PubMed · 413379
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J C Birnholz. 1977. On maps and comparing cross-sectional imaging methods.. https://doi.org/10.2214/ajr.129.6.1133
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An axisymmetric deformation of a viscoelastic sphere bounded by a prestressed elastic thin shell in response to external pressure is studied by a finite element method. The research is motivated by the need for understanding the passive behavior of human leukocytes (white blood cells) and interpreting extensive experimental data in terms of the mechanical properties. The cell at rest is modeled as a sphere consisting of a cortical prestressed shell with incompressible Maxwell fluid interior. A large-strain deformation theory is developed based on the proposed model. General non-linear, large strain constitutive relations for the cortical shell are derived by neglecting the bending stiffness. A representation of the constitutive equations in the form of an integral of strain history for the incompressible Maxwell interior is used in the formulation of numerical scheme. A finite element program is developed, in which a sliding boundary condition is imposed on all contact surfaces. The mathematical model developed is applied to evaluate experimental data of pipette tests and observations of blood flow.
The elastic behavior of closed multilayered membranes is analyzed with the assumption that the constituent layers are in close contact but are unconnected in the sense that they are free to slide by one another. The system exhibits three independent elastic deformation modes for any number of the constituent layers equal to or larger than two. These are the area expansivity of the membrane neutral surface, and the local and non-local membrane bending. The corresponding elastic moduli are expressed in terms of the elastic moduli of the constituent layers, their areas, and distances between their neutral surfaces. Closed multilayered membranes only differ from a closed bilayer membrane in that for any of their shapes some of the constituent layers are expanded and some compressed.
Dynamic mechanical analysis (DMA) has been used to characterize the viscoelastic properties of four suture materials, Prolene, Maxon, Vicryl and silk. The technique identified the main transitions for the sutures and frequency multiplexing was used to define the variation of modulus with time. The monofilament Prolene has a beta transition and the monofilament Maxon has an alpha transition close to room temperature, indicating that both materials exhibit a change in stiffness with time. The alpha transitions (Tgs) of silk and Vicryl occur at temperatures significantly above room temperature and frequency multiplexing data indicate that both materials exhibit stable moduli with respect to time at room temperature. Determination of the viscoelastic behaviour through DMS appears to be very useful in predicting the performance of suture materials.