307 - Designing with polymers

Stiffness

Resistance to deformation.

Strength

Ability of a material to withstand applied stress before yielding or failure.

Yield stress

Stress at which significant permanent deformation begins.

Ultimate tensile strength (UTS)

Maximum engineering stress reached during tensile loading.

Ductility

Ability to undergo significant plastic deformation before fracture.

Toughness

Energy required per unit volume to cause fracture, represented by the area under the stress-strain curve.

Impact strength

Ability to withstand rapidly applied loading.

Fatigue strength

Ability to withstand repeated cyclic loading.

Creep resistance

Ability to resist time-dependent deformation.

Stress-crack resistance

Ability to resist crack initiation and propagation under sustained stress.

Engineering stress

Force divided by the original cross-sectional area.

Cold drawing

Large localised deformation involving polymer-chain alignment.

High strain rate

Gives polymer chains less time to rearrange, generally increasing apparent modulus and strength.

Low strain rate

Gives polymer chains more time to rearrange and relax.

High temperature

Increases molecular mobility and promotes relaxation.

Low temperature

Restricts molecular mobility and promotes brittle behaviour.

Brittle failure

Failure at relatively low strain with little permanent deformation.

Ductile failure

Failure preceded by substantial permanent deformation.

Creep rupture

Failure resulting from prolonged loading.

Microvoiding

Formation of small voids during ductile deformation.

Craze

A highly oriented region containing microscopic voids and fibrils that can carry load.

Craze stability

Ability of a craze to remain load-bearing without rapidly becoming a crack.

Effect of molecular weight on crazing

Higher molecular weight increases craze stability because of increased chain entanglement.

Environmental Stress Cracking (ESC)

Premature brittle cracking caused by the combined effects of mechanical stress and an aggressive environment.

Environmental degradation

Deterioration of polymer properties due to environmental factors such as UV radiation, oxygen, ozone, chemicals or moisture.

UV degradation

Polymer degradation caused by ultraviolet radiation.

Ozone degradation

Polymer degradation caused by ozone attacking susceptible polymer structures.

ESC mechanism

An aggressive environment can enter porous craze regions, plasticise or swell the polymer and accelerate crack propagation.

Linear Elastic Fracture Mechanics (LEFM)

Fracture analysis based on predominantly elastic behaviour around a crack.

Commodity polymer

A high-volume, relatively low-cost polymer used for common applications.

Engineering polymer

A polymer designed for improved mechanical or environmental performance.

High-performance polymer

A polymer designed for demanding mechanical, thermal or chemical environments.

Processability

The ease with which a polymer can be manufactured into a desired shape.

Mechanical properties

Properties such as tensile strength, modulus, fatigue resistance and creep resistance.

Thermal properties

Properties such as Tg, Tm, heat-deflection temperature, thermal conductivity and thermal expansion.

Chemical properties

Properties describing resistance to chemicals, UV radiation, weathering, temperature and moisture.

Physical properties

Properties such as density, shrinkage and rheological behaviour.

Regulatory properties

Properties required to meet regulations such as flammability or electrical safety requirements.

Special material requirements

Requirements such as biocompatibility, transparency or outdoor durability.

Reverse engineering

Selecting a material by identifying polymers successfully used in a similar existing application.