Although existing continuum models for the elasto-viscoplastic response of amorphous polymeric materials phenomenologically capture the large deformation response of these materials in a reasonably acceptable manner, they do not adequately account for the creep response of these materials at stress levels below those causing “macro-yield”, as well as the Bauschinger-type reverse yielding phenomena at strain levels less than ≈ 30% associated with the macro-yield transient. Anand [1] has recently generalized the model of Anand and Gurtin [2] to begin to capture these important aspects of the mechanical response of such materials. In this work, we summarize Anand’s constitutive model and apply it to the amorphous polymeric solid poly(methyl methacrylate) (PMMA), at ambient temperature and compressive stress states under which this material does not exhibit crazing. We describe our compression-tension and creep experiments on this material from which the material parameters in the model were determined. We have implemented the constitutive model in the finite-element computer program ABAQUS/Explicit [3], and using this finite-element program, we show numerical results for some representative problems in micro-indentation of PMMA, and compare them against corresponding results from physical experiments. The overall predictions of the details of the load, P, versus depth of indentaion, h, curves are very encouraging. / Singapore-MIT Alliance (SMA)

Thèse de doctorat : Science et ingénierie des matériaux : INPL : 2008. / Titre provenant de l'écran-titre.

Detailed knowledge of tissue mechanical properties is widely required by medical applications, such as disease diagnostics, surgery operation, simulation, planning, and training. A new two degrees of freedom portable device, called Tissue Resonator Indenter Device (TRID), has been developed for measurement of regional viscoelastic properties of soft tissues at the Bio-instrument and Biomechanics Lab of the University of Toronto. As a device for clinical application, the accuracy and reliability of TRID is crucial. This thesis thus investigates the tissue samples’ mechanical properties through finite element analysis method after reviewing the experimental results of the same tissue samples using TRID. The accuracy of TRID is verified through comparing its experimental results with finite element simulation results of tissue mechanical properties. This thesis also investigates the reliability of TRID through experimental study of its indenter misalignment effect on the measurement results of tissue static stiffness, dynamic stiffness, and damping respectively.

Detailed knowledge of tissue mechanical properties is widely required by medical applications, such as disease diagnostics, surgery operation, simulation, planning, and training. A new two degrees of freedom portable device, called Tissue Resonator Indenter Device (TRID), has been developed for measurement of regional viscoelastic properties of soft tissues at the Bio-instrument and Biomechanics Lab of the University of Toronto. As a device for clinical application, the accuracy and reliability of TRID is crucial. This thesis thus investigates the tissue samples’ mechanical properties through finite element analysis method after reviewing the experimental results of the same tissue samples using TRID. The accuracy of TRID is verified through comparing its experimental results with finite element simulation results of tissue mechanical properties. This thesis also investigates the reliability of TRID through experimental study of its indenter misalignment effect on the measurement results of tissue static stiffness, dynamic stiffness, and damping respectively.

Finite element simulation of indentation is presented in this thesis. A rigid cylindrical indenter of flat end is used in all the cases, in which the simulation focuses on the effect of a hole on the indentation behavior of materials including elastic and elasto-plastic materials. In the simulation, the material is assumed to be a half-space. The relations between load and displacement are determined as a function of the hole size. Also indentation under cyclic loading is simulated for an elastic-perfect plastic half space. The influence of factors causing fatigue deformation like amplitude, median load and frequency is addressed. The propagation rate of plastic zone and Von Mises stress distribution at maximum and minimum loading are analyzed.

A combination of experimental and computational methods is used to explore the microstructure and mechanical behavior of cold sprayed 6061 aluminum alloy and Ti-6Al-4V alloy and their substrate materials. A variety of microscopic methods are used for characterization of the microstructure. The indentation size effect and characteristic length of strain gradient plasticity for the substrate materials are determined. An FEA simulation describes the behavior of a worn Berkovich nanoindenter. Stress strain is studied experimentally in the substrate materials for future comparison with bulk cold-sprayed materials. Abaqus FEA models are used to simulate a single particle impact for a particle with an oxide layer using a linear Johnson-Cook plasticity model and a bilinear Johnson-Cook plasticity model. The implications of the results are discussed for cold spray processes.

<p>There have been considerable requirements for improved products of sheet metal in automobile industry. A quick and economical route to new products is to design novel surface textures of varying scales for improved product enhancement in better optical appearance and formability. The critical deformation in the surface texturing is on the order of only a few microns, and can not be accurately predicted by the classical plasticity due to the size effect. The theory of strain gradient plasticity has been developed to capture the size effect based on the concept of geometrically necessary dislocations (GNDs). A selected strain gradient theory has been implemented into the finite element (FE) model to simulate the surface texturing process. A 3D FE model was developed to simulate the rolling process of sheet metal which has band-type feature on the original surface. The numerical results show that a textured roller can efficiently modify the band-type feature without changing the whole mechanical property of the sheet. Size effect has significant contribution to the magnitude of the rolling force. A FE model was developed to simulate the tensile test of the sheet with textured surface. A textured surface of the sheet is prepared through the indention on the sheet surface. The results show that the textured surface becomes harder due to the strain gradient effect, and finally improves the formability of the sheet.</p> / Master of Applied Science (MASc)

No description available.

Interest in superelastic (and shape memory) materials continues to rise, and there is a strong incentive to develop techniques for monitoring of their superelastic characteristics. This is conventionally done via uniaxial testing, but there are many advantages to having a capability for obtaining these characteristics (in the form of parameter values in a constitutive law) via indentation testing. Specimens can then be small, require minimal preparation and be obtainable from components in service. Interrogation of small volumes also allows mapping of properties over a surface. On the other hand, the tested volume must be large enough for its response to be representative of behaviour. Precisely the same arguments apply to more "mainstream" mechanical properties, such as yielding and work hardening characteristics. Indeed, there has been considerable progress in that area recently, using FEM simulation to predict indentation outcomes, evaluating the "goodness of fit" for particular sets of parameter values and converging on a best-fit combination. A similar approach can be used to obtain superelastic parameters, but little work has been done hitherto on sensitivities, uniqueness characteristics or optimal methodologies and the procedures are complicated by limitations to the constitutive laws in current use. The current work presents a comprehensive examination of the issues involved, using experimental (uniaxial and indentation) data for a NiTi Shape Memory Alloy. It was found that it is possible to obtain the superelastic parameter values using a single indenter shape (spherical). Information is also presented on sensitivities and the probable reliability of such parameters obtained in this way for an unknown material.

Many soft food materials, including vegetable shortening, exhibit complex rheological behaviour with properties that resemble those of a solid and a liquid simultaneously. The fundamental parameters used to describe the rheological response of vegetable shortening were obtained from uniaxial compression tests, including monotonic and cyclic compression, as well as creep and stress relaxation tests. The fundamental parameters obtained from the various compression tests were then used in two mechanical models (viscoelastic and elasto-visco-plastic) to predict the compression and conical indentation response of vegetable shortening. The accuracy of the two models was studied with the help of the commercially available finite element analysis software package Abaqus. It was determined that the viscoelastic model was not suitable for the prediction of the rheological response of shortening. On the other hand, the proposed elasto-visco-plastic model predicted with reasonable accuracy the uniaxial compression and indentation experimental response of vegetable shortening. / February 2009