Topic 6_Thermal Behavior_s

Topic 6_Thermal Behavior_s - Topic 6: Thermal Behavior MATE...

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1 1 Topic 6: Thermal Behavior MATE 210 Introduction to Materials Engineering 2 Topic 6 Outline Topic 6 Outline Composition Structure Performance Processing Atomic Vibration
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2 3 Application: Low Temperature O-Rings 4 Challenger Explosion • The Challenger failed as a result of both technical and organizational failures. • Among the technical failures was a lack of understanding about the cold temperature behavior of the elastomer o-rings.
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3 5 Typical Uses for O-Rings Rotating seal Static Seal 6 Designing O-Rings The elastic response to an applied stress is how o-rings provide a seal. Design objectives: 1. The key to a good seal, is to maximize the contact area between the o-ring and the mating surface. 2. Contact stresses in the o-ring must be less than the yield strength. 3. The o-ring must have a low yield strength so that it yields before the mating surfaces (e.g. σ y <100 MPa) to prevent the mating surfaces from being damaged.
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4 7 Critical Properties y R FE σ = 3 1 6 . 0 Objective #1: Contact area Objective #3: Limited Yield Stress Objective #2: Contact stresses 3 1 2 E FR b σ y < 100 MPa So elastic modulus (E) and yield strength ( σ y ) are critical properties for an o-ring. But what if we wanted an 0-ring that performed well at low temperatures? 8 Thermal Properties Composition Structure Performance Processing
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5 9 Melting Temperature (T m ) of Crystalline Solids • Temperature at which a pure crystalline solid becomes a liquid. • Atoms form solids to reduce their system energy. To break these atomic bonds, energy must be added to the system. When thermal energy (heat) is added the atoms vibrate. Atomic spacing at 0 K, r 0 Energy in bond at 0 K (energy well) Bond strength of A > B > C T mA > T mB > T mC Atomic vibration at elevated temperature 10 Glass Transition Temperature (T g ) : when an amorphous solid is heated it reaches a temperature at which the molecules begin moving easily and a viscous liquid is formed This isn’t melting because the material had no crystal structure!
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6 11 Melting of Solids • Crystalline solids undergo a state change at T m .
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Topic 6_Thermal Behavior_s - Topic 6: Thermal Behavior MATE...

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