Chapter9b_PI - Chapter 9 Performance Indices and Materials...

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Chapter 9 Performance Indices and Materials Charts for Materials Selection
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Reading Material • AERO3700 course pack p. 75 - 105
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erformance Indices and Materials Performance Indices and Materials Charts • Performance indices and materials charts are extremely useful in materials selection • They allow the prediction of performance ased on properties such as yield strength based on properties such as yield strength, fatigue and fracture, toughness etc. ou have already seen specific strength ( • You have already seen specific strength ( σ y / ρ ) and specific stiffness (E/ ρ ). There are other erformance indices that are also used performance indices that are also used.
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• Screening materials and ranking materials are key to optimized selection. • Consider an aircraft. Significant portions are dominated by compressive design requirements; e.g., the upper surface of the wing and the associated internal spars as well as the internal fuselage structure such as floor eams and support struts For these we need to avoid beams and support struts. For these we need to avoid elastic buckling and crippling (plastic collapse). bout half the aircraft requires design for stiffness About half the aircraft requires design for stiffness • Performance indices and material selection charts rovide a systematic way of approaching the problem. provide a systematic way of approaching the problem.
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Four Basic Steps in Material Selection z Translation of design requirements into a material ecification specification z Screening out of materials that do not meet nstraints constraints z Ranking by ability to meet objectives using material indices z Search for supporting information for promising candidates z Other considerations: cost, availability, form, manufacturability
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Design Requirements and Material Specification Design requirements “Translation” Analyze: What does the component do ? Function What essential conditions must be met ? Objectives Constraints What is to be maximized or minimized ? Which design variables are free ? Free variables From which we obtain … creening criteria: go / no- o criteria (usually many) Screening criteria: go / no go criteria (usually many) Ranking criteria: an ordering of the materials that “go”
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Function, objective, constraints • FUNCTION – what does the part do? (support a load, contain a pressure, transmit heat) BJECTIVE hat is to be minimized or • OBJECTIVE – what is to be minimized or maximized? ( the designer wants to make it light or safe) ONSTRAINTS hat non egotiable • CONSTRAINTS – what non-negotiable conditions must be met? (hard constraint e.g. , certain dimensions are fixed, it must carry the load without failure, it must function in certain temperature ranges ). What negotiable but desirable conditions must be met? (soft constraint)
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M. Ashby, Materials Selection and Mechanical Design, 2 nd Ed., Butterworth Function, Objective & Constraint Lead to Index
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Performance Index (PI or P ) or Material Index (M) I • The material index (or performance index) is a
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Chapter9b_PI - Chapter 9 Performance Indices and Materials...

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