1. Elastic Properties of Solids, Part III Topics Discussed in Kittel, Ch
Elastic Properties of Solids, Part III Topics Discussed in Kittel, Ch. 3, pages Another Lecture Found on the Internet!

2. Elastic and Complimentary Energy Density

3.  = Uo + Co

4. Uo= Uo(xx , yy , zz , xy , yz , zx , x, y, z, T)

5. Expressed in compliance matrix form

6. Expressed in stiffness matrix form
s = C· e

7. In general, stress-strain relationships such as these are known as constitutive relations
Note that the stiffness matrix is traditionally represented by the symbol C, while S is reserved for the compliance matrix!

8. Internal Energy

10. xy = 2xy yz = 2yz zx = 2zx
Strains
xy = 2xy yz = 2yz zx = 2zx

11. The 36 coefficients C11 to C66 are called elastic coefficients
Hooke’s Law (Anisotropic)
The 36 coefficients C11 to C66 are called elastic coefficients

12. Hooke’s Law

13. Hooke’s Law
The generalized Hooke’s law is an assumption, which is reasonably accurate for many material subjected to small strain, for a given temperature, time and location

14. Strain Energy Density

15. Isotropic material
Isotropic materials have only 2 independent variables (i.e. elastic constants) in their stiffness and compliance matrices, as opposed to the 21 elastic constants in the general anisotropic case.
Eg: Metallic alloys and thermo-set polymers

16. The two elastic constants are usually expressed as the Young's modulus E and the Poisson's ratio n.
Alternatively, elastic constants K (bulk modulus) and/or G (shear modulus) can also be used. For isotropic materials
G and K can be found from E and n by a set of equations, and vice-versa.

17. Hooke's Law in Compliance Form

18. Hooke's Law in Stiffness Form

19. Youngs Modulus from Uniaxial Tension
An isotropic material subjected to uniaxial tension in x direction, sxx is the only non-zero stress. The strains in the specimen are

20. The modulus of elasticity in tension, Young's modulus E, is the ratio of stress to strain on the loading plane along the loading direction.
2nd Law of Thermodynamics and understanding that under uniaxial tension, material must elongate in length implies:
E > 0

21. Shear Modulus for Pure Shear
Isotropic material subjected to pure shear, for instance, a cylindrical bar under torsion in the xy sense, sxy is the only non-zero stress. The strains in the specimen are

22. Shear modulus G:Ratio of shear stress to engineering shear strain on the loading plane

23. 2nd Law of Thermodynamics and understanding that a positive shear stress leads to a positive shear strain implies
G > 0

24. Since both G and E are required to be positive, the quantity in the denominator of G must also be positive. This requirement places a lower bound restriction on the range for Poisson's ratio,
n > -1
G=E/2(1+)

25. Bulk Modulus for Hydrostatic stress
For an isotropic material subjected to hydrostatic pressure s, all shear stress will be zero and the normal stress will be uniform

26. Under hydrostatic load, material will change its volume
Under hydrostatic load, material will change its volume. Its resistance to do so is termed as bulk modulus K, or modulus of compression.
hydrostatic pressure
K =
relative volume change
Also note: K > 0

27. The fact that both bulk modulus K and the elastic modulus E are required to be positive, it sets an upper bound of Poisson's ratio
n < 1/2
K=E/ 3(1-2)

28. Orthotropic material
Orthotropic material has at least 2 orthogonal planes of symmetry, where material properties are independent of direction within each plane.
Eg: Certain engineering materials, 2-ply fiber-reinforced composites, piezoelectric materials (e.g.Rochelle salt)
Orthotropic material require 9 independent variables (i.e. elastic constants) in their constitutive matrices.

29. 3 Poisson's ratios nyz, nzx, nxy, 3 shear modulii Gyz, Gzx, Gxy.
The 9 elastic constants in orthotropic constitutive equations are comprised of
3  Young's modulii Ex, Ey, Ez,
3 Poisson's ratios nyz, nzx, nxy,
3 shear modulii Gyz, Gzx, Gxy.
Note that, in orthotropic materials, there are no interaction between the normal stresses sx, sy, sz and the shear strains eyz, ezx, exy

30. Hooke’s law in compliance matrix form

31. Hooke’s law in stiffness matrix form
End of session 2