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1869 results in Biomedical engineering

Page 81 of 94

  • 8 - Causal Mechanisms

  • Roderic Lakes, University of Wisconsin, Madison
  • Book:
    Viscoelastic Materials
    Published online:
    21 January 2010
    Print publication:
    27 April 2009, pp 271-340

  • Summary

    Introduction

    Rationale

    The treatment of viscoelasticity thus far has dealt with phenomena, measurement of material properties, prediction of response to various load histories, and stress analysis. In this chapter we consider the physical causes of viscoelastic response. Study of causal mechanisms is motivated by (1) the desire for scientific understanding; (2) the intention to gain ability to choose or tailor materials with specified viscoelastic properties; and (3) the utility of viscoelastic measurements as a probe of microphysical processes that are causally linked to viscoelasticity. The conceptually simplest of the causal mechanisms are developed in detail here. These processes can occur in crystalline materials including metals, ceramics as well as in polymers. Examples of damping due to fluid–solid interaction in porous materials according to the Biot theory are given in §7.5, biological tissue. Viscoelasticity in tissue also results from stress induced motion at the molecular scale. The treatment is intended to be introductory, not exhaustive.

    Survey of Viscoelastic Mechanisms

    There are many causal mechanisms [1–3] responsible for viscoelastic response; several of these are as follows. Mechanisms indicated by a * are called fundamental mechanisms because they occur even in an ideal perfect single crystal, and they are not removable, even in principle. Many effects are named for the person who discovered them or who provided a clear analysis of the physical processes responsible for them. The word relaxation is used in these names to refer to viscoelasticity in a general sense, not necessarily a specific stress relaxation experiment.

  • 3 - Dynamic Behavior

  • Roderic Lakes, University of Wisconsin, Madison
  • Book:
    Viscoelastic Materials
    Published online:
    21 January 2010
    Print publication:
    27 April 2009, pp 55-90

  • Summary

    Introduction and Rationale

    The response of viscoelastic materials to sinusoidal load is developed in this section, and this response is referred to as dynamic. Dynamic in this context has no connection with inertial terms or resonance. The dynamic behavior is of interest because viscoelastic materials are used in situations in which the damping of vibration or the absorption of sound is important. The results developed in this section have a bearing on applications of viscoelastic materials, on experimental methods for their characterization, and on the development of physical insight regarding viscoelasticity.

    The frequency of the sinusoidal load on an object or structure may be so slow that inertial terms do not appear (the subresonant regime), or high enough that resonance of structures made of the material occurs. At a sufficiently high frequency, dynamic behavior is manifested as wave motion. This distinction between ranges of frequency does not appear in the classical continuum description of a homogeneous material, because the continuum view deals with differential elements of material.

    Oscillatory stress and strain histories are represented by sinusoid functions. Suppose we have α(ωt) = α0sinωt, in which t is time, α0 is the amplitude, and ω is the angular frequency. The sine function repeats every 2π radians. So α(ωt + 2π) = α(ωt). The time T required for the sine function to complete one cycle is obtained from ωT = 2π, or T = 2π/ω. T is called the period, the number of seconds required for one cycle.

  • 6 - Experimental Methods

  • Roderic Lakes, University of Wisconsin, Madison
  • Book:
    Viscoelastic Materials
    Published online:
    21 January 2010
    Print publication:
    27 April 2009, pp 145-206

  • Summary

    Introduction and General Requirements

    Experimental procedures for viscoelastic materials, as in other experiments in mechanics, make use of a method for applying force or torque, a method for measuring the same, and a method for determining displacement, strain, or angular displacement of a portion of the specimen. If the experimenter intends to explore a wide range of time, and/or frequencies, the requirements for performance of the instrumentation can be severe. Elementary creep procedures are discussed first, because they are the simplest. Methods for measurement and for load application are discussed separately, since many investigators choose to assemble their own equipment from components. Many procedures in viscoelastic characterization of materials have aspects in common with other mechanical testing. Therefore, study of known standard methods [1, 2] for mechanical characterization of materials is useful. As in other forms of mechanical characterization, it is important that the stress distribution in the specimen be well-defined. End conditions in the gripping of specimens are usually not well known. The experimenter often uses elongated specimens for tension, torsion, or bending, to appeal to Saint Venant's principle in using idealized stress distributions for the purpose of analysis. The determination of viscoelastic properties as a function of frequency is at times referred to as mechanical spectroscopy.

    Frequency response is an important consideration for instruments used in viscoelasticity; the transducers [3] for force generation and measurement and for measurement of deformation must respond adequately at the frequencies of interest.

  • 10 - Applications and Case Studies

  • Roderic Lakes, University of Wisconsin, Madison
  • Book:
    Viscoelastic Materials
    Published online:
    21 January 2010
    Print publication:
    27 April 2009, pp 377-440

  • Summary

    Introduction

    Viscoelasticity can enter in the application of materials in many ways. In some applications one must deal with natural materials, such as stone, earth, or wood in the case of building construction, or bone and soft tissue in the case of biomedical engineering. In these cases, the viscoelastic behavior of the natural materials should be known. Artificial materials used in engineering applications may exhibit viscoelastic behavior as an unintentional side effect. Finally, one may deliberately use the viscoelasticity of certain materials in the design process, to achieve a particular goal.

    A Viscoelastic Earplug: Use of Recovery

    A foam earplug [1, 2] was designed to be easily fitted into the ear by making use of the controlled viscoelastic behavior of the polymer from which it is made. The earplug serves to attenuate sound entering the ear to protect the ear from damage from excessive noise, and also to alleviate suffering and reduce human fatigue.

    To insert the earplug, the user rolls it into a narrow cylindrical shape, then inserts it into the ear canal. Insertion is easier if the outer ear is pulled outward and upward, since that straightens the ear canal. The earplug then gradually expands as a result of viscoelastic recovery to fill and contact the ear canal, and it then effectively blocks noise.

    The earplug is [2] cylindrical, somewhat larger than the ear canal, and made of a foamed polymeric material with a recovery from 60 percent compression to 40 percent compression occurring in 1 to 60 seconds and an equilibrium stiffness at 40 percent compression from 0.2 to 1.3 p.s.i. (1.4 kPa to 9 kPa).

  • Preface

  • Roderic Lakes, University of Wisconsin, Madison
  • Book:
    Viscoelastic Materials
    Published online:
    21 January 2010
    Print publication:
    27 April 2009, pp xv-xvi

  • Summary

    This book is intended to be of use in a one-semester graduate course on the properties, analysis, and uses of viscoelastic materials. A precursor book, Viscoelastic Solids, has been used as a text in such a course. This book contains many updates, expanded coverage of the materials science of the causes of viscoelastic behavior, and of the properties of materials of biological origin, and applications of viscoelastic materials. The objective is to make the subject accessible and useful to students in a variety of disciplines in engineering and physical science. To that end, the coverage is intentionally broad. For research scientists and engineers or graduate students who pursue the subject via self-study, many references have been included to provide links to the literature. The subject may be profitably studied by undergraduate students, particularly those who are interested in vibration abatement, biomechanics, and the study of materials. Most of the book should be accessible to people who have completed an intermediate or an elementary course on the mechanics of deformable bodies. Exposure to elasticity theory, materials science, and vibration theory is helpful but not necessary.

    A development of the theory is presented, including both transient and dynamic aspects, with emphasis on linear viscoelasticity. The structure of the theory is presented with the aim of developing physical insight. Methods for the solution of stress analysis problems in viscoelastic objects are developed and illustrated. Experimental methods for characterization of viscoelastic materials are explored in detail. Viscoelastic phenomena are described for a wide variety of materials, including polymers, metals, ceramics, geological materials, biological materials, synthetic composites, and cellular solids. High-damping alloys and composites are considered as well as materials that resist creep.

  • 14 - Frontiers of image processing in medicine

  • from Part IV - Medical applications and ongoing developments
  • Geoff Dougherty
  • Book:
    Digital Image Processing for Medical Applications
    Published online:
    12 September 2018
    Print publication:
    09 April 2009, pp 395-398

  • Summary

    Overview

    The recent rapid advances in medical imaging and automated image analysis will continue and allow us to make significant advances in our understanding of life and disease processes, and our ability to deliver quality healthcare. A few of the synergistic developments involving a number of disciplines are highlighted.

    Learning objectives

    After reading this chapter you will be able to:

  • • recognize the limitations of current imaging technology;

  • • appreciate the trends and ongoing developments in medical imaging.

    • Trends

    “A picture is worth a thousand words.”

    The rapid advances of the last two or three decades in medical imaging technology, which have delivered high-resolution, three-dimensional anatomical and physiological images, is continuing apace, enabling ever more powerful advances in diagnosis and intervention. Improved, miniature detectors are pushing spatial resolution below 1 mm, which will require large computer memories and storage capacities and improved software capabilities to visualize the larger data sets interactively. Advances in post-processing, especially in automated registration, segmentation, classification and rendering, will be required (Van Leemput et al., 1999; Huber and Hebert, 2003; Way et al., 2006). The availability of multimodality imaging, such as combined CT/PET scanners, is increasing, along with the means to share such images around the clinical setting and remotely, fueling improvements in PACS and telemedicine systems (Section 4.3).

    The inverse problem

    A basic aspect of most imaging modalities is to reconstruct an image based on minimally invasive measurements from a number of sensors. The inverse problem determines the properties of the unknown system from the observed measurements. The goal of the reconstruction can be either structural information, such as the anatomy that comes from CT or MRI imaging, or functional information from nuclear medicine imaging or electrical impedance tomography (EIT). An important key feature of inverse problems is their ill-posedness, i.e. they do not fulfil classical requirements of existence, uniqueness and stability under data perturbations. The last aspect is especially important since in the real world measurements always contain noise; approximation methods for solving inverse problems with minimal sensitivity to noise, so-called regularization methods, are being studied.

Page 81 of 94