The Mechanical Testing of Polymers

and their CompositesBy

Gordon Williams

Introduction

• Standard methods are vital in ensuring reliable data

• True in all fields – Thus ASTM & ISO• If a property is to be claimed it must be

backed by 1) a method and 2) statistical analysis ( means ,SD)

• For materials science ;materials development must be supported by proper results.

Introduction

• e.g. Toughened polymers (PA,PE),

• Filled polymers; glass filled ;particle filled

• Nano Composites

• Recent confusion in “Nano” field is often a result of poor data.

Test Types

• A range from direct product testing to basic properties ,e.g. Drop tests to Young’s Modulus.

• Intermediate type, Testing special plaques or bowls.

• Basic Tests measure material properties independent of size and method.

Test Types

• Two Main Types• Deformation behaviour; to find Young’s

Modulus ,Yield stress ,Poisson’s Ratio ,Elongation*,Strength*.

• Fracture Behaviour; to find Toughness,K1c,Izod*, Charpy*.

• All use Tension, Compression or Bending Tests.

• * See later; not usually a good idea.

Testing Machines

• A long history e.g. 1880

• Originally to test steel, e.g. Harbour Bridge

• Machines in Civil Eng. Dept. from 1908

• All work in a similar way

• Load applied via a screw ( Instron) or hydraulic rams (Servo Hydraulic)

• Load measured by load cells (must calibrate)

Test machine Instron 4302

Test machine Instron 1195

Test machine MTS 810

Testing Machines

• Deformations; Cross head ,Gauge length? Extensometers, videos.

• Modern machines are speed controlled, i.e. constant or exponential.

• Polymers are rate dependent, i.e. are visco-elastic.

Deformation Properties

• Measured in; Tension ,compression or bending.

• Tension; simplest but; end effects (dog bones) ,gripping (slipping) ,accurate strains difficult.

• Compression; no gripping problems

• However ,friction .

• Necessary for some yield stress tests.

Definition of Deformation

Strain,

Str

ess,

y

y F

F

2%0

E

Illustrations of Deformation

b

W

l

= /l =F/bW E= /=(F/)(l/bW)

Tension:

Illustrations of Deformation

Compression:

Illustrations of Deformation

E= /=(F/)(l/bW)(l 2/4W2)

Bending:

b

W

l

F

Deformation Properties

• Bending• Good for Modulus (magnification)• However; variable strain , inhomogeniety

(flexural modulus)• Interpretation of curves.• Young’s Modulus; elastic

property ,polymers are visco-elastic ,rate dependent ,unloading and hysterysis .Usually non- linear.

Deformation Behaviour

• Yield Stress; onset of permanent deformation, hard to define. 2% offset ,maximum stress.

• “Correct” or “definable” ?

• Fracture stress/strain ,strength .Variable (surface finish) .A property?

• Fracture Mechanics!! See next week.