1. Composite Materials Chapter 4. Interfaces

2. Behavior of fiber reinforced composites Fiber or reinforcing element Matrix Fiber/Matrix interface Ignoring the fiber ends, surface to volume ratio for cylindrical fiber Where r and l are the fiber radius and length Surface area or interfacial area per unit volume increases as r decreases Applied loads should be effectively transferred from matrix to fibers via interface Fibers not weakened by flaws because of an adverse interfacial reaction Wettability of fiber by matrix and type of bonding is primary consideration

3. 4. Interfaces matrix Reinforcement (fiber, … ) Interface : between any two phase

4. 4. Interfaces -Coherent : one-to-one correspondence between atomic sites -semicoherent : increasing size of the crystals → elastic strain energy > interfacial energy leading to a lowering of the free energy of the system by dislocation at the interface → interface containing dislocations to accommodate the large interfacial strains → Thus having only a partial atomic registry: semicoherent

5. 4. Interfaces -Incoherent :semicoherent still further increase in crystal sizes → dislocation density at the interface increases → it is no possible to specify individual atomic positions at the interface : incoherent A phase, lattice B phase Coherent semicoherent

6. 4. Interfaces a : ideal planar interface b : more likely real interface Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)

7. 4.1 Wettability and bonding - wettability is the term used to describe the extent to which a liquid would spread on a solid surface Q: surface energy Q LS +Q LV cos ⊖ = Q SV Example of partial wetting Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)

10. 4.1 Wet ability and bonding - 180 ⁰ : the drop assume a spherical and no wetting occurs - 0 ⁰ : contact angle represents perfect wetting - 0 ⁰ < ⊖ <90 ⁰ : a partial wetting occurs - Good bonding implies that atomic or molecular bonds are formed uniformly all along the interface !

11. 4.2 The interface in composite - The reason the interface in a composite is of great importance is that the internal surface area. Occupied by the interface is quite extensive.

12. 4.3 interactions at the interface - Interfacial zone having multiple interfaces resulting from the formation of different intermetallic Compounds, interdiffusion, and so. - The difference in the expansion coefficients of the two components can give rise to thermal stresses Of such a magnitude that the softer (generally the matrix) will deform plastically

13. 4.4 types of bonding at the interface - Mechanical bonding-any contraction of the matrix onto a central fiber would result in a gripping of the latter by the former. -Chemical bonding- dislocation and wettability bonding: the implies that surface should be treated to re-move any impurities  Reaction bonding : transport of atoms that is, this atomic transport is controlled by diffusional processes  Dissolution bonding

14. Mechanical Bonding Matrix penetrating the crevices on fiber surface by liquid or viscous flow: Radial gripping stress

15. 4.4 types of bonding at the interface Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)

16. 4.4 types of bonding at the interface -The reaction products as well the reaction rates can vary Depending on the matrix composition, reaction time, and temperature.

17. 4.5 tests for measuring interfacial strength -Flexural Test -3 pt Bending Test -4 pt. bending test -Short Beam Shear Test -Iosipescu Shear Test -Single Fiber Pullout Test -Instrumented Indentation Tests

18. Flexural Test

19. E=M/I X R (4.10) σ=M/I x у (4.11)

21. I=bh ^3/12 Y=h/2

22. Average shear stress : T =P/b h Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)

25. Single Fiber Pullout Test

26. Three types of indenters: Flat, Conical, with flat head, and pointed with a flat head Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)

27. An Indentation Method for Measuring Matrix-Fiber Frictional Stresses in Ceramic Composites

28. Abstract A simple method for measuring frictional stresses between the matrix and individual fibers in a ceramic composite is described. The frictional stress is calculated from measurement of the applied force and the amount of slipping between the fiber and matrix. A standard micro-hardness indenter

29. Main subject The strength and toughness of fiber composites are controlled to a large extent by the bonding between the fibers and matrix. Frictional force ▶ Mechanical link between the two components ▶ The magnitude of the frictional stress → Important micro-structural property.