H. Shen

Northwestern University, Evanston, IL, United States

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Publications (5)4.26 Total impact

  • H. Shen, L.C. Brinson
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    ABSTRACT: Porous titanium is being developed as an alternative orthopedic implant material to alleviate the inherent problems of bulk metallic implants by reducing the stiffness to be comparable to bone stiffness and allowing complete bone ingrowth. However, a porous microstructure is susceptible to local permanent plastic strain and residual stress under cyclic loading which reduces damage tolerance and therefore limits their application as orthopedic implants. The mechanical properties of porous titanium are governed by the microstructural configurations such as pore morphology, porosity, and bone ingrowth. To understand the influence of these features on performance, the macroscopic and microscopic responses of porous Ti are studied using three-dimensional finite element models. The models are generated based on simulated microstructures of experimental materials at porosities of 15%, 32% and 50%. The results show the effect of porosity and bone ingrowth on Young’s modulus, yield stress, and microscopic stress and strain distribution. Importantly, simulations predict that the bone ingrowth reduces the stress and strain localization under cyclic loading so significantly that it counteracts the concentration condition caused by the increased porosity of the structure.Highlights► Porous titanium is used as bone implant material. ► Intensive plastic bands connect closely located large pores. ► Bone ingrowth relieves the concentration of the plastic strain. ► Highly porous implant materials should consider a composite microstructure.
    Mechanics of Materials. 01/2011; 43(8):420-430.
  • H. Shen, H. Li, L.C. Brinson
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    ABSTRACT: The reduced stiffness, weight and open porosity of microporous titanium makes it an attractive material possibility for engineering applications ranging from medical implants to impact tolerant structures. To facilitate the design and application of this material, it is necessary to develop an understanding of the relationship between the porous microstructure and mechanical responses of the material. A factorial design of experiment methodology (DOE) is therefore used to systematically compare the effects that several microstructural features have on the mechanical responses via 2D and 3D finite element (FE) simulations. The FE models for the DOE study are all based on a titanium matrix of 12% porosity and the application to orthopedic implants. Five microstructural features are varied to create 32 test cases to study the effects of pore shape, size, orientation, and arrangement, and bone infiltration. The quantitative effects of the features are used to screen their relative importance for elastic modulus, yield stress, and stress concentration factor. The results of the DOE studies of both 2D and 3D numerical simulations demonstrate that bone infiltration into the pores is the most dominant factor for elastic modulus and yield stress. A random arrangement of pores has great effect on local stress concentrations where the local stress fields are primarily concentrated in the regions around closely spaced pores. Bone infiltration greatly reduces the stress concentration in such regions indicating an advantage of bone ingrowth beyond improved interface and attachment. Compared to bone infiltration and pore arrangement and orientation, relative pore size and shape have relatively small effect on the mechanical responses.
    Mechanics of Materials. 09/2008; 40(9):708–720.
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    ABSTRACT: Mechanical, thermal, and electrical properties of graphite/PMMA composites have been evaluated as functions of particle size and dispersion of the graphitic nanofiller components via the use of three different graphitic nanofillers: “as received graphite” (ARG), “expanded graphite,” (EG) and “graphite nanoplatelets” (GNPs) EG, a graphitic materials with much lower density than ARG, was prepared from ARG flakes via an acid intercalation and thermal expansion. Subsequent sonication of EG in a liquid yielded GNPs as thin stacks of graphitic platelets with thicknesses of ∼10 nm. Solution-based processing was used to prepare PMMA composites with these three fillers. Dynamic mechanical analysis, thermal analysis, and electrical impedance measurements were carried out on the resulting composites, demonstrating that reduced particle size, high surface area, and increased surface roughness can significantly alter the graphite/polymer interface and enhance the mechanical, thermal, and electrical properties of the polymer matrix. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2097–2112, 2007
    Journal of Polymer Science Part B Polymer Physics 07/2007; 45(15):2097 - 2112. · 2.22 Impact Factor
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    H. Shen, L. C. Brinson
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    ABSTRACT: The porous microstructures of metallic foams cause microscopic stress and strain localization under deformation which reduces the damage tolerance and therefore limits application of the materials. In this paper, the deformation of a relatively low porosity porous titanium is examined using two-dimensional (2D) plane strain and three-dimensional (3D) finite element models to identify the accuracy and limitations of such simulations. To generate the finite element models, a simulated microstructure was created based on micrographs of an experimental material. Compared to the 2D models, the 3D models require smaller model size to obtain convergent results. The macroscopic responses predicted by the 3D models are in reasonable agreement with experimental results while the 2D models underestimated the response. In addition, 3D models predicted more uniform microscopic field variable distributions. 2D models predicted higher probability of Von Mises stress and equivalent plastic strain exceeding a certain value and therefore overestimate the failure probability of the material.
    International Journal of Solids and Structures 01/2007; 44(1):320-335. · 2.04 Impact Factor
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    ABSTRACT: To facilitate the design and development of porous metals, simulation of their mechanical behavior is essential. As an alternative to complex tomography procedures, a methodology has been developed to construct a simulated microstructure that retains the essential features of the experimental material. The target material is a moderate porosity titanium foam that is being developed as a bone implant material. The methodology applies stereology theory to a foaming process based on growth of pressurized pores. Three-dimensional (3D) pore size and pore distribution information is derived from 2D sections for a sample with low porosity, early in the foaming process. A 3D microstructure is developed based on the 3D location and size distribution of the pores by use of a computational procedure. Pores are allowed to grow and coalesce in a simple simulated foaming process to achieve microstructures of higher porosity. These data have been used as inputs to write scripts of I-DEAS to create 3D finite element models which are then examined for basic global and local mechanical properties.
    Mechanics of Materials. 01/2006;

Publication Stats

73 Citations
4.26 Total Impact Points

Institutions

  • 2006–2011
    • Northwestern University
      • • Department of Mechanical Engineering
      • • Department of Materials Science and Engineering
      Evanston, IL, United States