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Primer onMaterials and Design for Stiffness

TEC 316Dr. Lou Reifschneider

Many figures taken from “Designing with Plastics by TICONA”

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The Ideal Material

• Endless and ready supply• Cheap to refine and produce• Energy efficient• Strong, stiff, & dimensionally stable at all temperatures• Lightweight• Corrosion resistant• No harmful effects on the environment or people• Biodegradable - when appropriate• Numerous secondary uses

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Important Material Properties• Strength (tensile, compressive, flexural, shear, and

torsional).• Resist deformation at elevated temperatures.• Fatigue resistance (repeated loading and unloading).• Toughness (resistance to impact).• Wear resistance (hardness).• Corrosion resistance (acids, oil, water, ..).• Electrical arc resistance (wiring).• Thermal insulation (where appropriate)

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from Design for Manufacturability Handbook, 2nd Ed. by James G. Bralla

Classification of Engineering Materials

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from Design for Manufacturability Handbook, 2nd Ed. by James G. Bralla

Classification of Engineering Materials

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from Plastics for Engineers by Domininghaus, H.

Stress/Strain Behavior of Metals and Plastics

Material Stiffness called Young’s Modulus, E

Initial slope of Stress/Strain curve is E

Beam Bending

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Beam during bending

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Common Constraints

Important Beam Geometry: Moment of Inertia, written as I

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A beam 10” long, 1” wide and 1” tall has an I of

121

1211

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bdI

“na” in above denotes the neutral axis.

Moment of Inertia of 2 x 4 board

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2 x 4 standing on 2” side has I

433

6.101242

12inbdI

2 x 4 standing on 4” side has I

433

6.21224

12inbdI

Vertical 2x4 is4 times stiffer than horizontal 2x4

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• The stiffer something is, the less it deflects under a given load.

• Material Stiffness ~ E• Geometric Stiffness ~ I• Flexural Rigidity ~ E x I

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RigidityLOAD

EILOADntDisplaceme

Design made stiffer by

• Increasing Modulus (E) by changing material.

OR• Increasing I by making part thicker,

wider, or moving material away from the bending axis.

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Want to stiffen this design.

As stiff, BUT THICKER (much more material)

Stiffer, keeping original thickness of product.

Other ways to stiffen while keeping original thickness

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