The matrix in a composite is the continuous phase that transfers stress to the dispersed phase. Common matrix materials include metals, ceramics, and polymers. There are two possible strengthening mechanisms for particle reinforced composites: load transfer from the matrix to the particles and restriction of dislocation movement. The Young's modulus of a large particle composite can be calculated using upper and lower bounds. The critical length (Lc) of a fiber depends on factors like fiber diameter and matrix-fiber bond strength. Fibers of different lengths experience different stress distributions relative to the critical fiber length.

2. Composites in Action

3. Composite Structures Particle, fiber, and structural composite.

4. 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 • Dispersed phase : --Purpose: enhance matrix properties. MMC: increase  y , TS, creep resist. CMC: increase Kc PMC: increase E,  y , TS, creep resist. --Possible Classifications: Particle , fiber , structural metal ceramic polymer 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.

5. Types of composites (MMC, PMC, CMC) Wood (cellulose fibers with stiffer lignin matrix) Bone (soft collagen and brittle apatite) Clay (particles and glass naturally form when fired) We will focus on artificial composites. Natural composites include:

7. Particle Reinforced Composites Particle-reinforced • Examples: Adapted from Fig. 10.10, Callister 6e . (Fig. 10.10 is copyright United States Steel Corporation, 1971.) Adapted from Fig. 16.4, Callister 6e . (Fig. 16.4 is courtesy Carboloy Systems, Department, General Electric Company.) Adapted from Fig. 16.5, Callister 6e . (Fig. 16.5 is courtesy Goodyear Tire and Rubber Company.)

9. COMPOSITE SURVEY: Particle-II • Elastic modulus, E c , of composites: -- two approaches. Particle-reinforced Adapted from Fig. 16.3, Callister 6e . (Fig. 16.3 is from R.H. Krock, ASTM Proc , Vol. 63, 1963.)

12. Large-Particle vs. Dispersion-Strengthened Composites Strong Particle >500 nm Strong Particle <100 nm Shear  Dislocation stopped Stress field of dispersion Dislocation shears through the dispersion Dispersion Strengthened Large-Particle