Mechanical Properties of Polymers

25.11.2014

Mechanical Tests

Polymer components, like other materials, may fail to perform their intended

functions in specific applications as a result of;

1.Excessive elastic deformation

Particularly in structural, load-bearing applications, due to inadequate rigidity or stiffness.

For such failure, the controlling material mechanical property is the elastic modulus.

2.Yielding or excessive plastic deformation Failure of polymers in certain applications to carry design loads or occasional accidental

overloads may be due to excessive plastic deformation resulting from the inadequate

strength properties of the polymer. For the quantification of such failures, the mechanical

property of primary interest is the yield strength and the corresponding strain.

3.Fracture Cracks constitute regions of material discontinuity and frequently precipitate failure

through fracture. Fracture may occur in a sudden, brittle manner or through fatigue

(progressive fracture).

Strain-Stress Experiments

Polymers exhibit a wide variation of

behavior in stress–strain

tests, ranging from hard and brittle to

ductile, including yield and cold drawing.

The utility of stress–strain

tests for design with polymeric materials

can be greatly enhanced if tests are

carried out over a wide range of

temperatures and strain rates.

Creep Experiments

In creep tests, a specimen is subjected to a constant load, and the

strain is measured as a function of time.

Creep tests are made mostly in tension, but creep experiments can also be

done in shear, torsion, flexure, or compression.

0

)t(J(t)

Compliance (J) is a time-dependent

reciprocal of modulus.

It is the ratio of the time-dependent

strain to the applied constant stress

Stress Relaxation Experiments

In stress relaxation experiments, the specimen is rapidly (ideally,

instantaneously) extended a given amount, and the stress required to

maintain this constant strain is measured as a function of time. The stress

that is required to maintain the strain constant decays with time.

Relaxation modulus (E(t,T)) is a function of

both time and temperature.

Impact Experiments

The most popular of these tests methods are the Izod and Charpy impact strength tests

Schematic representation of impact test.

Impact tests provide useful information in the selection of a polymer for a specific

application, such as determining the suitability of a given plastic as a substitute for glass

bottles or a replacement for window glass.

Stress-Strain Behaviour of Polymers

constant elongation rate

Engineering stress ();

0A

F where F=applied load

A0=the original cross sectional area

Engineering strain ();

L

L

L

LL

0

0

However,engineering stress–strain curves generally depend on the shape of the specimen. A more accurate

measure of intrinsic material performance is plots of true stress vs. true strain. True stress σt is defined as the

ratio of the measured force (F) to the instantaneous cross-sectional area (A) at a given elongation, that is,

A

Ft

True strain is the sum of all the instantaneous length changes, dL, divided by the instantaneous length L.

)1ln()L

LLln(

L

Lln

L

dL

0

0

0

L

Lt

o

)1(t

For small deformations, true stress and engineering stress are essentially equal. However, for large deformations

the use of true strain is preferred because they are generally additive while engineering strain is not.

L L0

Elastic Stress-Strain Relations

(Hooke`s law)

Elastic Modulus

h

x

For small strains, this is simply the tangent

of the angle of deformation.In pure shear,

Hooke`s law is expressed as;

G

Shear stress

Shear modulus

Shear strain

L L0

z

x

z

y

Poisson`s ratio

(1)

Metal Possion ratio Polymer Possion Ratio

Al 0,25 PS 0,33

Cu 0,31-0,34 Natural rubber 0,49

Steel 0,27-0,30 Nylon 6,6

LDPE

PMMA

0,4

0,4

0,33

Possion ratios of some metals and polymers

Adapted from Fig. 15.2,

Callister & Rethwisch.

For plastic polymers, the yield point is taken as a maximum on the curve, which occurs just beyond

the termination of the linear-elastic region. The stress at this maximum is the yield strength (y).

Ultimate tensile strength or tensile strength (TS) corresponds to the stress at which fracture occurs

TS may be greater than or less than y.

The strains associated with the yield point or the fracture point are referred to as the elongation at

yield and elongation at break, respectively.

Drawing stress

Yield stress

Elongation at yield

Elongation at break

Tensile strength

Elastic deformation

Stress

( )

Strain ()

Schematic tensile stress–strain curve for a semicrystalline polymer.

(Above Tg)

B C

D E

A

Compression versus Tensile Tests-1

The stress–strain curves for the amorphous polymers are characteristic of the

yield behavior of polymers.

Amorphous polymers

Compression versus Tensile Tests-2

Crystalline polymers

There are no clearly defined yield points for the crystalline polymers.

Compression versus Tensile Tests-2

Brittle polymer

In tension, polystyrene exhibited brittle failure, whereas in compression it

behaved as a ductile polymer.

Strength and yield stress are generally higher in compression than in tension.

Effect of Molecular Weight a schematic modulus–temperature curve for a linear amorphous polymer like atactic polystyrene.

Hard-glassy region

Transition from glassy to

rubbery region

Rubbery to melt flow transition

Rubbery region

If the Tg is above room temperature, the material will be a rigid polymer at room

temperature.

If, however, the Tg occurs below room temperature, the material will be rubbery and

might even be a viscous liquid at room temperature.

Effect of Cross-linking

G

RTMc

Average molecular weight between cross-links

density Shear modulus

Mc is a measure of the crosslink density; the smaller

the value of Mc, the higher the cross-link density.

In the glassy region, the increase in modulus due to cross-linking is relatively small.

The principal effect of cross-linking is the increase in modulus in the rubbery region and the

disappearance of the flow regions.

The crosslinked elastomer exhibits rubberlike elasticity even at high temperature.

Cross-linking also raises the glass transition temperature at high values of crosslink

density.

The glass-to-rubber transition is also considerably broadened.

Effect of Crystallinity

Crystallinity has only a small effect on modulus below the Tg but has a pronounced

effect above the Tg.

There is a drop in modulus at the Tg, the intensity of which decreases with increasing

degree of crystallinity. This is followed by a much sharper drop at the melting point.

Crystallinity has no significant effect on the location of the Tg, but the melting

temperature generally increases with increasing degree of crystallinity.

Effect of Copolymerization-1

Random and alternating copolymers

The copolymerization shifts the modulus-temperature curve as the same way as Tg

There is a broadening of the transition due to the polymer heterogeneity.

Effect of Copolymerization-2 Block and graft copolymers

The glass transition of the butadiene phase near –80°C and that for the styrene phase near

110°C are clearly evident.

Between the Tg of butadiene and the Tg of styrene, the value of the modulus is determined by

the amount of polystyrene; the rubbery butadiene phase is cross-linked physically by the hard and

glassy polystyrene phase.

Styrene–butadiene–styrene block copolymers have high tensile strength, butadiene–styrene–

butadiene copolymers have a very low tensile strength, showing that strength properties are

dictated by the dispersed phase.

Effect of Plasticizers

Plasticizers are low-molecular-weight, usually high boiling liquids that are capable of enhancing the

flow characteristics of polymers by lowering their glass transition temperatures.

Modulus, yield, and tensile strengths generally decrease with the addition of plasticizers to a polymer.

In general, on plasticization a polymer solid undergoes a change from hard and brittle to hard and tough to

soft and tough.

Plasticization and alternating or random copolymerization have similar effects on

modulus.

Effect of Temperature

• Decreasing T...

-- increases E

-- increases TS

-- decreases %EL

.

Effect of Strain Rate

Polymers are very sensitive to the rate of testing. As the strain rate increases, polymers in general sho

a decrease in ductility while the modulus and the yield or tensile strength increase.

The sensitivity of polymers to strain rate depends on the type of polymer: for britte polymers the effect is

relatively small, whereas for rigid, ductile polymers and elastomers, the effect can be quite substantial if

the strain rate covers several decades.