lecture 5b-flexural propwyj

8/10/2019 Lecture 5b-Flexural Propwyj

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CPE 667

ENGINEERING PROPERTIES OF POLYMER

FLEXURAL PROPERTIES

Lecture 5


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Many plastics parts are used in applications

where flexural properties are important

e.g. Plastic seating - must have minimum flexural

strength and modulus or the seat will sag

excessively

Determination of flexural modulus is important

to overcome certain practical problem in

measuring tensile strength of thermoplastics in

brittle region


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Performing Test

The flexural test method measures behavior of materials subjected to simple

beam loading.

The flexural sample is a simple rectangular shapes beam that is placed overtwo rests or supports and then loaded in the middle of the beam between thesupports.


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Most commonly the specimen lies on a

support span and the load is applied to the

center by the loading nose producing three

point bending at a specified rate.

The parameters for this test are the support

span; the speed of the loading; and the

maximum deflection for the test.

These parameters are based on the test

specimen thickness, and are defined

differently by ASTM and ISO.

The flexural properties are determined

using procedures given in ASTM D790.


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A flexural test produces tensile stress in the convex side of the specimen andcompression stress in the concave side.

This creates an area of shear stress along the midline. To ensure the primaryfailure comes from tensile or compression stress, the shear stress must be

minimized.

This is done by controlling the span to depth ratio; the length of the outer spandivided by the height (depth) of the specimen.

For most materials S/d=16 is acceptable. Some materials require S/d=32 to 64 tokeep the shear stress low enough.


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Maximum stress and maximum strain are calculated for increments of load.

Results are plotted in a load-deflection / stress-strain diagram.

Flexural Test Result


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Flexural Test Result


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Flexural Properties

Flexural strength is defined as the maximum stress in the outermost fiber.

This is calculated at the surface of the specimen on the convex or tensionside.

Flexural Strength is the ability of the material to withstand bending forces

applied perpendicular to its longitudinal forces

Flexural strength is the ability of the material to withstand bending forces

applied perpendicular to its longitudinal axis. The stresses induced by theflexural load are a combination of compressive and tensile stresses.

Flexural modulus is calculated from the slope of the stress vs. deflection

curve. If the curve has no linear region, a secant line is fitted to the curve to

determine slope. Flexural modulus is a measure of the stiffness during the first or initial part

of the bending process.

Flexural modulus is used as an indication of a materials stiffness when

flexed.


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The flexural stress is related to the load and sample dimensionsand is calculated using the following equation:

where :

f

=stress, P = Load, L = length of span,

b= width of specimen, d = thickness of specimen

P is obtained from the load at break.

For materials that do not break at outer fiber strain up to 5 %, Pis obtained from the maximum load.

2

3

2f

PL

bd


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The maximum strain in the outer fiber, which occurs at the

mid-span is calculated using the following equation:

where :

r =strain, D = max deflection at the center, L = length of

span and, d = thickness of specimen

2

6Ddr

L


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The tangent modulus of elasticity (modulus ofelasticity) is the ratio, within the elastics limit of stress

to corresponding strain:

where :

Ef= modulus of elasticity in bending

m = slope of the tangent to the initial straight

line of the load-deflection curve (N/mm of deflection)

3

34f

L mE

bd


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Factors Affecting Test Results

B. Temperature


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Factors Affecting Test Results

C. Test Conditions

The strain rate, which depends upon testing speed, specimen thickness, and

the distance between supports (span), can affect the results.

At a given span, the flexural strength increases as the specimen thickness is

increased.

The modulus of a material generally increases with the increasing strain rate.


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The test is initiated by applying the load to thespecimen at the specified crosshead speed.

Two basic method to determined flexural properties :-

a) Three point loading system utilizing center loading

on a simple supported beamb) Four point loading system utilizing two load point

equally spaced from their adjacent support point,with a distance between load point of one third ofthe support span.

For ASTM D790, the test is stopped when the specimenreaches 5% deflection or the specimen breaks before5% then load-deflection curve is plotted.

Flexural Test Procedures


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Three point loading system Four point loading system



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Specimen are bars of rectangular crosssection and are cut from sheet,plates ormolded shape.

Specimen size:A variety of specimen shapes can be used forthis test, but the most commonly used

specimen size for ASTM is 3.2mm x 12.7mm x125mm (0.125" x 0.5" x 5.0").

Test specimen and conditioning


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The parameters for this test are the support span, the speed of the

loading, and the maximum deflection for the test.

Data:Flexural stress at yield, flexural strain at yield, flexural stress atbreak, flexural strain at break, flexural stress 5.0% (ASTM) deflection,flexural modulus. Stress/Strain curves and raw data can be provided.



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The deflection is measured

either by gauge under specimenin contact with it in the centerof the support span or bymeasurement of the motion ofthe loading nose relative to thesupport.

Load deflection curve is plottedif the determination of flexuralmodulus value is desired.

Flexural strength = maximumstress in the outer fibers at the

moment of break. P is obtained from the load at

break.

22

3

bd

PLS

Test Procedures and calculation

S= stress

P= load

L= length of span

b= width of specimen

d= thickness


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If the material do not break

at outer strain up to 5%, theflexural yield strength usingthe same equation. P in thiscase is maximum load at

which there is no longer anincrease in load with anincrease in deflection.

Maximum strain in theouter fiber, which occurs atthe mid span is calculatedusing the followingequation :

2

)6(

L

Ddr

Test Procedures and calculation

r= strain

D= deflection

L= length of span

D= thickness


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Flexural test result is a plot of load versus

displacement or stress versus strain. From this data, a

number of properties can be calculated such as

flexural modulus and yield strength.


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Flexural modulus is a measure of thestiffness during the first initial part ofthe bending process. This value of theflexural modulus is, in many casesequal to the tensile modulus.

Flexural modulus represented by theslope of the initial straight line portion

of the stress-strain curves and iscalculated by dividing the change instress by the corresponding change instrain. The procedure to calculateflexural modulus is similar for tensile.

Maximum strain in the outer fiber

which occurs at the midspan iscalculated using the followingequation :

3

3

4bd

mLEB

Modulus of elasticity

EB= modulus elasticity inbending

M= slope of the tangent tothe initial straight line of

the load-deflection curve.


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If a material is used in the form of beamand if the service failure occurs in bending,then a flexural test is more relevant for

design or specification purpose than atensile test, which may give a strengthvalue very different from the calculatedstrength of the outer fiber in the beam.

Specimen is easy to prepare withoutresidual strain compare to tensile wherespecimen alignment is more difficult.

Advantage of flexural strength over tensile test


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Three factor affecting the flexural test result are

:-

Specimen preparation

Temperature

Test condition

Factor affecting test result


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Mechanical Properties

Impact Properties


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Impact properties for polymeric material are relatedto toughness of the material.

Toughness defined as the ability of the polymer toabsorb applied energy.

Area under the stress-strain curve is directlyproportional to toughness of a material.

Impact energy is measure of toughness.

Impact resistance is the ability of a material to resistbreaking under a shock loading or the ability to resistthe fracture under stress applied at high speed.

Introduction


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There are basically four types of failuresencountered due to the impact load :-

a) Brittle Fracture- The part fractures extensivelywithout yielding

b) Slight Cracking The part shows evidence ofslight cracking and yielding without losing itsshape or intergrity.

c) Yielding- The part actually yields showingobvious deformation and stress whitening butno cracking takes place.

d) Ductile Failure This type of failure ischaracterized by a definite yielding of material

along with cracking. Eg. Polycarbonate isconsidered a ductile material.

Introduction


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Factor affecting the impact strength :-

Rate of loading

Notch Sensitivity

Temperature

Orientation

Processing Condition and types

Degree of Crystalinity, Molecular Weight

Method of Loading

Factor affecting the impact strength


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Pendulum Impact Test


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Objective Izod-Charpy impact test is to measurethe relative susceptibility of a standard testspecimen to the pendulumtype impact load.

The result are expressed in terms of kineticenergy consumed by the pendulum in order tobreak the specimen.

The energy required to break a standard

specimen is actually the sum of energy needed todeform it, to initiate its fracture and to propagatethe fracture across it.

Pendulum Impact Test


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Izod Impact Test

Izod Test Specimen Polypropylene Matrix/Glass Fiber


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Most commonly used on polymericmaterials.

Specimen are clamped vertically as a

cantilever beam. The pendulum hammer is released ,

allowed to strike the specimen and swingthrough.

Impact strength are calculated by dividingimpact values obtained from the scale bythe thickness of the specimen.

Izod Impact Test


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Charpy most commonly used on metals, it isalso used on polymers, ceramics andcomposites.

The test conducted in very similar manner toIzod, the only differences is the positioning ofthe specimen.

Only the specimen that break completely are

considered acceptable.

Charpy Impact Test


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Charpy Impact Test


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Charpy Impact Test


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Impact strength is expressed in J/m2according to SI method or in J/maccording to ASTM D256

SI units : area of break section will betaken into account

ASTM D256 : width of the specimen will

be taken into account.

Impact Strength


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Impact strength are calculated by dividingimpact values obtained from the scale by thethickness of the specimen.

Example:

Energy values given by the pendulum is3Joule

Specimen dimensions = 6.35 x 12.7mm Depth of notch = 2.54m

Area behind notch = (12.7 -2.54)mm x6.35mm So impact strength ????

Impact Strength SI


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Impact Strength is

251045.6

3

mx

= 46.5kJ/m2

Impact Strength SI method


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Example:

Energy values given by the pendulum is 3Joule

Specimen dimensions = 6.35 x 12.7mm

Depth of notch = 2.54mm

So impact strength ????

mJ/4.47200635.0

3

Impact Strength ASTM D 256


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Notch

Specimen Thickness

Specimen preparation

Temperature

Fillers and other additives

Limitation

Effect of test variable and limitation


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Mechanical Properties

Hardness Properties


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What is Hardness?

Hardness is the property of a material that

enables it to resist plastic deformation,usually by penetration. However, the term

hardness may also refer to resistance to

bending, scratching, abrasion or cutting.

Introduction


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Hardness testing is important for plastic

material since :-

Determination is relatively simple

Data can be related to other bulk

properties such as yield stress and

modulus

Hardness is defined as the resistance of the

material to deformation and indentation.

Introduction


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Hardness testing of plastic is complexbecause of :

Elastic Recovery

Creep

Many types of testing have been devisedto measure hardness because plasticmaterials vary considerably with respectto hardness.

Introduction


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The Rockwell hardness test method consists of indenting the test material with a diamondcone or hardened steel ball indenter.

The indenter is forced into the test material under a preliminary minor load F0 (Fig. 1A)usually 10 kgf.

When equilibrium has been reached, an indicating device, which follows the movements of

the indenter and so responds to changes in depth of penetration of the indenter is set to adatum position. While the preliminary minor load is still applied an additional major load isapplied with resulting increase in penetration (Fig. 1B).

When equilibrium has again been reach, the additional major load is removed but thepreliminary minor load is still maintained. Removal of the additional major load allows apartial recovery, so reducing the depth of penetration (Fig. 1C).

The permanent increase in depth of penetration, resulting from the application and removalof the additional major load is used to calculate the Rockwell hardness number.

Rockwell hardness


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Measure the net increase in depth impressionas the load of the indenter is increased fromfixed minor load to a major load and thenreturned to minor load.

The hardness are without units.

Rockwell hardness are quoted scale symbolrepresenting the indenter size,load and dial

scale used. The hardness scale in order of increasing

hardness are R,L,M,E and K scales.

Rockwell hardness


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Rockwell hardness


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The higher the number in each scale the

harder the material

It is possible to get overlapping between

different scales

For a specific type of materials, correlation in

the overlapping is possible, but not general

one.

Rockwell hardness

B ll h d


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Barcol hardness is a method that a hardness value

obtained by measuring the resistance topenetration of a sharp steel point under a spring

load.

The instrument, called the Barcol impressor, gives

a direct reading on a 0 to 100 scale. The hardness

value is often used as a measure of the degree of

cure of a plastic.

Barcoll hardness


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ASTM D2583 Barcol Hardness test method is usedto determine the hardness of both reinforced andnon-reinforced rigid plastics.

The specimen is placed under the indentor of theBarcol hardness tester and a uniform pressure isapplied to the specimen until the dial indicationreaches a maximum. The depth of the penetrationis converted into absolute Barcol numbers.

Barcol hardness values are also used to determine

degree of cure of resin. Resin is considered curedwhen it has a hardness value greater than orequal to 90% of the surface hardness value.

Barcoll hardness


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Test Procedure

The specimen is placed under theindentor of the Barcol hardnesstester and a uniform pressure isapplied to the specimen until the

dial indication reaches amaximum. The depth of thepenetration is converted intoabsolute Barcol numbers.

Specimen size

Specimens are required to be aminimum thickness of 1/16th of aninch.

Barcoll hardness


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There are two types of durometres are used :

Type A (soft material)

Type D (harder materials)

The difference is in the shape and dimension

of the indenter

Durometer hardness


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There are two types of durometres are used :

Type A (soft material)

Type D (harder materials)

The difference is in the shape and dimension

of the indenter

Durometer hardness


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Quiz

Name the two common type of

methods used to measure the

impact

strength of polymers and explain.


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hank you

lecture 5b-flexural propwyj

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