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Mechanical Shape Correlation (MSC) is a novel Integrated Digital Image Correlation (IDIC) based technique used for parameter identification. Digital images taken during an experiment are correlated and coupled to a Finite Element model of the specimen, in order to find the correct parameters in this numerical model. In contrast to regular IDIC techniques , where the images consist of a grayscale speckle pattern applied to the sample, in MSC the images are projections based on the contour lines of the test specimen only. This makes the technique suitable in cases where IDIC cannot be used, e.g., when large deformations and rotations cause parts of the sample to rotate in or out-of-view, or when the speckle pattern degrades due to large or complex deformations, or when application of the pattern is difficult because of small or large specimen dimensions . The method targets problems for which the outline of the specimen is large with respect to the volume of the structure and changes significantly upon deformation. The technique is here applied to virtual experiments with stretchable electronic interconnects, for identification of both elastic and plastic properties. Furthermore, attention is paid to the influence of algorithmic choices and experimental issues. The method reveals good convergence and adequate initial guess robustness. The results are promising and indicate that the method can be used in cases of either large, complex or three-dimensional displacements and rotations on any scale.

For both single-phase and multiphase metallic materials, it is necessary to understand the mechanical behavior on the grain-size scale in detail to get information that is not obtainable from macro-scale mechanical characterizations. This paper presents a methodology for uniaxial tensile testing of micro-specimens isolated from a bulk material. The proposed concept of multiple parallel micro-tensile specimens at the tip of a macro-sized wedge reduces the alignment work and offers an easy way for specimen handling. The selection of site-specific specimens is based on detailed microstructural and crystallographic characterization. Three kinds of representative specimens are presented to illustrate the wide range of application of the methodology for a variety of materials. Accurate, reproducible measurement of force (2.5 μN resolution) and displacement (~10 nm resolution) is demonstrated, while accurate alignment (in-plane rotational and out-of-plane tilt misalignment of

This paper reveals potential micro mechanisms underlying time-dependent anelasticity observed in Al-(1 wt%)Cu thin films. The analyzed deformation mechanisms involve dislocation motion and interaction with solute diffusion, grain boundaries and precipitates. In order to investigate the role of these mechanisms, Al-(1 wt%)Cu alloy thin films are heat treated to systematically change the precipitation state, while characterizing the grain boundary distribution with electron backscatter diffraction. Micromechanical characterization is performed by microbeam bending, nano-tensile creep testing and nano-indentation. Results in microbeam bending reveal, for all precipitate and grain boundary states considered, a similar time-dependent evolution of the anelastic strain after load release. The magnitude of the recovered strain is also observed to be independent of the precipitate or grain boundary configuration. The nano-tensile creep test also indicates the same time-dependent anelastic evolution, indicating that the loading state does not affect the underlying mechanisms. Analysis of strain bursts in nano-indentation shows that pinning of dislocations by Cu solutes is unaffected by the precipitation state. Based on uniaxial creep and time-dependent anelasticity measurements in pure Al specimens, it is made plausible that the time-dependent anelasticity originates from diffusion-limited glide or climb of dislocation segments that are pinned at Cu solutes or in dislocation structures, which provide an internal driving force.

A novel adaptive isogeometric digital height correlation (DHC) technique has been developed in which the set of shape functions, needed for discretization of the ill-posed DHC problem, is autonomously optimized for each specific set of profilometric height images, without a priori knowledge of the kinematics of the experiment. To this end, an adaptive refinement scheme is implemented, which refines the shape functions in a hierarchical manner. This technique ensures local refinement, only in the areas where needed, which is beneficial for the noise robustness of the DHC problem. The main advantage of the method is that it can be applied in experiments where the deformation mechanisms are unknown in advance, thereby complicating the choice of suitable shape functions. The method is applied to a virtual experiment in order to provide a proof of concept. A second virtual experiment is executed with stretchable electronics interconnects, which entail localized buckles upon deformation with complex kinematics. In both cases, accurate results were obtained, demonstrating the beneficial aspects of the proposed method. Moreover, the technique performance on profilometric images of a real experiment with stretchable interconnects was demonstrated.

Well-defined uniaxial micro-tensile tests were performed on lath martensite with different types of substructure boundaries (block, sub-block and lath boundaries) tilted with respect to the loading direction. A characteristic deformation mechanism of lath martensite is hereby identified, i.e. sliding along the substructure boundaries. This boundary sliding can occur at all types of boundaries at relatively low stresses. Internal boundaries not only strengthen lath martensite, as well established in literature, but under favorable orientations also mitigate plastic deformation. The overall plastic deformation results from the competition of crystallographic slip with boundary sliding.

High resolution scanning electron microscopy (HR-SEM) is nowadays very popular for different applications in different fields. However, SEM images may exhibit a considerable amount of imaging artifacts, which induce significant errors if the images are used to measure geometrical or kinematical fields. This error is most pronounced in case of full field deformation measurements, for instance by digital image correlation (DIC). One family of SEM artifacts result from positioning errors of the scanning electron beam, creating artifactual shifts in the images perpendicular to the scan lines (scan line shifts). This leads to localized distortions in the displacement fields obtained from such images, by DIC. This type of artifacts is corrected here using global DIC (GDIC). A novel GDIC framework, considering the nonlinear influence of artifacts in the imaging system, is introduced for this purpose. Using an enriched regularization in the global DIC scheme, based on an error function, the scan line shift artifacts are captured and eliminated. The proposed methodology is demonstrated in virtually generated and deformed images as well as real SEM micrographs. The results confirm the proper detection and elimination of this type of SEM artifacts.

Full-field identification methods are increasingly used to adequately identify constitutive parameters to describe the mechanical behavior of materials. This paper investigates the more recently introduced, one-step method of Integrated Digital Image Correlation (IDIC) with respect to the most commonly used, two-step method of Finite Element Model Updating (FEMU), which uses a subset-based digital image correlation algorithm. To make the comparison as objective as possible, both methods are implemented in the most equivalent manner and use the same FE-model. Various virtual test-cases are studied to assess the performance of both methods when subjected to different error sources; (1) systematic errors, (2) poor initial guesses for the constitutive parameters, (3) image noise, (4) constitutive model errors, blueand (5) experimental errors. Results show that, despite the mathematical similarity of both methods, IDIC produces less erroneous and more reliable results than FEMU, particularly for more challenging test-cases exhibiting small displacements, complex kinematics, bluemisalignment of the specimen, and image noise. This article is protected by copyright. All rights reserved.

In digital image correlation (DIC), the unknown displacement field is typically identified by minimizing the linearized form of the brightness conservation equation, while the minimization scheme also involves a linearization, yielding a two-step linearization with four implicit assumptions. These assumptions become apparent by minimizing the non-linear brightness conservation equation in a consistent mathematical setting, yielding a one-step linearization allowing a thorough study of the DIC tangent operator. Through this analysis, eight different image gradient operators are defined, and the impact of these alternative image gradients on the accuracy, efficiency, and initial guess robustness is discussed on the basis of a number of academic examples and representative test cases. The main conclusion is that for most cases, the image gradient most common in literature is recommended, except for cases with: (1) large rotations; (2) initial guess instabilities; and (3) costly iterations due to other reasons (e.g., integrated DIC), where a large deformation corrected mixed gradient is recommended instead.

The introduction of flexible electronics calls for new ways of measuring the mechanical limits and failure mechanisms resulting from (large) bending loads during manufacturing and application. Therefore, an autonomous, miniaturized, pure bending test apparatus was developed to investigate material systems at the microscopic level. Improvements to conventional bending test methods are: (1) well-defined pure moment application without friction or local pressure contacts, through active feedback control of parasitic axial and transverse forces at the specimen edges, (2) continuous reversibility of the loading direction within an angular range of −114∘≤𝜃≤114∘ (minimum radius of 2 mm), (3) no view-obstruction from both the thickness and in-plane perspective, enabling in-situ optical and scanning electron microscopic studies of failure mechanisms under constant field-of-view. The pure bending test setup was validated by moment-curvature measurements of monocrystalline silicon. The setup’s continuous load-reversibility and strain determination were validated by cyclic tests and in-situ digital image correlation, respectively, on polyethylene napthalate specimens. Furthermore, in-situ microscopic failure analysis was demonstrated on multilayered flexible electronics, revealing fracture, delamination and buckling.

A novel global digital image correlation method was developed using adaptive refinement of isogeometric shape functions. Non-uniform rational B-spline shape functions are used because of their flexibility and versatility, which enable them to capture a wide range of kinematics. The goal of this work was to explore the full potential of isogeometric shape functions for digital image correlation (DIC). This is reached by combining a global DIC method with an adaptive refinement algorithm: adaptive isogeometric GDIC. The shape functions are automatically adjusted to be able to describe the kinematics of the sought displacement field with an optimized number of degrees of freedom. This results in an accurate method without the need of making problem-specific choices regarding the structure of the shape functions, which makes the method less user input dependent than regular global DIC methods, while keeping the number of degrees of freedom limited to realize optimum regularization of the ill-posed DIC problem. The method's accuracy is demonstrated by a virtual experiment with a predefined, highly localized displacement field. Real experiments with a complex sample geometry demonstrate the effectiveness in practice.

International audience; Metal-elastomer interfacial systems, often encountered in stretchable electronics, demonstrate remarkably high interface fracture toughness values. Evidently, a large gap exists between the rather small adhesion energy levels at the microscopic scale (‘intrinsic adhesion’) and the large measured macroscopic work-of-separation. This energy gap is closed here by unravelling the underlying dissipative mechanisms through a systematic numerical/experimental multi-scale approach. This self-containing contribution collects and reviews previously published results and addresses the remaining open questions by providing new and independent results obtained from an alternative experimental set-up. In particular, the experimental studies on Cu-PDMS (Poly(dimethylsiloxane)) samples conclusively reveal the essential role of fibrillation mechanisms at the micro-meter scale during the metal-elastomer delamination process. The micro-scale numerical analyses on single and multiple fibrils show that the dynamic release of the stored elastic energy by multiple fibril fracture, including the interaction with the adjacent deforming bulk PDMS and its highly nonlinear behaviour, provide a mechanistic understanding of the high work-of-separation. An experimentally validated quantitative relation between the macroscopic work-of-separation and peel front height is established from the simulation results. Finally, it is shown that a micro-mechanically motivated shape of the traction-separation law in cohesive zone models is essential to describe the delamination process in fibrillating metal-elastomer systems in a physically meaningful way.

Summary This paper discusses a method that provides the direct identification of constitutive model parameters by intimately integrating the finite element method (FEM) with digital image correlation (DIC), namely, directly connecting the experimentally obtained images for all time increments to the unknown material parameters. The problem is formulated as a single minimization problem that incorporates all the experimental data. It allows for precise specification of the unknowns, which can be, but are not limited to, the unknown material properties. The tight integration between FEM and DIC enables for identification while providing necessary regularization of the DIC procedure, making the method robust and noise insensitive. Through this approach, the versatility of the FE method is extended to the experimental realm, enhancing the analyses of existing experiments and opening new experimental opportunities. Copyright © 2015 John Wiley & Sons, Ltd.

International audience; High values for the work of separation have been reported in peel tests on fibrillating interfacial systems. The exact origin of these high values is not properly understood, since it remains unclear which dissipative mechanisms related to fibrillation cause a significant increase in the work of separation. In this paper, the contribution of fibril mechanics to the work of separation is quantified. To this end, a micromechanical model of a single fibril is used, in which the growth of a nucleated fibril up to the moment of fracture is described. The initial geometry is varied to reflect the variability in the substrate profile. It is observed that the stresses and strains that occur in the model are well beyond typical bulk values. Given the large variation in measured stress–strain curves for rubber materials reported in literature, a small scale single fibril experiment is performed to assess the applicability of reported bulk material parameters to this problem. The obtained experimental response falls well within the model bandwidth for the range of material parameters from literature. Furthermore, the fibril fracture stress, which serves as the failure criterion in the model, is extracted from the experiment. From the micromechanical model results, it appears that, for the considered range of material properties, the work of separation is mainly influenced by the fracture stress. Furthermore the initial geometry has a profound influence on the obtained work of separation. It is concluded that the work of separation determined from the micro-model is significantly larger than the intrinsic adhesion energy, yet it remains still an order of magnitude smaller than the values reported in literature. For the considered system this indicates that, although it has a significant contribution, fibril deformation is not the only contribution to the high work of separation.

Well-defined uniaxial micro-tensile tests were performed on lath martensite single block specimens and multi-block specimens with different number of block boundaries parallel to the loading direction. Detailed slip trace analyses consistently revealed that in the {110} slip system with the highest Schmid factor is activated. Both block and sub-block boundaries act as barriers to dislocation motion, whereby a Hall-Petch like behavior is observed. Sub-block boundary strengthening appears to be only slightly less effective than block boundary strengthening, even though fracture analyses indicates that dislocation motion can cross sub-block boundaries but not block boundaries.

To accurately predict ductile failures of new advanced metals, continuum damage models (CDMs) require experimental determination of material-specific damage parameter(s). While various experimental techniques are being used to determine these damage parameter(s), possible systematic errors due to methodological differences in damage definition have yet to be fully revealed. With the aim of finding the most reliable ductile damage quantification strategy for CDMs, this work provides an in-depth comparison of six theoretically equivalent methodologies by considering measurement accuracy, precision, damage spectrum, spatial resolution and practicality. It is found that the methodologies that quantify ductile damage from its geometry introduce significant systematic errors as they probe a very limited damage spectrum, whereas the methodologies that probe the degradation of a mechanical property suffer from low precision and high complexity, especially for high strains and material anisotropy.

Microstructural damage evolution is reported to influence significantly the failures of new high-strength alloys. Its accurate quantification is, therefore, critical for (1) microstructure optimization and (2) continuum damage models to predict failures of these materials. As existing methodologies do not fulfill the requirements, there is an active search for an accurate damage quantification methodology. In this article, a new, micropillar, compression-based methodology is presented, whereby damage evolution can be quantified successfully through the degradation of the modulus caused by previous deformation.

The reliability of metallic microelectromechanical systems (MEMS) depends on time-dependent deformation such as creep. Key to this process is the interaction between microstructural length scales and dimensional length scales, so-called size-effects. As a first critical step towards studying these size-effects in time-dependent deformation, a purely mechanical experimental methodology has been developed, which is presented here. The methodology entails the application of a constant deflection to a lm-sized free-standing aluminum-alloy cantilever beam for a prolonged period of time. After this load is removed, the deformation evolution is immediately recorded by acquiring surface topographies through confocal optical profilometry. Image correlation and an algorithm based on elastic beam theory are applied to the full-field beam profiles to correct drift and improve limited optical profilometry precision, yielding the tip deflection as function of time with a precision of 7% of the surface roughness. A proof-of-principle measurement reveals a remarkable time-dependent deflection recovery. Assumptions and errors of the methodology are analyzed. Finally, it is concluded that the methodology is most suitable for the investigation of creep due to the simplicity of specimen handling, preparation and setup design, while maximizing long term stability and deformation precision.

International audience; Interface delamination in metal–rubber-type stretchable electronic systems leads to early failure. This paper reports an investigation of metal–rubber interfaces through in situ scanning electron microscopy imaging of the progressing delamination front of 90° peel tests of rubber on copper samples. The results show that the energy dissipated in the forming, elongation and rupture of ∼50 μm long fibrils constitutes the major part of the work of separation. The experiments are characterized and modeled using a cohesive zone-enriched finite-element model.

Microstructural banding is commonly observed in commercial steels, but its effect on the global mechanical properties is still disputed in the literature. This letter investigates the influence of band morphology and banding phase properties from tensile tests for two limit cases (continuous hard band vs. discontinuous softer band), analysed from digital image correlation of in situ electron microscopy micrographs. The effects of band continuity and hardness are elucidated, yielding a clear detrimental influence especially for hard bands with a continuous morphology.

Efforts to implement continuum damage models in forming simulations are hampered by the absence of experimental techniques to provide reliable and accurate damage-evolution parameters. The widely used indentation approach, whereby deformation-induced damage is quantified through the degradation of indentation hardness and modulus, was shown to be intrinsically deficient (Tasan et al. [18] ). Here, a new indentation-based methodology is developed and evaluated, allowing damage-induced degradation of hardness and modulus as a reliable probe for damage quantification.

This paper contributes to the physical understanding of sheet metal micro-mechanics by addressing the influence of damage evolution on localization and eventually ductile fracture in different strain paths. For this purpose, two steels of different microstructure are deformed in different strain paths, along which forming and fracture limit curves are measured. Microstructural damage mechanisms are characterized and compared for different strain paths (i.e. uniaxial, plane strain and biaxial tension) and at different stages of deformation (i.e. before localization, after localization and at fracture). Interesting results are obtained revealing generic relationships between microstructure evolution (e.g. damage accumulation), localization (forming limit curve) and fracture (fracture limit curve). For single phase microstructures with limited damage sources, damage is initiated as a result of a developing plastic instability (localization), and therefore does not have a significant role on the forming limits. For microstructures with more damage mechanisms, however, the damage accumulates even before localization, and significantly affects both necking and fracture limits. © 2009 Elsevier Ltd. All rights reserved.

This paper addresses two important aspects of engineeringsize effects in miniaturizing metallic components and themodeling thereof. The critical role of processing induced sizeeffects is emphasized, which tend to be easily forgotten inexperimentalworkor engineeringinterpretations of size effects.The second aspect is related to the need for a rigorous straingradient enrichment in crystal plasticity modeling, as the resultof coarse graining the interaction between discrete dislocations.The physical origin of the energetic enrichment isdiscussed andits deterministic derivation is briefly confronted with theanalogous result obtained froma statistical mechanics analysis.

An improved experimental methodology is developed and successfully evaluated to visualize deformation-induced microevents in ductile sheet metal. This easy-to-use methodology consists in a well-controlled brittle separation of samples previously deformed in a ductile manner, whereby a coupled-analysis of the two fracture surfaces permits three-dimensional features to be recombined from the ductile predeformation. The results obtained show a wealth of information regarding grain deformations, microvoid nucleation sites and growth mechanisms, and other microevents induced by ductile deformation. © 2009 Acta Materialia Inc.

The introduction of advanced high strength steels, e.g., into the automotive industry initiated a huge interest in analyzing and understanding ductile fracture of sheet metals to greater details. This demands for the development of experimental methodologies that provide microvoid evolution parameters, which also serve as crucial input parameters for advanced forming simulation that can predict damage evolution. Therefore, this work scrutinizes the reliability and applicability of an increasingly popular damage characterization methodology, in which microindentation tests are carried out to measure hardness and elastic modulus degradation as a function of accumulated strain, relating this degradation to damage evolution. To accomplish this goal, this methodology is applied to several different sheet metals of different formability (an interstitial-free steel, a dual phase steel, an aluminum-magnesium-silicon alloy and a ferritic stainless steel). To analyze and verify the results of indentation based methodology, damage evolution in these metals is monitored also via different experimental techniques, i.e. scanning electron microscopy, micro-ct tomography and sensitive density measurement. Moreover, finite element simulations are carried out to understand the effect of void accumulation in the degradation of hardness and elastic modulus. In the case of using the hardness as a damage probe, the degradation due to damage is always coupled to other effects (strain hardening, grain shape change, texture development) causing an increase in the obtained hardness value for all of the sheet metals tested, thereby complete obscuring any degradation of the hardness due to damage. In the case of elastic modulus, all the sheet metals tend to pile-up upon indentation when they are severely deformed, leading to large systematic errors in the Oliver-Pharr methodology based modulus determination, whereas the elastic modulus is also intrinsically altered by the grain shape change and texture development seen for increasing deformation. Therefore, it can only be concluded that, contrary to the published results in the literature, neither the hardness degradation nor the elastic modulus degradation can be used as a precise probe for damage accumulation, at least when the indentation based methodology is carried out in the originally-proposed manner that is commonly used in the literature.

Metallographic techniques are crucial tools for establishing the connection between observed mechanical behaviour of metals and the underlying mechanisms in their microstructures. In this work, we propose a methodology that minimizes deformation and/or modification of the microstructure during specimen preparation, and provides a 3D representation of the deformed micromorphology. This methodology involves opening up fractured (tensile test) specimens under the ductile-to-brittle transition temperature of metals to yield two parts in a brittle manner. The occurrence of brittle fracture is validated by analyzing the detailed, unaltered microstructure from SEM characterization and surface profilometry mapping of sets of two parts. It is found that this technique yields additional valuable information regarding the size and morphology of deformed grains or nucleation mechanisms of ductile damage. Experiments with a number of different steels show that the methodology can be applied to sheet of different formability.

An original experimental approach is presented to automatically determine the average phase distribution around damage sites in multi-phase materials. An objective measure is found to be the average intensity around damage sites, calculated using many images. This method has the following benefits: no phase identification or manual interventions are required, and statistical fluctuations and measurement noise are effectively averaged. The method is demonstrated for dual-phase steel, revealing subtle unexpected differences in the morphology surrounding damage in strongly and weakly banded microstructures.

Full-field identification methods can adequately identify constitutive material parameters, by combining Digital Image Correlation (DIC) with Finite Element (FE) simulation. It is known that interpolation within the DIC procedure is an important error source for DIC-results. In this study, the influence of these errors on the eventual identification results is investigated. Virtual experiments are conducted from which constitutive parameters are identified by two approaches: the commonly used method of Finite Element Model Updating (FEMU) and the more recent method of Integrated Digital Image Correlation (IDIC), in which the utilized interpolation functions are varied, and the influence on the identified parameters is investigated. It was found that image-interpolation has a significant effect on the accuracy of both methods. However, the observed differences in results between the two methods of FEMU and IDIC cannot be explained by interpolation errors.

Dimensional stability issues may occur in paper sheets when subjected to moisture content variations. Paper is in fact mainly composed of hydrophilic wood fibres that experience significant dimensional changes upon humidity variations. The hygro-expansive deformations at the single fibre level and their effects on the meso-structural fibrous network govern the hygro-mechanical response of paper at the macroscopic, sheet scale. The present contribution aims at offering a relation between theoretical predictions and experimental measurements of the hygro-expansive behaviour of paper. Experimental observations of the hygroscopic and mechanical response at the sheet level have been performed, providing the anisotropic hygro-elastic properties of the material in the in-plane principal directions. To understand the multi-scale nature of the phenomenon, a periodic two dimensional meso-structural model of the fibrous network has been developed. The effective material response is extracted via homogenization. The influence of several meso-scale parameters (hygro-mechanical properties and geometry of fibres, inter-fibre bonds and of the network) on the overall behaviour of the material can be studied; in particular, the in-plane fibre orientation distribution along a preferential direction affects strongly the anisotropy of the macro-scale response. The proposed model has been adopted to represent the effective material response, focusing on the hygro-elastic response only and thus neglecting possible irreversible effects occurring at the different scales. A comparison of the experimental data with the results obtained from the model as a function of the fibre orientation distribution reveals a good accuracy of the prediction, thereby showing its applicability to estimate paper’s hygro-expansive response.

Multi-phase microstructures with high mechanical contrast phases are prone to microscopic damage mechanisms. For ferrite–martensite dual-phase steel, for example, damage mechanisms such as martensite cracking or martensite–ferrite decohesion are activated with deformation, and discussed often in literature in relation to their detrimental role in triggering early failure in specific dual-phase steel grades. However, both the micromechanical processes involved and their direct influence on the macroscopic behavior are quite complex, and a deeper understanding thereof requires systematic analyses. To this end, an experimental–theoretical approach is employed here, focusing on three model dual-phase steel microstructures each deformed in three different strain paths. The micromechanical role of the observed damage mechanisms is investigated in detail by in-situ scanning electron microscopy tests, quantitative damage analyses, and finite element simulations. The comparative analysis reveals the unforeseen conclusion that damage nucleation may have a beneficial mechanical effect in ideally designed dual-phase steel microstructures (with effective crack-arrest mechanisms) through microscopic strain delocalization.

The surface roughness of HWCVD deposited a-Si:H films has been monitored as a function of the film thickness for substrate temperatures between 70 and 450 °C using real-time spectroscopic ellipsometry. Information on the microstructural evolution of the a-Si:H films has been deduced from the data and different growth phases in this microstructural evolution are discussed in terms of the underlying surface processes such as nucleation and initial growth, surface smoothening and roughening processes, and surface diffusion. From the data, it is concluded that, for the specific conditions studied, the best material properties are obtained at ∼ 250–350 °C.

Time-resolved cavity ringdown spectroscopy (τ-CRDS) has been applied to determine the surface reaction probability β of Si and SiH3 radicals during plasma deposition of hydrogenated amorphous silicon (a-Si:H). In an innovative approach, our remote Ar-H2-SiH4 plasma is modulated by applying pulsed rf power to the substrate and the resulting time-dependent radical densities are monitored to yield the radical loss rates. It is demonstrated that the loss rates obtained with this τ-CRDS technique equal the loss rates in the undisturbed plasma and the determination of the gas phase reaction rates of Si and SiH3 as well as their surface reaction probability β is discussed in detail. It is shown that Si is mainly lost in the gas phase to SiH4 [reaction rate kr=(3.0±0.6)×10−16m3s−1], while the probability for Si to react at an a-Si:H surface is 0.95

For hydrogenated amorphous silicon (a-Si:H) film growth governed by SiH3 plasma radicals, the surface reaction probability b of SiH3 and the silicon hydride (–SiHx) composition of the a-Si:H surface have been investigated by timeresolved cavity ringdown and attenuated total reflection infrared spectroscopy, respectively. The surface hydride composition is found to change with substrate temperature from –SiH3-rich at low temperatures to BSiH-rich at higher temperatures. The surface reaction probability b, ranging from 0.20 to over 0.40 and with a mean value of b ¼ 0:30 � 0:03, does not show any indication of temperature dependence and is therefore not affected by the change in surface hydride composition. It is discussed that these observations can be explained by a-Si:H film growth that is governed by H abstraction from the surface by SiH3 in an Eley-Rideal mechanism followed by the adsorption of SiH3 at the dangling bond created. � 2003 Elsevier B.V. All rights reserved.

International audience; Remarkable toughnesses have been reported for metal–elastomer interfaces in stretchable electronics and attributed to surface roughness-induced fibrillation. Interestingly, this micro-mechanism was also observed for PDMS, known for its hyperelastic and thus non-dissipative material behavior. Here, the energy dissipation origin is elucidated by a thorough multi-scale experimental analysis of PDMS-copper delamination, for two distinct mode-mixities and surface roughnesses. The fracture process zone was observed by in situ and post-mortem optical and electron microscopy, and compared to cohesive-zone finite elements simulations: The PDMS deformation is indeed (predominantly) reversible and most energy is dissipated by release of elastically stored energy upon fibril fracture.

International audience; A remarkable high fracture toughness is sometimes observed for interfaces between materials with a large elastic mismatch, which is reported to be caused by the fibrillar microstructure appearing in the fracture process zone. In this work, this fibrillation mechanism is investigated further to investigate how this mechanism is dissipating energy. For that purpose, thermoplastic urethane (TPU)-copper interfaces are delaminated at various rates in a peel test experimental setup. The fracture process zone is visualized in situ at the meso-scale using optical microscopy and at the micro-scale using Environmental Scanning Electron Microscopy (ESEM). It is shown that the geometry of the fracture process zone is insensitive to the delamination rate, while the interface traction scales logarithmically with the rate. This research has revealed that, the interface roughness is shown to be pivotal in initiating the fibrillation delamination process, which facilitates the high fracture toughness. The multi-scale experimental approach identified two mechanisms responsible for this high fracture toughness. Namely, the viscous dissipation of the TPU at the high strain levels occurring in the fibrils and the loss of stored elastic energy which is disjointed from the propagation due to the size of the process zone.

Cohesive interfaces appear in many materials or structures, e.g. composites or adhesive bonds. Originally introduced to model crack tips in fracture mechanics, cohesive zone models are used to describe the constitutive behavior of cohesive interfaces since the early 1990s. In the present contribution, the concept of generalized standard materials (GSM) is transferred from the modeling of bulk behavior to cohesive zones. The potential-based framework obtained hereby is referred to as standard dissipative cohesive zones (SD-CZ). Within this framework, an irreversible interface model is developed for the one-dimensional case and subsequently extended to three-dimensional transverse isotropy. While the potential structure of the constitutive law may be required for certain applications, it also brings benefits with regard to the numerical implementation. To the best knowledge of the authors, this is the first approach to interface modeling in a GSM-like framework, where dissipative effects like damage and softening are considered as well as normal-tangential coupling for mixed-mode decohesion. Comparisons to experimental data and existing cohesive zone models outline the capabilities of the approach.

A procedure is proposed for the identification of spatial interfacial traction profiles of peel loaded Double Cantilever Beam (DCB) samples, from which the corresponding traction-separation relation is extracted. The procedure draws upon recent developments in the area of non-contact optical techniques and makes use of so-called Integrated Digital Image Correlation (I-DIC) concepts. The distinctive feature of the I-DIC approach proposed herein is that the unknown degrees of freedom are not displacements or rotations, but the set of interfacial fracture properties describing the traction profile. A closed-form theoretical model is developed to reconstruct a mechanically admissible displacement field representing the deformation of the adhering layers during debonding in the DCB fracture test. The proposed modeling accounts for the spatial traction profile along the interface between the adherends using few degrees of freedom, i.e. crack tip position, maximum stress and size of the process zone. By minimizing the correlation residual with respect to the degrees of freedom, the full set of interfacial fracture properties is obtained through a one-step algorithm, revealing a substantial gain in terms of computational efficiency and robustness. It is shown that the identified traction profile can be effectively combined with the crack opening displacement to extract the corresponding traction-separation relation, i.e. the key input data for any cohesive zone model (CZM). The proposed procedure is validated by post-processing virtually deformed images generated through the finite element method. The robustness with respect to noisy data, as well as the low sensitivity to the initial guess, are demonstrated. © 2014 Elsevier Ltd. All rights reserved.

Time-resolved cavity ring-down spectroscopy (CRDS) has been applied to determine gas phase and surface loss rates of Si and SiH 3 radicals during plasma deposition of hydrogenated amorphous silicon. This has been done by monitoring the temporal decay of the radicals densities as initiated by a minor periodic modulation applied to a remote SiH 4 plasma. From pressure dependence, it is shown that Si is reactive with SiH 4 [(1.4±0.6)×10 −16 m −3 s −1 reaction rate constant], while SiH 3 is unreactive in the gas phase. A surface reaction probability β of 0.9 β ⩽1 and β =0.30±0.05 has been obtained for Si and SiH 3 , respectively.

Spatially resolved SiH3 measurements are performed by cavity ring down spectroscopy on the SiH3 A2 A1←X2 A1 transition at 217 nm in a remote Ar–H2–SiH4 plasma used for high rate deposition of hydrogenated amorphous silicon. The obtained densities of SiH3 and its axial and radial distribution in the cylindrical deposition reactor are compared with simulations by a two-dimensional axisymmetric fluid dynamics model. The model, in which only three basic chemical reactions are taken into account, shows fairly good agreement with the experimental results and the plasma and surface processes as well as transport phenomena in the plasma are discussed. Furthermore, the SiH3 density determined by cavity ring down spectroscopy is in good agreement with the SiH3 density as obtained by threshold ionization mass spectrometry.

Cavity ring down absorption spectroscopy is applied for the detection of Si and SiH radicals in a remote Ar-H2-SiH4 plasma used for high rate deposition of device quality hydrogenated amorphous silicon (a-Si:H). The formation and loss mechanisms of SiH in the plasma are investigated and the relevant plasma chemistry is discussed using a simple one-dimensional model. From the rotational temperature of SiH typical gas temperatures of ∼1500 K are deduced for the plasma, whereas total ground state densities in the range of 1015–1016 m−3 for Si and 1016–1017 m−3 for SiH are observed. It is demonstrated that both Si and SiH have only a minor contribution to a-Si:H film growth of ∼0.2% and ∼2%, respectively. From the reaction mechanisms in combination with optical emission spectroscopy data, it is concluded that Si and SiH radicals initiate the formation of hydrogen deficient polysilane radicals. In this respect, Si and SiH can still have an important effect on the a-Si:H film quality under certain circumstances.

Time-dependent mechanical investigations of on-wafer specimens are of interest for improving the reliability of thin metal film microdevices. This paper presents a novel methodology, addressing key challenges in creep and anelasticity investigations through on-wafer tensile tests, achieving highly reproducible force and specimen deformation measurements and loading states. The methodology consists of a novel approach for precise loading using a pin-in-hole gripper and a high-precision specimen alignment system based on three-dimensional image tracking and optical profilometry resulting in angular alignment of?

International audience; A novel method is presented to experimentally quantify evolving surface profiles. The evolution of a surface profile is quantified in terms of in-plane and out-of-plane surface displacements, using a Finite Element based Global Digital Image Correlation procedure. The presented method is applied to a case study, i.e. deformation-induced surface roughening during metal sheet stretching. The surface roughness was captured in-situ using a confocal optical profiler. The Global Digital Image Correlation method with linear triangular finite elements is applied to track the three-dimensional material movement from the measured height profiles. The extracted displacement fields reveal the full-field kinematics accompanying the roughening mechanism. Local deviations from the (average) global displacements are the result of the formation, growth, and stretching of hills and valleys on the surface. The presented method enables a full-field quantitative study of the surface height evolution, i.e. in terms of tracked surface displacements rather than average height values such as Root-Mean-Square or height-height correlation techniques. However, the technique does require that an initial surface profile, i.e. contrast, is present and that the contrast change between two measurements is minimal.

International audience; This paper discusses an experimental method to characterize thin lms as they are encountered in micro-electronic devices. The method enables the measurement of the stress and strain of pressure deflected bulged membranes without using a priori defi ned bulge equations. An enrichment to the Global Digital Image Correlation method is detailed to capture the membrane strain and curvature while robustly dealing with acquisition noise. The accuracy of the method is analyzed and compared to the standard bulge test method. The method is applied to a proof of principle experiment to investigate its applicability and accuracy. Additionally, it is shown for two experimental cases that the method provides accurate results, although the bulge equations do not hold.

Interfacial delamination is a key reliability challenge in composites and micro-electronic systems due to (high-density) integration of dissimilar materials. Predictive finite element models are used to minimize delamination failures during design, but require accurate interface models to capture (irreversible) crack initiation and propagation behavior observed in experiments. Therefore, an Integrated Global Digital Image Correlation (I-GDIC) strategy is developed for accurate determination of mechanical interface behavior from in-situ delamination experiments.

While the industrial interest in sheet metal with improved specific-properties led to the design of new alloys with complex microstructures, predicting their safe forming limits and understanding their microstructural deformation mechanisms remain as significant challenges largely due to the inadequacy of the existing experimental tools. The investigation of the strain-path dependent failure mechanisms requires miniaturized testing equipment, which can be placed in a scanning electron microscope for in situ experiments. So far, such tests could only be carried out for a single strain path (uniaxial tension). In this work, in order to fill this gap, a miniaturized Marciniak test setup is designed, built and tested. With this setup real-time, multi-axial tests of industrial sheet metal can be carried out to the point of fracture within a scanning electron microscope. Proof-of-principle experiments demonstrate that a realm of information can be obtained, crucial for the understanding of the mechanical behavior of new alloys.

Bulge testing is known for its ability to quantify the mechanical behavior of homogeneous thin membranes. In this method the measured quantities are related to the averaged stress and strain using bulge equations that only exist for a very limited set of membrane geometries. A novel 3D Digital Image Correlation (DIC) method is proposed to directly measure the strain and curvature fields without using any closed form approximation of the deformation kinematics. Importantly, for membranes under pressure, the stress is directly related to the curvature.

International audience; The plane strain bulge test technique is a powerful and acknowledged technique for characterizing the mechanical behavior of thin films. In a bulge test analysis, the stress and strain are derived from the measured quantities using analytical approximations of the deformed geometry (bulge equations). To improve the bulge test, the systematic error introduced by these approximations is evaluated and quantified by scrutinizing the method on a finite element model of the bulge test, used as an idealized experiment.