Interfacial Fracture Mechanics Research Articles

Interfacial Stabilities, Electronic Properties and Interfacial Fracture Mechanism of 6H-SiC Reinforced Copper Matrix Studied by the First Principles Method

The interfacial mechanics and electrical properties of SiC reinforced copper matrix composites were studied via the first principles method. The work of adhesion (Wad) and the interfacial energies were calculated to evaluate the stabilities of the SiC/Cu interfacial models. The carbon terminated (CT)-SiC/Cu interfaces were predicted to be more stable than those of the silicon terminated (ST)-SiC/Cu from the results of the Wad and interfacial energies. The interfacial electron properties of SiC/Cu were studied via charge density distribution, charge density difference, electron localized functions and partial density of the state. Covalent C–Cu bonds were formed based on the results of electron properties, which further explained the fact that the interfaces of the CT-SiC/Cu are more stable than those of the ST-SiC/Cu. The interfacial mechanics of the SiC/Cu were investigated via the interfacial fracture toughness and ultimate tensile stress, and the results indicate that both CT- and ST-SiC/Cu interfaces are hard to fracture. The ultimate tensile stress of the CT-SiC/Cu is nearly 23 GPa, which is smaller than those of the ST-SiC/Cu of 25 GPa. The strains corresponding to their ultimate tensile stresses of the CT- and ST-SiC/Cu are about 0.28 and 0.26, respectively. The higher strains of CT-SiC/Cu indicate their stronger plastic properties on the interfaces of the composites.

Effects of temperature and water absorption on the interfacial mechanical properties of carbon/glass-reinforced thermoplastic epoxy hybrid composite rods

The effects of temperature and water absorption on the mechanical properties at the interface of a two-phase hybrid carbon/glass fiber reinforced thermoplastic epoxy composite rods were investigated. The push-out tests were performed on three types of these hybrid composite rods, with different carbon/glass ratios, at various temperatures and water immersion times to investigate their interfacial shear strength and fracture mechanism. Debonding occurred at the interface between glass fiber bundles and thermoplastic epoxy, followed by sinusoidal behavior during sliding, as revealed by the load–displacement curve. The interfacial debonding and sliding strengths decreased with increasing the temperature. The interfacial strengths also decreased with increasing water immersion time, but they were both restored after drying, for all the samples tested. In addition, the effects of temperature and water absorption on the thermoplastic epoxy matrix were also evaluated and the interfacial shear properties of hybrid composite rods were estimated.

The interfacial microstructure and fracture toughness of W/Ta multilayer composites

The application of tungsten (W) material is limited due to its intrinsic brittleness at low temperature. W-based multilayer composites are one of the solutions to improve the fracture toughness of W. In this study, a series of tungsten/tantalum (W/Ta) multilayer composites are prepared by diffusion bonding. The interfacial microstructure and fracture toughness under different deformation modes are investigated. It has been found that the flexural strength and fracture toughness loading perpendicular to the W/Ta interface are significantly higher than that loading parallel to the interface. The composites bonded at 1000 °C exhibits the highest flexural strength and fracture toughness. Both the flexural strength and fracture toughness decrease as bonding temperature increases. The decrease of flexural strength is due to recovery and recrystallization of the W layers. The decrease of fracture toughness is mainly attributed to the changes of the interfacial microstructure and fracture mechanism. For the composites bonded at 1000 °C, plastic deformation of the Ta layer and interfacial debonding are the main mechanisms for energy dissipation during crack propagation. As the bonding temperature increases, the decomposition of oxide scale and dynamic recrystallization occur at the vicinity of the W/Ta interface. A layer of fine recrystallization grains is developed at the W/Ta interface when the bonding temperature is higher than 1300 °C. As the interfacial bonding defects decreases, the interfacial bonding strength increases. Therefore, the interfacial debonding is inhibited in the composites bonded at a temperature higher than 1200 °C, instead, multiple tunnel micro-cracks appear in the W layers.

Interfacial mechanical properties of carbon/glass hybrid thermoplastic epoxy composite rods

Interfacial mechanical properties of carbon/glass hybrid thermoplastic epoxy composite rods play a crucial role in the determination of the composite strength and fracture behavior. In this study, push-out and push-back tests using three hybrid rods with different carbon/glass ratios were performed to investigate the interfacial shear strength and fracture mechanism. Debonding occurred at the interface between the glass fiber bundles and thermoplastic epoxy. After the onset of debonding, sinusoidal behavior during sliding was observed in the load displacement curve, which was identical with the interface roughness in terms of wavelength. Debonding and sliding strength were considered to be related to the interfacial bonding region between the carbon fiber bundle core and surrounding braided glass fiber bundles.

Nonlocal layerwise formulation for interfacial tractions in layered nanobeams

Interfacial tractions generated at the interface in two-layered nanobeams are studied through the stress-driven nonlocal theory of elasticity and an interface model. The model uses a layerwise description of the problem and satisfies the continuity conditions at the interface. The size-dependency are incorporated into formulation through a nonlocal constitutive law which defines the strain at each point as an integral convolution in terms of the stresses in all the points and a kernel. The Bernoulli-Euler beam theory is used separately for each layer to describe kinematic field, and to derive size-dependent system of coupled governing equations. The displacement components within the layers are derived and the interfacial tractions are obtained through the interfacial constitutive relations. Results are presented for the interfacial shear and normal tractions, exhibiting a different behavior at the nano-scale compared to those of the layered beams with large-scale dimensions including different maximum interfacial tractions and the location where maxima occur. A superior resistance of nanobeams against debondings and delaminations due to the interfacial normal tractions compared to that of the beams with large-scale dimensions is observed. The formulation and the understandings presented here are expected to stimulate further researches on multilayered nanobeams, including their interfacial fracture mechanics.

Investigation of failure mechanisms of nacre at macro and nano scales

Nacre is a natural composite featuring exceptional mechanical properties such as high strength and high toughness. Its unique structure is now universally applied in engineering bioinspired materials. On the other hand, it is still a technical challenge to investigate its interfacial strength and fracture mechanisms at micro or nano-scale. In this work, the interfacial strength and fracture mechanism of the ‘brick-mortar’ structure in nacre are investigated using micro-sized cantilever beam and bend samples. As compared to previous works, a high aragonite/biopolymer interfacial strength is observed (~298 MPa). The crack propagation path is investigated via experiment and finite element modelling and compared with the fracture mechanics analysis. It is confirmed that crack deflection to the aragonite/biopolymer interface contributes to a high overall toughness. This work provides a better understanding of the toughening mechanism in nacre and other bioinspired composites.

Influence of concrete strength on dynamic interfacial fracture behaviour between fibre reinforced polymer sheets and concrete

This study experimentally investigates the effect of concrete strength on the dynamic interfacial bond behaviour between basalt fibre reinforced polymer (BFRP) sheets and concrete under different loading speeds (i.e. 8.33E−6 m/s, 0.1 m/s, 1 m/s, 3 m/s, 5 m/s, and 8 m/s) by using single-lap shear tests. Three concrete strengths (i.e. C20, C30, and C40) were considered to examine the influence of concrete strength and strain rate on the interfacial bond-slip responses under dynamic loadings. The test results including the strain distributions, interfacial fracture energy, and bond-slip response were evaluated and discussed. The test results showed that the BFRP-concrete interface exhibited sensitivity to strain rate and the bond strength and shear resistance of the interface enhanced with strain rate. Compared with high strength concrete, low strength concrete showed higher strain rate sensitivity, which is induced by the different interfacial fracture mechanisms under different strain rates. An empirical bond-slip model incorporating strength effect of concrete and strain rate was proposed based on fracture mechanics.

Dynamic Observation of Interfacial IMC Evolution and Fracture Mechanism of Sn2.5Ag0.7Cu0.1RE/Cu Lead-Free Solder Joints during Isothermal Aging.

Dynamic observation of the microstructure evolution of Sn2.5Ag0.7Cu0.1RE/Cu solder joints and the relationship between the interfacial intermetallic compound (IMC) and the mechanical properties of the solder joints were investigated during isothermal aging. The results showed that the original single scallop-type Cu6Sn5 IMC gradually evolved into a planar double-layer IMC consisting of Cu6Sn5 and Cu3Sn IMCs with isothermal aging. In particular, the Cu3Sn IMC grew towards the Cu substrate and the solder seam sides; growth toward the Cu substrate side was dominant during the isothermal aging process. The growth of Cu3Sn IMC depended on the accumulated time at a certain temperature, where the growth rate of Cu3Sn was higher than that of Cu6Sn5. Additionally, the growth of the interfacial IMC was mainly controlled by bulk diffusion mechanism, where the activation energies of Cu6Sn5 and Cu3Sn were 74.7 and 86.6 kJ/mol, respectively. The growth rate of Cu3Sn was slightly faster than that of Cu6Sn5 during isothermal aging. With increasing isothermal aging time, the shear strength of the solder joints decreased and showed a linear relationship with the thickness of Cu3Sn. The fracture mechanism of the solder joints changed from ductile fracture to brittle fracture, and the fracture pathway transferred from the solder seam to the interfacial IMC layer.

Effects of electron-beam offsetting on microstructure and mechanical properties of joints between titanium alloy and stainless steel using Cu/Nb bilayer

Cu/Nb bilayer was utilized to join Ti-6Al-4V titanium alloy and AISI 316L stainless steel by electron-beam welding. Effects of electron-beam offsetting on microstructure and mechanical properties were studied. Microstructural characterization was carried out through optical microscopy, scanning electron microscopy with energy dispersive spectroscopy and x-ray diffraction. Results showed that dual-pass welding is better to achieve higher-quality joints than single-pass welding. When adopting dual-pass welding, the closer distance of first pass beam was to Nb layer side, the less contents of Fe and Cr were in the weld zone, and the phases changed from NbFe + NbFe2 + Cr1−xFex + NbCr2 + Cu based solid solution, NbFe + Cu3Ti + Cu based solid solution + Nb based solid solution to NbFe + Cu based solid solution + Nb based solid solution in weld zone. Owing to the formation of Cu-Fe and Nb-Ti solid solutions, the hardness of the weld zone was increased within 400 HV. Maximum average tensile strength of welded joints was 148.8 MPa, and fracture morphology changed from brittle mode to mixed fracture modes by adjusting first pass electron beam closer to the Nb side. Evolution of interfacial microstructure, formation and fracture mechanisms of the joints were summarized.

Effects of steel type and sandblasting pretreatment on the solid-liquid compound casting characteristics of zinc-coated steel/aluminum bimetals

Effects of zinc-coated steel type and steel surface sandblasting pretreatment in the solid-liquid compound casting of layered type steel/aluminum bimetals were investigated. The Zn coating behavior and its effects on interfacial microstructure evolution and fracture mechanism were also discussed. The aluminum fluidity in thin plate type flow channels formed by the steel insert and the mold wall primarily depended on the insert surface roughness and secondarily on the wettability. As-galvanized (GI) steel/aluminum bimetal joints showed the bonding strength of 20 MPa and more, while galvannealed (GA) steels showed poor bonding. The interfacial bonding zone consisted of most Al13Fe4, Al8Fe2Si, Al4.5FeSi intermetallic phases, as well as some Si phases. A low temperature and short time of the bonding reaction coupled with a high silicon content of the aluminum alloy suppressed the formation of Al5Fe2 phase. Oxide scales on the GA steel surface prevented the molten Zn coating from mixing with the aluminum melt. The Zn coating of GI steels was rapidly disappeared from the steel surface by the chemical affinity and surface energy-driven fluid flow as well as the diffusion, resulting in the formation of Zn-free intermetallic phases. The Zn coating of GI steels played a role in retarding the onset of bonding reaction. A long time sandblasting caused an excessive growth of intermetallic layers and the formation of Kirkendall voids on the steel side, resulting in the shift of main fracture sites and a slight decrease of the bonding strength.

Progressive damage through interface microcracking in cementitious composites: A micromechanics based approach

A multiscale model in the framework of micromechanics is developed to study the effect of microcracks present at the interface between coarse aggregates and mortar matrix in cementitious composites. The different stages of damage induced by the propagation of microcracks are analyzed at the mesoscale with the aid of solutions based on interface fracture mechanics. Using the solution of the interaction between an edge dislocation and a circular inclusion with an interface crack, the stress intensity factor of a kinked arc crack is derived numerically by the method of distributed dislocations. The macroscopic response under uniaxial tensile loading is seen to be affected considerably by the presence of microcracks. Various factors such as the inclusion size, inclusion volume fraction, initial size of microcrack and elastic properties of the individual phases are found to influence the overall constitutive behaviour of the composite. The findings of the study can be utilized to provide guidelines to tailor the material properties and enhance the performance of such composites.

Synergistic effects of fiber/matrix interface wear and fibers fracture on matrix multiple cracking in fiber-reinforced ceramic-matrix composites

ABSTRACTIn this paper, the synergistic effects of fiber/matrix interface wear and fibers fracture on matrix multiple cracking in fiber-reinforced ceramic-matrix composites (CMCs) are investigated using the critical matrix strain energy (CMSE) criterion. The shear-lag model combined with the fiber/matrix interface wear model, fibers fracture model and the fiber/matrix interface fracture mechanics debonding criterion is adopted to analyze the fiber and matrix axial stress distribution inside of the damaged composite. The relationships between the matrix multiple cracking, fatigue peak stress, applied cyclic number, fiber/matrix interface wear and debonding and fibers failure are established. The effects of fiber volume fraction, fiber/matrix interface shear stress, fiber/matrix interface debonded energy, cycle number, fatigue peak stress and fibers strength on matrix multiple cracking evolution are discussed. Comparisons of matrix multiple cracking with/without cyclic fatigue loading are analyzed. The experimental multiple matrix cracking of unidirectional SiC/CAS, SiC/CAS-II, SiC/Borosilicate and mini-SiC/SiC composites with/without cyclic fatigue loading are predicted.

Influence of ageing on glass and resin bonding of dental glass-ceramic veneer adhesion to zirconia: A fracture mechanics analysis and interpretation

Adhesion plays a major role in the bonding of dental materials. In this study the adhesion of two glass-ceramic systems (IPS e.max and VITABLOCS) to a zirconia sintered substrate using a glass (for IPS e.max) and resin (VITABLOCS) before and after exposure to ageing for 14 days in distilled water at 37 °C are compared using two interfacial fracture mechanics tests, the 3 point bend Schwickerath (Kosyfaki and Swain, 2014; Schneider and Swain, 2015) and 4 point bend (Charalambides et al., 1989) approaches. Both tests result in stable crack extension from which the strain energy release rate (G, N/m or J/m2) can be determined. In the case of the 3 PB test the Work of Fracture was also determined. In addition, the Schwickerath test enables determination of the critical stress for the onset of cracking to occur, which forms the basis of the ISO (ISO9693-2:2016) adhesion test for porcelain ceramic adhesion to zirconia. For the aged samples there was a significant reduction in the resin-bonded strengths (Schwickerath) and strain energy release rate (both 3 and 4 PB tests), which was not evident for the glass bonded specimens. Critical examination of the force-displacement curves showed that ageing of the resin resulted in a major change in the form of the curves, which may be interpreted in terms of a reduction in the critical stress to initiate cracking and also in the development of an R-curve. The extent of the reduction in strain energy release rate following ageing was greater for the Schwickerath test than the Charalambides test. The results are discussed in terms of; the basic mechanics of these two tests, the deterioration of the resin bonding following moisture exposure and the different dimensions of the specimens. These in-vitro results raise concerns regarding resin bonding to zirconia. Statement of SignificanceThe present study uses a novel approach to investigate the role of ageing or environmental degradation on the adhesive bonding of two dental ceramics to zirconia. This continues to be a major clinical problem but current approaches, till now, have relied upon a myriad of strength based tests to quantify the extent of environmental degradation with time. In this paper we use two fracture mechanics approaches, based upon simple 3 and 4 point bend testing procedures that enable stable debonding crack extension to occur. The paper provides a more critical approach to evaluate the role of environmental degradation of adhesion for dental materials.

Interfacial segregation and fracture in Mg-based binary alloys: Experimental and first-principles perspective

We investigated the effect of solute elements on interfacial segregation and fracture in Mg alloys by first-principles density functional theory calculations in conjunction with both experiment and interfacial fracture mechanics. Based on the assumption of brittle fracture in Mg alloys, the interfacial separation caused by segregated solutes in Mg can be efficiently described by the energy-based criterion of fracture, which is in good agreement with the fracture toughness obtained by experimental tests of Mg–M binary alloys. The electronic interaction—that is, the change in the electronic state between the interface and surface—mainly influences the ideal work of separation regardless of the type of interface. We found that IIIB (d1) and IVB (d2) solutes, such as Zr, show distinctive hybridization between the p band of Mg and the d band of the solute, which characterizes the strong fracture toughness of Zr-doped Mg alloys in both the calculations and experiments.

Fracture mechanics based assessment of manufacturing defects laying at the edge of CFRP-metal bondlines

This study presents a simplified approach for the evaluation of the bond line load capacity of CFRP-metal hybrid structural parts, bearing manufacturing defects. The proposed approach overcomes the need of modeling debonding with the use of advanced non-linear FEM, and relies on the post-processing of the stress and strain fields developed at the free edges of the hybrid material (critical locations in the presence of manufacturing defects), with the use of interfacial fracture mechanics. First, the mathematics, related to the analytical calculation of the bi-material fracture toughness, is presented and applied to a simple CFRP-metal geometry. Corresponding FE simulations are performed and the J-integral is employed for the numerical calculation of the SERR magnitude of different defect sizes. In the following, a simplified calculation of the SERR magnitude that sources from FE results and implemented in geometries modeled by shell finite elements that typically require advanced modeling techniques, is provided for the simulation of the de-bonding process.

Bending-driven failure mechanism and modelling of double-ceramic-layer thermal barrier coating system

Bending-driven failure test is an effective and efficient method to evaluate the quality and load-bearing capacity of a thermal barrier coating (TBC) system. In this study, the failure mechanism of a double-ceramic-layer TBC (DCL-TBC) system was investigated experimentally and analytically. First, a series of in-situ four-point bending tests were conducted on DCL-TBC system with the two ceramic layers having different thickness ratios. A cracking phenomenon was observed, i.e. vertical cracks (visible on the surface) and interfacial cracks (invisible unless sectioned) simultaneously evolved, from which a concept of critical vertical crack density associated with interfacial crack propagation was proposed to evaluate the bending-driven failure of DCL-TBC system. Second, an analytical model was developed to predict the critical crack density, which was based on a modified shear-lag model and interfacial fracture mechanics. The analytical prediction agreed well with the experimental results. Third, the analytical model was used to establish the failure maps of DCL-TBC system with respect to a series of physical and geometrical parameters in dimensionless form, considering both multiple surface cracking (i.e., vertical cracks) and interfacial cracking. The mechanism-based analytical approach, which has been validated by experimental results, may be used to provide a better evaluation of the damage of DCL-TBC system under bending.

Toughening of polyamide 12/nanoclay nanocomposites by incompatible styrene‐butadiene‐styrene rubber through tailoring interfacial adhesion and fracture mechanism

AbstractThe main drawback of polyamide 12 (PA 12)/nanoclay nanocomposites in their wide range of potential engineering applications known as diminished toughness was overcome using incompatible styrene‐butadiene‐styrene block copolymer (SBS) rubber. Applying optimal processing parameters for co‐rotating twin‐screw extrusion resulted in strong interfacial adhesion of matrix and rubber and intercalated and delaminated nanoclay stacks confined in the matrix. The formulation of compatibilized PA 12/SBS/nanoclay nanocomposites was optimized by employing response surface Box–Behnken methodology. A combination of microscopic methods, that is, atomic force microscopy, transmission and scanning electron microscopy, and transmission optical microscopy, along with single‐edge‐double‐notch four‐point‐bending technique, was exploited to exhaustively explore the toughening mechanisms and microstructure. The findings verify that by tailoring the rubber/matrix interfacial strength and nanocomposite morphology, the Izod impact strength of PA 12/nanoclay nanocomposites is noticeably improved even by incorporating an incompatible rubber and tough fracture mechanisms including rubber particle cavitation and large‐scale shear yielding are attained.

Analysis of semi-elliptical surface cracks in the interface of bimaterial plates under tension and bending

A semi-elliptical surface crack widely exists in the multilayered electronic devices, fibre reinforced laminated composites and solder joints. To ensure the safety of these structural components, three-dimensional interface fracture mechanics analysis is required. In this study, three-dimensional finite element analyses have been conducted to calculate the fracture mechanics parameters, including the stress intensity factors (SIFs), the strain energy release rate G and two phase angles ψ and φ for semi-elliptical interface surface cracks in finite thickness plates. A wide range of crack aspect ratios and relative depths are considered. The SIF computations are presented along the front of a three-dimensional bimaterial interface cracks with a/t values of 0.2, 0.4, 0.6 or 0.8 and a/c values of 0.2, 0.4, 0.6 or 1.0. In particular, far-field loads i.e., remote tension and bending, and various combinations of bimaterials are investigated. These fracture mechanics parameters’ solutions are suitable for fracture and fatigue life prediction for semi-elliptical bi-material interface surface cracks in engineering components.

Non‐oscillatory and non‐singular asymptotic solutions to stress fields at interface cracks

AbstractThis work concerns the complex oscillatory singularities revealed in Williams's asymptotic solutions to stress fields around arbitrary interface cracks, which are the foundation of phenomenological interface fracture mechanics. First, we highlight the fatal discrepancy between the asymptotic stress fields for cracks in a homogeneous material obtained by assigning an identical material on both regions embracing an interface crack, and the solutions directly derived from cracks in a single material. Next, following a brief introduction to Williams's formulation process, we adopt the method of repeatedly eliminating variables instead of solving the determinant equation for the coefficient matrix to reformulate the asymptotic analysis of stress fields at arbitrary interface cracks. The resultant stresses get rid of oscillatory character. Further, under two specific loading conditions, namely, remotely uniaxial tension or shear, non‐oscillatory and non‐singular asymptotic solutions to stress fields around interface cracks are obtained.

An Accurate Thermomechanical Model for Laser-Driven Microtransfer Printing

A recently developed transfer printing technique, laser-driven noncontact microtransfer printing, which involves laser-induced heating to initiate the separation at the interface between the elastomeric stamp (e.g., polydimethylsiloxane (PDMS)) and hard micro/nanomaterials (e.g., Si chip), is valuable to develop advanced engineering systems such as stretchable and curvilinear electronics. The previous thermomechanical model has identified the delamination mechanism successfully. However, that model is not valid for small-size Si chip because the size effect is ignored for simplification in the derivation of the crack tip energy release rate. This paper establishes an accurate interfacial fracture mechanics model accounting for the size effect of the Si chip. The analytical predictions agree well with finite element analysis. This accurate model may serve as the theoretical basis for system optimization, especially for determining the optimal condition in the laser-driven noncontact microtransfer printing.