On the Mutual Effect of Viscoplasticity and Interfacial Damage Progression in Interfacial Fracture of Lead-Free Solder Joints

The main goal of this paper is to shed light on the effect of strain rate and viscoplastic deformation of bulk solder on the interfacial failure of lead-free solder joints. For this purpose, interfacial damage evolution and modeI fracture behavior of the joint were evaluated experimentally by performing stable fracture tests at different strain rates employing an optimized tapered double cantilever beam (TDCB) design. The viscoplastic behavior of the solder was characterized in shear, and the constitutive parameters related to the Anand model were determined. A rate-independent cohesive zone damage model was identified to best simulate the interfacial damage progression in the TDCB tests by developing a three-dimensional (3D) finite-element (FE) model and considering the viscoplastic response of the bulk solder. The influence of strain rate on the load capability and failure mode of the joint was clarified by analyzing the experimental and simulation results. It was shown how, at the lower strain rates, the normal stress generated at the interface is limited by the significant creep relaxation developed in the bulk solder and thus is not sufficiently high to initiate interfacial damage, whereas at higher rates, a large amount of the external energy is dissipated into interfacial damage developmen

On the Mutual Effect of Viscoplasticity and Interfacial Damage Progression in Interfacial Fracture of Lead-Free Solder Joints

The main goal of this paper is to shed light on the effect of strain rate and viscoplastic deformation of bulk solder on the interfacial failure of lead-free solder joints. For this purpose, interfacial damage evolution and mode I fracture behavior of the joint were evaluated experimentally by performing stable fracture tests at different strain rates employing an optimized tapered double cantilever beam (TDCB) design. The viscoplastic behavior of the solder was characterized in shear, and the constitutive parameters related to the Anand model were determined. A rate-independent cohesive zone damage model was identified to best simulate the interfacial damage progression in the TDCB tests by developing a three-dimensional (3D) finite-element (FE) model and considering the viscoplastic response of the bulk solder. The influence of strain rate on the load capability and failure mode of the joint was clarified by analyzing the experimental and simulation results. It was shown how, at the lower strain rates, the normal stress generated at the interface is limited by the significant creep relaxation developed in the bulk solder and thus is not sufficiently high to initiate interfacial damage, whereas at higher rates, a large amount of the external energy is dissipated into interfacial damage development

Influence of the moisture content on the fracture characteristics of welded wood joint. Part 2: Mode II fracture

As a second part of this series, the present study also addresses the water resistance of joints obtained by friction welding. Here, the mode II fracture is in focus, that is, 4-points end-notched flexure specimens (4-ENF) were investigated with various moisture contents (MCs). The critical energy release rate was decreasing at higher MCs. The maximal shear strength of the joining material, as determined by torsion tests, was also affected by high MCs. The experimental data were implemented in a finite element model (FEM) based on the cohesive law to simulate the behavior of welded connection in 4-ENF tests. The FEM results describe well the experimental load-displacement curves

Influence of the moisture content on the fracture characteristics of welded wood joint. Part 1: Mode I fracture

Friction welding is a joining technique for wood materials. The positive aspects of this technique are the speed of processing and the absence of chemical or mechanical agents, but the welded joints are not water resistant. To understand better the effect of moisture on the fracture behavior of welded joints, their fracture characteristics have been investigated. The double cantilever beam specimens were tested, which permit to compute the mode I energy release rate of a welded joint. The results confirm the negative effect of moisture on the fracture properties of the joint. The data concerning the maximal tensile strength of the joining material were collected by uniaxial tests and implemented in a finite element model to establish a cohesive law, which describes the behavior of welded pieces in terms of moisture content

Probing crystalline phases in cubic boron nitride as a function of boron content by massive nanoindentation and microsample testing

Polycrystalline cubic boron nitride (cBN) is a super-hard multiphase composite is extensively used in highly demanding applications, where improved and consistent performances together with high reliability are required. The remarkable mechanical properties of these materials result from a two-fold effectiveness associated with its composite character. On the one hand in terms of composite nature: combination of a brittle cBN particles and a ceramic TiN binder with optimal interface properties, as given by a very low interfacial energy and very good adhesion between cBN and TiN. Information on the small-scale mechanical response mainly for superhard materials is rather scarce in the literature This is particularly true regarding experimental data and analysis on the influence of phase and/or chemical nature and interfacial adhesion on hardness. It is clear that knowledge of these issues is crucial not only to improve the performance of this superhard materials but also to designer of new PCBN systems, which will lead to highly desirable improvements in the cost and time on the materials development cycle. The present work aims to evaluate the boron effect on the cBN particles by doing a systematic micro- and nanomechanical study of the mechanical integrity for different superhard systems, with different binder and reinforcement content. In doing so, different micromechanical approaches are followed: i) Assessment of the micromechanical properties by using the statistical approach, ii) evaluation of the fracture toughness by microcantilever deflection, strength by micropillar compression, and iii) finite element modelling based on 3D FIB tomography is performed by using the acquired micromechanical data in order to correlate micromechanical behavior with macroscopic response of the material. From the obtained results by the statistical method it is found that the boron content strongly modifies the cBN hardness; which produces a modification of this superhard particles being this tetragonal or octhoedrical depending the amount of the boron content dissolved inside the parti

Microstructure-based modeling of the ageing effect on the deformation behavior of the eutectic micro-constituent in SnAgCu lead-free solder

The eutectic micro-constituent in SnAgCu solder governs the deformation behavior of the joint as it shows better deformation resistance than the Sn dendrites and occupies a high volume percentage of the whole solder. The main scope of this study is to develop a three-dimensional (3-D) homogenization model taking into account the microstructural evolution in the eutectic micro-constituent of SnAgCu solder in order to simulate the change in mechanical behavior of the joint caused by isothermal ageing. For this purpose, 3-D configurations of Ag3Sn and Cu6Sn5 intermetallic compounds (IMCs) in near-eutectic SnAgCu solder are visualized in the as-soldered condition and after ageing by focused ion beam/scanning electron microscopy tomography. The tomographic images are used to generate feature-preserving finite element meshes of the actual microstructures. The representative volume element size and constitutive behavior of the eutectic mixture in the two conditions are determined by a numerical homogenization procedure. The results show a considerable reduction in the yield stress level of the eutectic micro-constituent after ageing of the solder joint. It is shown that the increase in the interparticle spacing and decrease in the aspect ratio of IMCs due to ageing cause a significant change in the strain distribution in the tin matrix, which leads to a lower contribution of IMCs in load-sharing and yield strength of aged solder. The elastic plastic properties of as-soldered and aged eutectic mixtures are determined by nanoindentation. The results of homogenization are validated through comparison with experimental results and prediction of the dislocation detachment theory. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Multi-scale modeling of elasto-plastic response of SnAgCu lead-free solder alloys at different ageing conditions: Effect of microstructure evolution, particle size effects and interfacial failure

In microelectronics applications, SnAgCu lead-free solder joints play the important role of ensuring both the mechanical and electrical integrity of the components. In such applications, the SnAgCu joints are subjected to elevated homologous temperatures for an extended period of time causing significant microstructural changes and leading to reliability issues. In this study, the link between the change in microstructures and deformation behavior of SnAgCu solder during ageing is explained by developing a hybrid multi-scale microstructure-based modeling approach. Herein, the SnAgCu solder alloy is seen as a three phase metal matrix composite in which Ag3Sn and Cu6Sn5 hard intermetallics play the role of reinforcements and Sn the role of a ductile matrix. The hardening of the Sn matrix due to fine intermetallics in the eutectic mixture is modeled by incorporating the mean field effects of geometrically necessary dislocations. Subsequently, a two level homogenization procedure based on micromechanical finite element (FE) models is used to capture the interactions between the different phases. For this purpose, tomographic images of microstructures obtained by Focused Ion Beam (FIB) and synchrotron X-Ray in different ageing conditions are directly used to generate statistically representative volume elements (RVE) using 3D FE models. The constitutive behavior of the solder is determined by sequentially performing two scales of numerical homogenization at the eutectic level and then at the dendrite level. For simplification, the anisotropy of Sn as well as the potential recovery processes have been neglected in the modeling. The observed decrease in the yield strength of solder due to ageing is well captured by the adopted modeling strategy and allows explaining the different ageing mechanisms. Finally, the effects of potential debonding at the intermetallic particle-matrix interface as well as particle fracture on the overall strength of solder are discussed based on simplified unit cell FE models and experimental observations. (C) 2016 Elsevier B.V. All rights reserved

Multi-factorial models of a carbon fibre/epoxy composite subjected to accelerated environmental ageing

Among materials being introduced in the aerospace industry, the carbon fibre reinforced plastics (CFRP) have a place of privilege because of their exceptional stiffness-to-mass ratio. However, the polymer-based matrix is vulnerable to damages by environmental conditions. This work exposes the experimental results of several accelerated environmental ageing protocols on CFRP panels. The main concern is to justify or reject by statistical means that a significant degradation of mechanical properties does occur over the time, and to establish a basic model to quantify the effects of different environmental factors of the composite ageing. The results considered here are the elastic properties evaluated over several weeks of accelerated artificial ageing. The stiffness degradation of the samples subjected to the aforementioned ageing protocols is statistically described by a non-linear multi-factorial model inspired by the Design of Experiments (DoE) theory. The evolution of constitutive properties (namely mass and elastic properties) over the time exhibits an asymptotic exponential increasing (or decreasing) pattern over the time. The usefulness of these mathematical models is their predictability, based only on theoretical considerations on moisture absorption. This path is further investigated in this paper, clearing up the way to a methodical prediction of ageing models. (C) 2013 Elsevier Ltd. All rights reserved

Vibration-based characterization of impact induced delamination in composite plates using embedded FBG sensors and numerical modelling

Dynamic strain signals from embedded fiber Bragg grating sensors are used for experimental modal analysis in carbon fiber reinforced polymer plates. Throughout a series of impact tests, the change of eigen-frequencies due to damage is evaluated for different impact energies. A detailed numerical model including the damage pattern obtained from X-ray computed tomography images demonstrates that most of the frequency changes can be explained by a delamination type of damage, whereas the total delamination surface has an affine relation to the absorbed impact energy. A homogenized damage model, including two damage factors, allows to predict the change of natural frequencies for a known damage size. (C) 2011 Elsevier Ltd. All rights reserved