A bulge test based methodology for characterizing ultra-thin buckled membranes

Buckled membranes become ever more important with further miniaturization and development of ultra-thin film based systems. It is well established that the bulge test method, generally considered the gold standard for characterizing freestanding thin films, is unsuited to characterize buckled membranes, because of compressive residual stresses and a negligible out-of-plane bending stiffness. When pressurized, buckled membranes immediately start entering the ripple regime, but they typically plastically deform or fracture before reaching the cylindrical regime. In this paper the bulge test method is extended to enable characterization of buckled freestanding ultra-thin membranes in the ripple regime. In a combined experimental-numerical approach, the advanced technique of digital height correlation was first extended towards the sub-micron scale, to enable measurement of the highly varying local 3D strain and curvature fields on top of a single ripple in a total region of interest as small as approximately 25 microns. Subsequently, a finite element (FE) model was set up to analyze the post-buckled membrane under pressure loading. In the seemingly complex ripple configuration, a suitable combination of local region of interest and pressure range was identified for which the stress-strain state can be extracted from the local strain and curvature fields. This enables the extraction of both the Young's modulus and Poisson's ratio from a single bulge sample, contrary to the conventional bulge test method. Virtual experiments demonstrate the feasibility of the approach, while real proof of principle of the method was demonstrated for fragile specimens with rather narrow ( approximately 25 microns) ripples

Thermo-mechanical analysis of flexible and stretchable systems

This paper presents a summary of the modeling and technology developed for flexible and stretchable electronics. The integration of ultra thin dies at package level, with thickness in the range of 20 to 30 μ m, into flexible and/or stretchable materials are demonstrated as well as the design and reliability test of stretchable metal interconnections at board level are analyzed by both experiments and finite element modeling. These technologies can achieve mechanically bendable and stretchable subsystems. The base substrate used for the fabrication of flexible circuits is a uniform polyimide layer, while silicones materials are preferred for the stretchable circuits. The method developed for chip embedding and interconnections is named Ultra Thin Chip Package (UTCP). Extensions of this technology can be achieved by stacking and embedding thin dies in polyimide, providing large benefits in electrical performance and still allowing some mechanical flexibility. These flexible circuits can be converted into stretchable circuits by replacing the relatively rigid polyimide by a soft and elastic silicone material. We have shown through finite element modeling and experimental validation that an appropriate thermo mechanical design is necessary to achieve mechanically reliable circuits and thermally optimized packages

An isogeometric analysis framework for ventricular cardiac mechanics

The finite element method (FEM) is commonly used in computational cardiac simulations. For this method, a mesh is constructed to represent the geometry and, subsequently, to approximate the solution. To accurately capture curved geometrical features many elements may be required, possibly leading to unnecessarily large computation costs. Without loss of accuracy, a reduction in computation cost can be achieved by integrating geometry representation and solution approximation into a single framework using the Isogeometric Analysis (IGA) paradigm. In this study, we propose an IGA framework suitable for echocardiogram data of cardiac mechanics, where we show the advantageous properties of smooth splines through the development of a multi-patch anatomical model. A nonlinear cardiac model is discretized following the IGA paradigm, meaning that the spline geometry parametrization is directly used for the discretization of the physical fields. The IGA model is benchmarked with a state-of-the-art biomechanics model based on traditional FEM. For this benchmark, the hemodynamic response predicted by the high-fidelity FEM model is accurately captured by an IGA model with only 320 elements and 4,700 degrees of freedom. The study is concluded by a brief anatomy-variation analysis, which illustrates the geometric flexibility of the framework. The IGA framework can be used as a first step toward an efficient workflow for an improved understanding of, and clinical decision support for, the treatment of cardiac diseases like heart rhythm disorders

From Fibrils to Toughness: Multi-Scale Mechanics of Fibrillating Interfaces in Stretchable Electronics

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

Homogenisation of structured elastoviscoplastic slids

viii+96hlm.;24c

Assessing the performance of Data-based and Physics-based Model Order Reduction techniques for Geometrically nonlinear problems

Despite many advancements in computational resources, the cost of using them for simulating high- fidelity (Finite Element) models is still high. Model order reduction aims to reduce this by projecting the entire system of equations onto a lower-dimensional subspace through a projection function. In this contribution, we look at two ways of generating these projection functions, data-based and physics- based approaches. In the data-based method, called Proper Orthogonal Decomposition (POD), Singular Value Decomposition (SVD) is applied to a training data set (generated from varying parameters of the same high-fidelity problem) to obtain the projection function. For the physics-based approach, named Linear Manifold (LM), the dynamic eigenmodes of the system are extended using modal derivatives that can capture the effect of nonlinear kinematics. These so-called modal derivatives and dynamic eigenmodes form the projection function. In this contribution, we intend to model quasi-statics of the same high-fidelity problem (that can be extrapolated to dynamics, if necessary) to observe the difference between these methods. To this extent, we propose a residual parameter in the reduced space for both these methods and an additional mode selection algorithm for the physics-based method (LM). As a start, we assess the performance of both these methods on problems involving geometric nonlinearity. The results showed that the displacement error for these methods for model problems involving simple loading scenarios falls way below 1% and a computational time gain of approximately 25 − 30% compared to the original FE calculation. The difference in these methods has been visible in complex loading scenarios, where LM takes less number of modes compared to POD to reach an error below 1%, but the time gain remains the same

Computational modeling of braided venous stents - effect of design features and device-tissue interaction on stent performance

Designing venous stents with desired properties is challenging due to the partly conflicting performance criteria, e.g., enhancing flexibility may be at odds with increasing patency. To evaluate the effect of design parameters on the mechanical performance of braided stents, computational simulations are performed using finite element analysis. Model validation is performed through comparison with measurements. Considered design features are stent length, wire diameter, pick rate, number of wires, and stent end-type, being either open-ended or closed looped. Based on the requirements of venous stents, tests are defined to study the effect of design variations with respect to the following key performance criteria: chronic outward force, crush resistance, conformability, and foreshortening. Computational modeling is demonstrated to be a valuable tool in the design process through its ability of assessing sensitivities of various performance metrics to the design parameters. Additionally, it is shown, using computational modeling, that the interaction between a braided stent and its surrounding anatomy has a significant impact on its performance. Therefore, taking into account device-tissue interaction is crucial for the proper assessment of stent performance