Thesis (Ph. D.)--Lehigh University, 1997. / Includes vita. Includes bibliographical references (leaves 253-260).

No description available.

Rheology is a discipline of continuum mechanics that is concerned with the mechanical properties of matter as it flows. Key to the study of rheology is the concept that materials do not behave as Newtonian liquids of as heterogenous, homogenous mateirials, but as a combination of the two. This combination and blurring of the line between liquid and solid peoperties is knows as viscosity. Furthermore, the viscosity of a material or liquid will not necessarily remain constant when it is subjected input forces or stresses at different frequencies. This consideration brings with it the idea of viscoelasticity which can account for the variations in the characteristics of a sample medium.Magnetic tweezers are tools that allow examination of and investigation into the viscoelastic properties of a sample on the mesoscopic scale. Magnetic matter can be inserted into and bound onto a sample. This magnetic matter can then be manipulated using an external magnetic or electromagnetic field. Calibrated magnetic tweezers apparatus can be used to investigate the mechanical and viscoelastic properties of a material with the novel application of time-variant forcesand stresses. The resultant behaviour, or response, of the sample can be observed using a microscope and analysed further.Fibrin is the highly extensible, fibre-like protein that makes up blood clots. Its particularly high levels of extensibility combined with interesting material properties such as viscoelasticity can strain-hardening make it an ideal test sample for magnetic tweezers experiments. The high elastic limit of fibrin ensures that plastic deformation does not usually occur under the range of input forces and stresses exerted by magnetic tweezers. This allows non-destructive and repeatable tests to be performed.Magnetic tweezers have been developed and used in a series of experiments on fibrin to produce a viscoelastic characterisation of the fibrous networks. The key results in this work are the design of high-sensitivity apparatus for the experiments and associated techniques for high-frequency analysis, use of the tweezers with a high-speed CCD attached to a microscope and the analysis of the viscoelastic properties of fibrin over a several decades of frequency.

To ensure the shelf life of a semiconductor, the integrated circuits are usually embedded in an epoxy molding compound (EMC) to shield it from stress and corrosion. The EMC is a viscoelastic material and is categorized as a thermoset. Working with viscoelastic material appose a challenge due to the cure-induced warpage. Warpage is a deformation that reduces stress resistance and can also make for assembly issues. Being able to predict the warpage accurately is an essential part of the electronics industry. To simulate the warpage, the finite element method is usually used. The Research Institutes of Sweden (RISE) in Piteå, which focuses on composites, have created a solver and corresponding material definition for simulating the curing process of a viscoelastic material. In this thesis, the solver created by RISE was investigated to see how well it could predict the warpage during the curing of an EMC. The investigation consisted of reproducing the simulating conducted by Lin et al. in the article "Modeling and Characterization of Cure-Dependent Viscoelasticityof Molded Underfill in Ultrathin Packages" where the curing of an ultrathin flip-shipChip-Scale Package (fcCSP) was simulated. The result will be reproduced using the simulating program LS-Dyna and both the RISE material definition and an build-in material definition will be simulated to see the difference. From the investigation, we could conclude that although the result from the article was unobtainable, the RISE model could predict the warpage more accurately which shows the importance of the cure shift factor. The investigation also found that the RISE model has some implementation errors for the stress-strain calculations in the RISE model. In conclusion, the RISE model was able to predict the warpage in a desired way, but more studies need to be created to ensure the model’s accuracy for the correct warpage magnitude.

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In this paper, the complete double layer boundary integral equation formulation for Stokes flows is extended to viscoelastic fluids to solve the mobility problem for a system of particles, where the non-linearity is handled by particular solutions of the Stokes inhomogeneous equation. Some techniques of the meshless method are employed and a point-wise solver is used to solve the viscoelastic constitutive equation. Hence volume meshing is avoided. The method is tested against the numerical solution for a sphere settling in the Odroyd-B fluid and some results on a prolate motion in shear flow of the Oldroyd-B fluid are reported and compared with some theoretical and experimental results. / Singapore-MIT Alliance (SMA)

Nanoindentation has a high load resolution, depth sensing capabilities, and can be used to characterize the local mechanical behavior in material systems with heterogeneous microstructures. Recently nanoindentation has been used to extract useful stress-strain curves, primarily in hard materials such as metals and ceramics. To apply these indentation stress-strain methods to polymer composites, we have to first develop analysis techniques for materials that exhibit viscoelasticity. In a lot of current research the viscoelastic material properties are extracted after the material has been deformed enough to initiate plasticity and in some cases the time dependence of the deformation is ignored. This doesn’t give an accurate representation of the material properties of the undeformed sample or the local deformation behavior of the material. This dissertation develops analysis protocols to extract stress-strain curves and viscoelastic properties from the load-displacement data generated from spherical nanoindentation on materials exhibiting time-dependent response at room temperature. Once these protocols are developed they can then be applied, in the future, to study viscoelastic and viscoplastic properties of various mesoscale constituents of composite material systems. These new protocols were developed and tested on polymethyl methacrylate, polycarbonate, low-density polyethylene, and the bio-polymer chitosan. The properties extracted were consistent under different conditions and we were able to produce stress-strain curves for different loading rates and different indenter tip sizes. This dissertation demonstrates that a set of protocols can be used to reliably investigate the mechanical properties and deformation behavior of time-dependent materials using nanoindentation.

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

Recent development of additive manufacturing technologies has led to lack of information on the base materials being used. A need arises to know the mechanical behaviors of these base materials so that it can be linked with macroscopic mechanical behaviors of 3D network structures manufactured from the 3D printer. The main objectives of my research are to characterize properties of a material for an additive manufacturing method (commonly referred to as 3D printing). Also, to model viscoelastic properties of Procast material that is obtained from 3D printer. For this purpose, a 3D CAD model is made using ProE and 3D printed using Projet HD3500. Series of uniaxial tensile tests, creep tests, and dynamic mechanical analysis are carried out to obtained viscoelastic behavior of Procast. Test data is fitted using various linear and nonlinear viscoelastic models. Validation of model is also carried out using tensile test data and frequency sweep data. Various other mechanical characterization have also been carried out in order to find density, melting temperature, glass transition temperature, and strain rate dependent elastic modulus of Procast material. It can be concluded that melting temperature of Procast material is around 337°C, the elastic modulus is around 0.7-0.8 GPa, and yield stress is around 16-19 MPa.

Dans le calcul des structures, la prise en compte des effets dus aux déformations différées des matériaux, spécialement le fluage des matériaux, est un aspect important. Il provoque des déplacements au cours du temps pour les systèmes statiques et des redistributions des efforts intérieurs dans les systèmes hyperstatiques. Dans le cas des matériaux hétérogènes, il y a généralement une phase caractérisée par un fluage beaucoup plus important. Le cas extrême est celui de l'eau dans le béton au jeune age, où l'on peut considérer que la viscosité vient principalement de la présence de l'eau. Toutefois, l'obtention des propriétés effectives d'un tel matériau hétérogène est un cas particulier d'un matériau hétérogène comportant des phases viscoélastiques. La prédiction des propriétés effectives d'un matériau hétérogène composé de phases viscoélastiques a fait l'objet de plusieurs travaux. Ce mémoire entre dans ce cadre. Il existe différentes méthodes pour traiter ce problème. L'utilisation de méthodes utilisant des variables cachées permet par exemple de traiter le cas de matériaux vieillissants. Toutefois, de nombreux travaux ont été réalisés en utilisant des solutions élastiques grâce à la similitude des équations de la viscoélasticité dans le domaine de Laplace Carson avec les équations de l'élasticité. Cette similitude, connue sous le nom de principe de correspondance (Maldel 1966 [46] ; Lee 1961 [42] ; Yves Rougier, Claude Stolz et André Zaoui 1993 [66]; Stéphan Beurthey et André Zaoui [4]) permet d'obtenir des relations explicites des lois de relaxation et de fluage, dans le cas où l'inverse de la transformée de Laplace-Carson est explicite. Le cas où le spectre dans ledomaine de Laplace-Carson est continu conduit à une expression peu pratique des lois de relaxation et de fluage. Ce mémoire est donc limité au cas où la transformée de Laplaceest traitée pour un spectre discret. En restant dans ce cadre, on peut noter que le problème a été traité d'abord dans le cas du modèle de Mori-Tanaka (Y.M.Wang et G.J.Weng [75] ; L.C.Brinson et W.S.Lin [9] ; Le QV [41]) qui permet d'obtenir une expression explicite de la transformée de Laplace. Compte tenu des limitations de ce modèle, plusieurs travaux ont porté sur le Schéma Auto Cohérent Généralisé (ACG). Toutefois dans ce cas, on obtient un spectre continu et la transformée de Laplace inverse comporte une partie qui n'est pas analytique et se présente sous forme d'intégrale (Yves Rougier [66] ; Beuthey et Zaoui [4]). Une possibilité pour approcher l'inverse de Laplace est d'utiliser l'approximation de Padé comme décrit par Mikhail F. Selivanov et Yuri A. Chernoivan [69]. Le schéma autocohérent généralisé rend compte de façon approximative de la structure du matériau à l'échelle microscopique. Aussi, nous nous sommes intéressés aux méthodes utilisant la Transformée de Fourier, méthodes permettant de prendre de façon explicite la géométrie de la microstructure. Le mémoire est structuré de la façon suivante : les deux premiers chapitres comportent peu de résultats originaux mais présentent les principaux aspects liés aux techniques d'homogénéisation qui seront étendues dans la suite du mémoire (chapitre 1) et au traitement des problèmes liés à la viscoélasticité par utilisation du principe de correspondance. Le chapitre 3 traite de deux extensions du Schéma Auto Cohérent Généralisé (ACG). La première extension porte sur une approximation simple permettant de rendre explicite la transformée de Laplace inverse de la solution obtenue pour le schéma ACG. La deuxième extension porte sur la prise en compte d'interfaces imparfaites. Les deux derniers chapitres portent sur la mise en oeuvre de méthodes reposant sur la transformée de Fourier qui permettent de prendre explicitement en compte la géométrie de la microstructure (...) / Pas de résumé en anglais