1.5 Solids under stress Flashcards
Hooke’s law
The tension in a spring or wire is proportional to its extension from its natural length, provided the extension is not too great.
Spring constant, k
The spring constant is the force per unit extension. Unit: Nm-1
Stress, σ
Stress is the force per unit cross-sectional area when equal opposing forces act on a body.
Unit Pa or N m-2
Strain, ε
Strain is defined as the extension per unit length due to an applied stress. Unit: none
Young modulus, E
Young modulus = tensile stress / tensile strain
Unless otherwise indicated this is defined for the Hooke’s law region. Unit: Pa or N m-2
Crystalline solid
Solid consisting of a crystal, or of many crystals, usually arranged randomly. The latter is strictly a polycrystalline solid. Metals are polycrystalline.
Crystal
Solid in which atoms are arranged in a regular array. There is a long range order within crystal structures.
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 which is made up of chain-like molecules.
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 (or inelastic) strain
This is strain that decreases only slightly when the stress is removed. In a metal it arises from the movement of dislocations within the crystal structure.
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.
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.
Grain boundaries
The boundaries between crystals (grains) in a polycrystalline material.
Ductile fracture (necking)
The characteristic fracture process in a ductile material. The fracture of a rod or wire is preceded by local thinning which increases the stress.
Brittle material
Material with no region of plastic flow, which, under tension, fails by brittle fracture.
Brittle fracture
This is the fracture under tension of brittle materials by means of crack propagation.
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.
