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The Design Core Market Assessment Specification Concept Design Detail Design Manufacture Sell DETAIL DESIGN A vast subject. We will concentrate on: Materials Selection Process Selection Cost Breakdown

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Materials Selection with Shape FUNCTION MATERIAL PROCESS SHAPE SHAPES FOR TENSION, BENDING, TORSION, BUCKLING SHAPE FACTORS PERFORMANCE INDICES WITH SHAPE

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Common Modes of Loading

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Moments of Sections: Elastic Section Shape A (m 2 ) I (m 4 ) K (m 4 ) A = Cross-sectional area I = Second moment of area wherey is measured vertically b y is the section width at y K = Resistance to twisting of section ( Polar moment J of a circular section) whereT is the torque L is the length of the shaft θ is the angle of twist G is the shear modulus

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Moments of Sections: Elastic Section Shape A (m 2 ) I (m 4 ) K (m 4 )

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Moments of Sections: Failure Section Shape Z (m 3 ) Q (m 3 ) Z = Section modulus where y m is the normal distance from the neutral axis to the outer surface of the beam carrying the highest stress Q = Factor in twisting similar to Z where is the maximum surface shear stress

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Moments of Sections: Failure Section Shape Z (m 3 ) Q (m 3 )

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Shape Factors: Elastic BENDING Bending stiffness of a beam where C 1 is a constant depending on the loading details, L is the length of the beam, and E is the Youngs modulus of the material Define structure factor as the ratio of the stiffness of the shaped beam to that of a solid circular section with the same cross- sectional area thus: so, TORSION Torsional stiffness of a beam where L is the length of the shaft, G is the shear Modulus of the material. so, Define structure factor as the ratio of the torsional stiffness of the shaped shaft to that of a solid circular section with the same cross-sectional area thus:

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Shape Factors: Failure/Strength BENDING Define structure factor as the ratio of the failure moment of the shaped beam to that of a solid circular section with the same cross-sectional area thus: so, The beam fails when the bending moment is large enough for σ to reach the failure stress of the material: The highest stress, for a given bending moment M, experienced by a beam is at the surface a distance y m furthest from the neutral axis: TORSION The highest shear stress, for a given torque T, experienced by a shaft is given by: so, Define structure factor as the ratio of the failure torque of the shaped shaft to that of a solid circular section with the same cross-sectional area thus: The beam fails when the torque is large enough for to reach the failure shear stress of the material:

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Shape Factors: Failure/Strength Please Note: The shape factors for failure/strength described in this lecture course are those defined in the 2nd Edition of Materials Selection In Mechanical Design by M.F. Ashby. These shape factors differ from those defined in the 1st Edition of the book. The new failure/strength shape factor definitions are the square root of the old ones. The shape factors for the elastic case are not altered in the 2nd Edition.

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Comparison of Size and Shape Rectangular sections I-sections SIZE

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Shape Factors Section Shape StiffnessFailure/Strength

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Shape Factors contd Section Shape StiffnessFailure/Strength

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Efficiency of Standard Sections ELASTIC BENDING Shape Factor: Rearrange for I and take logs: Plot logI against logA : parallel lines of slope 2

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Efficiency of Standard Sections BENDING STRENGTH Shape Factor: Rearrange for I and take logs: Plot logI against logA : parallel lines of slope 3/2

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Efficiency of Standard Sections ELASTIC TORSION TORSIONAL STRENGTH N.B. Open sections are good in bending, but poor in torsion

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Performance Indices with Shape ELASTIC BENDING Bending stiffness of a beam: ELASTIC TORSION Torsional stiffness of a shaft: f1(F) · f2(G) · f3(M) So, to minimize mass m, maximise Shape factor: so, So, to minimize mass m, maximise Shape factor: so, f1(F) · f2(G) · f3(M)

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Performance Indices with Shape FAILURE IN BENDING Failure when moment reaches: FAILURE IN TORSION Failure when torque reaches: So, to minimize mass m, maximise Shape factor: so, f1(F) · f2(G) · f3(M) So, to minimize mass m, maximise f1(F) · f2(G) · f3(M) Shape factor: so,

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Shape in Materials Selection Maps Engineering Alloys Polymer Foams Woods Engineering Polymers Elastomers Composites Ceramics Search Region A material with Youngs modulus, E and density, ρ, with a particular section acts as a material with an effective Youngs modulus and density Performance index for elastic bending including shape, can be written as EXAMPLE 1, Elastic bending Φ=1 Φ=10

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Shape in Materials Selection Maps A material with strength, σ f and density, ρ, with a particular section acts as a material with an effective strength and density Performance index for failure in bending including shape, can be written as EXAMPLE 1, Failure in bending Engineering Alloys Polymer Foams Ceramics Composites Search Region Woods Elastomers Engineering Polymers Φ=1 Φ=10

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Micro-Shape Factors Material Micro-Shape + Macro-Shape, φ + Macro-Shape from Micro-Shaped Material, ψφ = Up to now we have only considered the role of macroscopic shape on the performance of fully dense materials. However, materials can have internal shape, Micro-Shape which also affects their performance, e.g. cellular solids, foams, honeycombs. Micro-Shaped Material, ψ =

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Micro-Shape Factors Prismatic cells Concentric cylindrical shells with foam between Fibres embedded in a foam matrix Consider a solid cylindrical beam expanded, at constant mass, to a circular beam with internal shape (see right). Stiffness of the solid beam: On expanding the beam, its density falls fromto, and its radius increases from to The second moment of area increases to If the cells, fibres or rings are parallel to the axis of the beam then The stiffness of the expanded beam is thus Shape Factor:

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Function Tie Beam Column Shaft Mats. Selection: Multiple Constraints Objective Minimum cost Minimum weight Maximum stored energy Minimum environmental impact Constraint Stiffness Strength Fatigue Geometry Mechanical Thermal Electrical….. Index

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Materials for Safe Pressure Vessels DESIGN REQUIREMENTS FunctionPressure vessel =contain pressure p ObjectiveMaximum safety Constraints(a)Must yield before break (b)Must leak before break (c)Wall thickness small to reduce mass and cost Yield before break Leak before break Minimum strength

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Materials for Safe Pressure Vessels Search Region M 3 = 100 MPa M 1 = 0.6 m 1/2 MaterialM 1 (m 1/2 ) M 3 (MPa) Comment Tough steels Tough Cu alloys Tough Al alloys Ti-alloys High strength Al alloys GFRP/CFRP > Standard. OFHC Cu. 1xxx & 3xxx High strength, but low safety margin. Good for light vessels.

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1.Express the objective as an equation. 2.Eliminate the free variables using each constraint in turn, giving a set of performance equations (objective functions) of the form: where f, g and h are expressions containing the functional requirements F, geometry M and materials indices M. 3.If the first constraint is the most restrictive (known as the active constraint) then the performance is given by P 1, and this is maximized by seeking materials with the best values of M 1. If the second constraint is the active one then the performance is given by P 2 and this is maximized by seeking materials with the best values of M 2 ; and so on. N.B. For a given Function the Active Constraint will be material dependent. Multiple Constraints: Formalised

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Multiple Constraints: A Simple Analysis A LIGHT, STIFF, STRONG BEAM The object function is Constraint 1: Stiffnesswhereso, Constraint 2: Strengthwhereso, If the beam is to meet both constraints then, for a given material, its weight is determined by the larger of m 1 or m 2 or more generally, for i constraints MaterialE (GPa) σ f (MPa) ρ (kgm -3 ) m 1 (kg) m 2 (kg) 1020 Steel 6061 Al Ti Choose a material that minimizes

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Multiple Constraints: Graphical log Index M 1 log Index M 2 Construct a materials selection map based on Performance Indices instead of materials properties. The selection map can be divided into two domains in each of which one constraint is active. The Coupling Line separates the domains and is calculated by coupling the Objective Functions: where C C is the Coupling Constant. Coupling Line M 2 = C C ·M 1 M 1 Limited Domain M 2 Limited Domain A B Materials with M 2 /M 1 >C C, e.g., are limited by M 1 and constraint 1 is active. Materials with M 2 /M 1

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Multiple Constraints: Graphical Coupling Line M 2 = C C ·M 1 Search Area C log Index M 1 log Index M 2 M 1 Limited Domain M 2 Limited Domain A B C C A box shaped Search Region is identified with its corner on the Coupling Line. Within this Search Region the performance is maximized whilst simultaneously satisfying both constraints.are good materials. M 1 Limited Domain M 2 Limited Domain A B Coupling Line M 2 = C C ·M 1 log Index M 1 log Index M 2 C Search Area AC Changing the functional requirements F or geometry G changes C C, which shifts the Coupling Line, alters the Search Area, and alters the scope of materials selection. Nowandare selectable.

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Windings for High Field Magnets DESIGN REQUIREMENTS FunctionMagnet windings ObjectiveMaximize magnetic field Constraints(a)No mechanical failure (b)Temperature rise <150°C (c)Radius r and length L of coil specified 2r2rdd L N Turns Current i B Upper limits on field and pulse duration are set by the coil material. Field too high the coil fails mechanically Pulse too long the coil overheats ClassificationPulse Duration Field Strength Continuous Long Standard Short Ultra-short 1 s ms-1 s ms µs µs <30 T T T T >100 T

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Windings for High Field Magnets The field (weber/m 2 ) is whereμ o = the permeability of air, N = number of turns, i = current, λ f = filling factor, f(α,β) = geometric constant, α = 1+(d/r), β = L/2r CONSTRAINT 1: Mechanical Failure Radial pressure created by the field generates a stress in the coil σ must be less than the yield stress of the coil material σ y and hence So, B failure is maximized by maximizing

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Windings for High Field Magnets CONSTRAINT 1: Overheating So, B heat is maximized by maximizing The energy of the pulse is(R e = average of the resistance over the heating cycle, t pulse = length of the pulse) causes the temperature of the coil to rise by whereΩ e = electrical resistivity of the coil material C p = specific heat capacity of the coil material If the upper limit for the change in temperature is ΔT max and the geometric constant of the coil is included then the second limit on the field is

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Windings for High Field Magnets Material σ y (MPa) ρ (Mg/m 3 ) C p (J/kgK) Ω e (10 -8 Ωm) B failure (wb/m 2 ) B heat (wb/m 2 ) High conductivity Cu Cu-15%Nb composite HSLA steel Pulse length = 10 ms In this case the field is limited by the lowest of B failure and B heat : e.g. Thus defining the Coupling Line

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Search Region: Ultra-short pulse Search Region: long pulse Search Region: short pulse HSLA steels Cu Al-S150.1 Cu-4Sn Cu-Be-Co-Ni Be-Coppers GP coppers HC Coppers Cu-Nb Cu-Al 2 O 3 Cu-Zr Windings for High Field Magnets MaterialComment Continuous and long pulse High purity coppers Pure Silver Short pulse Cu-Al 2 O 3 composites H-C Cu-Cd alloys H-C Cu-Zr alloys H-C Cu-Cr alloys Drawn Cu-Nb comps Ultra short pulse, ultra high field Cu-Be-Co-Ni alloys HSLA steels Best choice for low field, long pulse magnets (heat limited) Best choice for high field, short pulse magnets (heat and strength limited) Best choice for high field, short pulse magnets (strength limited)

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