Materials Option Flashcards
Crystal
Solid in which atoms are arranged in a regular array.
Crystalline Solid
Solid consisting of a crystal, or of many crystals, usually arranged randomly. The latter is strictly a polycrystalline solid. Metals are polycrystalline.
Amorphous solid
A truly amorphous solid would have atoms arranged quite randomly. Examples are rare. In practice we include solids such as glass or brick in which there is no long range order in the way atoms are arranged, though there may be ordered clusters of atoms.
Polymeric Solid
Solid made up of chain-like molecules.
Hookes Law for wires and rods
The tension in a rod or wire is proportional to its extension from its natural length, provided the extension is not too great.
Stress sigma
Stress is the force per unit cross-sectional area when equal opposing forces act on a body.
Unit: pascal (Pa) or Nm-2.
Strain epselon
Strain is defined as the extension per unit length due to an applied stress. Unit: none
The Youngs Modulus
Young Modulus = Stress / Strain
Unless otherwise indicated this is defined for the Hooke’s Law region. Unit: Pa or Nm-2
Ductile Material
A material which can be drawn out into a wire. This implies that plastic strain occurs under enough stress.
Elastic Strain
This is strain that disappears when the stress is removed, that is the specimen returns to its original size and shape.
Plastic Strain
This is strain that decreases only slightly when the stress is removed. In a metal it arises from the sliding of layers of atoms over each other in the crystals.
Elastic Limit
This is the point at which deformation ceases to be elastic. For a specimen it is usually measured by the maximum force, and for a material, by the maximum stress, before the strain ceases to be elastic.
Dislocations in crystals
Certain faults in crystals which (if there are not too many) reduce the stress needed for planes of atoms to slide. The easiest dislocation to picture is an edge dislocation: the edge of an intrusive, incomplete plane of atoms.
Grain Boundaries
The boundaries between crystals (grains) in a polycrystalline material.
Ductile Fracture (necking)
The characteristic fracture process in a ductile material. Fracture of a rod or wire is preceded by local thinning, increasing the stress.
Creep
This is an increase of strain with time, which may sometimes occur even if the stress is kept constant.
Fatigue Failure
If a metal specimen is subjected to many cycles of increasing and decreasing stress, even if the stress is always below the elastic limit, the metal is likely to develop cracks and eventually break.
Work- Hardening (Cold Working)
This is the process of causing inelastic strain in a metal, through at least one application of stress. It raises the elastic limit, but reduces the extent of the ductile region between elastic limit and fracture.
Annealing
This is the process of heating a metal, in order to alter its mechanical properties. In the case of copper, it will restore most of the ductility destroyed by any previous work-hardening.
Quench- Hardening
This is the heating and rapid cooling of certain steels to produce changes in the crystal structure. The effects include raising of the elastic limit.
Brittle Material
Material with no region of plastic flow, which, under tension, fails by brittle fracture.
Thermoplastic Polymers
When heated, thermoplastic polymers soften, and at higher temperatures melt. They solidify on cooling. The process can be repeated.
Thermosetting Polymers
After one episode of heating and cooling, usually during manufacture of an object, any further heating of a thermosetting polymer may result in cracking and/or charring, but not melting.
Elastic hysteresis
When a material such as rubber is put under stress and the stress is then relaxed, the stress-strain graphs for increasing and decreasing stress do not coincide, but form a loop. This is hysteresis (literally, lag).
Brittle Fracture
This is the fracture under tension of brittle materials by means of crack propagation.
