, MAO–Al/MAO–Al, and Al/MAO–Al joints were determined after direct active soldering in air with the Sn3.5Ag4Ti(Ce) active solder at 250°C for 30s. During active soldering, Al dissolves into SAT solder to form a coarse Al–Ag–Sn solid solution around the solder. Also, the active element Ti concentrates to and reacts with the MAO–Al layer to form both Ti-oxidized (e.g., TiO and TiO2) or rich-Ti(Al,Sn)3, and subsequently Ag3Sn nanoparticles are adsorbed at the solder/MAO–Al interfaces. The shear-tested bonding strengths were 15.3±1.38MPa for Al/Al, 10.45±1.53MPa for MAO–Al/Al, and 8.25±1.53MPa for MAO–Al/MAO–Al joints. In the Al/Al specimen, the fracture occurred in Al–Ag–Sn compounds of the active matrix after shear testing. In the MAO–Al/MAO–Al and MAO–Al/Al specimens, the fracture occurred in the MAO–Al/active solder interface.

Nanoindentation-induced interfacial fracture of ZnO thin films deposited on Si(111) substrates by atomic layer deposition

The structural and nanomechanical characteristics of ZnO thin films grown on Si(1 1 1) substrates by using the ALD process are investigated by combining XRD, AFM and Berkovich nanoindentation techniques. Results indicated that the ZnO/Si(1 1 1) thin films also exhibited clear evidences of fracture behaviors-induced pop-in phenomenon in the load–displacement curve during nanoindentation. Moreover, based on the analysis of the energy release in cracking, the fracture toughness of ALD-derived ZnO thin films deposited on Si(1 1 1) substrates is calculated. • ZnO thin films are deposited on Si(1 1 1) substrates using atomic layer deposition. • The interfacial fracture behaviors for ZnO thin films are characterized by Berkovich nanoindentation. • The fracture toughness of ZnO/Si(1 1 1) thin films is about 3.1 MPa m 1/2 . In this study, the structural and nanomechanical characteristics of ZnO thin films are investigated by mans of X-ray diffraction (XRD), atomic force microscopy (AFM) and nanoindentation techniques. The ZnO thin films are deposited on Si(1 1 1) substrates by using atomic layer deposition (ALD). The interfacial fracture behaviors for ALD-derived ZnO thin films are characterized by Berkovich nanoindentation, and the morphologies of indentations are revealed by using scanning electron microscopy (SEM). During nanoindentation, the interfacial fracture of ZnO thin films is significantly observed as the indentation load reaches 50 mN and, the corresponding loading segment in the load–displacement curve displays an obvious discontinuity event. Based on the analysis of the energy release in cracking, the fracture toughness of ALD-derived ZnO thin films deposited on Si(1 1 1) substrates is calculated.

Effect of strain rate on interfacial fracture behaviors of Sn-58Bi/Cu solder joints

The interfacial reactions and mechanical properties of Sn-58Bi/Cu solder joints reflowed at different temperatures ranging from 180 to 220 °C for constant time of 10 min were investigated with various strain rates. Only a continuous Cu6Sn5 intermetallic compound (IMC) layer was formed at the interface between the Sn-58Bi solder and the Cu substrate during reflow. The equivalent thickness of the Cu6Sn5 layer increased with increasing reflow temperature, and the relationship between Cu6Sn5 layer equivalent thickness (X) and reflow temperature (T) is obtained by using method of linear regression and presented as \( X = 0.01 \times T + 0.187 \). For the tensile property, the tensile strength of solder joint gradually decreased as the reflow temperature it increased, whereas it increased with increasing strain rate. Moreover, the fracture behavior of Sn-58Bi/Cu solder joint indicated the ductile fracture with low strain rate (5 × 10−4 and 1 × 10−3 s−1), while toward brittle fracture with high strain rate (2 × 10−3 and 1 × 10−2 s−1). The strain rate sensitivities of the solder joints fractured with various modes were also investigated, and it is found that the tensile strength of the solder is more sensitive to the strain rate than that of the IMC layer.

Overview of interfacial fracture energy predictions for stacked thin films using a four-point bending framework

Developing a robust predicting methodology on the interfacial fracture energy of dissimilar materials is urgently needed to meet the mechanical strength requirements for investigating novel materials and subsequent device structures with the bonded types of multi thin films. Thus, this paper provides an overview of useful approaches according to linear-elastic interfacial fracture mechanics, such as J-integral method, modified virtual crack closure technique (MVCCT), and analytical solutions derived from the relationship between the stress field of delaminated tip and crack tip opening displacements to compare their estimated capability of cracking energy. A popular technique for determining the interface adhesion of thin coatings is the four-point bending test (4-PBT). The testing vehicles of 4-PBT allow a more thorough validation of the foregoing predicted approaches based on finite element analysis compared with the simulated results with related experimental data. The critical energies of 18J/m2, 78.53J/m2, 52.01J/m2, and 71.02J/m2 for the bonded interfaces of SiO2/SiLK, Ta/SiLK, TaN/SiLK, and Si3N4/SiLK stacked nano-scaled thin films are precisely judged, respectively. The predicted results of the J-integral method match well with the MVCCT estimation. The results reveal that the opening fractured mode is dominant at the beginning of the crack advances along the bonded interface of Ta/SiLK, TaN/SiLK, and Si3N4/SiLK stacked films as the interfacial crack length increases. Moreover, the essential trends of dependences for the modulus ratio and thickness ratio of stacked thin films on interfacial energy release rate are logically identified and investigated. Therefore, this paper could be regarded as a design guideline of interfacial fracture behavior for stacked films utilized in the development of advanced device technologies.

Interfacial microstructure and fracture behavior of laser welded–brazed Mg alloys to Zn-coated steel

Laser lap welding–brazing of AZ31B Mg alloy to Zn-coated dual phase 980 steel using Mg-based filler was performed. The microstructures at the brazed side were characterized. Fracture behavior was then investigated by in situ scanning electron microscopy observation. The results indicated that heterogeneous reaction layers formed along the interface between the seam and the steel, which had a significant influence on the fracture behavior. Thick and diverse morphologies of reaction products were observed at the seam head and the seam root, where the crack initiated during in situ tensile test. The crack from the seam head then propagated along the Mg-Zn reaction layer/the original Fe-Al phase smoothly due to their weak bonding. When the crack propagated to the direct laser irradiation zone, the newly formed Fe-Al phase at the interface played a positive role in inhibiting the crack propagation, making the crack deflect into the seam zone. Owing to this, the joint strength was ultimately enhanced to 200 N/mm. While the crack from the seam root just propagated into the seam and contributed little to the failure of joint.

The cylindrical interface crack in a layered tubular composite of finite thickness under torsion

The present work investigates the problem of a cylindrical interface crack in a bi-layered tubular composite of finite thickness under torsion. Theoretical derivation is performed by the methods of Fourier integral transform and Cauchy singular integral equations. Numerical results of the stress intensity factor are discussed to reveal the coupled effects of the geometrical and physical parameters on the interfacial fracture behavior. The preferred values for the thickness ratio and the stiffness ratio are obtained, which provide necessary reference to the optimal design in engineering. To prevent interfacial fracture, it is better for the two layers to have identical thickness, and under this condition a stiffer inner layer plus a softer outer layer is a preferred choice. However, if the outer layer is enough thin, a softer inner layer plus a stiffer outer layer becomes optimal instead.

Effects of bondline thickness on Mode-I nonlinear interfacial fracture of laminated composites: An experimental study

Delamination or interfacial fracture is one of the most important issues for laminated composite structures. For the recent two decades, cohesive zone models (CZMs) have been receiving intensive attentions for modeling interfacial fracture of composite structures. Numerous global fracture tests have been conducted to measure the interfacial toughness of laminated composite plates. Some local tests have also been conducted to determine the interfacial traction–separation laws in adhesively bonded joints. However, most of these local studies focused on the interfacial fracture between metal adherends. Limited studies have addressed the local test of interfacial fracture between laminated composite plates. While it was well known that the bondline thickness has an important effect on interfacial fracture, very few studies investigated its effects on the local interfacial traction–separation laws of laminated composite plates in the literatures. In this work, both global and local tests are employed to investigate the effect of bondline thickness on the interfacial energy release rate, interfacial strength, and shape of the local interfacial traction–separation laws. Basically, the measured laws in the present work reflect the equivalent and lumped interfacial fracture behaviors which include the cohesive fracture, damage and plasticity. The experimentally determined traction–separation laws provide valuable baseline data for parameter calibrations in numerical models. The experimental results may also facilitate the understanding of bondline thickness dependent delamination of composite structures.

The interfacial strength and fracture characteristics of ethanol and polymer modified carbon nanotube fibers in their epoxy composites

Carbon nanotube (CNT) fibers spun from CNT arrays were used as the reinforcement for epoxy composites, and the interfacial shear strength (IFSS) and fracture behavior were investigated by a single fiber fragmentation test. The IFSS between the CNT fiber and matrix strongly depended on the types of liquid introduced within the fiber. The IFSS of ethanol infiltrated CNT fiber/epoxy varied from 8.32 to 26.64MPa among different spinning conditions. When long-molecule chain or cross-linked polymers were introduced, besides the increased fiber strength, the adhesion between the polymer modified fiber and the epoxy matrix was also significantly improved. Above all, the IFSS can be up to 120.32MPa for a polyimide modified CNT fiber, one order of magnitude higher than that of ethanol infiltrated CNT fiber composites, and higher than those of typical carbon fiber/epoxy composites (e.g. 60–90MPa). Moreover, the composite IFSS is proportional to the tensile strength and modulus of the CNT fiber, and decreases with increasing fiber diameter. The results demonstrate that the interfacial strength of the CNT fiber/epoxy can be significantly tuned by controlling the fiber structure and introducing polymer to optimize the tube–tube interactions within the fiber.

Influences of reflow time and strain rate on interfacial fracture behaviors of Sn-4Ag/Cu solder joints

In this study, the influences of reflow time and strain rate on interfacial fracture behaviors of the Sn-4Ag/Cu solder joints were investigated through in-situ observation. The interfacial microstructures of the solder joints reflowed for different times were first observed by scanning electronic microscope and measuring laser confocal microscope. Then tensile tests of the solder joints were conducted using in-situ tensile stage at different strain rates, and the interfacial fracture processes were in-situ observed. The observation results reveal that the thickness and surface roughness of the interfacial Cu6Sn5 layers increase linearly with increasing square root of the reflow time. Due to the serious strain concentration, fractures of the solder joints occur around the solder/Cu6Sn5 interface. The solder joints reflowed for a short time usually fracture inside the solder close to the joint interface, while the long-time reflowed solder joints are more apt to fracture in the interfacial Cu6Sn5 layer, because the stress applied on the Cu6Sn5 grains increases with increasing reflow time and grain size of Cu6Sn5. The solder joints reflowed for moderate time have the highest fracture resistance. At higher strain rate, the solder can apply a higher stress on Cu6Sn5 layer prior to its fracture, making the latter more apt to fracture.

Interface characteristics and fracture behavior of brazed polycrystalline CBN grains using Cu–Sn–Ti alloy

Polycrystalline CBN grains were brazed onto AISI 1045 steel matrix using Cu–Sn–Ti brazing alloy. Brazing temperature was 880°C, 900°C, 920°C respectively, and the holding time was 8min. Interface characteristics, elemental distribution and fracture behavior were analyzed in detail by means of scanning electron microscopy, energy dispersive spectroscopy and testing machine for grain compressive strength. The maximum value of the mean compressive strength of brazed PCBN grains reached 879.3MPa when the brazing temperature was 900°C. The resultants layer composed of TiN, TiB2, TiB and TiAl3 was formed at grain/alloy interface. Elemental distribution showed that Ti mainly concentrated within the interface zone. The intercrystalline fracture along the CBN/CBN particle boundary was the main fracture modes of brazed polycrystalline CBN grains.

Size and constraint effects on interfacial fracture behavior of microscale solder interconnects

The fracture behavior of solder joints has long been an important issue in the reliability evaluation of electronic components and packages. In this study, experimental and finite element methods were used to characterize the interfacial fracture behavior of “Cu-wire/solder/Cu-wire” sandwich-structured microscale Sn–3.0Ag–0.5Cu solder joints with different diameters (100–575μm) and thicknesses (35–325μm) under quasi-static micro-tension loading, with systematic comparison to Sn–37Pb solder joints. Dynamic stress intensity factors for the microcracks in Cu, Sn–3.0Ag–0.5Cu, and Sn–37Pb under pulse tensile loading and high speed propagation were calculated. The numerical simulation results showed that the interface stress intensity factors KII and KI for the crack set at the solder/IMC interface increased with increasing the thickness of the joints from 35μm to ∼125μm, and thereafter decreased and then became stable. With increasing the diameter of the joints, basically KI increases while KII decreases. Under the quasi-static micro-tension loading, the crack driving forces in Sn–3.0Ag–0.5Cu solder joints are lower than that in Sn–37Pb joints, and this means that fracture is less likely to occur in Sn–3.0Ag–0.5Cu solder joints than in Sn-joints. The rapid expansion of a high hydrostatic pressure region may enhance the mechanical performance of interconnects when the diameter-to-thickness ratios of the joints are very large. As the diameter of the solder joints increases, the evolution of the energy release distribution results in a change of the fracture position and mechanism in the interconnects. Compared to Sn–37Pb solder, the Sn–3.0Ag–0.5Cu lead-free solder shows a lower resistance to rapid crack propagation under constant tensile loading.

Effects of solder dimension on the interfacial shear strength and fracture behaviors of Cu/Sn–3Cu/Cu joints

Shear fracture behaviors of Cu/Sn–3Cu/Cu joints with different solder dimensions were investigated. The experimental results demonstrate that, with decreasing ratio R of the solder thickness to length, the interfacial shear strength increased and the fracture mode changed from ductile to a mixture of ductile and brittle. Based on the interfacial stress analysis across the solder joint, a hyperbolic formula expressing the relationship between the interfacial shear strength and the ratio R was proposed.