Lecture 13
- 1. Today’s objectives-Composite Applications and Discontinuous (short) fibers Be able to calculate the longitudinal strength for long, short, and very short fiber composites. Be able to calculate the Young’s modulus for discontinuous fibers oriented in 3 dimensions, 2 dimensions, or 1 dimension. Know about GFRP, CFRP, and AFRP PMC’s—increased modulus/weight ratio. Know the strengthening mechanisms for fibers in a composite. Recall transformation toughened ceramics (Yttria stabilized zirconia). Be familiar with carbon-carbon, hybrid, and structural composites. Understand fiber composite fabrication: protrusion, prepreg, and winding.
- 3. Optimal fiber length Long fibers are for L > 30L c Short (discontinuous) fibers are for L c < L < 30*L c Very short (also discontinuous) fibers are for L < L c . Poorer fiber efficiency Better fiber efficiency fiber diameter shear strength of fiber-matrix interface or the matrix itself (whichever is smaller) fiber strength in tension
- 4. Review for Long Fibers (L>L c ): Longitudinal Slope changes once the matrix begins to yield. When the fibers are stressed sufficiently, the worst one fails. Longitudinal tensile strength can be calculated. The load transfers to the matrix with each fiber failure. Overall part doesn’t fail until all fibers and the matrix fail. Transverse The transverse tensile strength is usually at least one order of magnitude less than the longitudinal strength. The matrix properties will dominate, along with the fiber/matrix bond strength.
- 5. Problem 16.16 In an aligned and continuous glass-fiber-reinforced nylon6,6 matrix composite, the fibers are to carry 94% of a load applied in the longitudinal direction. a) determine the volume fraction of fibers necessary. b) What will be the tensile strength of the composite. Assume matrix stress at fiber failure is 30 MPa. And, solving for Vf yields, Vf = 0.418 = 1440 MPa 76 3 Nylon 6,6 3400 72.5 Glass fiber σ * (MPa) E (GPa) And, to calculate the transverse strength:
- 6. Fiber composite strengthening mechanisms Improvements are similar to those for particle dispersions (hindering crack propagation). Redistributing stress near a crack tip and/or deflect cracks. Form bridges across crack faces. Absorb energy as whiskers are pulled out of the matrix by an advancing crack. Absorb energy upon fracture of whiskers.
- 7. Fiber composite failure as a f(load orientation) Fibers and matrix may fracture Interface may fracture Interface may shear http://www.matter.org.uk/matscicdrom/manual/co.html#_Toc360435310
- 8. What if fibers are short (<30L c =“discontinuous”)? They can still be partially oriented (in plane) It is even cheaper if they are totally randomly oriented Discontinuous, random 2D fibers • Examples: disk brakes, gas turbine exhaust flaps, nose cones. Adapted from F.L. Matthews and R.L. Rawlings, Composite Materials; Engineering and Science , Reprint ed., CRC Press, Boca Raton, FL, 2000. (a) Fig. 4.24(a), p. 151; (b) Fig. 4.24(b) p. 151. (Courtesy I.J. Davies) Reproduced with permission of CRC Press, Boca Raton, FL. (b) (a)
- 9. Strength for aligned fibers of various lengths Aligned shorter fibers can still be beneficial (and much cheaper). Chopped glass, carbon, and aramid fibers are employed in tennis rackets, automobile parts, etc. Fracture strength ( σ c * ) up to ½ that of continuous fiber composites. The longitudinal strength for short, aligned fibers with L c <L<30L c : The longitudinal strength for very short, aligned fibers with L<L c : Compare with the longitudinal strength for optimally long aligned fibers with L>30L c from earlier: F F F
- 10. E for Discontinuous and random fibers For very short fibers (L<L c ) the elastic modulus follows a rule of mixtures (as if the fibers were simply particles). The fibers may be oriented or random, dictating the composite properties (and the manufacturing cost). So, for uniaxial loads, aligned fibers are optimal. For isotropic loads, fibers should be uniformly randomly oriented either in plane or in all directions. 1 Parallel to fibers Parallel 0 Perpendicular Parallel 3 / 8 Any in-plane direction Random, uniform, in plane 1 / 5 Any direction Random, uniform, in all directions K Stress Direction Fiber Orientation
- 11. Summary of fiber orientation effects For uniform properties, the fiber distribution should be uniform. Continuous fibers should be aligned to take advantage of them. Discontinuous fibers benefit from parallel or random alignment depending on the application (uniaxial, biaxial, or arbitrary loading). For enhanced properties in many orientations: random orientations may be used, or multiple orthogonal layers can be stacked (structural composite).
- 12. Polymer matrix composites Primary composite type Glass Fiber Reinforced Polymer (GFRP, used in car bodies, boat hulls, pipes, containers, and flooring) Fiberglass with fibers of 3-20 um diameter are typically used. High strength to weight ratio (specific strength). Relatively chemically inert if the surface is protected from environment. Easily drawn into high strength fibers (flawless surfaces). Processing is straightforward. Some disadvantages of gfrp’s too: Polymer matrix begins to soften or deteriorate around 200C. not rigid enough (E is small). Carbon Fiber Reinforced Polymer (CFRP, used in sporting goods, aero/astro) Advantages of carbon fibers over glass: Higher E and strength of all reinforcing materials. Better high temperature response, except to oxidation. Less sensitive to environment, fluids, etc. Fiber properties can be tailored over a wide range of properties. Reasonable manufacturing options
- 13. More polymer matrices Kevlar, Spectra Excellent strength to weight ratio Great longitudinal tensile strength Excellent roughness, impact resistance Great resistance to creep and fatigue failure Resistant to combustion Stable from -200 to 200C. Inert unless strong acids or bases. But: Poor in compression Poor transverse tensile strength Expensive Aramid fiber reinforced polymer composites (AFRP, used for bulletproofing, asbestos replacement, sporting goods)
- 14. Carbon-Carbon composites Advantages High tensile modulus and strength up to 2000C Resistant to creep Large fracture toughness Low coefficient of thermal expansion High thermal conductivity Thus, thermal shock is relatively unimportant Disadvantages High temperature oxidation Very expensive Complex and low volume processing/manufacturing Others fiber types include SiC, B, Al 2 O 3
- 15. Ceramic Matrix Composites Fracture toughness is the important parameter for CMC’s. K Ic ranges from 1-5 for most ceramics, 15-150 for most metals, and 5-10 for ceramic matrix composites containing ceramic dispersants (particles, fibers, or whiskers).
- 16. Transformation Toughening Stress promotes transformation from one phase to another with a higher volume, causing a compressive stress. Partially stabilized zirconia particles (tetragonal phase made to be stable at ambient conditions instead of the expected and higher volume monoclinic phase). An approaching crack can be pinched shut.
- 17. (Mostly) uniaxial fiber composite fabrication Prepreg (preimpregnated with resin) Resin added to fibers, then rolled (pressed) and heated into sheets, rolled and sold. Cures at or just above room temperature, so generally must be stored at 0C. http://www.netcomposites.com/news.asp?2714 Designed for record solo non-stop round-world flight. Composite prepreg materials: carbon fibre and epoxy. stiff carbon fibres in long wings sandwich of carbon and epoxy with aramid honeycomb for skin. Same materials as SpaceShipOne, X Prize winner for “tourist” space flight.
- 18. Start to finish http://www.owenscorning.com/ http://www.acp-composites.com/ACP-CAT.HTM
- 19. More (mostly) uniaxial fiber composite fabrication Pultrusion For continuous shape structures (rods, beams, pipes) Start with fibers, impregnate with polymer resin, pass through a die of near final shape, pass through a die of final shape with simultaneous heating to cure (fix) the part, roll to finish, cut and sell. Structures are high strength to weight, corrosion resistant, non conductive, electromagnetically transparent, have better thermal expansion than steel and aluminum, and work from -70 to 80 F. http://www.acmanet.org/pic/products/description.htm
- 20. Fiber composite fabrication, not uniaxial Filament Winding Wind either fibers or narrow prepreg structures around a mandrel using various patterns. High strength to weight. “ Most economically attractive.” http://www.owenscorning.com/
- 21. Hybrids If we want to improve properties of a given part, we can make a composite. If we want to improve the composite, we can tweak the particle or fiber size (length), distribution, material, interface, or orientation. If we want to improve it still further, we might combine benefits from multiple composite types Glass and carbon fibers http://www.owenscorning.com/
- 22. Structural Composites Laminar composites 2-d sheets or panels Stacked and bound with orientation in altering directions Improved strength in 2 or more directions in 2d, but not 3d. Sandwich Panels Face sheets with uniform strength are separated by a core or honeycomb.
- 23. SUMMARY Be able to calculate the longitudinal strength for long, short, and very short fibere composites. Be able to calculate the Young’s modulus for discontinuous fibers oriented in 3 dimensions, 2 dimensions, or 1 dimension. Know about GFRP, CFRP, and AFRP PMC’s—increased modulus/weight ratio. Know the strengthening mechanisms for fibers in a composite. Recall transformation toughened ceramics (Yttria stabilized zirconia). Be familiar with carbon-carbon, hybrid, and structural composites. Understand fiber composite fabrication: protrusion, prepreg, and winding.