Elastic Properties of Solids Part III Topics Discussed

Elastic Properties of Solids, Part III Topics Discussed in Kittel, Ch. 3, pages 73 -85 Another Lecture Found on the Internet!

Elastic and Complimentary Energy Density

= Uo + Co

Expressed in compliance matrix form = S·

Expressed in stiffness matrix form = C·

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!

Internal Energy

Strains xy = 2 xy yz = 2 yz zx = 2 zx

Hooke’s Law (Anisotropic) The 36 coefficients C 11 to C 66 are called elastic coefficients

Hooke’s Law

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

Strain Energy Density

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. anisotropic Eg: Metallic alloys and thermo-set polymers

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.

Hooke's Law in Compliance Form

Hooke's Law in Stiffness Form

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

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. 2 nd Law of Thermodynamics and understanding that under uniaxial tension, material must elongate in length implies: E>0

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

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

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

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+ )

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

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

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 )

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.

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 yz, zx, xy

Hooke’s law in compliance matrix form

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