Hybrid spectral/finite element analysis of dynamic delamination of patterned thin films

A combined spectral and finite element analysis is performed to investigate the dynamic edge delamination of patterned thin films from a substrate. The analysis is motivated by an emerging experimental technique in which high-amplitude laser-induced stress waves initiate progressive interfacial debonding of thin film interfaces. The numerical method relies on the spectral representation of the elastodynamic solutions for the substrate and the finite element model for the thin film. A cohesive model is introduced along the interface of the bimaterial system to capture the decohesion process. The important role of the film inertia on the crack extension and the appearance of the mixed-mode failure are demonstrated by observing the traction stress evolution at various points along the bond line. Parametric studies on the effect of film thickness, interface fracture toughness, loading pulse shape and amplitude on the debonding process are performed. A semi-analytical investigation on the inertial effect is carried out to predict the final crack length as a function of the film thickness and pulse amplitude.     

Application of the Blister Test in Study of Epoxy Adhesive

Shaft-loaded blister test technique is used as an effective quantitative tool to measure adhesion strength. Investigation on conductive adhesive was done by modified blister test. It is found that shaftloaded blister test can be a good solution for the debonding of thin film adhesion. The intrinsic stable interface debonding process has been proved an attractive alternative to the conventional adhesion measurement techniques. In our study, epoxy matrix adhesive was studied using blister test technique in comparison with the traditional test-lap shear test. Adhesion strength was studied as a function of surface treatment and the metallization of substrate. It was found that surface conditions of substrate have significant impact on adhesion behaviour. The oxidation of surface is responsible for the poor adhesion. Activating chemical treatment and Plasma cleaning on substrate surface has been found to be a way of dreamatically improving adhesion strength of electronic conductive adhesive.     

Effect of interfacial adhesion and statistical fiber strength on tensile strength of unidirectional glass fiber/epoxy composites. Part I: experiment results

The effects of fiber surface treatment on ultimate tensile strength (UTS) of unidirectional (UD) epoxy resin matrix composites are examined experimentally. The interfacial shear strength (IFSS) and statistical fiber strength are significantly altered by five different kinds of surface treatments, which are: (a) unsized and untreated; (b) γ-glycidoxypropyltrimethoxysilane (γ-GPS); (c) γ-methacryloxypropyltrimethoxysilane (γ-MPS); (d) mixture of γ-aminoxypropyltrimethoxysilane (γ-APS), film former (urethane) and lubricant (paraffin); and (e) urethane-sized. The maximum UTS is obtained for the relatively strong interfacial adhesion (glass/γ-MPS/epoxy) but not for the strongest interfacial adhesion (glass/γ-GPS/epoxy). The governing micro-damage mode around a broken fiber and the interface region is matrix cracking for γ-GPS treated fibers, and a combination of interfacial debonding and matrix cracking for γ-MPS treated fibers. The micro-damage mode related to the interfacial adhesion strongly affects the fracture process, and thus the UTS of UD composites. The results also indicate that the interfacial adhesion can be optimized for effective utilization of fiber strength for fiber composites. A parameter called “efficiency ratio” of fiber strength in UD composites is proposed to examine and distinguish different effects of IFSS and fiber strength on the UTS of UD composites. The experimental results show that improved UTS of UD composites due to surface treatments mainly result from the increase in fiber strength but not from the modified interface.     

Investigation of combined stress state failure criterion for glass fiber/epoxy interface by the cruciform specimen method

The interfacial failure criterion under combined stress state in a glass fiber/epoxy composite is investigated by the cruciform specimen method. Experiments were conducted by using specimens with a fiber whose angle from the loading direction is varied in order to make various stress state of normal and shear at the interface. Finite element analysis is performed to calculate the interfacial stress distribution. By combining the experimental measurement of the specimen stress at the interfacial debonding initiation and the finite element stress analysis, it is possible to obtain the interfacial stress state at interfacial failure. A method to determine the interfacial failure criterion and the interfacial failure initiation location simultaneously is proposed in the present study. We conclude the value of the interfacial shear strength is higher than that of the interfacial normal strength for the material system used in the present study.     

Fracture mechanics of a shaft-loaded blister of thin flexible membrane on rigid substrate

A shaft-loaded blister test has been developed to measure the interfacial energy W of a thin flexible polymeric film adhered to a rigid substrate. A theoretical analysis is given of an axisymmetric debond (blister) in terms of an external applied load P, tensile stretching modulus E and thickness h of the adhering layer. The fracture mechanics model presented considers both elastic and elastoplastic deformations in the thin film. The intrinsic stable interface debonding process provides an attractive alternative to the conventional adhesion measurement techniques.     

The deformation response of sol-gel-derived zirconia thin films on 316L stainless steel substrates using a substrate straining test

Thin (78 ± 4 nm), well-bonded zirconia films were formed on 316L stainless steel substrates by dip-coating in an alkoxide precursor solution followed by annealing in air to achieve film densification. X-ray diffraction showed the film to be either metastable cubic or tetragonal zirconia. A substrate-straining test was used to investigate the mechanical characteristics of the film and interface; this protocol has been used previously to estimate interfacial shear strength through a shear-lag model. At strain levels of about 1.5%, 15 times the yield strain of the substrate, through-thickness cracking of the film was observed. These cracks were driven by deformation localized at slip bands on the substrate surface and the cracking pattern reflected the slip band pattern of the underlying substrate; the propagation of long cracks transversely to the applied stress, as observed in similar experiments previously, was not seen and consequently the shear-lag model was not applicable. As a qualitative indication of good adhesion, film debonding was not observed even at high strain levels. A non-quantitative model was proposed which examined stress transfer across the film-substrate interface on a microscopic scale, and suggested how film, substrate and interface properties affect competition between transverse and slip-band-induced modes of film cracking. This model was then used to reconcile the observations of this study with the transverse cracking observed by others.     

Critical discussion of the single-fibre pull-out test: does it measure adhesion?

With a comprehensive finite-element model the interface failure process of the single-fibre pull-out test, for the measurement of fibre/matrix adhesion, is investigated on the basis of a fracture-mechanics debonding criterion. Special emphasis is placed on the interface local mixed-mode load, which is shown to have an important influence on the debonding process and is taken into account by a fracture ellipsoid criterion. Additional features investigated are residual thermal stresses, specimen geometrical details (wetting meniscus, drop shape) and a simplistic model of fibre/matrix interfacial friction. For medium debonding lengths the energy release rate runs through a plateau range that can be approximated by a simple analytical approach and can be observed experimentally with a very stiff loading configuration. The mixed-mode state in the plateau range is uniform and dominated by mode 2, but its actual value is quite uncertain. From experimental experience the actual adhesion failure is closely connected with the interface local normal load, while local shear load induces submicroscopic friction and matrix inelasticity which strongly reduce the interface sensitivity, resulting in G_1c<G_2c. G_1c seems to be more significant for adhesion. The interpretation of the plateau range may provide the total critical energy release rate, G_c, for the debonding process, but from a region where mode II prevails. G_c will therefore be far from G_1c, reducing the significance of the tests results for characterization of adhesion.     

Evaluation of film adhesion to substrates by means of surface acoustic wave dispersion

For thin film structures acoustically classified as slow-on-fast systems, modeling and evaluation of their interfacial condition are known to be very complex and difficult due to dispersion and multi-mode excitation of acoustic waves. This paper presents a quantitative model and a reliable measurement procedure established for adhesion evaluation of such film structures. An effective interface model employing a virtual intermediate layer is utilized for the dispersion prediction of the surface acoustic wave, which is affected by various interfacial conditions. Through acoustic microscopy experiments, this model presents a potential method to classify the bonding condition. Comparisons with a destructive scratch test and an acoustic imaging verify the failure mode of the film structure.     

A hybrid experimental/numerical approach to characterize interfacial adhesion in multilayer low-κ thin film specimens

The strength of multilayer low dielectric (low-κ) constant organo-silicate glass (OSG) and silicon-carbon-nitride (SiCN) thin film interfaces is characterized by a laser spallation technique. Two specimen sets with different OSG/SiCN stacking sequences are evaluated. The effect of helium (He) pretreatment is also investigated. The stress at the low-κ interface is enhanced through the use of a fused silica backing layer to shape the incident pulse and the addition of a thin gold (Au) top layer to increase inertial force during the dynamic failure event. The weakest interface in the multilayer stack (first to fail) is identified through optical images, profilometry and scanning electron microscope images of the spallation zone. The strength of the failed interface is inferred from the incident stress history in combination with a one dimensional dynamic wave propagation analysis. The adhesion strength of the OSG/SiCN interface (372 MPa) is 32% larger than that of Si/OSG (282 MPa) for specimens with no He pretreatement. The interfacial strength of both interfaces is significantly increased by a He pretreatment, making failure by spallation difficult.     

Surface effects on delamination of a thin film bonded to an elastic substrate

The delamination of a circular thin film at micro/nanoscale is studied using the Kirchhoff plate theory incorporating surface effects in this paper. Bending of a clamped circular nanoplate subjected to a concentrated force at the center or a uniformly distributed force over a lateral surface is solved. The bending deflection is derived in closed form. The adhesion energy and its release rate for delamination are determined when surface effects are taken into account. The influences of surface residual stress and surface elasticity along with the film’s size on the energy release rate of debonding advance or interfacial adhesion of a thin film bonded to an elastic substrate are analyzed for applied loading or given displacements. Analytic results are compared with experimental data and satisfactory agreement is confirmed.     

Debonding of a thin rubberised and fibre-reinforced cement-based repairs: Analytical and experimental study

An analytical approach for the prediction of debonding initiation between a rubberised cement-based overlay and old concrete substrate under monotonous mechanical loading was applied. Based on the linear elastic fracture mechanics, a model has been developed taking into account the interlocking between two crack surfaces in the overlay. Assuming that the debonding initiation just occurs after the crack cutting the overlay layer reaches the overlay–substrate interface, the stress intensity factor of the debonding tip can be calculated, allowing prediction of stress fields near the interface debonding tip. Then with a criterion of debonding initiation and propagation depending on the interface tensile strength, the load associated could be determined and might be interesting for the design of thin bonded cement-based overlays. The adequateness of this analytical approach was verified by both experimental data and finite element calculations. It has been used to show the relevance of a cement-based material with low modulus of elasticity combined with a high residual post crack strength to achieve sustainable repairs.     

Mechanical Characterisation of Interface for Steel/Polymer Composite Using Pull-out Test： Shear-Lag and Frictional Analysis

Fibre-matrix interface is known to have contribution to the mechanical performance of fibre-reinforced composite by its potential for load transfer between the fibre and the matrix. Such load transfer is of great importance in dentistry when a post is used for fixing a ceramic crown on the tooth. In this study, a pull-out test was carried out to analyse the interfacial properties of a steel fibre embedded in a polyester and epoxy matrices. It was found that the fibre-matrix interface is debonded on the whole embedded length when the fibre stress reached the debonding stress. Then, the fibre stress fell down to the initial extraction stress required to pulling out the debonded fibre from the matrix. Both debonding stress and initial extraction stress initiated a linear increase with the implantation length after the debonding stress reached horizontal asymptotes. To analyse the fibre-matrix load transfer before debonding, an analytical shear-lag model was adopted to in this test conditions. Fitting the experimental results with the analytical model provided the interfacial shear strength. By considering the Coulomb friction at the fibre-matrix interface during the fibre extraction process, an analytical model which considers Poisson＇s effects on both fibre and matrix, was developed. In this model, knowledge of the initial extraction stress of the fibre provides the residual normal stress at the fibre-matrix interface.     

Interfacial adhesion of polymeric adhesive film on different surfaces in the fabrication of polymer photonic devices

One main critical issue in the fabrication of polymer optical devices is the adhesion strength of polymeric layer to the substrate. High adhesion strength is desirable and critical in order to avoid peeling out of polymeric layer from the substrate due to stress generated during fabrication, handling and lifetime. Therefore, the aim of this study is to investigate the interfacial adhesion of polymeric adhesive film on different possible substrate surfaces such as pure silicon wafer, silica on silicon wafer, and thin metal layer (Chromium–Cr) on silicon wafer under different processing conditions. Surface morphology of the substrates before deposition was characterized by atomic force microscope (AFM). Adhesive shear button was made on those substrates by using photolithography process and the interfacial adhesion was measured by using a Dage D2400 shear tester. The effect of exposing in high temperature and typical damp heat condition on the interfacial adhesion was also studied. We found that the best adhesion performance was obtained for the case using Cr thin in all processing conditions, especially under heat treatment and damp heat test. From this study, we suggest that a thin layer of metal film on silicon wafer can be use to improve the adhesion and the reliability of the polymer photonic devices. The oxidized silica on silicon wafer is an alternative choice at the expense of reducing adhesion performance. Moreover, using silica layer has the advantage over Cr layer that one fabrication step can be reduced since the silica layer itself can effectively act as the lower cladding of the devices.     

Determination of adhesion strength between a coated particle and polymer matrix

The aim of the paper is to provide the theoretical basis for a test to determine the interfacial adhesion strength between a coated particle and a polymer matrix material. The specimen has a notched (neck-like) geometry and contains a single coated particle in the centre. A non-linear relation between the true stress and logarithmic strain is considered for the interphase. Tensile axial loading of such a necked sample concentrates stresses in the smallest cross-section and causes a multiaxial loading situation in the centre of the specimen, i.e., in the vicinity of the enclosed particle. By variation of sample curvature the distribution of normal tensile stresses at the interface between particle and coating can be changed. This enables the variation of the interface area which is under tensile stress. A finite-element analysis provides the stress field within the whole specimen and especially in the vicinity of the coated particle. The motivation for the calculations is to determine the maximum radial stress at the particle surface as a function of applied load. Assuming that normal stresses at the interface are responsible for debonding, the adhesion strength can be obtained from the experimentally determined critical load at debonding initiation.     

Fracturing behaviors of FRP-strengthened concrete structures

In this paper, we focus on the study of concrete cracking behavior and interfacial debonding fracture in fiber reinforced polymer (FRP)-strengthened concrete beams. An experimental program is systematically reviewed according to the observed failure modes, in which it is found that the interfacial debonding may propagate either within the adhesive layer or through concrete layer in the vicinity of bond interface. A finite element analysis is performed to investigate the different types of debonding propagation along FRP-concrete interface and crack distribution in concrete. For the numerical fracture models, interfacial debonding that initiates and propagates in adhesive layer is modeled by fictitious interfacial crack model. And concrete cracking, including the debonding fracture through interfacial concrete, is modeled by smeared crack model. Properties of the interfacial adhesive layer and concrete are considered to significantly influence the debonding propagation types and crack distribution. The interactions between interfacial bond strength, interfacial fracture energy of bond adhesive layer and tensile strength, fracture energy of concrete are discussed in detail through a parametric study. According to the results, the effects of these properties on different types of interfacial debonding, concrete cracking behavior and structural load-carrying capacity are clearly understood.     

Adhesion of polymer thin-films and patterned lines

The adhesion of interfaces in thin-film structures containing ductile polymer blanket films and patterned lines is reported. The intent of the study was to demonstrate that both the film thickness and the aspect ratio of patterned lines have a significant effect on the interfacial fracture energy of interfaces adjacent to the ductile polymer. In particular, there is currently limited understanding of the effect of dimensional constraint in the plane of the film on local plasticity and associated interfacial fracture energies. Accordingly, the interfacial adhesion of patterned structures containing arrays of polymer/SiO₂ lines with varying aspect ratios was investigated. Macroscopic adhesion values were determined by measuring the critical strain energy release rate, G _c, for debonding of the patterned interface. The yield properties of the polymer films as a function of film thickness was also investigated. Decreasing aspect ratio of the polymer lines was found to significantly increase interface fracture energy and is rationalized in terms of the effect of stress state on the extent of plastic deformation in the polymer line.     

Evaluation of the interfacial strength of carbon-fiber-reinforced temperature-resistant polymer composites by the micro-droplet test

This paper presents an evaluation method for the fiber/matrix interfacial strength. The interfacial strength is determined by comparing experimental data with numerical simulations. The micro-droplet test is conducted, and the fiber axial stress at the point of interface debonding is obtained. A numerical simulation is performed with ABAQUS using an axisymmetric finite-element model. In the numerical simulation, an accurate value of the thermal residual stress based on the thermo-viscoelasticity and the damage to the resin around the blade-contacting point is considered to simulate the experimental phenomena ideally. In the thermal residual stress analysis, the actual thermal residual stress is calculated by considering the relaxation modulus and the time–temperature superposition principle for the resin. Damage initiation criteria for both dilatational and shear cases, based on continuum damage mechanics, are considered for the resin. Interfacial debonding is simulated using a cohesive zone model, and the interfacial strength is taken as the strength of the cohesive zone element at the simulated fiber maximum stress corresponding to the experimental value.     

Stress distribution in a segmented coating film on metal fibre under tensile loading

When a coating film on a metal fibre or wire is brittle, it exhibits multiple-fracture under loading. In order to describe the exerted tensile stress on the segments of a coating film as a function of the distance from the end of the segments and as a function of applied stress, a new approximate calculation method is presented, assuming that the interfacial bonding strength is high enough and no interfacial debonding occurs. Using the present calculation method, effects of geometrical factors such as fibre diameter, thickness of coating film and length of segment as well as those of mechanical factors such as Young's modulus, shear modulus and the yield stress of the fibre and the coating film on the exerted tensile stress on the segments and also on the exerted shear stress at the interface are described in a quantitative manner.     

Delamination of thin bonded cement-based overlays: analytical analysis

This paper deals with an analytical approach for the prediction of debonding initiation between cement-based overlay and old concrete substrate under monotonous mechanical loading. Based on the linear elastic fracture mechanics, an available analytical model has been used. The calculations take into account the interlocking between two crack surfaces in the overlay. To validate the model, three point static bending tests on composite specimens were carried out. Assuming that the debonding initiation just occurs after the crack cutting the overlay layer reaches the overlay–substrate interface, the stress intensity factor of the debonding tip can be calculated, allowing prediction of stress fields near the interface debonding tip. Then with a criterion of debonding initiation and propagation depending on the interface tensile strength, the load associated could be determined and might be interesting for the design of thin bonded cement-based overlays. The adequateness of this analytical approach was verified by both experimental data and finite element calculations.     

A modeling study on residual stress-induced interfacial debonding and stress–strain behavior of weakly bonded UD composites

Residual stress-induced interfacial debonding and its influence on stress–strain behavior of unidirectional fiber-reinforced brittle matrix composites with weak interface were studied using mini-composite model by means of the two-dimensional shear lag analysis combined with a Monte Carlo method. Damages (fracture of fiber, matrix and interface) were accumulated intermittently, resulting in serrated stress–strain curve. In this process, the residual stresses changed the strain, order and location of occurrence of damages, and consequently the shape of stress–strain curve and strength of composite. Under the existence of compressive and tensile axial residual stresses in fiber and matrix, respectively, the fracture of the matrix and the debonding from the fracture-ends of matrix were enhanced, while the fracture of fiber and the debonding from the fracture-ends of fiber were suppressed. The residual stress-induced premature fracture of the matrix, followed by debonding, reduced the strength of composite.