E MCH 213D Material Selection - 1 Material Selection Tutorial • Selecting an appropriate material is a critical part of almost all engineering designs • There are many factors to consider – Strength, stiffness, durability, corrosion, cost, formability, etc • Methods – Experience: how do you get it? limiting – Ashby selection charts (http://www-materials.eng.cam.ac. uk/mpsite/DT .html ) – Quantitative ranking of options (described here)
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E MCH 213D Material Selection - 1
Material Selection Tutorial• Selecting an appropriate material is a critical part of
almost all engineering designs• There are many factors to consider
• Methods– Experience: how do you get it? limiting– Ashby selection charts(http://www-materials.eng.cam.ac.uk/mpsite/DT.html)– Quantitative ranking of options (described here)
Quantitative Ranking of Options forMaterial Selection*
• Objective: develop a rational method to select the bestmaterial for an application based upon known materialparameters and the requirements of the application
• Use a 5-step method1. Select a quantity, Q, to minimize or maximize2. Classify the variables3. Determine the relationship between the geometry variable, the
requirements, and material properties4. Determine Q as a function of requirements and material
properties5. Rank candidate materials based upon function f2
* Based on N.E. Dowling, Mechanical Behavior of Materials, section 3.8
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Step 1: Select a quantity, Q, tominimize or maximize
• Mass (weight), m• Cost, C
are the most common and the only ones that we willconsider
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Step 2: Classify the variables
• Requirements – variables that have prescribed valuesthat will not change
• Geometry – variables that define the dimensions of thecomponent and depend implicitly upon the materialproperties
• Material Properties – variables used to define thematerial in terms of physical behavior, mechanicalbehavior, and cost
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Step 3: Determine the relationshipbetween the geometry variable, the
requirements, and materialproperties
• Strength– Bar, axial stress– Beam, flexural stress
• Stiffness– Bar, deformation– Beam, deflection
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Step 4: Determine Q as a functionof requirements and material
properties
• Q = f1(requirements)* f2(material props)
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Step 5: Rank candidate materialsbased upon function f2
• If both weight and cost are important then separaterankings can be generated and results combined
• Calculate geometry variable after ranking materials– Adjustments may be necessary if calculated
dimensions are impractical (either too large or toosmall)
• There may be multiple requirements such as strengthand serviceability– Often material can be selected based on strength and
then the serviceability requirements checked
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Sample Problem• We must bridge a gap of L = 8’• The bridge must have a width of b = 4”• A load P = 300 lb can be applied at any point• There must be a safety factor X = 1.5 for strength• The deflection, v, must not exceed 1”• Weight (mass) and cost have equal importance
OBJECTIVE: select the best candidate material from…AISI 1020 steel AISI 4340 steel7075-T6 aluminum Ti-6Al-4V (titanium alloy)Polycarbonate Loblolly pineGFRP (glass fiber reinforced polymer)CFRP (carbon fiber reinforced polymer)
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Step 1: Select a quantity, Q, tominimize
Here, mass and cost have equal importance• Mass, m• Cost, CSelect Q to be the sum of the normalized mass and cost• Q = m/min(m) + C/min(C)
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Step 2: Classify the variables
• Requirements: L = 8’, b = 4”, P = 300 lb, X = 1.5, v = 1”• Geometry: restrict analysis to a rectangular cross-
section, h = height• Material Properties (need step 3 & 4 results here):ρ = mass density, E = Young’s modulus, S = strength,Cm = cost index
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Step 3: Determine the relationshipbetween the geometry variable, the
requirements, and materialproperties
• We have a simply supported beam with a rectangularcross-section
• The worst case occurs when the concentrated load, P, isapplied at the center
E MCH 213D Material Selection - 13
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Strength – elastic flexural formula shows the maximum stress occurs at the extreme fibers of the beam at midspan
Deflection – from integration, is found to be maximum at midspan
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Step 4: Determine Q as a functionof requirements and materialproperties – strength basis
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Step 5: Rank materials basedupon function f2 – strength basis