Stress-Strain Curve 3. Short Term Mechanical Properties
Stress-Strain Curve 3. Short Term Mechanical Properties
Stress-Strain Curve 3. Short Term Mechanical Properties
Mechanical Properties
CONTENTS
1. INTRODUCTION
2. STRESS-STRAIN CURVE
3. SHORT TERM MECHANICAL PROPERTIES
3.1 Tensile Properties
3.2 Flexural Properties
3.3 Impact Properties
3.4 Compressive Properties
3.5 Shear Strength
3.6 Tear Strength
A. Initiation
B. Propagation
3.7 Stiffness Test
3.8 Burst Strength
A. Long- term method
B. Short- term method
Topics Covered
1. Introduction
2. Definition
3. Significance
4. Test Method
5. Specimen Preparation and Condition
6. Test Procedure
7. Observation / Calculation/Result
8. Formula
9. Factors Influencing
10. Test Results
11. Safety Precautions
12. References
1. INTRODUCTION
The mechanical properties are the most important properties because all service
conditions and the majority of end-use applications in involve some degree of mechanical
loading. The material selection for a variety of applications is quite often based on mechanical
properties such as tensile strength, modulus, elongation and impact strength. Although the
voluminous data on these engineering properties are available, this is still not sufficient in view
of the rapid development of new polymers and their formulations. Available data on
mechanical properties are not sufficient for material selection since these are dependent on
temperature, humidity, time, loading conditions, rate of loading etc.
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.
The mechanical properties such as tensile strength, modulus, elongation and
impact strength are normally derived from the technical literature. Definition of these
properties are as follows:
a) Stress: The force applied to produce deformation in a unit area of a test specimen.
Stress is a ratio of applied load to the original cross sectional area expressed in lbs/in 2.
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 expressed as the dimensionless
quantity.
c) Gauge Length: The original length between two marks on the test piece over which
the change in length is determined.
d)
tensile load.
e) Percentage Elongation at Yield: The percentage elongation produced in the gauge
length of the test piece at the yield stress.
f)
maximum load, produced in the gauge length of the test piece, expressed as a percentage of
the gauge length. a)
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 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 (Youngs modulus):
corresponding strain below the proportional limit. The stress-strain relationship of many
plastics does not conform to Hookes law throughout the elastics 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
D yield
Stress
E
break
Strain
iv)
superimposed on each other. The bonding & the stretching of the interactive
bonds are almost instantaneous. The molecular uncoiling is relatively slow.
vi)
The polymers are broadly classified as per their strain behavior, which is the indication of
softness, brittleness, hardness and toughness. A hard & strong material has high modulus,
high yield stress, usually high ultimate strength & low elongation e.g. acetal. A hard & tough
material characterized by high modulus, high yield stress, high elongation at break & high
ultimate strength Polycarbonate is considered hard & tough material.
Maxwell
Model
Ratio of stress to corresponding strain with in the range of greatest stress that the Maxwell is
capable to sustaining with any deviation of proportionality of stress to strain.
The initial portion of the stress-strain curve between curve point A to C is linear and it
follows Hooks law, which states that the stress is proportional to the strain. The point at which
the actual curve dewaks from
the straight line is called the Proportional limit, meaning that only upto this point is stress
proportional to strain. The behavior of the plastics material below the proportional limit is elastic
in nature and therefore the deformations are recoverable.
3. 1 TENSILE PROPERTIES
3.1.1 INTRODUCTION
The study of stress in relation to strain in tension depicts the tensile properties of the
material. The tensile elongation and modulus measurements are the most important
indications of strength in a material and are the most widely specified properties of plastics
material. The tensile properties is measurement of the ability of material to with stand forces
that tend to pull it apart and to determine to what extent material stretches before breaking.
Tensile modulus, an indication of the relative stiffness of a material, which is calculated from
a stress-strain curve. Plastics materials are compared on the basis of tensile strength,
elongation and tensile modulus data. These data are useful for the propose of engineering
design and understanding the characteristics of materials. The properties of material changes
by various factors like rate of straining, environmental conditions, the addition of additives
like fillers, Plasticizer etc.
3.1.2 DEFINITION
3.1.2 (a) Tensile strength.
The maximum Tensile stress ( nominal) sustained by a test piece during a tension test or
Ultimate strength of a material subjected to tensile loading otherwise, it is a measurement of
the ability of a material to withstand forces that to pull it apart and to determine to what extent
the material stretches before breaking.
UNITS:
Tensile strength / Modulus = Kgf/cm2
Percentage of Elongation = %
3.1.3 SIGNIFICANCE:
(1) This test method is designed to produce tensile property data for the control and
specification of plastics materials. These data are also useful for qualitative
characterization purpose and for research and development. .
(2) Tensile properties may provide useful data for plastics engineering design
purposes. However, because of the high degree of sensitivity exhibited by many
plastics to rate at straining and environmental conditions.
Dumb-bell shaped
Universal testing machine for testing of the specimen in either Tension or compression
Universal testing machine for testing of the specimen in either Tension or compression
Strip Chucks Grip It is suitable for vary strong laminates and sheet materials.
Self Tightening Jaws Grip- It is suitable for soft rubber and plastics sheeting for example
Plasticised PVC.
Proof Ring
Strain Gauge
3.1.6(d) ELONGATION MEASUREMENTS:
The requirement for measuring elongation in highly extensible materials gives various grades of
extensometer with differing degree of precision. The accuracy requirement range from 1mm
down to 0.05mm. These are two types one is Contact & Non contact type. The contact types
rely on the physical contact between extensometer and the specimen to sense the change in
length during test. Non contact extensometer light being are used to track the movement of
contrasting color gauge marks on the test piece, servomechanisms is used to drive the optical
heads in the appropriate direction.
Accurate measurement of width and thickness of the test specimen in the narrow
parallel portion at several places to the nearest 0.025mm. The gauge length on the specimen
is marked appropriately according to the according to the test standard.
Fixing the specimen between the grips of the machine while maintaining the alignment.
Selection of the lowest test speed that is produced rupture in to 5 min. the speed is
Difference in stress
(4) Tensile Modulus =
Difference in corresponding strain
Change in length (elongation)
(5) Elongation at yield, Strain () =
Original length (gauge length)
(6) Percent Elongation = 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.
b) Test Speed 0.05 mm/min. to 500mm/min. 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.
c) Method of specimen Preparation Molecular Orientation has a significant effect on tensile
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.
d) Effect of Plasticizer and filler Soften the material, brings down the Tensile Strength and
increase Elongation.
e) Crystallinity With the increase of Crystallinity, Tensile Strength increases.
f) Rate of Straining- As the strain rate increases, Tensile Strength and modulus increases.
Elongation is inversely proportional to the strain rate.
g) Molecular Weight and Molecular Weight Distribution With increase in molecular weight,
Tensile Strength also increases. Smaller molecules in polymer work as plasticizer. So with
increase of Molecular Weight Distribution, Elongation decrease and Tensile Strength increases.
3.2.2 DEFINITION:
3.2.2(a) 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
3.2.3 SIGNIFICANCE:
1. Flexural properties determined by test method 1 are especially useful for quality Control
and specification purposes.
2. Materials do not fail at the points of maximum stress under test method 1 is test by test
method 2. Flexural properties are determined by second method, are also useful for quality
control and specification purposes. The basic difference between the two types of method is
the location of maximum bending moment and maximum axial fiber stress.
3.2.7(b) 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.
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 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 ) ]
(ii)
(iii)
(iv)
(v)
(vi)
(vii)
3.3.2 DEFINITION
3.3.2 (a) IMPACT TEST:
Impact test is a method of determining the behavior of material subjected to
shock loading in bending or tension. The quantity usually measured is the energy absorbed
in fracturing in a single blow.
3.3.2. (b) IMPACT STRENGTH:
3.3.3 SIGNIFICANCE:
(1) The excess energy pendulum impact test indicates the energy to break std. Test
specimen of specified size under stipulated conditions of specimen mounting, notching and
pendulum velocity at impact.
(2) The energy lost by the pendulum during the breakage of the specimen is the sum of
energy required,
(i) To initiate fracture of the specimen
(ii) To propagate the fracture across the specimen
(iii) To through the free end of the broken specimen
(iv) To bend the specimen
(v) To produced vibration in the pendulum arm
(vi) To produced vibration or horizontal movement of the machine frame or
base
(vii) To overcome friction in the pendulum bearing and in the excess energy
indicating mechanism and to overcome pendulum air drag (wind age).
(viii) To indent or deformed plastically the specimen at the line of impact.
3.3.4(a) CONDITIONING:
Condition on the test specimens at 23 2C and 50 5% relative humidity for not less
than 40h. prior to test in accordance with procedure A of method D 618 for those test,
where conditioning is required. In case of disagreement, the tolerances shall be 1C and
2% relative humidity.
(7) The test specimen is supported against to rigid envil in such a position that its center of
gravity and the center of notch shall lie on the tangent to the arc of travel of the center of
percussion of the pendulum drawn at the position of the impact.
(8) Means shall be provided for determining energy remaining in the pendulum after the
breaking specimen, usually this will consist a pointer and a dial mechanism which indicate
the height of rise of the pendulum beyond the point of impact, in terms of energy remove
from the specific pendulum.
(9) The vise-pendulum and frame shall be rigid to assure correct alignment of the hammer
and the specimen, both at the movement of impact and during the propagation and to
minimize the energy loss due to vibration.
(10) A check of calibration of an impact machine is difficult to make under dynamic
condition. The basic parameters are normally checked under static condition. If the
machine passes the static test then it is assumed to be accurate.
3.3.7 PROCEDURE:
(1) Estimating the breaking energy for the specimen and select the pendulum of suitable
energy.
(2) Before testing the specimen, perform the following operation on the machine.
(a) With the excess energy indicating pointer in its normal starting portion but without a
specimen in the vise release the pendulum from its normal starting position and note
the position the pointer attains after the swings as one reading of factor A.
(b) Without resetting a pointer, raise the pendulum & release again.
(c) Repeat the above two operations several time, and calculate the record the average
A & B readings.
(3) Check the specimen for conformity.
(4) Position the specimen precisely and rigidly but not the lightly clamped in the vise.
(5) Calculate the machine correction for indicating breaking strength of the specimen and
factor A & B using table or the graph-describing appendix X2.
(6) Calculate the average impact strength of the group of the specimen.
Diagram illustrating izod impact test specimen properly positioned in test fixture
method
ASTMD 244
Impact tester specifically designed for impact testing pipe & fitting
Thickness (m)
3.3.11 RESULTS:
The result is including following.
(1) Complete identification of the material tested, including type source, and previous
history.
(2) A statement of how the specimen work prepared the testing condition used.
(3) The capacity of the pendulum in joules.
(4) The nominal width of the specimen.
(5) Should specify clearly Izod / Charpy method.
(6) Should report test results with notch or un-notch along with test method.
The test specimen should be impacted within 10 seconds after removing from
conditioning chamber.
3) The sample should be striked on at least four different places on center.
4) The mass of weight and its height of fall should be carefully calculated.
5) The mass should be 25mm hemispherical striking end.
6) Notch dimensions should be maintained precisely in samples.
3.4.2 DEFINITIONS:
3.4.2 (a) COMPRESSIVE STRENGTH: The maximum load sustained by a test
specimen in a compressive test divide by the original cross section area of the specimen.
3.4.2 (b) 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.
3.4.2 (c) 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.
3.4.2 (d) SLENDERNESS RATIO:- The ratio of the length of a column of uniform cross
section to its least radius of gyration known as slenderness ratio.
3.4.2 (e) 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.
3.4.2 (f) COMPRESSIVE YIELD POINT:- The fist point of stress-strain diagram at which
an increase in strain occurs without an increase in stress.
Unit :- kg/cm2
3.4.3 SIGNIFICANCE:
1) Compressive test provides information about compressive properties of plastics when
employed under conditions approximating these under which the tests are made.
2) Compressive properties include the modulus of elasticity, yield stress; deformation
beyond yield point at the compressive strength, material processing a low order of ductility
may not exhibit yield point.
3) Compression tests provide a standard method of obtaining data for research and
development, quality control, acceptance or rejection under specifications and special
purposes.
Compressive strength =
Original cross sectional area (cm2)
Maximum load recorded (N)
Rate of Straining
Unit is lb / inch.
3.5.3 SIGNIFICANCE:
Shear strength data is of great importance to a designer of film and sheet products
that tends to be subjected to such shear loads. Most large molded and extruded products
are usually not subjected to shear loads.
3.5.4 TEST METHOD:
Test Method for shear strength of plastics by punch tool (ASTM D 732)
3.5.5 TEST SPECIMEN
3.5.5(a) Sample preparation method and Dimensions of Test specimen:
The specimen shall consists of a 50mm (2inch) square or a 50mm (2inch) dia disk
cut from sheet material or moulded into this form. The thickness of the specimen is from
0.127 to12.7mm (0.005 to 0.500inch). The upper and lower surface is parallel to each
other and reasonably 11mm (7/16inch) in diameter is drilled through the specimen at its
centre.
40h prior
to test in accordance with procedure A of methods D618, for those tests where conditioning
is required. In case of disagreement, the tolerance shall be 1C(1.8F) and 2% relative
humidity.
+ 2% relative humidity.
3.5.6(c) Micrometers
Suitable micrometers for measuring the testing thickness of the test specimen
to an incremental discrimination of at least 0.025 mm are used.
3.5.9 FORMULA:
Shear strength is calculated as follows:
Force required to shear the specimen
Shear strength (psi) =
Area of sheared edge
Area of sheared edge = (circumference of punch) x (thickness of specimen)
Calculate shear strength in Mpa determined by dividing the load required to shear the
specimen by the area of the sheared edge, which shall be taken as the product of the
thickness of the specimen by the circumference of the punch.
3.6(A). 2 DEFINITIONS:
The test method covers the determination of the average force to propagate tearing to
a specified length of plastic film or non-rigid sheeting. After the tear has been started using
an element of types tearing tester to specimen are cited a rectangular type and one with a
constant radius testing length. The latter shall be preferred are free specimen.
3.6(A). 3 SIGNIFICANCE:
This test method is of value in ranking the tearing resistance of plastic films and
thin sheeting of comparable thickness. Variable elongation and oblique tearing effects on
the more extensible films preclude its use as a precise production tool for this type of
plastic. This test method is used for specification acceptance testing method.
3.6(A).9FORMULA:
1. Calculate the average resistance to tearing from all specimens tested in each
principal direction of orientation. The tearing resistance is denoted as gf or pounds.
16 * 9.81* average scale reading
Average tearing force, mN =
n
16 * average scale reading
Average tearing force, gf =
n
Where
n =1
16 * 9.81* average scale reading * gf capacity
Average tearing force, mN =
n * 1600 gf
3.6(A).10FACTORS INFLUENCING:
a. Temperature and Humidity
b. Test Speed
c. Method of specimen Preparation
d. Effect of Plasticizer and filler
e. Molecular Weight and Molecular Weight Distribution
f.
Crystallinity
g. Rate of Straining
Stationary clamp
Movable clamp
Stop watch
Indicating device
Capacity instruments
Template, die or shear type
Razor blades
Thickness measuring device
3. 6 (B). 8FORMULA:
1. Calculate the average tearing force, if the standard 1600 gf instrument with a 0 to
100 scale is used
16 x 9.81 x average scale reading
Average tearing force, mN =
n
Where n =1
16 x average scale reading
Average tearing force, gf =
n
Where n = 1, or number of piles, if used
2.Calculate the average tearing force, if an instrument has direct reading scale in
millinewtons,
9.81 x average scale reading
Average tearing force, mN =
n
Where, n =1
Average scale reading
Average tearing force, gf =
n
Where n = 1, or number of piles, if used
Crystallinity
g. Rate of Straining
3. 6 (B). 11 SAFETY PRECAUTION:1. Selection of scales should be more-than 15% of approx test value
2. Specimen should be proper in shape & size.
3. Notch should be straight and middle in the specimen
4. Dont touch the knife when specimen notch
5.
Dont touch pendulum when test perform (close the chamber of machine)
4.1. 2 DEFINITION:
4.1.2 (a) 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
Data from creep and creep rupture tests are necessary to predict the creep
modulus and strength of material under long-term loads and to predict dimensional
changes that occurs as a result at such loads.
(ii)
Data from these tests methods are used to compare material in that design at
fabricated parts to characterized plastic for long term performance under constant load
and under certain condition for specification purpose.
4.1.5 (c ) CONDITIONING:
(i) Condition the test specimen at 23 2C and 50 5% relative humidity for not less than
40 h prior to test in accordance with procedure A of test methods D 618 for those tests
where conditioning is required.
(ii) The specimen shell be preconditioning in the test environment for at least 40 h prior to
being tested.
4.1.6 EQUIPMENT DETAILS:
(i) Tensile Creep:
loading of the specimen. It is recommend that grips permit the final Centring of the
specimen prior to applying the load.
(ii) Flexural creep:
1. Test rack
2. Stripping
3. Loading system
4. Extension, compression and deflection measurement device
5. Time measurement device
6. Temperature control and measurement device
7. Vibration control device.
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)
gauge.
(iii)
The electrical resistance gauges may also be used in place of a dial indicator.
(iv)
For each test temperature plot log creep strain in percent versus long time in
etc.
2.
3.
4.
Temperature is maintained.
4.2. 3 SIGNIFICANCE:
Stress relaxation behavior of the polymers is overlooked
by many design engineers and researchers, partly because the creep data is much easier
to obtain and is readily available. However, many practical applications dictate the use of
stress relaxation data. For example, extremely low stress relaxation is desired in the case
of threaded bottle closure, which may under constant strain for a long period.
Stress-Time curve
4.2.9 FORMULA AND CALCULATION:a) For Stress relaxation measurements calculate the maximum fibre stress for each specimen
in megapascals as follows:
S = 3PL
2bd2
Where
S = Stress, mpa
P = Load, N (Ibf)
L = Span, mm (in)
B = Width, mm (in) and
D = depth, mm (in)
b) Calculate
6Dd
The max. strain r =
L2
Where
r
D
d
L
=
=
=
=
4.2.13 REFERENCES:
1. Handbook of plastics testing technology VISHNU SHAH
5. 1(a). 2 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
5. 1(a). 3 SIGNIFICANCE:
1. Transparent plastics material when used as enclosures are subjected to wiping and
cleaning hence the maintenance of optical quality of a material after abrasion is
important.
2. Although this test method does not provide fundamental data. It is suitable for grading
materials relative to this type of abrasion in a manner that co-relates with service.
5.1(a). 6 CONDITIONING:
Condition the test specimens at 23 + 2C and 50 5% relative humidity or not
less than 40 hours prior to test in accordance with procedure A of test methods D 618 for
these tests where conditioning is required. In case of disagreement the tolerances shall be
1C and 2% relative humidity .
5. 1(a). 7 EQUIPMENT:
1. Abrader
2. Refacing stone
3. Abrasive wheels
4. Abraser turn table
5. Photometer
6. Stops
7. Specimens holder.
Abrasion tester
Reface new wheels for 100 cycles reface previously used wheels for 25 cycles.
1.3
Discard the ST-11 refacing stone when grooves or ridges first become evident.
2. Mount the specimen on the specimen holder and subject it to abrasion for a selected no.
Of cycles.
3. Using an integrating sphere photometer that is properly adjusted.
4. Place the specimen in the holder and measure the percentage of transmitted light that is
diffused by abraded track on at least four equally spaced intervals along the track.
5. The abraded track is against the entrance window of the photometer.
6. The specimen holder is positioned so that no portion of the light beam is with in 1mm of
the inside or outside edge of the track.
x 100
5. 1(a). 12 RESULT:
1. The result shall include the following.
1.1
Percentage of transmitted light that is scattered by the abrased specimens averaged for
Plot the percentage of light scattered verses cycles abraded, if more than one number
Description of the integrating sphere photometer including, sphere geometry, exit light
beam diameter with and without the diaphragm inserted, and location of the diaphragm in
the light beam.
5.1(a).13 SAFETY PRECAUTIONS:1. Specimen should be free from burrs and other defects like grease, oil, voids and
other specimen preparation defects.
2. Weight of the specimen takes properly before and after test.
3. Dont touch Abrasion wheals and rotating table during machine run
4. Use the brush when clean the specimen after test.
5. After test specimen should be clean properly.
6. Before and after test machine should be clean.
5.1(b). 2 DEFINITION:
Resistance to abrasion - The ability of material to withstand mechanical action such as
rubbing scrapping or erosion, that tends progressively to remove material from its
surface.
Unit :- mg/1000 cycles
A specimen,
Abrasives revolving counter are added weight of 4.5 kg and suitable mechanism
for driving the disk at 23.5 rpm and the specimen holder at 32.5 rpm.
Abrasion tester
5.1(b).5.1 CONDITIONING:
(1) Condition the specimen at 23 2C and 50 5% relative humidity for not less than
40 h. prior to test in accordance with procedure A of methods D 618 for those tests where
conditioning is required. In case of disagreement, the tolerances shall be 1C and 2%
relative humidity.
(2) Test conditions- conduct tests in the standard laboratory atmosphere of 23 2C and
50 5% relative humidity, unless otherwise specified in the test methods. In case of
disagreement, the tolerances shall be 1C and 2% relative humidity.
5.1(b). 10 RESULTS
(1) The resistance to abrasion is the abrasion loss in volume at 1000 revolutions.
(2) The average volume loss in cm3 at 1000 revolution for the three specimens tested in
duplicate.
(3) The 95% confidence limits.
(4) The name and grade of abrasive grit employed in making the test.
5.1(b). 11 REFERENCES:
(1)- ASTM Standards
D 618 Method of conditioning plastic and electrical insulating materials for testing.
E 11 specification or wire cloth sieves for testing purpose
(2)- ASTM Standards special technical publication ASTM manual on quality control of
materials
ASTM STP 15-C. 1951.
5.2.2
(b) 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.
2F
Brinell Hardness =
D2 {1-[1-(d/D)2 ]}
Where
F = Load in Kg
D = Diameter of indenter
d = Diameter of impression produced
5.2. 3 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.
5.2. 4. TEST METHODS: Test Method for Rockwell Hardness of Plastic &
Electrical Insulating Material. ASTM D 785, JISK-7112, ASTM D 2240,
ASTM D 2583
At least five hardness tests are made on isotropic material for anistropic materials, at
least five hardness test is made along each principle axis of anisotropy, provided the
sample size permits.
5.2. 5 TEST SPECIMEN
5.2. 5 (a) Specimen Preparation Method
The standard test specimen shall have a minimum thickness of 6mm (1/4 in) unless it
is verified that, for the thickness used, the hardness values are not affected by the
supporting surface and that no imprint shows on the under surface of the specimen after
testing.
The specimen is a piece cut from a moulding or sheet or composed of a file-up of
several pieces of the same thickness, provided that precaution is taken that the surface of
the pieces are in total contact and not held apart by sink marks burrs from saw cuts or
other protrusions.
Care is taken that the test specimen has parallel flat surface to ensure good seating on
the anvil and thus avoid the deflection that is caused by poor contact.
For Rockwell hardness testing, it is necessary that the major load, when fully
applied, be completely supported by the specimen and not held by other limiting elements
of the machine. To determine whether this condition is satisfied, the major load is applied
to the test specimen. If an additional load is then applied, by means of hand pressure on
the weights, the needle should indicate an additional indentation.
(2)
If this is not indicated, the major load is not being applied to the specimen, and a
long-stroke machine or less severe scale is used. For the harder materials with a modulus
around 5500Mpa (8 x 105 Psi) or over, a stroke equivalent to 150 scale divisions, under
major load application, is adequate; but for softer materials the long-stroke (250 scale
divisions under major load) machine is required.
(3)
A V- block anvil or double roller anvil is used if solid rods are tested.
Calculate the arithmetic mean for each series of tests on the same material and
at the same set of test conditions. Report the results as the average value
rounded to the equivalent of one dial division.
(2) Calculate the standard deviation (estimated) as follows, and report it to two significan
figures:
S=
( x2 n x2 / (n-1)
Where:
S = estimated standard deviation
X = value of a single observation,
X = arithmetic mean of a set of observation ,and,
n = number of observation in the set.
5.2. 11 RESULT:
The report shall include the following:
(1) Material identification,
(2) Filler identification and particle size, if possible,
(3) Total thickness of specimen,
(4) The number of pieces in the specimen and the average thickness of each piece,
(5) Surface conditions, for example, molded or machined, flat or round,
(6) The procedure used (procedure A or B),
(7) The direction of testing,
(8) A letter indicating the Rockwell hardness scale used,
(9) An average Rockwell hardness number calculated by procedure A or B,
(10) The standard deviation.
5.2.12 SAFETY PRECAUTIONS:1) The load should be taken according to correct scale.
2) Specimen should be flat.
3) The diameter of ball should be according to the scale.
Sled
2.
Plane
3. Scissors or cutter
4. Adhesive tape
5. Beaded chain
6. Low-Friction pulleys
7. Force- Measuring Device
8. Supporting Base
9. Driving or pulley Device for sled or Plane
For plane specimen cut 250 mm in the machine direction and 130 mm in the
transverse direction
2. For film specimen cut 120 mm square
3. For sheet specimen cut 63.5 mm square
5.3.7 FORMULAE:
(1)
(X
X2 n X2 / (n-1)
(n-1)
Where
S = sample standard deviation
X = value of a single observation
n = number of observation and
5.3.9 RESULTS
The report shall include the following
(1) Complete description of the plastic sample, including manufacturers code designation,
thickness, material, production, surfaces tested, principal direction tested, approximate age of
sample after manufacturer.
(2) Description of second substance if used
(3) Apparatus used
(4) Average static and kinetic coefficient of friction together with the standard deviation, and
(5) Number of specimen tested for each coefficient of friction.
5.3.11SAFETY PRECAUTIONS:
1. Level of Instrument should be horizontal
2. Slides should be smooth and free from rust etc.
3.