Interfacial Crack Propagation Research Articles

Effects of Welded Mechanical Heterogeneity on Interface Crack Propagation in Dissimilar Weld Joints

The stress and strain status associated with the material properties is one of the main factors affecting stress corrosion cracking (SCC) of structural components in nuclear power plants (NPPs). In many SCC prediction models, the stress intensity factor calculated with homogeneous materials is used to characterize the crack tip stress state. However, the mechanical and material properties in weld joints are heterogeneous, which will produce the discontinuous distribution of stress and strain nearby crack tip and affect the crack propagation. To understand the material mechanical heterogeneity effects on interface crack propagation, the specimens with ultimate tensile strength mismatch and elastic modulus mismatch were studied by using an extended finite element method (XFEM). The results indicate that the interface crack extension is easy to occur in the specimens with larger ultimate tensile strength mismatch, while the elastic modulus mismatch has little effects on crack extension. The different interface cracks in the dissimilar metal weld joints of the reactor pressure vessel used in NPPs tend to deviate from the initial direction into alloy 182, and the interface crack propagation path fluctuation is small.

Failure characterization of solid structures based on an equivalence of cohesive zone model

Cohesive zone models have been widely used to model interface crack initiation and propagation both in single-material media and bi-material systems. For single-material media with cohesive elements inserted into interface among segments, in order to ensure that the introduction of interface cohesive zone models does not affect the mechanical properties of single-material media before the softening stage of cohesive zone models, a selection criterion of stiffness of cohesive elements is proposed theoretically firstly based on the properties’ equivalence. Taking the softening stage into account, the mechanical responses of the overall stress-strain relationship of single-material media, for the cases of stable increase of strain and snap-back instability of strain, are both obtained, and the related energy mechanism are investigated. For bi-material systems with cohesive elements at interface between two materials, the thickness-dependent failure characteristics of systems in uniaxial tension are found, which is attributed to the difference of the releasing rate of elastic strain energy in the materials with different thicknesses. Furthermore, as a more complex application of cohesive elements, based on the selection criterion proposed, failure behaviors of the ceramic coating/substrate systems under three-point bending are modeled by finite element method and inserting cohesive elements into the coating segments and the coating/substrate interface simultaneously. The simulation results indicate the transition of dominated failure mode from coating cracking to interface delamination with increasing coating thickness, and show faster damage of thick coating systems, agreeing with experimental results. The effects of interface strength and toughness of cohesive elements on failure are also revealed. These results can provide guidance for the application of cohesive elements, and help us better understand the overall failure behaviors of interface systems.

Energy Partitioning During Subcritical Mode I Crack Propagation Through a Heterogeneous Interface

AbstractThe energy budget during the propagation of tensile fractures typically includes two major energy dissipation modes: the fracture energy to create a new interface and the radiated energy. Since seismic hazard is related to the amount of seismic energy released, it is important to evaluate precisely the energy budget during fracture propagation and see in particular if it is constant or not. However, two aspects are typically limiting a precise estimate of the energy budget: first, the measurement of the nonseismic energy release and second, the rock heterogeneity‐like asperities or barriers. We conducted here laboratory experiments, using an analog material (Polymethyl methacrylate (PMMA)), of a stable mode I interfacial crack propagation close to brittle‐creep transition through a heterogeneous interface for different macroscopic rupture velocities to evaluate carefully their energy budget. Both acoustic and optical advances of the crack front were measured simultaneously, providing precise estimates of both types of dissipated energy. We computed the radiation efficiency ηR (ratio of the radiated energy to the available energy for driving the fracture) and observed a nonlinear increase of ηR with the average fracture propagation velocity v over 2 orders of magnitude independently of the initial quenched disorder. The experimental observations are supported by a model based on the fluctuations of the local rupture velocity induced by the crack front pinning on local asperities which leads to ηR ∝ v0.55. We discuss implications for slow shear rupture modes, seismicity rate evolution, and induced seismicity.

Magnetomechanical model for coating/substrate interface and its application in interfacial crack propagation length characterization

During the service process of remanufactured components, the stress and crack may occur at the interface between coating and substrate due to their mechanical property mismatch. Based on the spontaneous magnetization phenomenon in ferromagnetic materials, the stress distribution and crack propagation length along the interface can be characterized. In this paper, the magnetomechanical constitutive model for interface is established according to Timoshenko beam theory and Jiles' stress-magnetization model. The distributions of stress and residual magnetization along the interface are analyzed under the effect of typical three-point bending (TPB) load. The results show that the interfacial residual magnetization Mr can reflect the stress distribution very well and its peak value at supporting seat Mr3 is closely related to interfacial crack propagation length. In order to verify the theoretical model, the metal magnetic memory (MMM) technique is used to collect the residual magnetic field along the interface in the TPB testing. The experimental results are consistent with the theoretical predictions. They indicate that the magnetomechanical model for coating/substrate interface established in this paper can provide the theoretical support for magnetic non-destructive testing and the calculation results can be applied to the interfacial crack propagation length characterization in MMM.

Composite Double-Network Hydrogels To Improve Adhesion on Biological Surfaces.

Despite the development of hydrogels with high mechanical properties, insufficient adhesion between these materials and biological surfaces significantly limits their use in the biomedical field. By controlling toughening processes, we designed a composite double-network hydrogel with ∼90% water content, which creates a dissipative interface and robustly adheres to soft tissues such as cartilage and meniscus. A double-network matrix composed of covalently cross-linked poly(ethylene glycol) dimethacrylate and ionically cross-linked alginate was reinforced with nanofibrillated cellulose. No tissue surface modification was needed to obtain high adhesion properties of the developed hydrogel. Instead, mechanistic principles were used to control interfacial crack propagation. Comparing to commercial tissue adhesives, the integration of the dissipative polymeric network on the soft tissue surfaces allowed a significant increase in the adhesion strength, such as ∼130 kPa for articular cartilage. Our findings highlight the significant role of controlling hydrogel structure and dissipation processes for toughening the interface. This research provides a promising path to the development of highly adhesive hydrogels for tissues repair.

Multifold interface and multilevel crack propagation mechanisms of graphene oxide/polyurethane/epoxy membranes interlaminar-toughened carbon fiber-reinforced polymer composites

Graphene oxide/polyurethane/epoxy (GO/PU/EP) membranes were directly fabricated by functionalization of graphene oxide with epoxy-grafted polyurethane (GO-UE), and the interface correlation and crack propagation mechanisms in GO/PU/EP membranes interlaminar-toughened carbon fiber-reinforced polymer composites were investigated. The functionalized GO-UE with corrugation and scrolling nature of graphene sheets was evenly dispersed in GO/PU/EP membranes below 0.50 wt% loading. Mode I fracture toughness, flexural properties and interlaminar shear strength of GO/PU/EP membranes-toughened composites were enhanced in comparison with untoughened composites and PU/EP membranes-toughened composites, which was ascribed to the multifold interface bonding between the GO-UE layers, epoxy matrix and carbon fiber. Schematic models of multilevel crack propagations were proposed based on different crack extension directions to GO-UE and the morphology evolutions of GO-UE in the interlaminar region and at the carbon fiber interface in toughened composites, which highlighted the toughening mechanisms of crack pinning, crack deflection and separation between GO-UE layers.

Effect of addition of fine sub-micron sized and thick glass fibers into epoxy matrix on fatigue life of plain-woven textile carbon fiber composite

This study investigated the mechanism of the effect of addition of fine sub-micron sized (close to nano-sized) and thick (11μm) glass fibers into epoxy matrix on the fatigue life of plain-woven textile carbon fiber composite. Static bending strength and fatigue life of the coupon shaped composite with the modified matrix were investigated by three-point static and cyclic loading test. The resistance of interlaminar delamination was characterized with double cantilever beam of Mode-I fracture test and ENF (end notch flexure) of Mode-II fracture test. The effect of the size of the added glass fibers on those mechanical properties was discussed. To explain the effect of the modification on the observed interfacial strength around the intertwining point of the reinforcing textile cloth, model specimens were prepared where an embedded carbon fiber bundle was put between other two carbon fiber bundles oriented to the perpendicular direction in the modified resin. Test results in current study showed that the addition of glass fibers with fine size was rather effective to dramatically improve the fatigue life. The fatigue life was dramatically increased 10-100 times by the addition of nano-sized glass fibers into the matrix in contrast to the static case. Fracture toughness in Mode-II was also improved by the modification of the matrix, while that in Mode-I was improved slightly. Macroscopic crack propagation in fatigue was totally prevented when the matrix was modified with fine glass fibers of 500nm in diameter. Those effects in improvement of mechanical properties of the textile carbon fiber composite were explained by the prevention of interfacial crack propagation with large resistance between carbon fiber bundles in Mode-II and modified matrix around the texture points where comparatively large strain energy would be released, if the epoxy matrix was adequately modified with the fine glass fibers.

An experimental study on stick-slip of gelatin hydrogels using fracture mechanics

Stick-slip (SS) is a mechanical instability which occurs on sliding interfaces in low velocity regimes in form of jerky motions. In this article, SS behaviour of gelatine hydrogels sliding on a smooth glass substrate is investigated in direct shear experiment using fracture mechanics. It is observed that average value of shear stress amplitudes and corresponding energy release rate (ERR) decrease with increase in pulling velocity upto the critical velocity. Time of relaxation is also found to be sliding rate dependent in the experiment. The video analysis of interfacial crack propagation reveals that crack length increases to a critical value and becomes unstable at the last phase of ‘stick’ period, but the same crack begins to restore to its original length just after ‘slip’ phase. The hysteresis present during the stick-slip motion diminishes at the critical velocity. Finally, this present study establishes that stick-slip is basically governed by the competition between rate dependent crack growth and crack healing along the sliding interface.

Study on Failure Length of Cementing Interface in Horizontal Wells During Fracturing

The cementing interface of oil and gas wells is often the weak link between oil and gas turbulence. Due to the low cementation strength at the fracturing interface, the two interfaces have been crushed to form turbulence channels before the target layer is opened during fracturing. If the closure is not good, there will be inter-layer channeling. Therefore, the pressure bearing capacity of the fracturing interface is an important indicator for designing the fracturing construction parameters. The pressure capacity of the two interfaces during the fracturing process is the key to evaluating the success of the fracturing construction. This paper establishes the calculation model for the stress distribution of horizontal wells in horizontal wells under the effect of non-uniform stress. At the same time, the influence of the pressure change in the wellbore during the fracturing process on the stress distribution in the borehole wall was analyzed. The calculation model of the interfacial stress distribution in the horizontal well during the fracturing process was established, and the debonding pressure and debonding length of the two interfaces under different cementing strengths were calculated. After the establishment of the horizontal well fracturing two interface crack propagation mechanics model, calculate the pressure required for cracks along the two interfaces to expand at different failure lengths.

Trapped fluid in contact interface

We study the mechanical contact between a deformable body with a wavy surface and a rigid flat taking into account pressurized fluid trapped in the interface. A finite element model is formulated for a general problem of trapped fluid for frictionless and frictional contact. Using this model we investigate the evolution of the real contact area, maximal frictional traction and global coefficient of friction under increasing external pressure. Elastic and elasto-plastic material models, compressible and incompressible fluid models and different geometrical characteristics of the wavy surface are used. We show that in case of incompressible fluid, due to its pressurization, the real contact area and the global coefficient of friction decrease monotonically with the increasing external pressure. Eventually, the contact opens and the fluid occupies the entire interface resulting in vanishing of static friction. An asymptotic analytical result for the critical trap-opening pressure is found and shown to be independent of the surface slope if it is small. In case of compressible fluids with pressure-dependent bulk modulus we demonstrate a non-monotonous behaviour of the global coefficient of friction due to a competition between non-linear evolution of the contact area and of the fluid pressure. However, for realistic compressibility of solids and fluids, contact-opening cannot be reached at reasonable pressures. On the other hand, in case of elastic-perfectly plastic materials, we again observe fluid permeation into the contact interface. Finally, we study the distribution of frictional tractions during the depletion of the contact area under increasing external pressure. This process leads to emergence of singularity-like peaks in tangential tractions (bounded by the Coulomb’s limit) near the contact edges. We point out the similarity between the processes of trap opening and interfacial crack propagation, and estimate the complex stress intensity factor in the framework of linear elastic fracture mechanics.

Rock-Concrete Interfacial Crack Propagation under Mixed Mode I-II Fracture

AbstractExperimental tests were conducted on composite rock-concrete specimens with four roughness profiles to investigate the propagation of interfacial cracks under three-point bending and four-p...

Stable discontinuous space–time analysis of dynamic interface crack growth in orthotropic bi-materials using oscillatory crack tip enrichment functions

ABSTRACT The problem of dynamic interface crack propagation in layered composites is addressed in this paper. Recently developed dynamic orthotropic bi-material enrichment functions are utilized to allow for accurate capturing of the oscillatory stress and displacement fields near the interface crack tip and proper evaluating of the crack tip velocity. The time discontinuous finite element and balance of recovery methods are employed during the crack propagation process in order to increase the stability and accuracy of the solution in dynamic crack propagation problems. Also, a suitable crack propagation criterion and an iterative algorithm are used to determine the interface crack tip velocity in each time step. Fracture mechanics parameters, such as dynamic stress intensity factor, phase angle and energy release rate are accurately predicted using the interaction integral method and the crack velocity dependent auxiliary fields. Numerical simulations of dynamic crack propagation in a homogeneous isotropic material, dynamic interface crack propagation in an isotropic bi-material, and dynamic interface crack propagation in an orthotropic bi-material are performed and the predictions are compared with the reference experimental and analytical results and the numerical data available in the literature.

Relevance of 3D simulations and sandwich core topology for the modeling of honeycomb core sandwich panels undergoing interfacial crack propagation

A recently developed cohesive zone traction-separation law, which includes the effects of fiber bridging in a novel way, is extended from 2D to 3D. The proposed cohesive model is applied to low fidelity (i.e. homogenized core representation) and high fidelity (i.e. directly accounting for the core topology) finite element models of a composite panel comprised of carbon fiber reinforced plastic facesheets and a honeycomb sandwich core. This enables the investigation of width-dependent effects such as 1.) the stress concentrations at the specimen edges, 2.) the stress concentrations due to the honeycomb core topology, 3.) the thumbnail-shaped crack growth, and 4.) the conditions under which the honeycomb core topology causes discontinuous crack propagation, which in turn begets oscillations of the global load-displacement curve. To this end, a cohesive parameter transfer procedure from 2D to 3D (low and high fidelity) is developed. Normal stresses of the 3D high and low fidelity predictions are qualitatively similar, whereas shear stresses in the sandwich core are significantly higher and very localized in the 3D high fidelity model. Crack propagation becomes noticeably unstable and global load-displacement curve oscillations occur for certain core topologies. The numerical predictions are compared to experimental load-displacement and R-curves.

Fracture criterion of mixed-mode crack propagation along the interface in nanoscale components

To investigate the fracture behavior of mixed-mode interfacial cracking in nanoscale components, three different types of nano-specimens, including the nano-cantilever specimen with a horizontal Cu/SiN interface, the nano-cantilever specimen with a vertical Cu/SiN interface, and the modified double-cantilever-beam (DCB) specimen, are adopted to conduct experiments of crack propagation from a pre-crack tip along the Cu/SiN interface, where the mixed-mode, mode-I dominated and qusai-mode-I interface cracking in nanoscale components are successfully achieved, respectively. Crack propagations from the pre-crack tip along Cu/SiN interface with different mode mixities are in situ TEM observed, and critical load/displacement for the starting of crack propagation are precisely measured. FEM analysis reveals that the interfacial crack propagation is dominated by a nanoscale singular stress field in all specimens, and the mixity of fracture modes varies from 0.27 to 1.28. Critical energy release rates for the starting of crack propagation along the Cu/SiN interface of all specimens are obtained with the compliance method, and the energy release rate linearly increases with the transition of fracture mode from the qusai-mode-I status to the mixed-mode one. Furthermore, the critical normal stress and shear stress show a circular relation, indicating the criterion for evaluating the mixed-mode crack propagation along interface in nanoscale components.

Thermal failure mechanism of multilayer brittle TiN/CrAlN films

The thermal failure mechanism of multilayer brittle TiN/CrAlN films at high annealing temperature has been investigated. Firstly, multilayer brittle TiN/CrAlN films (about 2.1 µm thick) were deposited on high speed steel by using metallic Ti and Al-Cr alloy (Al/Cr at% ratio=70:30) targets by RF-pulsed magnetron sputtering. Then, the multilayer TiN/CrAlN films were subjected to large heating-cooling shock during the high temperature process. The thermal expansion mismatch between the substrate-film, film-film and oxide-nitride interfaces causes large thermal stress. Finally, large thermal stress results in crack initiation, where the defects are the preferential sites for crack nucleation. During the heating-cooling process, the deformation results in stress concentration and strain mismatch, generates surface cracks and interface cracks due to the different mechanical properties between the substrate and multilayer films. The propagation of the interface cracks ultimately brings about the film delamination and loses thermal protection capability under high temperature service. Additionally, multiple surface cracks, to some extent, cause a reduction of the driving force for interface crack nucleation via relieving some tensile stress, thus inhibiting interface crack propagation and delamination failure.

A Robust in situ TEM Experiment for Characterizing the Fracture Toughness of the Interface in Nanoscale Multilayers

A novel in situ transmission electron microscopy (TEM) bending method using a nano-cantilever specimen that includes a naturally sharp pre-crack at the interface between a 500 nm-thick SiN layer and a 200 nm-thick Cu layer on a Si substrate is developed in order to precisely characterize the fracture toughness of the interface in nanoscale multilayers. By fabricating a perpendicular nanoscale notch in the SiN layer close to the horizontal Cu/SiN interface, a sharp pre-crack is successfully introduced at the Cu/SiN interface. In addition, by changing the relative position of the notch with respect to the fixed end of the specimen, both the instant and continuous interface crack propagation behaviors could be in situ observed using TEM. Finite element analysis shows that the crack propagation from the sharp pre-crack is dominated by a singular stress field within a region 100 nm from the crack tip under a mixed-mode state in all specimens. On the other hand, the fracture toughness represented by the critical energy release rate for the start of crack propagation along the Cu/SiN interface in all specimens is determined through a compliance method and shows good agreement with an average value of 7.1 J/m2. This indicates the robust reliability and high precision for characterizing the fracture toughness of the interface in nanoscale multilayers.

Numerical study on notched steel beams strengthened by CFRP plates

Carbon fibre reinforced polymer (CFRP) externally bonding is an efficient and effective method to strengthen damaged steel beams, and thereby prolong their service life. However, debonding failure, which requires accurate predictions to ensure safety, can occur before the full usage of CFRP. In this study, the notched steel beams strengthened with CFRP plate were simulated by finite element method where the mixed-mode cohesive law was employed to determine the interfacial stress. The load–deflection curves and strain development at different load levels from experimental study were used to verify the validity of the numerical model. The interfacial stress distribution with increasing load was analysed, and good correlation with theoretical calculations at elastic stage was observed. In contrast to the previous elastic analytical study, the plastic behaviour of the CFRP strengthened notched steel beams was revealed. More importantly, interfacial crack initiation, propagation and debonding were accurately simulated. This simulation method can be used to predict debonding process in actual engineering applications. In addition, parametric analysis was conducted to assess the effects of notch depth, CFRP elastic modulus and CFRP thickness. The ultimate load and ductility decreased substantially with increasing notch depth. Furthermore, although increased bearing capacity was achieved by increasing the CFRP elastic modulus and thickness, ductility decreased and premature debonding failure occurred more easily.

Experimental Study on Fatigue Behaviour of BFRP‐Concrete Bond Interfaces under Bending Load

Basalt fiber reinforced polymer (BFRP) composites are increasingly being used to retrofit concrete structures by external bonding. For such strengthened members, the BFRP‐concrete interface plays the crucial role of transferring stresses. This study aims to investigate the fatigue behaviour of the interface under bending load. A series of tests were conducted on BFRP‐concrete bonded joint, including static, fatigue, and postfatigue loading. The fatigue failure modes, the development of deflection, the evolution of BFRP strains, and the propagation of interfacial cracks were analysed. In addition, the debonding‐induced fatigue life of BFRP‐concrete bonded joints was studied. Finally, a new model of fatigue life was proposed by defining the effective fatigue bond stress. The results showed that the fatigue experience has a significant effect on the BFRP strength especially near the root of concrete transverse crack and on the bond performance of the adhesive near the interface crack tip. There are two main fatigue failure modes: BFRP rupture and BFRP debonding. The fatigue damage development of the bond interface has three stages: rapid, stable, and unstable growth. The proposed model for the debonding‐induced fatigue life is more conservative for the BFRP‐concrete bonded joints under pure shear load than for those under bending load.

Modelling of dynamic crack propagation on concrete matrix-aggregate interface

This paper is focused on the formulation of a numerical model for dynamic crack propagation on concrete aggregate interface. The concept of energy release rate is incorporated in the algorithm that is conducted in Abaqus through Python script interface. The proposed model is capable of manipulating free propagation of interface cracks. Thus, the on-surface growth of the interface crack, as well as the crack penetration into the concrete matrix, are successfully implemented. A case in point is the rupture behavior of concrete matrix containing one-single aggregate. Simulation of one matrix containing an isolated aggregate was conducted. Influence of the side-edge constraint, the aggregate direction as well as the fracture energy of the interface, was investigated. The results show that, tensile constraint on the side edge, a smaller angle between tensile axis and aggregate, and higher fracture energy could lead to a higher rupture strength of the interface. Once the interface starts to grow, it immediately and unstably propagates to the two ends of the aggregate major axis, and further enters the matrix. The three factor influences less on the character of above rupture path. Though the conclusion is prudently stipulated to concrete matrix with single aggregate, the numerical model can be also further modified to study the trans-scale propagations of multiple cracks in concrete materials or components.

An experimental and simulation study of interface crack on zinc coating/304 stainless steel

The three-point bending tests were carried out on zinc coating/304 stainless steel with different film thicknesses by means of a universal testing device equipped with an acoustic emission technique to obtain crack-initiation threshold and interface fracture toughness. Tensile test was used to obtain the relationship between crack spacing and the film thickness. Then, the critical fracture toughness and the interface shear stress were studied and utilized to evaluate the mechanical properties of interface layer. At last, a finite element method (FEM) was employed to verify the accuracy of acoustic emission method by the comparison of results between experiment and FEM analysis. Both interface crack propagation length and stress phase angle were analyzed. The results illustrate that the propagation length of interface crack increases with increasing film thickness and external load. However, it decreases with the increase of the interface fracture toughness and the elastic modulus of the film. The stress phase angle at the crack tip declines as the elastic modulus of the film increases. With uniform condition, the interface crack length and stress phase angle change little when the Poisson’s ratio increases.