T and Strain Rate: Thermoplastics C 20 C 40 C

Transcription

1 T and Strain Rate: Thermoplastics Decreasing T... --increases E --increases TS --decreases %EL Increasing strain rate... --same effects as decreasing T. σ(mpa) C 20 C 40 C Data for the semicrystalline polymer: PMMA (Plexiglas) 20 to C Adapted from Fig. 15.3, Callister 6e. (Fig is from T.S. Carswell and J.K. Nason, 'Effect of Environmental Conditions on the Mechanical Properties of Organic Plastics", Symposium on Plastics, American Society for Testing and Materials, Philadelphia, PA, 1944.) ε

2 Elastic, Viscoleastic and Viscous Behavior Input: Constant Stress Response: Elastic Response: Viscoelastic Response: Viscous

3 Time Dependent Deformation Stress relaxation test: --strain to εο and hold. --observe decrease in stress with time. ε o tensile test time Relaxation modulus: E r (t) = σ(t) ε o strain σ( t) Data: Large drop in Er for T > Tg Er(10s) in MPa (amorphous polystyrene) T( C) Tg Sample Tg(C) values: PE (low Mw) PE (high Mw) PVC PS PC rigid solid (small relax) transition region viscous liquid (large relax) Adapted from Fig. 15.7, Callister 6e. (Fig is from A.V. Tobolsky, Properties and Structures of Polymers, John Wiley and Sons, Inc., 1960.) Selected values from Table 15.2, Callister 6e.

4 More on the Relaxation Modulus Schematic E r (t) for a thermoplastic E r (t) for amorphous polystyrene

5 More (again!) on the Relaxation Modulus A: Crystalline isotactic B: Lightly cross-linked atactic C: Amorphous Relaxation Modulus: Polystyrene

6 Fracture of Polymers Crazing (thermoplastics) Formation of localized yielding Fibrillar bridges between microvoids Molecular chain reorientation + bridge fibers = increased fracture toughness Crazing in Polyeythylene Oxide

7 Fatigue in Polymers Fatigue in polymers Less studied than in metals Strong dependence on testing frequency (why?)

8 Melting and the Glass Transition Temperature Melting Temperature T m Transition to a viscous liquid Can occur over a range of T Depends on sample history! Glass Transition Temperature T g Amorphous or semicrystalline polymers Transition Rubbery solid to a rigid solid

9 Factors Influencing T m and T g Melting Temperature T m as chain stiffness e.g., add double bonds on chain T m with bulky side chains or polar side groups e.g., polypropylene: T m = 175 C; polyethylene: T m = 175 C e.g., polyvinyl chloride: T m = 175 C; polyethylene: T m = 175 C Tm with significant side branching Weaker interchain interactions, low density Similar trends for the Glass Transition Temperature Cross-linking: T g

10 Thermoplastics: --little cross linking --ductile --soften w/heating --polyethylene (#2) polypropylene (#5) polycarbonate polystyrene (#6) Thermoplastics vs. Thermosets Thermosets: --large cross linking (10 to 50% of mers) --hard and brittle --do NOT soften w/heating --vulcanized rubber, epoxies, polyester resin, phenolic resin T mobile liquid crystalline solid viscous liquid Callister, rubber Fig tough plastic partially crystalline solid Molecular weight Adapted from Fig , Callister 6e. (Fig is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed., John Wiley and Sons, Inc., 1984.) Tm Tg

11 Liquid Crystalline Polymers

12 Summary General drawbacks to polymers: -- E, σy, Kc, Tapplication are generally small. -- Deformation is often T and time dependent. -- Result: polymers benefit from composite reinforcement. Thermoplastics (PE, PS, PP, PC): -- Smaller E, σy, Tapplication -- Larger Kc -- Easier to form and recycle Elastomers (rubber): -- Large reversible strains! Thermosets (epoxies, polyesters): -- Larger E, σy, Tapplication -- Smaller Kc Table 15.3 Callister 6e: Good overview of applications and trade names of polymers.

13 Chapter 16: Composite Materials ISSUES TO ADDRESS... What are the classes and types of composites? Why are composites used instead of metals, ceramics, or polymers? How do we estimate composite stiffness & strength? What are some typical applications?

14 Terminology/Classification Composites: --Multiphase material w/significant proportions of ea. phase. Matrix: --The continuous phase --Purpose is to: transfer stress to other phases protect phases from environment --Classification: MMC, CMC, PMC metal ceramic polymer Dispersed phase: --Purpose: enhance matrix properties. MMC: increase σy, TS, creep resist. CMC: increase Kc PMC: increase E, σy, TS, creep resist. --Classification: Particle, fiber, structural woven fibers 0.5mm cross section view 0.5mm Reprinted with permission from D. Hull and T.W. Clyne, An Introduction to Composite Materials, 2nd ed., Cambridge University Press, New York, 1996, Fig. 3.6, p. 47.

15 Composite Survey: Particle-I Particle-reinforced Fiber-reinforced Structural Examples: -Spheroidite matrix: steel ferrite (α) (ductile) 60µm particles: cementite (Fe3C) (brittle) Adapted from Fig , Callister 6e. (Fig is copyright United States Steel Corporation, 1971.) -WC/Co cemented carbide matrix: cobalt (ductile) Vm: 10-15vol%! 600µm particles: WC (brittle, hard) Adapted from Fig. 16.4, Callister 6e. (Fig is courtesy Carboloy Systems, Department, General Electric Company.) -Automobile tires matrix: rubber (compliant) particles: C (stiffer) Adapted from Fig. 16.5, Callister 6e. (Fig is courtesy Goodyear Tire and Rubber Company.) 0.75µm

16 Composite Survey: Particle-II Particle-reinforced Fiber-reinforced Elastic modulus, Ec, of composites: -- two approaches. Data: Cu matrix w/tungsten particles E(GPa) upper limit: rule of mixtures E c = V m E m + V p E p lower limit: 1 = V m + V p E c E m E p Structural Adapted from Fig. 16.3, Callister 6e. (Fig is from R.H. Krock, ASTM Proc, Vol. 63, 1963.) vol% tungsten (Cu) (W) Application to other properties: -- Electrical conductivity, σe: Replace E by σe. -- Thermal conductivity, k: Replace E by k.

17 Composite Survey: Fiber-I Particle-reinforced Fiber-reinforced Aligned Continuous fibers Examples: --Metal: γ'(ni3al)-α(mo) by eutectic solidification. matrix: α (Mo) (ductile) Structural --Glass w/sic fibers formed by glass slurry Eglass = 76GPa; ESiC = 400GPa. 2µm fibers:γ (Ni3Al) (brittle) From W. Funk and E. Blank, Creep deformation of Ni3Al-Mo in-situ composites", Metall. Trans. A Vol. 19(4), pp , Used with permission. (a) (b) fracture surface From F.L. Matthews and R.L. Rawlings, Composite Materials; Engineering and Science, Reprint ed., CRC Press, Boca Raton, FL, (a) Fig. 4.22, p. 145 (photo by J. Davies); (b) Fig , p. 349 (micrograph by H.S. Kim, P.S. Rodgers, and R.D. Rawlings). Used with permission of CRC Press, Boca Raton, FL.

18 Composite Survey: Fiber-II Particle-reinforced Fiber-reinforced Discontinuous, random 2D fibers Example: Carbon-Carbon --process: fiber/pitch, then burn out at up to 2500C. --uses: disk brakes, gas (b) turbine exhaust flaps, nose cones. Other variations: --Discontinuous, random 3D --Discontinuous, 1D (a) view onto plane Structural C fibers: very stiff very strong C matrix: less stiff less strong fibers lie in plane Adapted from F.L. Matthews and R.L. Rawlings, Composite Materials; Engineering and Science, Reprint ed., CRC Press, Boca Raton, FL, (a) Fig. 4.24(a), p. 151; (b) Fig. 4.24(b) p (Courtesy I.J. Davies) Reproduced with permission of CRC Press, Boca Raton, FL.

19 Composite Survey: Fiber-III Particle-reinforced Fiber-reinforced Structural Critical fiber length for effective stiffening & strengthening: fiber strength in tension fiber diameter fiber length > 15 σ f d τ c Ex: For fiberglass, fiber length > 15mm needed shear strength of fiber-matrix interface Why? Longer fibers carry stress more efficiently! Shorter, thicker fiber: fiber length < 15 σ f d τ c σ(x) Adapted from Fig. 16.7, Callister 6e. Longer, thinner fiber: fiber length > 15 σ f d τ c σ(x) Poorer fiber efficiency Better fiber efficiency

20 Composite Survey: Fiber-IV Particle-reinforced Estimate of Ec and TS: --valid when Fiber-reinforced fiber length > 15 σ f d τ c -- Elastic modulus in fiber direction: Structural E c = E m V m + KE f V f efficiency factor: --aligned 1D: K = 1 (anisotropic) --random 2D: K = 3/8 (2D isotropy) --random 3D: K = 1/5 (3D isotropy) --TS in fiber direction: Values from Table 16.3, Callister 6e. (Source for Table 16.3 is H. Krenchel, Fibre Reinforcement, Copenhagen: Akademisk Forlag, 1964.) (TS) c = (TS) m V m + (TS) f V f (aligned 1D)

21 Composite Survey: Structural Particle-reinforced Fiber-reinforced Structural Stacked and bonded fiber-reinforced sheets -- stacking sequence: e.g., 0/90 -- benefit: balanced, in-plane stiffness Sandwich panels -- low density, honeycomb core -- benefit: small weight, large bending stiffness face sheet adhesive layer honeycomb Adapted from Fig , Callister 6e. Adapted from Fig , Callister 6e. (Fig is from Engineered Materials Handbook, Vol. 1, Composites, ASM International, Materials Park, OH, 1987.

22 Composite Benefits CMCs: Increased toughness Force particle-reinf un-reinf fiber-reinf Bend displacement MMCs: Increased creep resistance εss (s -1 ) Al E(GPa) Al w/sic whiskers σ(mpa) PMCs: Increased E/ρ PMCs G=3E/8 K=E ceramics metal/ metal alloys polymers Density, ρ [Mg/m 3 ] Adapted from T.G. Nieh, "Creep rupture of a silicon-carbide reinforced aluminum composite", Metall. Trans. A Vol. 15(1), pp , Used with permission.

23 Summary Composites are classified according to: -- the matrix material (CMC, MMC, PMC) -- the reinforcement geometry (particles, fibers, layers). Composites enhance matrix properties: -- MMC: enhance σy, TS, creep performance -- CMC: enhance Kc -- PMC: enhance E, σy, TS, creep performance Particulate-reinforced: -- Elastic modulus can be estimated. -- Properties are isotropic. Fiber-reinforced: -- Elastic modulus and TS can be estimated along fiber dir. -- Properties can be isotropic or anisotropic. Structural: -- Based on build-up of sandwiches in layered form.