1. MECHANICAL PROPERTIES

1. MECHANICAL PROPERTIES 2.1 INTRODUCTION The data on mechanical properties have been the foremost requirement for a plastics designer to select the best suited material for a desired application. Although voluminous data on engineering properties is available, this is still not sufficient in vioew of rapid development of polymers and their formulations. Furthermore, the material properties being dependent on temperature, humidity, time & loading conditions, the available data may not be sufficient for material selection for a particular application. The material selection for a variety of applications is quite often based on mechanical properties such as tensile strength, modulus, elongation and impact strength. The judgment on the choice of the plastic material therefore requires an appreciation of the actual significance of a test as well as the quantitative information it provides. The mechanical properties of plastics can be broadly classified as short-term, long-term and surface properties. The short-term properties are measured at a constant rate of stress or strain different modes like tension, compression, flexural, shear etc. The long-term properties are measurements of deformation or stress decay with respect to time in static conditions e.g. creep and stress relaxation.

Definition:

a) Stress: Stress is the ratio of applied load to the original cross sectional area and

expressed in kg/ cm2 or lbs/in2. b) Strain: The ratio of elongation to the gauge length of the test specimen or

simply stated, change in length per unit of the original length. It is a dimensionless quantity.

c) Gauge Length: The original length between two marks on the test piece over which the change in length is determined.

d) Percentage Elongation: The increase in the length of a specimen produced by a tensile load.

e) Percentage Elongation at Yield: The percentage elongation produced in the gauge length of the test piece at the yield stress.

f) Percentage Elongation at break, or at maximum load: The elongation at break, or at maximum load, produced in the gauge length of the test piece, expressed as a percentage of the gauge length.

g) Elongation: The increase in the length of a specimen produced by a tensile

load. h) Yield point: The first point of stress-strain curve at which an increase in the

strain occurs without the increase in stress. i) Yield strength: The stress at which a material exhibits a specified limiting

deviation from the proportionality of stress to strain. j) Proportional limit: The greatest stress at which a material is capable of

sustaining the applied load without any deviation from proportionality of stress to strain.

k) Elastic Modulus in tension (Young’s modulus): The ratio of tensile stress to corresponding strain below the proportional limit. The stress-strain relationship of many plastics does not conform to Hooke‟s law throughout the elastic range

but deviates their form even at stress well below the yield stress. For such materials the slope of the tangent to the stress-strain curve at low strain is usually taken as the elastic modulus.

l) Secant modulus: The ratio of total stress to corresponding strain at any specific

point on the stress-strain curve. It is also expressed in F/A „or‟ lb/in2 m) Resilience: It is defined as the ratio of energy given up on recovery from

deformation to the energy required to produce the deformation. Area under stress-strain curve upto yield point gives the resilience of the material.

n) Toughness: It is defined as the extent to which the material absorbs energy. The area under stress-strain curve is a measure of toughness of the material.

o) Plasticity: It is defined as the property which enables a material to be deformed continuously and permanently with out any rupture by the application of force that exceeds the yield value of the material.

Stress – Strain Curve:

The mechanical behaviour of plastics is defined by using the stress-strain curve obtained in different modes like tension, flexure, compression or shear. The curves obtained in different loading conditions broadly resemble each other. Typical stress and strain curve of plastics obtained in tension for constant rate of loading is given below in figure.

The

deformation behavior is summarized in different stages as discussed below: i) Polymer molecules rest in random fashion in the solid state (A) whereas on

initial application of stress at a constant rate, bending and stretching of

Strain

Stress

A

E C

D

B

F Break

Yield

interatomic bonds takes place (A-B). This results in smaller deformation at molecular level. On relieving the stress, this deformation is recoverable instantaneously.

ii) The stretching of molecules continues from B to C, where in C is „proportionality limit‟ as stress is proportional to strain from A to C (Hooke‟s law) on relieving the stress, this anywhere up to the point C deformation is recoverable. This region is known as elastic behaviour of polymer materials. The ratio of stress to strain from A to C gives the value of Young‟s modulus or modulus of elasticity, which indicates the material‟s stiffness.

iii) The straightening of molecules continues from C to D but without their slippage. The molecular deformations in this region are recoverable but not instantaneous. Point D in the curve is the Yield Point. As the material stretches, its dimensions orthogonal to the axis of applied force decreases, thus the cross-section area decreases and the material thus starts necking.

iv) The deformation from D to E takes place without increase in stress. Strain hardening starts from point E. This hardening of material is a necessary prerequisite for cold drawing phenomena of material deformation when the rate of deformation is constant. Molecules slippage and orientation in the direction of applied stress continuous in E-F region. The deformation of this nature determines the viscous behaviour of the material and is irrecoverable. At point F the material fails to withstand the applied stress and breaks.

v) Area under curve i.e. A to E known as toughness of the material. The polymers can be broadly classified in terms of their relative softness, brittleness, hardness and toughness.

Soft & weak material: A soft & a weak material is characterized by Low modulus, losw

yield stress & moderate elongation at break Eg. PTFE

Hard and brittle material: Hard and brittle materials are characterized by high modulus and low elongation. It may or may not yield before breaking. Ex: general purpose phenolics. Hard & strong material: A hard and strong has high modulus, high yield stress, usually

high ultimate strength & low elongation. Ex: Polyacetal. Soft & tough material: A soft & tough material is characterized by low modulus and

low yield stress but very high elongation (20 to 1000) and high stress. Ex: PE, Rubber and plasticized PVC. Hard and tough material: A hard & tough material is characterized by high modulus, high yield stress, high elongation at break & high ultimate strength Polycarbonate is considered hard & tough material. Ex: PC 2.2 TENSILE PROPERTIES Definition: (a)Tensile Strength: It is the ability of a material to withstand forces that to pull it apart

and to determine to what extent the material stretches before breaking. Unit:Kgf/cm2 (b) Tensile Modulus: The ratio of tensile stress to corresponding strain at the maximum load within the proportional limit is known as tensile modulus. Unit: Kg/ cm2 (c) Percentage of Elongation at Yield: It is defined as the percentage elongation produced in the gauge length of the test piece at the yield tensile stress. Unit: % (d) Percentage of Elongation at Break: It is defined as the elongation at break or at maximum load, produced in the gauge length of the test piece. Unit: % Test Method: ASTM-D 638, ISO-527 Specimen Dimension: Dumb-bell shaped

The most common specimen for ASTM D638 is a Type I tensile bar. The most common specimen for ISO 527 is the ISO 3167 Type 1A multipurpose specimen. ASTM D882 uses strips cut from thin sheet or film.

Tensi

le test

piece ISO / DIS 527

Type 1(Broad-waisted Dumb-bell)

Tensile test piece ISO / DIS 527 Type 2 (Narrow-waisted Dumb-bell)

Conditioning: Prior to the testing, samples are conditioned at 23 ± 2°C and 50 ± 5% or

27 ± 2°C and 65 ± 5% relative humidity for 48 hours in accordance with procedure as per ASTM D 618. Test Procedure:

Specimens are placed in the grips of the tensile tester or Universal Testing Machine (UTM) at a specified grip separation and pulled until failure. For ASTM D638 the test speed is determined by the material specification. For ISO 527 the test speed is typically 5 or 50mm/min for measuring strength and elongation and 1mm/min for measuring modulus. An extensometer is used to determine elongation and tensile modulus. Calculations:

Force (load) (N) (1) Tensile strength =

Cross-section area of the specimen (mm²) Maximum load recorded (N) (2) Tensile strength at yield (N/mm²) =

Cross section area (mm²)

Load recorded at break (N) (3) Tensile strength at break (N/mm²) =

Cross section area (mm²)

Difference in stress (4) Tensile Modulus =

Difference in corresponding strain Change in length (elongation) (5) Elongation at yield, Strain (ε) =

Original length (gauge length) (6) Percent Elongation = ε x 100

If the specimen gives a yield load that is larger than the load at break, calculate

“percent elongation at yield” otherwise; calculate “percent elongation at break”. Factors Influencing:

Specimen preparation: Injection molded specimens have high molecular

orientation which yield higher tensile strength and low elongation than compression molded specimen.

Temperature and Humidity: Tensile strength decreases as temperature increases.

Moisture works as plasticizer, so it causes the decreases in tensile strength and increase the elongation.

Rate of strain: Tensile strength and modulus increases where as percentage of

elongation decreases with the increase in rate of straining and vice versa.

Molecular Weight and Molecular Weight Distribution: Tensile strength increases with the increase in molecular weight. Smaller molecules in polymer work as plasticizer. So with increase in molecular weight distribution, elongation decrease and tensile strength increases.

Crystallinity: Tensile strength increases with increase in crystallinity.

2.3 FLEXURAL PROPERTIES Definition: Flexural Strength: Flexural strength is the ability of the material to withstand bending

forces applied perpendicular to its longitudinal axis. The stresses induced due to the flexural load are a combination of compressive and tensile stresses. Unit-Kg/cm2 Flexural Modulus: Within the elastic limit, the ratio of the applied stress on a test

specimen in flexure to the corresponding strain of the specimen is known as flexural modulus. Unit-Kg/cm2 Test Method: ASTM-D-790, ISO 178, JIS-7203-1982, BS-2782. Specimen Dimensions Specimen are rectangular bar of 127mm x 12.7mmx 3 mm size.

Conditioning: Prior to the samples are conditioned at 23 ± 2°C and 50 ± 5% or 27 ±

2°C and 65 ± 5% relative humidity for 48 hours. Test Procedure:

There are two test methods for flexural test. 1. Three point bending method 2. Four point bending method

1. Three Point Bending Method (Test Method-I): It is a three-point loading system

utilizing center loading on a simple supported beam. A bar of rectangular cross section rest on two supports & is loaded by means of a loading nose midway between the supports the maximum axial fiber stresses occur on a line under loading nose.

2. Four Point Bending Method (Test Method-II): It is four -point loading system

utilizing two load points equally spaced from their adjacent supports point, with a distance between load points of one-third of the support span. In this method, the test bar rests on two supports & is loaded at two point (by means of two loading noses), each on equal distance from the adjacent support point. This method is very useful in testing materials that do not fail at the point of maximum stress under a three-point loading system the maximum axial fiber stress occurs over the area between the loading noses.

Calculation: 1) Calculate the rate of cross-head motion as follows and set the machine for the

calculated rate, or as near as possible to it, R = Zl2 / 6d Where, R = rate of cross-head motion (mm/min) l = support span (mm) d = depth of beam (mm) Z = rate of straining of entire fiber (mm/min)

2) Terminate the test when the maximum strain in the outer fiber has reached 0.05

mm/min. The deflection at which distortion occurs are calculated by „r‟ equal to 0.05 mm/min as follows:

D= rl2 / 6d Where, D = midspan deflection (mm) r = strain (mm/mm) l = support span d = depth of beam (mm) 3) Max.fiber stress- test method 1

S = 3PL / 2 bd2

Where, S = stress in the outer fiber at midspan (Mpa) P = load at given point on the load deflection curve (v) L= support beam (mm) b= width of beam tested (mm) d = depth of beam tested in (mm)

4) Maximum fiber stress for beam tested at large support spans (test method 1)

S = (3PL / 2 bd2 ) 1+ 6(D/L)2 – 4(d/l) (D/L)

5) Max.fiber stress-test (method „2‟) S = PL / bd2

For a load span of ½ of the support span S = 3PL / 4 bd2

6) Maximum fiber stress test (method 2) for beam tested at large support span:-

S = (PL / bd2 ) 1 + (4.70 D2 / L2 – (7.04 Dd / L2 )] For a span of one-half of the support Span: S = (3PL / 4bd2 ) * [ 1- (10.91 Dd / L2 ) ]

Factors Influencing:

Specimen Preparation: Injection moulded specimen usually shows a higher flexural

value than a compression moulded specimen.

Temperature: Flexural Strength and Modulus value are inversely proportional with

temperature.

Test Conditions: The strain rate, which depends upon testing speed; specimen

thickness and distance between supports (span), can affect the results. At a given span, Flexural Strength increases as the specimen thickness is increased. Modulus of a material generally increases with increasing strain rate.

2.4 COMPRESSIVE PROPERTIES Definitions: Compressive Strength: It is the ratio of the maximum load sustained by a test

specimen in a compressive test to the original cross section area of the specimen.Unit: kg/cm2 Compressive Deformation: The decrease in length produced in the gauge length of the test specimen by a compressive load. It is expressed in unit of length. Compressive Strain: The ratio of compressive deformation to the gauge length of the test specimen, i.e., the change in length per unit of original length along the longitudinal axis. It is expressed as dimension ratio. Slenderness Ratio: The ratio of the length of a column of uniform cross section to its

least radius of gyration known as slenderness ratio. Modulus of Elasticity: The ratio of stress to corresponding strain below the

proportional limit of a material. It is expressed as force per unit area, based on the average initial cross- sectional area. Compressive Yield Point: The fist point of stress-strain diagram at which an increase in strain occurs without an increase in stress. Test method: ASTM D 695, ISO 75-1 AND 75-2 Specimen Dimension:

For cylindrical specimen: 12.7mm dia x 25.4mm long.

For rectangular specimen: 12.5mm x 12.5mm x 25.4mm. Conditioning: Test specimens are conditioned at 23 ± 2°C and 50 ± 5% or 27 ± 2°C

and 65 ± 5% relative humidity for 48 hours. Test procedure:

1) Measure the width and thickness of the specimen to the nearest 0.01 mm at several points along its length.

2) Calculate and record the minimum value of the cross-sectional area. 3) Measure the length of specimen and record the value. 4) Place the test specimen between the surfaces of the compression tool, taking care

to align the center of its long axis with the centreline of the plunger and to ensure that the ends of the specimen are parallel with the surfaces to the compression tool.

5) Place thin specimens in the jig so that they are flush with the base and centered. 6) If only compressive strength, then set the speed control at 1.0 mm / min and start

the machine. 7) If strain- stress data are desired then,

a) Attach compressor b) Record load and corresponding compressive strength at appropriate intervals of

strain.

c) After the yield point reach, it is desirable to increase the speed from 5 to 6 mm/min. and allow machine to run at this speed until the specimen breaks.

Calculation

Load (kg) 1. Compressive strength = Original cross sectional area (cm2) Maximum load recorded (N) 2. Compressive strength at yield (N/mm²) = Cross-section area (mm²) Load recorded at break (N) 3. Compressive strength at break (N/mm²) = Cross-section area (mm²) Difference in stress 4. Compressive Modulus = Difference in corresponding strain Change in length (Deformation) 5. Deformation at yield, Strain (ε) = Original length (gauge length) 6. Percent Deformation = ε x 100

If the specimen gives a yield load that is larger than the load at break, calculate

“percent Deformation at yield” otherwise; calculate “percent Deformation at break”. Factors Influencing:

Temperature and Humidity: Compressive strength decreases as temperature

increase. Moisture acts as plasticizer, there by decreasing the compressive strength and increase the deformation.

Method of specimen Preparation – Molecular Orientation has a significant

effect on Compressive Strength values. A load-applied parallel to the direction of molecular Orientation may yield higher value than the load applied perpendicular to the orientation. The opposite is true for elongation.

Rate of Straining- As the strain rate increases, Compressive Strength and

modulus increases, and deformation is inversely proportional to the strain rate.

Molecular Weight and Molecular Weight Distribution: Increase in molecular

weight increases the compressive strength. Smaller molecules in polymer acts as plasticizer. Hence increase in molecular weight distribution, decreases deformation and increases compressive strength.

Crystallinity Increase in crystallinity increases in compressive strength. 2.5 IMPACT PROPERTIES

Definition Impact strength: Impact strength is defined as the ability of material to absorb applied

energy. Unit: J/m. Resilience: It is defined as the ratio of energy given up on recovery from deformation to

the energy required to produce the deformation. Area under stress-strain curve upto yield point gives the resilience of the material. Notch sensitivity: The ratio of impact strength of un-notched specimen to the notched specimen of same material is called notch sensitivity. Test Method: ASTM D 256 A (Izod) & B (Charpy), ASTMD1822, ISO 180 (Izod), ISO 179 (Charpy), JISK-7111 &7112,

The impact test methods are as following: (1) Pendulum impact tests

(i) Izod impact test (ii) Charpy impact test (iii) Chip impact test (iv) Tensile impact test

(2) High-rate tension test (3) Falling weight impact test

(a) Drop weight (top) impact test (b) Dart Impact test

(4) Instrumented impact tests (5) High- rate impact testers.

(a) High speed ball impact tester (b) High speed plunger impact tester

(6) Miscellaneous impact test. Conditioning: 23 ± 2°C and 50 ± 5% relative humidity for 48 hours. IZOD IMPACT TEST: Specimen Dimension: 63.5 x12.7 x 3.2 with notch angle 45o and notch depth 2.54mm. Test Procedure:

The test specimen is clamped in a position so that the notched end of the specimen is facing the striking edge of the pendulum.

Check for properly positioned the test specimen.

The pendulum hammer is released, allowed to strike the specimen and swing through.

If the specimen does not break, more weights are attached to hammer and the test is repeated until failure is observed.

The impacts values are read directly in in- lbs are ft- lbs from the scale

CHARPY IMPACT TEST: Specimen Dimension: 63.5 x12.7 x 3.2 with notch angle 45o and notch depth 2.54mm. Test Procedure:

This test is conducted in a very similar manner to the Izod impact test.

The only difference is the positioning of the specimen.

In this test the specimen is mounted horizontally and supported at both ends. Only the specimens that break completely are considered acceptable.

The Charpy impact strength is calculated by dividing the indicator reading by the thickness of the specimen.

The results are reported in ft-lbs/in. of notch for notched specimen and ft-lbs /in. for unnotched specimens.

FALLING-WEIGHT IMPACT TEST: Test Procedure:

The falling impact test, also known as the drop impact test or the variable-height impact test, employs a falling weight.

This falling weight is a tup with a conical nose, a ball, or a ball-end dart.

The energy required to fail the specimen is measured by dropping a known weight from a known height onto a test specimen.

This test is also very suitable for determining the impact resistance of plastic films, sheets and laminated materials.

Three basic ASTM tests are commonly used depending upon the application: ASTMD 3029 Impact resistance of rigid plastics sheeting ASTMD 1709 Impact resistance of polyethylene film by the free falling

dart method ASTMD 244 Test for impact resistance of thermoplastics pipe and fittings by means of a tup.

DROP IMPACT TEST: Test Procedure:

The test is carried out by raising the weight to a desired height manually or automatically with the use of motor-driven mechanism & allowing it to fall freely on to the other side of the round- nosed punch.

The punch transfers the impact energy to the flat test specimen, which is positioned, on a cylindrical die or a part lying on the base of the machine.

The kinetic energy possessed by the falling weight at the instant of impact is equal to the energy used to raise to the height of drop and is the potential energy possessed by the weight as it is released.

Since the potential energy is expressed as the product of weight and height, the guide tube is marked with a linear scale representing the impact range of the instrument in inch-lb.

Thus, the toughness or the impact strength of a specimen or a part is read directly off the calibrated scale in in-lb.

The energy loss due to the friction in the tube or due to the momentary acceleration of the punch is negligible

Calculations:

Energy required breaking the sample (J) Impact strength (J/m) = (Izod / Charpy) Thickness (m) Dart Impact Test: Calculate Wf = WL - [ ΔW (S/100 – ½)] Where, Wf = impact failure weight, gms, ΔW = uniform weight increment used, gms, WL = lowest missile weight, gms, according to the particular ΔW used, at which 100% failure occurred and S = sum of the percentages of breaks at each missile weight (from a weight corresponding to no failures upto and including WL ) Factors Influencing:

Rate of Loading: The speed at which the specimen or part is struck with an object has a significant effect on the behavior of the polymer under impact loading. At low rate of impact, relatively stiff materials can have good impact strength however at high rates of impact, even rubbery materials may exhibit brittle failure.

Preparation of Test Specimen - Specimen prepared by injection moulding have

high impact strength in the direction of flow than the horizontal compression molded specimen

Temperature of testing: At lower temperature the impact strength is reduced

whereas at high temperature, it is significantly improves conversely.

Notch sensitivity: Radius of Notch: Impact strength is directly proportional to radius of notch. Angle of Notch: Impact strength is directly proportional to angle of notch.

Notch Depth: Impact strength is indirectly proportional to notch depth.

Processing conditions: High processing temperature, presence of voids and

weld lines reduces the impact strength. High process temperature causes thermal degradation which reduces the impact strength.

Degree of Crystallinity and Molecular Weight: Increase the percentage of

crystallinity, decreases the impact strength and increase probability of brittle failure. A reduction in average molecular weight tends to reduce impact strength.

Glass transition temperature:

Effect of addition of filler/ reinforcement: Impact strength increases with the

addition of filler/reinforcement. 2.6 TEAR STRENGTH Definitions:

The average force to require propagating tearing to a specified length of plastic film or non-rigid sheeting is known as tear strength. Unit: kg Test Method: ASTM D 1004 Specimens Dimension: The dimensions are 101.60 mm x 19.05mm and 0.750” rad.

Conditioning: Test specimens conditioned at 23 2C and 50 5% relative humidity

for 48 hours prior to test in accordance with procedure A of method D 618. Test procedures:

Measure and record the thickness of each specimen at several points to the accuracy limits of the measuring devices and the average thickness.

Place the specimen in the grips of the testing machine so that the long axis of the enlarged ends of the specimen is in line with the points of attachment of the grips to the machine.

Apply the load at 51 mm/ min. rate of grip separation. After complete rupture of the specimen, the maximum tearing load is noted from the dial scale or recorder chart and recorded.

Calculation: Force required to shear the specimen (Kg)

Shear strength (kg/cm2) = Area of sheared edge (cm2) Area of shear edge = circumference of punch x thickness of specimen = 2p (R – r) x t Where r = radius of hole initially drilled in specimen. R= radius of hole after shear, t= thickness of specimen. Factors Influencing:

Temperature and Humidity: Tear resistance decreases as temperature increases.

Test Speed: Elongation is high when test speed is minimum i.e. 0.05 mm/min and is lower, when test speed is maximum i.e. 500 mm/min.

Rate of Straining- As the strain rate increases, tear resistance and modulus

increases. Elongation is inversely proportional to the strain rate.

Method of specimen Preparation: A load-applied parallel to the direction of

molecular Orientation may yield higher value than the load applied perpendicular to the orientation. The opposite is true for elongation.

Molecular Weight and Molecular Weight Distribution: Increase in molecular

weight increases tear resistance.

Crystallinity: Increase of crystallinity increases tear resistance.

2.7 CREEP PROPERTIES Definition: Creep: When a plastic material is subjected to a constant load, it deforms quickly to a strain roughly predicted by its stress-strain modulus, and then continues to deform slowly with time indefinitely or until rupture or yielding causes failure. This phenomenon under load with time is called creep. Because of viscoelastic nature, a plastic subjected to a load for a period of time tends to deform more than it would from the same load released immediately after application and the degree of this deformation is dependent on the load duration. Creep is the permanent deformation resulting from prolonged application of stress below the elastic limit. Creep at room temperature is sometimes called cold flow. Creep Modulus (Apparent modulus): It is defined as the ratio of initial stress to creep

strain. Creep rupture strength: Stress required causing fracture in a creep test within a

specified time. Unit: Flexural Creep = lb/inch2

Test Methods: ASTM D 2990, JIS-7115 Specimen Dimension: (i) The test specimen for tensile creep measurement is either type1 or type 2 as

specified in test method D 638 (ii) Specimen for unconfined compressive creep test is suitably prepared in the

manner described in test method D 695, except that the length is increased so that the slenderness ratio lies between 11 and 15.

(iii) Test specimen for flexible creep is a rectangular bar conforming to the requirements of section 5 at test method D-790.

(iv) Test specimen is made by injection or compression moulding or machining from sheets or other fabricated form.

(v) Specimen prepared from sheets is cut in the same direction.

Conditioning: Test specimens conditioned at 23 2C and 50 5% relative humidity for 48 hours. Test Procedure: Tensile Creep:

Mount a properly conditioned and measured specimen in the grips, tensile creep fixture.

Attach the deformation-measuring device to the specimen.

Apply the constant load to a tensile test specimen and smoothly to the specimen and measuring its extension as a function of time.

The extension measurement is carried out by taking two gauge marks on the tensile specimen and measure the distance between the marks at specified interval time.

Measure temperature, relative humidity and other environmental variables and deformation of control specimen with the some schedule as that for deformation of the test specimen.

Flexural Creep (i) Flexural creep measurements are also made by applying a constant load to the

standard flexural test specimen and measuring its deflection as a function of time. (ii) The deflection of the specimen at mid-span is measured using a dial indicator

gauge. (iii) The electrical resistance gauges may also be used in place of a dial indicator. (iv) The deflections of the specimen are measured at a predetermined time interval. Calculation:

(i) For flexural measurements calculate the maximum fibre stress for each specimen in MPa as follows:

3PL

S = 2bd2 Where

S = Stress, N/mm² (MPa) P = Load, N (lbf) L = Span, mm (in) b = Width, mm (in) and d = depth, mm (in)

(ii) Calculate the percent flexural creep strain, 6Dd

r = x 100 L2

r = maximum percent creep strain, mm /mm (in/in) D = maximum deflection at mid-span, mm (in) d = depth, mm (in) and L = span, mm (in) Initial applied stress (iii) Creep (Apparent) modulus at time t = Creep strain Factors Influencing:

Temperature of testing: The creep resistance is inversely proportional to the

Temperature.

Humidity of Environment: The creep Resistance is inversely proportional to the Humidity level in moisture sensitive material

Effect of Reinforcing Filler: Creep resistance is directly proportional to the

loading of Reinforcing Filler.

Effect of Plasticizer: Creep resistance is inversely proportional to plasticizer.

Aromatic Content & Cross-linking of Polymer: Creep resistance is directly

proportional to the Aromatic Content & Cross-linking of Polymer. 2.8 STRESS RELAXATION Definition: Stress Relaxation: Stress relaxation is defined as a gradual decrease in stress with

time, under a constant deformation (strain). Stress optical sensitivity: The ability of some material to exhibit double refraction of

light when placed under stress is referred to as stress-optical sensitivity. Stress Concentration: The magnification of the level of applied stress in the region of

a notch, crack, void, inclusion, or other stress rises. Test Method: ASTM D 2991 Specimen Dimension: The test specimen may be round, rectangular, square or in dumbbell shape as used for tensile test in ASTM D 638. Conditioning: Test specimen is conditioned at 23 ± 2°C and 50 ± 5% relative humidity

for not less than 48 hours. Test Procedure:

Stress relaxation a measurement is carried out by using a tensile testing machine such as that describes earlier.

The equipment for a stress relaxation test must be capable of measuring very small elongation accurately, even when applied high speeds. Many sophisticated pieces of equipment now employ a strain gauge or a difference.

The strain is applied to the specimen at a constant rate to achieve the desired elongation.

Once the specimen reaches the desired elongation, the strain is held constant for predetermined amount of time, the stress decay, which occurs due to stress relaxation, is observed as function of time.

If a chart recorder is not available the stress values at different time intervals are recorded and the results are plotted to obtain a stress versus time curve.

The stress data obtained from stress relation experiment is converted to a more meaningful apparent modulus data by simply dividing stress at a particular time by the applied strain.

The curve is re-plotted to represent apparent modulus as a function of time.

The use of logarithmic co-ordinates further simplifies the stress relaxation data by allowing us to use standard extrapolation methods such as the one used in creep experiments.

Calculation: For Stress relaxation measurements calculate the maximum fibre stress for each specimen in mega pascal (MPa) as follows:

3PL

S = 2bd2

Where, S = Stress, mpa P = Load, N (Ibf) L = Span, mm (in) b = Width, mm (in) and d = depth, mm (in) ii) Calculate

6Dd

The maximum strain r = L2

r = maximum strain, mm /mm (in/in) D = maximum deflection at mid-span, mm (in) d = depth, mm (in) and L = span, mm (in) Factors Influencing:

Temperature of testing: The stress relaxation is directly proportional to the

temperature.

Humidity of Environment: The stress relaxation is directly proportional to the

Humidity level in moisture sensitive material.

Effect of Reinforcing Filler - Stress relaxation is inversely proportional to the loading of reinforcing filler.

Effect of Plasticizer: The stress relaxation is directly proportional to plasticizer.

Aromatic Content & Cross-linking of Polymer: Stress relaxation is directly

proportional to the aromatic Content & cross-linking of polymer. 2.9 FATIGUE RESISTANCE Definition: Fatigue: The failure or decay in mechanical properties due to repeated cycle loading in terms of flexing, stretching, compressing or twisting is known as fatigue Fatigue Endurance (life): Fatigue endurance is defined as the number of cycles of deformation required to bring about the failure of the test specimen under a given set of oscillating conditions. Fatigue failure: The failure or rupture of a plastic article under repeated cyclic stress,

at a point below the normal static breaking strength. Fatigue Limit: The stress below which a material can be stressed cyclically for an

infinite number of times without failure. Fatigue strength: The maximum cyclic stress a material can withstand for a given

number of cycles before failure. Test Method: ASTM D 671, JISK7118, JISK7119

Specimen Dimension:

For flexural fatigue two geometry‟s (Type A or Type B).

For tensile fatigue, the specimen is dumbbell-shaped of about 2 inch long with cylindrical cross section.

Conditioning: Test specimen is conditioned at 23 ± 2°C and 50 ± 5% relative humidity for not less than 48 hours. Test Procedure: Flexural Fatigue Test

The ability of a material to resist deterioration from cyclic stress is measured in this test by using a fixed cantilever-type testing machine capable of producing a constant-amplitude-of – force on the test specimen each cycle.

The specimen is held as a cantilever beam in a vice at one end and bent by a concentrated load applied through a yoke fastened to the opposite end.

A counter is used to record the number of cycles along with a cutoff switch to stop the machine when the specimen fails.

Tensile fatigue Test

Tensile fatigue test is conducted under constant load (stress) conditions.

The test is conducted by mounting both ends of the dumbbell specimen in the testing machine.

The specimen is rotated between two spindles and stress in the form of tension and compression is applied.

The specimen is subjected to the number of cycles of stress specified or until fracture occurs.

Tuning machine: Tune machine by using a thin material specimen having a

natural frequency of 1800 ± 4 cpm when vibrating as a free cantilever beam.

Measurements: Test three or more specimens at each of atleast four different stress amplitudes. Choose stress amplitudes to yield a mean log of cycles to failure (log N) of about 4,5,6 and 7.

Calculation of effective mass of test specimen: Calculate the effective mass (more 2) of the standard specimen types A and B.

For type A specimen, W = kDd For type B specimen, W = k‟Dd

Attachment of tuning weight: Determine the proper amount of tuning weight, by subtracting the effective mass at the test specimen.

Calculation: calculate the load to be applied to the specimen from the desired

(known) alternating stresses. 3Sbd2

P =

6L Where P=Load (N), S=Stress (N/mm2), b=width (mm), d= depth (mm), L= span (mm) FACTORS INFLUENCING:

Moulded Conditions- Fatigue resistance is inversely proportional to moulded

conditions.

Temperature of testing -The stress relaxation is inversely proportional to the lifetime

of test specimen.

Humidity of Environment – Humidity level in moisture sensitive material is inversely

proportional to fatigue endurance limit.

Frequency of Stress - Frequency of stress is inversely proportional to fatigue

resistance

Thermal Conductivity - Thermal conductivity is inversely proportional to fatigue endurance limit.

2.10 ABRASION RESISTANCE Definition Abrasion Resistance: Abrasion Resistance is defined as the ability of a material to withstand mechanical action (such as rubbing, scrapping or erosion) that tends progressively to remove material from its surface. Unit:- mg/1000 cycles Test method: ASTM-D 1044, ISO 9352, JIS-7205-77 Specimen Dimension: 102 mm square or disc specimen of thickness 3mm and 6.3mm

hole in the centre. Conditioning: Test specimen is conditioned at 23 ± 2°C and 50 ± 5% relative humidity

for not less than 48 hours. Test Procedure:

(1) Determine the density of material to be tested in accordance with standard analytical procedure.

(2) Fill the abrasive container with the grit and adjust the rate at feed to 44 ± 2 g/min. (3) Weigh the mounted test specimen to the nearest 0.1 g, and attach to the specimen

plate holder by means of the endcam provided. (4) Place the weight on the specimen shaft bush at grid under the specimen, place the

1.6 mm spacer under the specimen with the largest dimension on the surface at the rotating disk, and adjust the cam follower to this max. Tilt. Remove the spacer.

(5) For continuing runs reattach the specimen mounting plate to the specimen plate holder. Read just to a 1.6mm lift.

Calculation: Calculate the resistance to abrasion or the abrasion loss as the in volume at 1000 revolution as follows:

W1 – W2 ------------------------> (1) Volume loss, (cm3) = D Where, W1 = initial weight (gm) W2 = final weight after abrasion (gm) D = Density of the material being abraded.

Factors Influencing:

Temperature of testing: The abrasion resistance is inversely proportional to the

temperature.

Humidity of Environment: The abrasion resistance is inversely proportional to the humidity level in moisture sensitive material.

Effect of Reinforcing Filler: The abrasion resistance is directly proportional to the

loading of reinforcing filler.

Surface Conditions of the specimen: Surface finish has significant effect on the

abrasion resistance results. A smooth moulded surface yield higher value than a machined surface.

2.11 HARDNESS PROPERTIES Definition: Hardness:- Hardness is defined as the “resistance of a material to deformation” particularly permanent deformation, indentation or scratching. Unit: Rockwell Hardness – Number reading in M, L or R scales Durometer Hardness – Number Types: Following are some of the method used for measuring the hardness of plastics-

(1) Shore durometers: These tests measure the depth of penetration under load

when a hardness steel indenter is forced into a surface by calibrated spring. (2) Rockwell hardness tester: - Rockwell hardness number is not a measure of

total indentation but of the non-recoverable indentation after a major load applied for 15 second is reduced to a 10 kg minor load for 15 second. Measurement is made from the increase in depth of impression when load on a ball indenter is increased from a fixed minimum to a specified maximum then returned to the minimum load.

(3) Barcol Tester: Barcol tester is a hand pressed one with a spring loaded plunger.

Indenter is a frustum of 26 cone with flat tip 0.0062 inch surrounded by concentric sleeve. The indenter is mob and hardened steel. The hardness value is the initial highest dial reading.

(4) Brinell Hardness method: The brinell test for plastics generally used loads of

500 Kg and a 10 mm diameter steel ball applying the load for 30 second specimen should be 0.125” thick.

2 F Brinell Hardness = π D2 {1-[1-(d/D)2 ]½} Where F = Load in Kg D = Diameter of indenter d = Diameter of impression produced

Significance: (1) A Rockwell hardness number is a number derived from the net increase in depth

impression as the load on an indenter is increased from a fixed minor load to a major load and then returned to a minor load. Indenters are round steel balls of specific diameters. Rockwell hardness numbers are always quoted with a scale used. This test method is based on test method E18. Each Rockwell scale division represents 0.002-mm (0.00008in) vertical movement of the indenter.

(2) A Rockwell hardness number is directly related to the indentation hardness of a plastic material, the higher the reading the harder the material. Hardness number is equal to minus the instrument reading. Due to a short overlap of Rockwell hardness scales, two different dials reading on different scales are technically correct.

Rockwell Hardness

Rockwell hardness number derived from the net increase in depth impression as the load on an indentor is increased from a fixed minor load a major load and then returned to minor load. Test Methods: ASTM D 785, ISO 2039, JIS K 7202, DIN 53426 Specimen Dimensions: Smooth flat sheet of 25 x 25 x 6 Conditioning: Test specimen is conditioned at 23 ± 2°C and 50 ± 5% relative humidity for not less than 48 hours. Test Procedures:

The specimen is placed on the anvil of the apparatus and minor load is applied by lowering the steel ball onto the surface of the specimen.

The minor load indents the specimen slightly and assures good contact.

The dial is adjusted to zero under minor load and the major load is immediately applied by releasing the trip lever.

After 15 sec the major load is removed and the specimen is allowed to recover for an additional 15 sec.

Rockwell hardness is read directly off the dial with the minor load still applied. Durometer Hardness Test Methods: ASTM D 2240, ISO 868, JIS K 7215, DIN 53505. Specimen Dimensions: minimum 3mm thick sheet Conditioning: Test specimen is conditioned at 23 ± 2°C and 50 ± 5% relative humidity

for not less than 48 hours. Test Procedures:

The test is carried out by placing specimens on a hard flat surface.

The pressure foot of the instrument is pressed onto the specimen, making sure that it is parallel to the surface of the specimen.

The durometer hardness is read within 1sec after the pressure foot is in firm contact with the specimen.

Two types of durometers are most commonly used –Type A & Type D.

The difference between two types is the shape and dimension of the indentor.

Type A durometer is used for soft material.

Type D durometer is used for hard material. Factors Influencing:

Temperature of testing: The hardness is inversely proportional to the Temperature.

Humidity of Environment: The hardness is inversely proportional to the Humidity

level in moisture sensitive material.

Effect of Reinforcing Filler: The hardness is directly proportional to the loading of

Reinforcing Filler.

Surface Conditions of the specimen: Surface finish has significant effect on the

Hardness results. A smooth moulded surface yield higher value than a machined surface.

An-isotropy: Plastic material with the An-isotropic characteristic may cause

indentation hardness to vary with the direction testing. 2.12 COEFFICIENT OF FRICTION Definitions: Coefficient of friction- It is the ratio of the frictional force to the force, usually

gravitational acting perpendicular to the two surfaces in contact. This coefficient is a measure of the relative difficulty with which the surface of one material will slide over an adjoining surface of itself or of another material. Friction: The resisting force that arises when a surface of one substance slides or

tends to slide over an adjoining surface of itself or of another substance. There are two types of friction: (1) The resistance opposing the force required to start to move one surface over

another. (2) The resistance opposing the force required to move one surface over another at a

variable, fixed or predetermined speed. Slip: It denotes the lubricity of two surfaces sliding in contact with each other. Significance: (1) Measurements of frictional properties are made on a film or sheeting specimen

when sliding over itself or another substance. The coefficients of friction are related to the slip properties of plastic films that are of wide interest in packaging applications.

(2) Slip properties are generated by additives in some plastic films for e.g. Polyethylene. These additives have varying degrees of compatibility with the film matrix. Some of them bloom, or exude to the surface, lubricating it and make it more slippery.

(3) Frictional and slip properties of plastic film and sheeting are based on measurement of surface phenomena where products have been made by the same process, their surface is dependent on the equipment or its running conditions.

(4) The measurement of the static coefficient of friction is highly dependent on the rate of loading and on the amount of blocking occurring between the loaded sled and the platform due to variation in time before motion is initiated.

Test Methods: ASTM D 1894, Specimens Dimension: 1. For plane specimen – 250 mm in the machine direction and 130 mm in the

transverse direction 2. For film specimen – 120 mm square 3. For sheet specimen – 63.5 mm square

Conditioning: Test specimen is conditioned at 23 ± 2°C and 50 ± 5% relative humidity for not less than 48 hours. Test Procedures:

(1) Tape the 250/130 mm film or sheet specimen to plane with the machine direction at the specimen in the 250mm direction.

(2) Smooth the film specimen to eliminate wrinkles. Care must be taken, not to alter the specimen surface through finger oils etc.

(3) Start the driving mechanism which is adjusted previously provides a speed at 150 ± 30mm/mm per min.

(4) Record the visual average reading during the run at app. 130mm where the surfaces are sliding uniformly over one month.

(5) This is equivalent to the kinetic force require to sustain motion between the surface normally lower than the static force required to initiate motion, after the sled has traveled over 130mm over stop the apparatus.

(6) Remove the film or sheeting from the sled and the horizontal plane. (7) The apparatus is now ready for the next set of specimens. (8) A new set of specimens is used for each run. Specimen surface(s) is tested more

than once unless such tests constitute one of the variables to be studied. Calculation: (1) Calculate the static coefficient of friction μs, follows

As μs = B

Where, As = initial motion scale reading, gms and

B = sled weight, gms. (2) Calculate the kinetic coefficient of friction μk, follows

Ak μk =

B Where, Ak = average scale reading obtained during uniform sliding of the

film surfaces, gms, and B = sled weight, gms Factors Influencing:

Surface Conditions of the specimen: Surface finish has significant effect on the

coefficient of friction results. A smooth moulded surface yield lower value than a machined surface.

Test Speed: Higher the test speed is directly proportional to the values coefficient of

friction.

Sled Weight: Sled Weight inversely proportional to the values coefficient of friction