13.Materials%20Selection - MSE 280: Engineering Materials...

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1 MSE 280: Engineering Materials Reading: Callister Ch. 21 Materials Selection and Design MSE280 © 2007-2009 Moonsub Shim Materials Selection and Design For selection, one must establish a link between materials and function , with shape and processing also playing possibly important roles (ignored for now) Materials Attributes: physical, function AREAS OF DESIGN CONCERN Function- support a load, contain a mechanical, thermal, electrical, economic, environmental. shape process pressure, transmit heat, etc. What does the component do? Objective- make things cheaply, light weight, increase safety, etc., or combinations of these. What needs to be maximized or minimized? Constraints- make things within given budget, max required weight safety requirements MSE280 © 2007-2009 Moonsub Shim max required weight, safety requirements, etc., or combinations of these. What are non-negotiable conditions to be met? What are negotiable but desired conditions? Following “Materials Selection in Mechanical Design”, M. Ashby Modified from D. Johnson
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2 Design & Selection: Materials Indices Material index ( performance index ) is a combination of materials properties that characterizes the Performance of a material in a given application. Performance of a structural element may be specified by the functional requirements, the geometry, and the material’s properties. Performance [ (Functional needs, F); (Geometric, G); (Material Property, M) ] For OPTIMUM design , we need to MAXIMIZE or MINIMIZE the Performance. MSE280 © 2007-2009 Moonsub Shim Consider only the simplest cases where these factors form a separable equation . Performance = f 1 (F) f 2 (G) f 3 (M) From D. Johnson Examples of Materials Indices Function, Objective, and Constraint Index Tie, minimum weight, stiffness E/ ρ Beam, minimum weight, stiffness E 1/2 / ρ Beam, minimum weight, strength s 2/3 / ρ Beam, minimum cost, stiffness E 1/2 /C m ρ Beam, minimum cost, strength s 2/3 /C m ρ Column, minimum cost, buckling load E 1/2 /C m ρ C m =cost/mass MSE280 © 2007-2009 Moonsub Shim Spring, minimum weight for given energy storage s YS 2 /E ρ Thermal insulation, minimum cost, heat flux 1/(a C m ρ ) Electromagnet, maximum field, temperature rise k C p ρ α =thermal cond κ =elec. cond Modified from D. Johnson Why different dependences? – linear vs. square root etc.
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3 Example 1: Material Index for a Light, Strong, Tie-Rod A Tie-rod is common mechanical component. Functional needs: F, L, σ f • Tie-rod must carry tensile force , F . • NO failure . Stress must be less than σ f . (f=YS, UTS) L is usually fixed by design, can vary Area A . A = x-area F L • While strong, needs to be lightweight, or low mass . F A σ f S -Strength relation: Tie-Rod has to be able to withstand applied tensile load Safety facto f A F Applied tensile stress = Including safety factor: MSE280 © 2007-2009 Moonsub Shim Safety factor - Mass of rod: m = ρ LA - Eliminate the "free" design parameter, A : A = m/ L S L m F f / Example 1: Material Index for a Light, Strong, Tie-Rod minimize for small m m ( FS )( L ) f - Rearrange: ** Compare to Performance = f1(F) f2(G) f3(M) Functional Needs (how much
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This note was uploaded on 03/15/2010 for the course MSE 280 taught by Professor Johnson during the Fall '08 term at University of Illinois, Urbana Champaign.

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13.Materials%20Selection - MSE 280: Engineering Materials...

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