A shaft-loaded blister test for elastic response and delamination behavior of thin film-substrate system

The elastic response of a thin film of photoresist deposited on a silicon wafer is studied by using a shaft-loaded blister test method developed recently. Experiment data are compared with an analytical solution. Results demonstrated that under shaft loading, the thin film underwent a pure bending mode at small deformation and gradually transformed to a pure stretching mode at larger deformation. The effect of residual stress on elastic response is also studied. The delamination of thin film from substrate can be successfully measured under displacement control mode by the shaft-loaded blister test.     

鼓膜实验法测试薄膜基体界面结合强度的进展

评述了用鼓膜实验方法测试薄膜与基体界面结合强度的现状和最新进展，分析了该方法针对圆孔、膜内无残余应力柔性膜试样和矩形孔或圆孔、膜内有残余应力试样（薄膜为柔性膜或刚性膜）所用解析解力学模型建立的薄膜力学性能假设前提条件、能量平衡思路和推导过程．介绍了用硅微技术制备Si基体试样的过程，指出了制备方法和解析解力学模型的特点及所存在的问题．给出了该实验方法的应用实例，指出了需要解决非硅基体的制样和薄膜塑性变形阶段力学模型等问题．     

Analysis of mode mixity in blister tests

The transition from plate to membrane behaviour in the pressurised blister test is analysed by a coupled membrane/boundary layer analysis. The analysis is verified by comparing with numerical results. Recent progress in analysis of interface fracture mechanics is emphasized by analysing the near tip phase angles of loading. This revised version was published online in July 2006 with corrections to the Cover Date.     

Cohesive-zone modelling of the deformation and fracture of spot-welded joints

The deformation and failure of spot‐welded joints have been successfully modelled using a cohesive‐zone model for fracture. This has been accomplished by implementing a user‐defined, three‐dimensional, cohesive‐zone element within a commercial finite‐element package. The model requires two material parameters for each mode of deformation. Results show that the material parameters from this type of approach are transferable for identical spot welds in different geometries where a single parameter (such as maximum stress) is not. The approach has been demonstrated using a model system consisting of spot‐welded joints made from 5754 aluminium sheets. The techniques for determining the cohesive fracture parameters for both nugget fracture and nugget pullout are described in this paper. It has been demonstrated that once the appropriate cohesive parameters for a weld are determined, quantitative predictions can be developed for the strengths, deformations and failure mechanisms of different geometries with nominally identical welds.     

Magnetically actuated peel test for thin films

Delamination along thin film interfaces is a prevalent failure mechanism in microelectronic, photonic, microelectromechanical systems, and other engineering applications. Current interfacial fracture test techniques specific to thin films are limited by either sophisticated mechanical fixturing, physical contact near the crack tip, or complicated stress fields. Moreover, these techniques are generally not suitable for investigating fatigue crack propagation under cyclical loading. Thus, a fixtureless and noncontact experimental test technique with potential for fatigue loading is proposed and implemented to study interfacial fracture toughness for thin film systems. The proposed test incorporates permanent magnets surface mounted onto micro-fabricated released thin film structures. An applied external magnetic field induces noncontact loading to initiate delamination along the interface between the thin film and underlying substrate. Characterization of the critical peel force and peel angle is accomplished through in situ deflection measurements, from which the fracture toughness can be inferred. The test method was used to obtain interfacial fracture strength of 0.8-1.9 J/m² for 1.5-1.7 μm electroplated copper on natively oxidized silicon substrates.     

Models of Interface Separation Accompanied by Plastic Dissipation at Multiple Scales

Two continuum mechanical models of interface fracture for interfaces joining materials where at least one undergoes plastic deformation are reviewed and examined critically. The embedded process zone model (EPZ model) has an adhesive zone, characterized by a work of separation and an interface strength, embedded within a continuum model of the adjoining materials. The SSV model imposes an elastic, plasticity-free layer of prescribed thickness between the interface and the surrounding elastic-plastic continuum. Crack advance requires the work of separation to be supplied by the local elastic crack tip field. The objective of each model is to provide the relation between the macroscopic interface toughness (the total work of fracture) and the work of separation. Under steady-state crack growth, the total work of fracture is the work of separation plus the work of plastic dissipation, the latter often far exceeding the former. It will be argued that each model has its own domain of validity, subject to the accuracy of conventional continuum plasticity at small scales, but neither is able to capture the dramatic trends which have been observed in macroscopic toughness measurements stemming from alterations in interface bonding conditions. A unified model is proposed which coincides with the two models in their respective domains of validity and provides a transition between them. Interface separation energy and interface strength (the peak separation stress) each play a central role in the unified model. Strain gradient plasticity is used to illustrate the effect of plastic deformation at the micron scale on the link between interface and macroscopic properties. This revised version was published online in July 2006 with corrections to the Cover Date.     

Nonlinear elastic mechanics of the ball-loaded blister test

The nonlinear elastic mechanics of spherically capped shaft or ball-loaded blister tests is presented. In the test model, a thin film is attached to a substrate with a circular hole running through the thickness of the substrate. A central load is applied to the film through the hole by a spherically capped shaft or a ball with a finite radius. The deformed blister is divided into two parts: a circular region in contact with the sphere of the cap or ball and an outer noncontact annulus. The Reissner’s plate theory is employed to describe the deformation of the contact part and the von Kármán plate theory for the noncontact annulus. A constitutive equation of coupled linear springs is obtained to quantify the effect of the substrate deformation on the blister deflection. For small deflection, the analytical solution of load-deflection is derived. For large deflection, an iteration approach is adopted to predict numerically the load-deflection curve. Finite-element analysis is conducted to verify the analytical and numerical solutions. The influence of the substrate deformation, residual stress, radius of the spherical cap or ball and the friction between the film and ball on the load-deflection relation is investigated.     

Modelling scale failure in tension (fracture and spallation)

Abstract

The paper addresses some important issues that relate to the prediction of through-thickness cracking and spallation that can occur in oxide layers subject to local tensile stresses that arise during cooling following periods in service where the oxide layers form and thicken. The issues are addressed in the context of steam corrosion of ferrous substrates that leads to the formation of three corrosion layers, namely, spinel, magnetite and haematite (on the outer exposed surface). For this system, the magnetite layer develops tensile stresses that lead to through-thickness cracking in this layer. The first issue concerns the failure criteria that should be used when predicting the formation of through-thickness cracking. A popular approach is to assume that an oxide layer develops through-thickness cracks when a critical tensile stress (the oxide strength) or strain (the oxide strain to failure) is encountered. Another approach applies fracture mechanics principles to defects that are assumed to exist in the oxide layer, although there is great uncertainty regarding the relevant defect size distributions that control behaviour. A third lower bound (and conservative) approach, that is discussed in some detail, is to consider the energetics of steady state through-thickness cracking that avoids the fracture energy issue of needing to know the defect size that initiates through-thickness cracking. The applicability of the three approaches is discussed with regard to predicting the progressive growth of through-thickness cracking in the magnetite layer, and the importance of residual stresses. Example predictions are made using a proven analytical stress transfer model that enables simulations to be made of progressive through-thickness cracking in the magnetite layer.

The second issue that is discussed concerns the development of interface cracking from through-thickness cracks that can lead to the spallation of oxide layers. One key factor is the influence of the spacing of through-thickness cracks that can determine the size of oxide fragments that might be released during spallation. Another key factor is the determination of conditions for steady-state debond growth that is a critical factor when considering conditions for spallation of oxide layers.

Issues that are considered to require further investigation are highlighted.     

Adhesion and adhesion changes at the copper metal-(acrylonitrile-butadiene-styrene) polymer interface

It is known that the adhesive strength of metallic films on polymer substrates often changes in the course of time. To study this effect in more detail, the adhesion energy of sputtered and galvanically strengthened copper coatings on acrylonitrile-butadiene-styrene polymer substrate was determined as a function of storage time. The adhesion energy is initially only about 6 J/m², because of water introduced to the interface during the galvanic deposition process. Within about 48 hours most of the water disappears leading to a relatively rapid increase of the adhesion energy to a value of approximately 23 J/m². Thereafter, an increase, albeit at a lower rate, was still measured and at the end of the study (1008 hours) adhesion energy of about 54 J/m² was found. The increase during the latter period is attributed to structural rearrangements taking place near the interface.     

Fracture toughness measurement of thin films on compliant substrate using controlled buckling test

Thin films and multilayered structures are increasingly used in the industry. One of the important mechanical properties of these thin layers is the fracture toughness, which may be quite different from the known value of the bulk sample due to microstructural difference. In the design towards device flexibility and scratch resistance, for example, fracture toughness is an important parameter of consideration. This work presents a testing scheme using controlled buckling experiment to determine the fracture toughness of brittle thin films prepared on compliant substrates. When the film is under tension, steady-state channelling cracks form in parallel to each other. Critical fracture strain can be calculated by the measuring the displacement of the buckled plate. The fracture toughness can then be obtained with the help of finite element calculation. When the substrate experiences plastic deformation, the energy release rate is increased by the degree of plasticity. Fracture toughness measurement of two types of thin film Cu-Sn intermetallic compounds has been given to illustrate the merits of such a test scheme.     

Experimental analysis and finite element modelling of nano-scratch test applied on 40-120 nm SiCN thin films deposited on Cu/Si substrate

In this work, the nano-scratch test is used to characterize the interfacial adhesion of amorphous SiCN thin films deposited by plasma enhanced chemical vapour deposition on Cu/Si substrates. The experimental results show that the critical load F_c is directly related to the rupture of the SiCN/Cu interface. A strong linear dependence of F_c to the SiCN thickness independently to the adhesion is also put in evidence. A three-dimensional finite element model of the test is then built. The results show a clear relation between the stresses into the coating and the cracking and buckling of the film observed experimentally. We then discuss how the interfacial tensile stresses can explain the increase of F_c with the film thickness.     

Theory and numerics for finite deformation fracture modelling using strong discontinuities

A general finite element approach for the modelling of fracture is presented for the geometrically non‐linear case. The kinematical representation is based on a strong discontinuity formulation in line with the concept of partition of unity for finite elements. Thus, the deformation map is defined in terms of one continuous and one discontinuous portion, considered as mutually independent, giving rise to a weak formulation of the equilibrium consisting of two coupled equations. In addition, two different fracture criteria are considered. Firstly, a principle stress criterion in terms of the material Mandel stress in conjunction with a material cohesive zone law, relating the cohesive Mandel traction to a material displacement ‘jump’ associated with the direct discontinuity. Secondly, a criterion of Griffith type is formulated in terms of the material‐crack‐driving force (MCDF) with the crack propagation direction determined by the direction of the force, corresponding to the direction of maximum energy release. Apart from the material modelling, the numerical treatment and aspects of computational implementation of the proposed approach is also thoroughly discussed and the paper is concluded with a few numerical examples illustrating the capabilities of the proposed approach and the connection between the two fracture criteria. Copyright © 2005 John Wiley & Sons, Ltd.     

物医用不锈钢表面大面积规则排列微孔PLGA高分子涂层的形成

通过垂直提拉工艺利用水辅助自组装法在生物医用不锈钢表面制备出大面积规则排列微孔丙交酯．乙交酯共聚物（PLOA）涂层，实验证明溶液浓度和PLGA的分子量对涂层的微孔排列形貌有较大影响。高浓度和高分子量的PLGA溶液有利于获得大面积规则排列的微孔涂层；反之，则易出现多边形胞状的不均匀微孔排列结构。分析认为多边形胞状结构的形成与表面张力梯度引发的Marangoni对流效应有关。通过该方法可实现复杂轮廓金属器械表面的高分子微孔涂层生物改性。     

Adhesion investigation of low-k films system using 4-point bending test

This research presents a simulation-based methodology to accurately predict interfacial adhesion behaviors of heterostructures. Validation of the proposed approach is achieved through comparison of 4-point bending test results on interfaces of multiple stacked low-k films with those of theoretical solutions from the finite element analysis. Impact induced by the compliance of 4-point bending test system can be neglected using the averaged energy release rate of various crack lengths in simulations. On the basis of precise predictions drawn from the considered analyses, uncertainty of experimental tests for the nano-scale fractured strength could be promptly observed and estimated.     

Preparation and photochromic behavior of crosslinked polymer thin films containing polyoxometalates

A series of reversible photochromic nanocomposite films were prepared by entrapping phosphotungstic acid (PWA) and molybdenumphsophoric acid (PMoA) into P(VP-BVA), which was a crosslinked polymer based on N-vinylpyrrolidone (VP) and bisvinyl-A (BVA). The microstructure, photochromic behavior and mechanism of the films were studied with transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), ultraviolet-visible spectra (UV-vis) and electron resonance spectra (ESR). The TEM image showed that the polyoxometalates particles had regular microstructure with narrow size distribution (average diameter of 30 nm) in hybrid films. FT-IR results showed that the Keggin geometry of polyoxometalates (POM) was still preserved inside the composites and strong coulombic interaction between POM and crosslinked polymer matrix was built. Irradiated with ultraviolet light, the transparent films changed from colorless to blue and showed reversible photochromism. Oxygen plays an important role during the bleaching process. PMoA/P(VP-BVA) film had higher photochromic efficiency and slower bleaching reaction than PWA/P(VP-BVA) film. The characteristic signals of W (V) or Mo (V) in ESR spectra indicated that electron transfer occurred between the organic substrates and heteropolyanions under UV irradiation, which induced heteropolyanions to heteropolybules with simultaneous oxidation of the organic substrates.     

Fracture mechanics‐based progressive damage modelling of adhesively bonded fibre‐reinforced polymer joints

A quasi‐static progressive damage model for prediction of the fracture behaviour and strength of adhesively bonded fibre‐reinforced polymer joints is introduced in this paper. The model is based on the development of a mixed‐mode failure criterion as a function of a master R‐curve derived from the experimental results obtained from standard fracture mechanics joints. Consequently, the developed failure criterion is crack‐length and mode‐mixity dependent, and it takes into account the contribution of the fibre‐bridging effect. Energy release rate values for adhesively bonded double‐lap joints are obtained by using the virtual crack closure technique method in a finite element model, and the numerically obtained strain energy release rate is compared to the critical strain energy release rate given by the mixed‐mode failure criterion. The entire procedure is implemented in a numerical algorithm, which was successfully used for predicting the strength and R‐curve response of adhesively bonded double‐lap structural joints made of pultruded glass fibre‐reinforced polymers and epoxy adhesives.     

Abrasion of thin films deposited onto glass by the Taber test

The mechanism of abrasion of sputtered metal nitride thin films on glass by the Taber abrasion test and the effects of mechanical and tribological properties of thin films on Taber abrasion were investigated. The abrasion occurred in a peeling mode: the film was peeled off the substrate and the peeled area increased with increasing number of Taber abrading cycles. The peel rate per Taber cycle, a quantitative measure to represent the abrasion resistance of the thin films, was small in the film with greater adhesion strength, higher hardness and lower frictional resistance of the surface. The effect of these properties on the peel rate was represented with a mathematical equation of exponential form. The peel rate was found to vary most sensitively with the hardness.     

Choice of standard fracture test for concrete and its statistical evaluation

The main characteristics of the cohesive (or fictitious) crack model, which is now generally accepted as the best simple fracture model for concrete, are (aside from tensile strength) the fracture energies G _F and G _f corresponding to the areas under the complete softening stress-separation curve and under the initial tangent of this curve. Although these are two independent fracture characteristics which both should be measured, the basic (level I) standard test is supposed to measure only one. First, it is argued that the level I test should measure G _f, for statistical reasons and because of relevance to prediction of maximum loads of structures. Second, various methods for measuring G _f (or the corresponding fracture toughness), including the size effect method, the Jenq-Shah method (TPFM), and the Guinea et al. method, are discussed. The last is clearly the most robust and optimal because: (1) it is based on the exact solution of the bilinear cohesive crack model and (2) necessitates nothing more than measurement of the maximum loads of notched specimens of one size, supplemented by tensile strength measurements. Since the identification of material fracture parameters from test data involves two random variables, f_t (tensile strength) and G _f, statistical regression should be applied. But regression is not feasible in the original Guinea et al.'s method. The present study proposes an improved version of Guinea et al.'s method which reduces the statistical problem to linear regression thanks to exploiting the systematic trend of size effect. This is made possible by noting that, according to the cohesive (or fictitious) crack model, the zero-size limit _N0 of nominal strength _N of a notched specimen is independent of F _f and thus can be easily calculated from the measured f_t. Then, the values of _N0 obtained from the measured f_t values, together with the measured _N-values of notched specimens, are used in statistical regression based on the exact size effect curve calculated in advance from the cohesive crack model for the chosen specimen geometry. This has several advantages: (1) the linear regression is the most robust statistical approach; (2) the difficult question of statistical correlation between measured f_t and the nominal strength of notched specimens is bypassed, by virtue of knowing the size effect trend; (3) the resulting coefficient of variation of mean G _f is very different and much more realistic than in the original version; (4) the coefficient of variation of the deviations of individual data from the regression line is very different from the coefficient of variation of individual notched test data and represents a much more realistic measure of scatter; and (5) possible accuracy improvements through the testing of notched specimens with different notch lengths and the same size, or notched specimens of different sizes, are in the regression setting straightforward. For engineering purposes where high accuracy is not needed, the simplest approach is the previously proposed zero-brittleness method, which can be regarded as a simplification of Guinea et al.' method. Finally, the errors of TPFM due to random variability of unloading-reloading properties from one concrete to another are quantitatively estimated.     

Fabrication and modelling of the macro-mechanical properties of cross-ply laminated fibre-reinforced polymer composites using artificial neural network

The mechanical behaviour of fibre-reinforced polymer composites (FRPCs) is considered very complex due to many factors such as composition, material type, manufacturing process and end user applications. This article presents the mechanical properties and artificial neural network (ANN) modelling results of cross-ply laminated FRPCs. Twenty composite samples were fabricated by varying the number of layers of carbon fibre and glass fibre as reinforcement and polyphenylene sulphide and high-density polyethylene as matrix. Mechanical properties were measured in terms of flexural modulus, hardness, impact and transverse rupture strength. Multilayer feed-forward backpropagation ANN approach was used to predict the mechanical properties by using material type, composition and number of reinforcement and matrix layers as input variables. From 20 data patterns, 16 were used for network training and remaining 4 were used to test the models. Furthermore, trend analysis was also performed to understand the influence of inputs on developed models. It is evident from the ANN prediction results that there is good correlation between predicted and actual values within acceptable mean absolute error. The outcomes of this research will help to reduce cost and time by eliminating tedious composite property measurements and to fabricate tailored composites meeting application requirements.