1.5 Solids Under Stress Flashcards
What are the three different types of solids?
Crystalline, Amorphous, Polymeric
Crystalline
Atoms are arranged in a regular array (lattice). There is long and short order. They can be made from a single crystal or be polycrystalline
Amorphous
Atoms arranged randomly such that they have no long range order, but may have short range order
Polymeric
Made up of long chain-like molecules. There is no long range order between molecules but there is order within each molecule
Hooke’s law
The tension in a spring or wire is directly proportional to its extension from its natural length, provided the extension is not too great. F = kx
Spring constant
Is the force per unit extension
Stress, σ = … / …
F
A
Strain, ε = … / …
ΔL
L
Young’s Modulus, E = … / …
Stress, σ
Strain, ε
The work done in a deforming solid is equal too …
the area under a force extension graph
On a Stress-Strain graph for brittle materials the point P, is called …
The Limit of proportionality
On a Stress-Strain graph for brittle materials the point E, is called …
The Elastic Limit
Brittle Material
A material with no region of plastic deformation. Under tension, it breaks by brittle fracture, caused by crack propagation
What happens for the first section of a Force-Extension graph for rubber?
The stretching of the rubber means the breaking of cross-link bonds, this makes it stiff, which means it requires more force per unit of extension
What happens for the second section of a Force-Extension graph for rubber?
Stretching further only involves breaking weak Van Der Waals forces between the long chain molecules, this makes it less stiff, which means it requires less force per unit extension
What happens for the third section of a Force-Extension graph for rubber?
Stretching even further now means trying to stretch the actual long chain molecules which is not easy due to the strong covalent bonds within these molecules, this makes it very stiff, which means it requires exponentially more force per unit extension until it eventually breaks
What are the two points about a Stress-Strain graph for brittle materials that can be noted?
- The material doesn’t stretch much before fracturing
- The material pretty much obeys Hooke’s law until the fracture
Dislocations
Are small gaps of atoms missing in the lattice array of a crystalline solid, it allows the material to extend further then normal buy gradually moving the each of the dislocations to the edge of the material. This leads to a part of the Force-Extension graph having small increases in force but larger increases in extension.
