Extraction of Superelastic Parameter Values from Instrumented Indentation Data

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.National Council for Scientific and Technological Development (CNPq) - Brazi

Determination of Local Elastic Modulus of Soft Biomaterial Samples Using AFM Force Mapping

Conventional methods of mechanical testing cannot measure properties of soft materials at the nanoscale. In fields such as tissue engineering, it is important to distinguish the bulk elastic modulus from the surface elastic modulus and to characterize the spatial distribution of material with non-uniform stiffness. One of the important new methods of testing is the force mapping using the atomic force microscope. The existing force mapping approaches often suffer from omitting important effects that might result in artifacts. The most important effects include taking into account sample’s adhesion and viscoelasticity and considering more realistic probe shapes. In this work, we have applied a method that take into consideration probe shape and sample adhesion and developed elastic modulus mapping. This inclusive approach can be applied to samples with adhesion that exhibit indentation large enough that requires indenter shape models more sophisticated than the traditional paraboloid shape model. Sample viscoelasticity can be revealed by comparing parameters obtained in opposite scanning directions. The application developed methodology is illustrated with the study of composite biomaterial scaffolds that are used to support the differentiating cells. Samples are composed of four groups: uncrosslinked electrochemically aligned collagen (ELAC), uncrosslinked Bioglass incorporated ELAC (BG-ELAC), genipin crosslinked-ELAC and crosslinked electrochemically compacted collagen (ECC, unaligned). The force mapping on BG-ELAC sample did not exhibit any area with high elastic modulus (measured modulus was around 0.6 MPa), indicating that there is no Bioglass particle protrusion observed at the BG-ELAC surface. Elastic modulus of ELAC molecules appears softer (~ 0.1 MPa) for samples without Bioglass added. Adding genipin crosslinker to collagen threads made the ELAC and ECC samples stiffer even more than uncrosslinked BG-ELAC with characteristic values of elastic modulus approximately 0.97 MPa and 1.28 MPa for crosslinked-ELAC and crosslinked ECC respectively. More extensive studies are necessary to fully investigate effect of crosslinking ratio and adding Bioglass at different concentrations on the elastic modulus values of collagen samples. Methodology reported here is a suitable tool for such studies and can be applied to other soft and potentially heterogeneous biomaterial samples

Discrete Particle Dynamics Models: Computational Aspects And Applications

Particle dynamics considers discrete particles as Newtonian point masses in diverse domains such as, pedestrian particles in pedestrian movement modeling and molecules in material science modeling. These dynamic modeling approaches share a commonality in computational approach where a series of ordinary differential equations are solved for the particle’s motion and equilibrium under active interactions and boundary conditions. In general, particle dynamics approaches are computationally intensive and address problems that require a vast parameter design space. In this dissertation, a computational paradigm is developed for large-scale particle dynamics modeling and demonstrated by three process design applications, (a) infection risk modeling for mitigation policy design, (b) powder spread modeling for metal additive manufacturing design, and (c) energy characterization of nanomaterials modified fiber-reinforced composite for interface design. All these problems require large computational resources to parametrically address inherent uncertainties. Three major strategies are incorporated to address the inherent uncertainties and the resulting computational cost issues. (a) Infection risk modeling incorporates conventional parameter sweeps and empirical data to address the stochastic human behaviors. The incorporation of pedestrian movements suggests the need for more stringent guidelines to reduce transportation-related infectious disease. (b) Powder spread modeling formulates a strategic parametric analysis approach combining a novel algorithmic parameter sweep and artificial intelligence methods to capture the correlation between modeling parameters and outcomes for parameter space interpolation and engineering process design. Such analysis strategy requires at least ten times fewer computational resources than conventional methods while achieving a comprehensive understanding of the parameter space. Modeling outcomes provide efficient and insightful iv guidance for metal additive manufacturing process design. (c) Interface characterization of nanomaterials at fiber-reinforced composite interface utilizes data fusion approaches to address the unknown parameters of particulate systems. A composite interface composing jute fibers and hydroxyapatite nanocrystals is designed to demonstrate the data fusion approach integrating experimental data with modeling calibration. The nanoparticle enhanced interface endows the hybrid composite with superior mechanical performance and functionalizes jute fibers for additive manufacturing. Data assimilation from experiments enable model calibration and validation. The calibrated model allows for comprehensive parametric and sensitivity analysis, which aims to provide reliable insights for future composite interface design. Overall, the computational strategies for discrete particle dynamics models demonstrated in this dissertation are readily expandable for other systems and process design tasks involving vast design space

Modelling and in vitro evaluation of customised Ti6Al4V and PEEK hip implants for improving osseointegration and reducing stress shielding

Following total hip arthroplasty (THA), a considerable level of natural mechanical loading is shielded from the cortical bone and is transferred to the hip stem. This stress shielding effect caused by the underloaded femur post THA, over time, lead to bone loss. This consequently weakens the implant support and increases the risk of elevated micromotion at the bone-implant interface, leading to aseptic loosening and potentially femoral fracture, requiring a revision surgery. Many studies have investigated the development of different types of hip stems with various geometrical/structural designs and material properties to reduce the stress shielding. However, current approaches in the literature do not show a significant reduction in stress shielding and bone resorption, and often do not evaluate these parameters in Gruen zones, where such considerations are very important for clinicians when evaluating the performance of a hip implant. Furthermore, no fatigue performance has been performed on any of the suggested low stiffness hip stems in the literature. The overall aim of this thesis was to design and develop a low stiffness hip stem that can simultaneously minimize bone resorption and implant instability. Two custom tailored stems were developed and evaluated. One had a low material stiffness made from polyether ether ketone (PEEK) and one had a variable stiffness based on graded lattice structures made from 3D printed titanium (i.e. Ti6Al4V). In this study, stress shielding and bone resorption were evaluated across the Gruen zones using experimental and validated computational models. The overall stiffness and fatigue life of the developed hip stems were measured. Results demonstrated that the developed low stiffness porous Ti6Al4V and PEEK hip stems considerably reduced the level of stress shielding and bone resorption compared to a solid Ti6Al4V hip stem with identical geometry. This reduction was more evidenced in the proximal femur

Morphology and mechanical properties of electrospun polymeric fibers and their nonwoven fabrics

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2011.Cataloged from student submitted PDF version of thesis.Includes bibliographical references.Electrospinning is a straight forward method to produce fibers with diameter on the order of a few tens of nanometers to the size approaching commercial fibers (on the order of 10 prm or larger). Recently, the length scale effect on physical properties has attracted great attention because of the potential to produce new materials with unique behavior. In general, the behavior of commercial fibers can be investigated by traditional experiments, and that of nanofibers can be studied by molecular dynamics simulation or Monte Carlo technique. However, the transition of their properties from the bulk to the nanoscale materials is not well understood. Electrospinning provides us a bridge to understand the properties of fibers transiting from the behavior of the bulk material to that of the nanofibers. Among these areas, I am interested in the possible remarkable changes in mechanical properties that may occur in electrospun fibers due to the size effect, where the comprehensive understanding is still lacking. My research objectives are to understand mechanical properties of electrospun polymeric fibers as a function of their size, structure and morphology. The first part of my research is to study internal structures and external topographies of electrospun fibers, and to understand their effect on mechanical properties. Amorphous polystyrene (PS) and semicrystalline polyacrylonitrile (PAN) were dissolved in a high boiling point solvent, dimethylformamide (DMF), for electrospinning. When electrospun in a high-humidity environment, the interior of these fibers was found to be highly porous rather than consolidated, despite the smooth and nonporous appearance of the fiber surfaces. The formation of interior porosity is attributed to the miscibility of water, a nonsolvent for the polymers in solution, with DMF. The resulting morphology is a consequence of the relatively rapid diffusion of water into the jet, leading to a liquid-liquid phase separation that precedes solidification due to evaporation of DMF from the jet. When electrospun in a low humidity environment, the fibers exhibit a wrinkled morphology that can be explained by a buckling instability. Understanding which structures and morphology form under a given set of conditions is achieved through the comparison of three characteristic times: the drying time, the buckling time and the phase separation time. The structures and morphology have important consequences for the properties of the fibers such as their mechanical strength and stiffness.(cont.) Secondly, we studied the size effects of single electrospun fibers on their stiffness and strength. The Young's modulus and yield strength of individual electrospun fibers of amorphous poly(trimethyl hexamethylene terephthalamide) (PA 6(3)T) have been obtained in uniaxial extension. The Young's modulus is found to exhibit values in excess of the isotropic bulk value, and to increase with decreasing fiber diameter for fibers with diameter less than roughly 500 nm. The yield stress is also found to increase with decreasing fiber diameter. These trends are shown to correlate with increasing molecular level orientation within the fibers with decreasing fiber diameter. Using Ward's aggregate model, the correlation between molecular orientation and fiber modulus can be explained, and reasonable determinations of the elastic constants of the molecular unit are obtained. Finally, we identified a relation of stiffness between single electrospun fibers and their nonwoven fabrics. This is of interest because adequate mechanical integrity of nonwoven fabrics is generally a prerequisite for their practical usage. The Young's modulus of electrospun PA 6(3)T nonwoven fabrics were investigated as a function of the diameter of fibers that constitute the fabric. Two quantitative microstructure-based models that relate the Young's modulus of these fabrics to that of the fibers are considered, one assuming straight fibers and the other allowing for sinuous fibers. This study is particularly important for meshes comprising fibers because of our recent discovery of an enhanced size effect on their Young's modulus as well as the tendency towards a curved fiber topology between fiber junctions. The governing factors that affect the mechanical properties of nonwoven mats are the fiber network, fiber curvature, intrinsic fiber properties, and fiber-fiber junctions. Especially for small fibers, both the intrinsic fiber properties and fiber curvature dominate the mechanical behavior of their nonwoven fabrics. This thesis helps us to understand the mechanism behind the enhanced mechanical behavior of small fibers, and to identify determining parameters that can be used to tailor their mechanical performance.by Chia-Ling Pai.Ph.D

Embodied politics and extreme disgust: an investigation into the meanings of bodily order and bodily disorder, with particular reference to the work of William Burroughs and David Cronenberg

This thesis is an analysis of the ways in which images of bodily disgust function in social conflicts. It considers the necessary embodiment of political struggle: that is, the ways in which inequalities are sustained and contested through the material forms taken by human bodies and the meanings attached to bodily states. In chapter one I map out the theoretical grounding for an inquiry into embodiment, by showing how the physical forms taken by bodies are produced by social practices. I argue that ‘the body’ should be seen as a biological product, a ‘body project’, regulated and transformed by its environment. This in turn leads me to a consideration of how such body-shapings sustain regimes of power through constructing for subjects physical forms which are designed to maintain existing systems of inequality. Through a reading of Michel Foucault’s work, I show how such bodies are also able to resist power by making use of the material and discursive structures which seek, but fail, to render them wholly submissive. In chapter two I look at the ways in which the body acts as a map of the psyche, producing a subject which understands itself in terms of its experience of its body parts. I also consider how the body acts as a social symbol, encoding anxieties about the society that it inhabits. By considering both psychoanalytic accounts, and the work of Mary Douglas, I interrogate how concepts of order, form, and integrity become central to embodied subjectivity. In chapter three I consider how, in the Naked Lunch Quartet, William Burroughs represents the body as under threat from repulsive external substances, and how his depiction of such substances in fact relies on a notion of body matter itself as repulsive. I will show how this results from his conceptualization of bodily materiality as antithetical to freedom, and I argue that by demonstrating the impossibility of escaping from acts of invasion and possession, Burroughs's texts in fact undermine the libertarian position that he adopts. In chapter four I develop this argument through a comparison with Julia Kristeva's concept of abjection. I suggest that his representation of abject bodies enables Burroughs to critique the invasive mechanisms of authority, but requires that he collude with the stigmatizing discourses of authority in order to adopt such a position. In particular I consider how this affects his representation of gender. In chapter five I show how David Cronenberg's Shivers may be read as a film that both sustains and critiques the notion of innate bodily disorder. I argue that this is derived from his reliance upon notions of a hierarchy of bodies derived from inequalities of race and class. In chapter six I develop this critique with a reading of Cronenberg's The Fly. I suggest that this film is much more explicit about the fact that bodily chaos is in fact a state experienced by the socially excluded. It offers a critique of the processes by which we are made to feel disgust at our bodies, suggesting that disgust inaugurates a logic of paranoid purification, which in fact impedes the possibilities of the acceptance of those bodies which fall outside certain social limits. Finally, in my conclusion, I look at how Cronenberg's Rabid might be seen as a compendium of the issues of embodied politics, and use this to suggest possible directions in which the work of this thesis might be extended