Rapid Prototyping Tensile Test The most common static test is the uniaxial tensile test, which provides information about a variety of properties. As.

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Tensile Test• The most common static test is the

uniaxial tensile test, which provides information about a variety of properties.

• As a load is applied to a test specimen, the elongation is recorded.

• The ordered pairs are graphed in a stress/strain curve.

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Universal Testing Machine

Specimenmounting

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Schematics of the Process

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Units of Strength: Stress• Strength of materials are recorded in units

of Force/Area, and are referred to as various stresses.

• The symbol for stress is the Greek letter sigma:

• Stress is synonymouswith strength

F

A

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Examples of Normal Stress• Cables on suspension bridges.

• Legs of chairs: dent the pine floors? (compressive strength of pine vs oak)

• Skyboxes at the ball park.

• Cutting tool operations.

• Forming and fabricating operations.

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Units of Deformation: Strain• Tension equates to elongation• Units of elongation are Strain:

• Note that strain is dimensionless: merely a ratio

o

o

o o

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The Stress-StrainDiagram(fig A.5)

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Region I

The modulus of Elasticity (E) is calculated in stress/strain up to the proportionallimit.Designers live here

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Young’s Modulus (E)• Is in the Elastic region: strain is directly

proportional to stress. (Hooke’s Law)• Units are in psi or MPa• Yield stress is the strength of the material

when permanent deformation takes place.• The larger value of E, the more resistant a

material is to deformation. This is a measure of material stiffness in response to an applied load: a quantified property: E

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Elastic Deformation• Airplane wings: sway with fracture

• 1950’s accident: B17 into Empire State building: swayed 12’?

• Bridge swaying

• glass vs steel

• Design safety factors

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Region II: Plastic Deformation

Malleability: the ability to plasticallydeform in compression without breaking

Ductility: the ability to plasticallydeform in tension without breaking

Fabrication processes: forming

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Region III: Failure

Material thinsfrom the stress,work-hardeningleads to fracture

Cutting processes occur in this region

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Specific Tensile Properties• Modulus of Elasticity E

• Yield stress (strength)

• Ultimate Tensile strength (stress)(UTS)

• Ductility: elongation percentage

final original

original

100

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Strength of Materials• Strength is the property determined by the

greatest stress the material can withstand prior to failure, as failure is determined by the designer. It may be defined by the proportional limit, the yield point, or ultimate strength. No single value is adequate to define strength, since the behavior under load differs with the kind of stress, and the nature of the loading.

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Stiffness• The property that enables a material to

withstand high stress without great strain (elongation). It is a resistance to any form of deformation, and is a function of the modulus of elasticity.

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Elasticity & Ductility• Elasticity is the property of material enabling it

to regain its original dimensions after removal of a deforming load.

• Ductile materials can undergo considerable plastic deformation under tension, before breaking. % elongation is the measurement. Wires, extrusions as examples

• Malleability is material ability to deform plastically in compression

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Brittleness• Brittleness implies the absence of any

plastic deformation prior to sudden failure. This means the breaking strength (UTS) is usually equal to the yield stress. In general, poor in tension, and tested in compression: concrete, cast iron, ceramics.

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Toughness & Resilience • Toughness is that property enabling it to

endure high impact loads or shock loads. A body that can be both highly stressed, and greatly deformed without failure is tough.

• Resilience is that property enabling it to endure high impact loads without inducing a stress in excess of the elastic limit (yield).

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Creep• Permanent deformation at stress less than yield.• Materials with higher resilience are more

susceptible to creep.• Temperature is the most contributing

environmental factor.• For example: Creep limit for LEXAN at 73oF is

2000psi. (GE, p.5)• Easiest to beat by adding reinforcing materials.

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Comparison of Materials

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The Nature of the Load

Shear LoadsShear StressesShear strengths can be approximated from tensile strengths

Determine the stresson the 1.0” Ø rod.

If, by design, the stress onthe rod cannot exceed 10,000psidetermine the minimum diameter

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Torsion Stress

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Brazing Application

allow psi6 000,

L

0.75 6 ton load

Determine L