AdvMat Glossary

7/28/2019 AdvMat Glossary

http://slidepdf.com/reader/full/advmat-glossary 1/4

The University of Edinburgh

School of Engineering and Electronics

Session 2002-2003

Advanced Materials

Deformation, fracture and failure

Glossary of terms

Anelastic deformation. Deformation processes where a material assumes its orig-

inal shape after the load is removed, but with a time delay.

Anisotropy. Directional differences in physical properties in a material.

Brittleness. Absence of ductility: that is, the material does not deform plasti-

cally since fracture by crack propagation occurs before plastic yielding takes

place.

Composite material. A solid material which consist of a combination of two or

more constituents, in which the individual components retain their separate

identities. The main interest technologically is in obtaining materials with

superior physical (usually mechanical) properties to those of the compos-ite’s component materials.

Constitutive equation. An equation which connects stress, elastic and/or plastic

strain and possibly strain rate, experimental and material parameters. The

simplest example of a constitutive equation is Hooke’s law.

Creep. Plastic deformation under constant load.

Diffusion. Motion of atoms with the help of thermal fluctuations. In most materi-

als, diffusion is accomplished by the generation and movement of vacancies.

Diffusion may involve only one atom species (self-diffusion) or in an alloy,

may involve more than one type of atom (solute diffusion).

1



Dislocation. A lattice imperfection in a crystal structure which exerts a profound

effect on a structure-sensitive properties such as strength, hardness, ductility

and toughness. A dislocation is located at the boundary of a region where

a lattice plane has slipped with respect to the plane below. By moving the

dislocation, the slipped region can extend and plastic deformation can take

place. The dislocation is characterized by a Burgers vector which represents

the amount and direction of slip when the dislocation moves.

Dissipation. Conversion of mechanical energy into heat. Materials which dis-

sipate large amounts of energy during deformation have usually a large

fracture toughness, since the dissipated energy is not available for crack

propagation.

Dissociation. The separation of a dislocation line into two smaller dislocations.

This occurs because the energy of a dislocation is proportional to the square

of the Burgers vector and therefore a single dislocation of large Burgers

vector may have a larger energy than two dislocations of smaller Burgersvectors. The two dislocations so formed are generally partial dislocations

(their Burgers vectors are less than a crystal lattice vector) and the region

between them is a stacking fault.

Ductility. The degree to which a material can be plastically deformed before fail-

ure.

Elastic deformation. Deformation processes where the material instantaneously

reverts to its initial shape after the load is removed. The associated stress-

strain curves may be linear ( Hooke’s law) or nonlinear (for example in poly-

mers at large strains).

Fatigue. The failure of a structure subjected to repeated loading at stress levels

below those required to cause yielding. Fatigue fractures begin as minute

cracks that grow under the action of the fluctuating stress.

Fracture toughness. A measure of the resistance offered by a material to the

propagation of crack within it. Characterized by critical stress intensity

factor, K

c

(combination of applied stress and a crack length) at which the

stress concentration at the crack tip is sufficient to allow crack advance.

Isotropy. Uniformity of physical properties measured in different directions in a

material.

2



Plastic deformation. Deformation which remains after the load is removed.

Liquid. A material which cannot support static shear stresses.

Slip. (also: Glide) Plastic deformation of crystals by parallel lattice planes shear-

ing against each other. Such planes are called a slip planes, and the dis-

placement of two adjacent planes during slip is called the slip vector. In

crystals, the slip vector must be a lattice vector. Usually slip takes place by

motion of dislocations under the action of shear stresses. In this case the

slip vector equals the Burgers vector of the dislocations.

Solid. A material which can support static shear stresses

Strain. The ratio of the total deflection or change in dimension of a body under

load expressed as the ratio of the total deflection or change in dimension to

the original unloaded dimension. The three simplest strains are as follows:

linear strain: the change in length per unit length.

volume (bulk) strain: the change in volume per unit volume.

shear strain: angular deformation without change in volume. The angle at

one corner is a measure of the strain.

Stress. A system of forces in equilibrium producing strain or deformation in a

body or part of a body. The stresses may be regarded as the forces applied

to deform the body or as the equal and opposite forces with which the body

resists. In all cases a stress is measured as a force per unit area.

Strength. A measure of a material’s resistance to failure. It depends on how it is

measured, specimen geometry and, for brittle materials, on the presence of

flaws. The tensile strength, is the maximum tensile force in the test divided

by the original cross-sectional area. The shear strength and compressive

strength are equivalents of tensile strength in their respective loading modes.

Testing. Experimental determination of deformation properties. Tests can be

characterized according to deformation geometry (deformation in tension,

torsion, bending) and according to the testing mode: In a stress-controlled

test (Creep test), the stress is kept fixed and the strain (or equivalently strain

rate) is recorded as a function of time. In a strain-controlled test, the strain

rate is kept fixed and the stress is recorded as a function of time (or, equiva-

lently, strain). In a fatigue test, an oscillating load is applied. Again, either

the stress amplitude or the strain amplitude can be controlled.

3



Toughness. Capacity of a material to absorb energy during deformation.

Vacancy. A site in a crystal unoccupied by an atom. Vacancies are important in

a number of processes, not least diffusion, in which they provide a means

for atoms to move relative to the lattice. The concentration of vacancies

increases exponentially with temperature.

Viscosity. A measure of a material’s dynamic resistance to flow when a shear

stress is applied. The coeffcient of viscosity is shear stress divided by shear

rate. It is a characteristic quantity of liquids.

Viscoelasticity. The property of some materials (polymers) to exhibit time-dependent

mechanical properties. For example, the rate of loading has influence upon

stiffness and ultimate strength. A viscoelastic material behaves both like a

liquid (in the limit of small deformation rates) and an elastic solid (in the

limit of large deformation rates).

Yielding. The point where for the first time marked deviations from elastic/anelastic

behavior occur in a strain-controlled test. Often the yield stress is defined as

the stress where the actual stress-strain curve deviates from the elastic curve

by a fixed amount, e.g. 0.2 per cent strain.

Revised 07 October 2002

4

Documents

AdvMat Glossary