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Glossary

Anisotropic

A material is anisotropic when it exhibits different values of a property in different directions.

Cermets

Clad Metals

Columnar Grains

These are elongated grains that form as a result of competitive growth between chill crystals during the solidification of a casting.

Covalent

Degree of Cold Work

%Cold Work = 
A0 - Af
  x 100
 
A0
 

A0 = original cross-sectional area
Af = final cross-sectional area after deformation.

Ductile

A ductile material is one which can undergo a large amount of plastic deformation without fracturing.

Engineering strain

This is defined by the formula, where l0 is the original length, l is the instantaneous length, Dl is the elongation or change in length.

Engineering stress

This is defined in the formula, where F is the applied load, and A0 is the cross-sectional area before any load is applied.

Equiaxed Grains

These are randomly oriented grains that form in the centre of a casting. They are roughly rounded in shape.

Extensometer

A small device that is fitted to the specimen to measure the change in gauge length when the elongation is very small. Gives an accurate measure of strain for the calculation of elastic modulus.

Factor of safety

In design, the permissible load is kept below the calculated maximum load that the structure can withstand to allow for unforseen circumstances. The ratio of calculated load to the permissible load is the factor the safety.

Fatigue

A progressive mode of failure which occurs from cyclic loading, usually at stresses below the yield stress.

Ferrous alloy

Alloys with iron as the major constituent.

Fibrous Composites

Grain Growth

The increase in average grain size of a material. Usually an elevated temperature is required for this to occur.

Hot work

Working of a metal above the recrystallisation temperature

Inoculants

Inoculants are materials that are added to molten metal to promote a high rate of nucleation during solidification. This results in a fine grain size in the casting.

Ionic

Isostrain

This condition exists when all components undergo equal strain. This occurs in fibre-reinforced composites when the fibres are parallel to the applied load.

Isostress

This condition exists when all components are subjected to equal stress. This occurs in fibre-reinforced composites when the fibres are perpendicular to the applied load.

Laminar

Laminates

Macrostructure

The fine structure of a material which can be seen at a macroscopic level, ie, without using a microscope.

Microstructure

The fine structure of a material which can only be seen at a microscopic level, ie, when using a microscope.

Mode I

Crack is loaded solely in tension

Modulus

see Young's Modulus

Particulate

A composite that has equiaxed particles as the dispersed phase.

Plane Strain

This is the condition where under tensile stress there is zero strain in a direction perpendicular to the stress axis and the direction of crack propagation. This exists in thick plates - the zero-strain direction is perpendicular
to the plate surface.

Plastic Deformation

Leads to permanent deformation of a material after the release of the applied load. This occurs after the elastic limit, and is related to permanent atomic displacements.

Recovery

Eliminates residual stresses introduced during deformation, usually by a low temperature heat treatment, without reducing strength.

Recrystallisation

Formation of new strain-free grain structure in a previously cold-worked structure by heat treatment above the recrystallisation temperature.

Reduce

Residual Stress

Stresses that are introduced during deformation that persist when there is no longer any external force or temperature gradient present.

Rule of Mixtures

It states that the modulus of a unidirectional fibre composite (Ec) is proportional to the volume fractions (f) of the materials in the composite. It may also be used to determine the density of a composite.

Sandwich Materials

Segregation

The presence of composition differences in a material caused by insufficient time for diffusion during solidification.

Solute

The minor element (ie, added element) in a solid solution alloy. For example, in a solid solution containing 70% copper and 30% zinc, zinc is the solute.

Solvent

The major element (ie, host element) in a solid solution alloy. For example, in a solid solution containing 70% copper and 30% zinc, copper is the solvent.

Specific Strength

The ratio of strength to density of a material.

Specific Stiffness

(Specific Modulus) The ratio of elastic modulus to density of a material.

Strain

see Engineering strain

Stress

see Engineering stress

Stress amplitude

This is defined as one half of the difference between the maximum stress and the minimum stress during constant amplitude stress cycling.

Thermal Conductivity

Thumbnail crack

A thumbnail crack at the surface of a failed component:
thumbnail crack

Transition

Uniaxial

This means 'one axis'. A uniaxial tensile test is one in which the test piece is stretched in one direction only, along its axis.

Work Hardening

The increase in strength and hardness of a material by increasing the number of dislocations by plastic deformation or cold working.

Young's Modulus

(Modulus of elasticity) The ratio of the applied stress to the strain in a material that is deformed elastically. It is also a measure of the stiffness of a material.

 

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