1. EXPERIMENTAL
Chapter-III
3. Experimental
3.1 Tensile strength
3.1.1 Definition
The maximum load applied in breaking a tensile test piece divided by the original cross-
sectional area of the test piece. Originally quoted as tons/sq.in. It is now measured as
Newtons/sq.mm. Also termed Maximum Stress and Ultimate Tensile Stress.
3.1.2 Specimen Shape
The specimen's shape is usually defined by the standard or specification being utilized,
e.g., ASTM D638. Its shape is important because you want to avoid having a break or
fracture within the area being gripped. So, standards have been developed to specify the
shape of the specimen to ensure the break will occur in the "gage length" (2 inches are
frequently used) by reducing the cross sectional area or diameter of the specimen
throughout the gage length. This has the effect of increasing the stress in the gage length
since stress is inversely proportional to the cross sectional area under load,
Vertical alignment of the specimen is an important factor to avoid side loading or
bending moments created in the specimen. Mounting the specimen in the upper grip
assembly first then allowing it to hang freely will help to maintain alignment for the test.
There are three definitions of tensile strength:
1. Yield strength The stress at which material strain changes from elastic deformation to
plastic deformation, causing it to deform permanently.
2. Ultimate strength The maximum stress a material can withstand.
Breaking strength
3. Rupture The stress coordinate on the stress-strain curve at the point of rupture.
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2. EXPERIMENTAL
Figure 3.1 ASTM D638 Lloyds UTM
Metals including steel have a linear stress-strain relationship up to the yield point, as
shown in the figure. In some steels the stress falls after the yield point. This is due to the
interaction of carbon atoms and dislocations in the stressed steel. Cold worked and alloy
steels do not show this effect. For most metals yield point is not sharply defined. Below
the yield strength all deformation is recoverable, and the material will return to its initial
shape when the load is removed. For stresses above the yield point the deformation is not
recoverable, and the material will not return to its initial shape. This unrecoverable
deformation is known as plastic deformation. For many applications plastic deformation
is unacceptable, and the yield strength is used as the design limitation.After the yield
point, steel and many other ductile metals will undergo a period of strain harening, in
which the stress increases again with increasing strain up to the ultimate strength. If the
material is unloaded at this point, the stress-strain curve will be parallel to that portion of
the curve between the origin and the yield point. If it is re-loaded it will follow the
unloading curve up again to the ultimate strength, which has become the new yield
strength.After a metal has been loaded to its yield strength it begins to "neck" as the
cross-sectional area of the specimen decreases due to plastic flow. When necking
becomes substantial, it may cause a reversal of the engineering stress-strain curve, where
decreasing stress correlates to increasing strain because of geometric effects. This is
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3. EXPERIMENTAL
because the engineering stress and engineering strain are calculated assuming the original
cross-sectional area before necking. If the graph is plotted in terms of true stress and true
strain the curve will always slope upwards and never reverse, as true stress is corrected
for the decrease in cross-sectional area. Necking is not observed for materials loaded in
compression. The peak stress on the engineering stress-strain curve is known as the
ultimate strength. After a period of necking, the material will rupture and the stored
elastic energy is released as noise and heat. The stress on the material at the time of
rupture is known as the tensile strength. Ductile metals do not have a well defined yield
point. The yield strength is typically defined by the "0.2% offset strain". The yield
strength at 0.2% offset is determined by finding the intersection of the stress-strain curve
with a line parallel to the initial slope of the curve and which intercepts the abscissa at
0.002.
3.2 Flexural strength
3.2.1 Definition
The strength of a material in bending, expressed as the stress on the outermost fibers of a
bent test specimen, at the instant of failure. In a conventional test, flexural strength
expressed in Psi is equal to
Where
P= the load applied to a sample of test length L, width b, and thickness d.
The flexure test method measures behavior of materials subjected to simple beam
loading. It is also called a transverse beam test. Maximum fiber stress and maximum
strain are calculated for increments of load. Results are plotted in a stress-strain diagram.
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 tension side. Flexural modulus
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4. EXPERIMENTAL
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 A flexure test produces
tensile stress in the convex side of the specimen and compression stress in the concave
side. This creates an area of shear stress along the midline. To ensure the primary failure
comes from tensile or compression stress the shear stress must be minimized and
controlling the span to depth ratio does this; the length of the outer span divided by the
height (depth) of the specimen. For most materials S/d=16 is acceptable. Some materials
require S/d=32 to 64 to keep the shear stress low enough.
Figure 3.2 ASTM D790 Three Point Bending
Flexure testing is often done on relatively flexible materials such as polymers, wood and
composites. There are two test types; 3-point flex and 4-point flex. In a 3-point test the
area of uniform stress is quite small and concentrated under the center loading point. In a
4-point test, the area of uniform stress exists between the inner span loading points
(typically half the outer span length). The 3-point flexure test is the most common for
polymers. Specimen deflection is usually measured by the crosshead position. Test
results include flexural strength and flexural modulus.
3.3 Impact strength
Energy per unit thickness required to break a test specimen under flexural impact. Test
specimen is held as a vertical cantilevered beam and is impacted by a swinging
pendulum. The energy lost by the pendulum is equated with the energy absorbed by the
test specimen. Several methods are used to measure the impact resistance of plastics -
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5. EXPERIMENTAL
Izod, Charpy, Gardner, tensile impact, and many others. These impact tests allow
designers to compare the relative impact resistance under controlled laboratory conditions
and, consequently, are often used for material selection or quality control a pendulum
swings on its track and strikes a notched, cantilevered plastic sample. The energy lost
(required to break the sample) as the pendulum continues on its path is measured from
the distance of its follow through. Sample thickness is usually 1/8 in. (3.2mm). The test
method of ASTM D256. The result of the Izod test is reported in energy lost per unit of
specimen thickness (such as ft-lb/in or J/cm) at the notch ('t' in graphic at right).
Additionally, the results may be reported as energy lost per unit cross-sectional area at
the notch (J/m² or ft-lb/in²). In ISO 180 methods the results reported based only on the
cross-sectional area at the notch (J/m²). Polymeric materials that are sensitive to the stress
concentrations at the notch ('notch-sensitive') will perform poorly in the notched izod test.
Engineers use this knowledge to avoid using such polymers in designs with create high
stress concentrations such as sharp corners or cutouts.
Figure 3.3 ASTM D256. The Izod impact strength test apparatus.
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6. EXPERIMENTAL
Unnotched specimens are also frequently tested via the Izod impact method to give a
more complete understanding of impact resistance. Izod impact tests are commonly run at
low temperatures - down to -40°F (-40°C) or occasionally lower - to help gauge the
impact resistance of plastics used in cold environments. The impact resistance of a given
grade of polymer is a function of the base resin plus the presence of any impact modifiers
(such as Elastomers) and reinforcing agents that may be added by the
manufacturer/compounder. Environmental factors other than temperature also play a role
in impact resistance For example, nylons (polyamides) generally experience higher
impact strength in the conditioned state (in equilibrium with atmospheric moisture) than
in a dry-as-molded state because of the plasticizing effect of absorbed moisture.
Figure 3.3 Notched Impact Strength Specimen
3.4 Heat deflection temperature
The deflection temperature is a measure of a polymer's resistance to distortion under a
given load at elevated temperatures. The deflection temperature is also known as the
'deflection temperature under load' (DTUL), 'heat deflection temperature', or 'heat
distortion temperature' (HDT). The two common loads used are 0.46 MPa (66 psi) and
1.8 MPa (264 psi). The Heat Distortion Temperature is determined for ASTM D648. The
test specimen is loaded in three-point bending in the edgewise direction.
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7. EXPERIMENTAL
Figure 3.4 ASTM D648 Heat Deflection Apparatus
The outer fiber stress used for testing is either 0.455 MPa or 1.82 MPa, and the
temperature is increased at 2 °C/min until the specimen deflects 0.25 mm.The deflection
temperature is the temperature at which a test bar, loaded to the specified bending stress,
deflects by 0.010 inch (0.25 mm). The value obtained for a specific polymer grade will
depend on the base resin and on the presence of reinforcing agents. Deflection
temperatures of glass fiber or carbon fiber reinforced engineering polymers will often
approach the melting point of the base resin. The deflection temperature rest results are a
useful measure of relative service temperature for a polymer when used in load-bearing
parts. However, the deflection temperature test is a short-term test and should not be used
alone for product design. Other factors such as the time of exposure to elevated
temperature, the rate of temperature increase, and the part geometry all affect the
performance.
Limitations that are associated with the determination of the HDT is that the sample is
not thermally isotropic and, thick samples in particular, will contain a temperature
gradient. The HDT of a particular material can also be very sensitive to stress
experienced by the component which is dependent on the component’s dimensions. The
selected deflection of 0.25 mm (which is 0.2% additional strain) is selected arbitrarily
and has no physical meaning.
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8. EXPERIMENTAL
3.5 Melt Flow Index
The Melt Flow Index is a measure of the ease of flow of the melt of a thermoplastic
polymer. It is defined as the mass of polymer in grams flowing in 10 minutes through a
capillary of specific diameter and length by a pressure applied via prescribed alternative
gravimetric weights for alternative prescribed temperatures. The method is given in
ASTM D1238 .Melt flow rate is an indirect measure of molecular weight, high melt flow
rate corresponding to low molecular weight. At the same time, melt flow rate is a
measure of the ability of the material's melt to flow under pressure. Melt flow rate is
inversely proportional to viscosity of the melt at the conditions of the test, the viscosity
for any such material depends on the applied force. Ratios between two melt flow rate
values for one material at different gravimetric weights are often used as a measure for
the broadness of the molecular weight distribution.Melt flow rate is very commonly used
for polyolefins, polyethylene being measured at 190°C and polypropylene at 230°C. The
plastics converter should choose a material with a melt index so high that he can easily
form the polymer in the molten state into the article intended, but on the other hand so
low that the mechanical strength of the final article will be sufficient for its use.
Figure 3.5 ASTM D1238 Melt Flow Index Apparatus
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9. EXPERIMENTAL
1. A small amount of the polymer sample (around 4 to 5 grams) is taken in the
specially designed MFI apparatus which is nothing but a miniature extruder. The
apparatus consists of a small die inserted into the extruder, with the diameter of
the die generally being around 2 mm.
2. The material is packed properly inside the extruder barrel to avoid formation of
air pockets.
3. A piston is introduced which acts as the medium that causes extrusion of the
molten polymer.
4. The sample is preheated for a specified amount of time: 5 min at 190°C for
polyethylene and 6 min at 230°C for polypropylene.
5. After the preheating a specified weight is introduced onto the piston. Examples of
standard weights are 2.16 Kg, 5 Kg, etc.
6. On account of the weight shear is exerted on the molten polymer and it
immediately starts flowing through the die.
7. A sample of the melt is taken after desired period of time and is weighed
accurately.
8. MFI is expressed as grams of polymer/10 minutes of flow time.
Melt Flow Index are Melt Flow Rate and Melt Index. More commonly used are their
abbreviations: MFI, MFR and MI.
3.6 Differential scanning calorimeter
Differential scanning calorimetry or DSC is a thermoanalytical technique in which the
difference in the amount of heat required to increase the temperature of a sample and
reference are measured as a function of temperature. Both the sample and reference are
maintained at nearly the same temperature throughout the experiment. Generally, the
temperature program for a DSC analysis is designed such that the sample holder
temperature increases linearly as a function of time. The reference sample should have a
well-defined heat capacity over the range of temperatures to be scanned.The basic
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10. EXPERIMENTAL
principle underlying this technique is that, when the sample undergoes a physical
transformation such as phase transitions, more or less heat will need to flow to it than the
reference to maintain both at the same temperature. Whether more or less heat must flow
to the sample depends on whether the process is exothermic or endothermic. For
example, as a solid sample melts to a liquid it will require more heat flowing to the
sample to increase its temperature at the same rate as the reference. This is due to the
absorption of heat by the sample as it undergoes the endothermic phase transition from
solid to liquid. Likewise, as the sample undergoes exothermic processes (such as
crystallization) less heat is required to raise the sample temperature. By observing the
difference in heat flow between the sample and reference, differential scanning
calorimeters are able to measure the amount of heat absorbed or released during such
transitions. DSC may also be used to observe more subtle phase changes, such as glass
transitions. DSC is widely used in industrial settings as a quality control instrument due
to its applicability in evaluating sample purity and for studying polymer curing.The result
of a DSC experiment is a curve of heat flux versus temperature or versus time. There are
two different conventions: exothermic reactions in the sample shown with a positive or
negative peak; it depends by the different kind of technology used by the instrumentation
to make the experiment. This curve can be used to calculate enthalpies of transitions. This
is done by integrating the peak corresponding to a given transition. It can be shown that
the enthalpy of transition can be expressed using the following equation:
ΔH = KA
where ΔH is the enthalpy of transition, K is the calorimetric constant, and A is the
areaunder the curve. The calorimetric constant will vary from instrument, and can be
determined by analyzing a well-characterized sample with known enthalpies of transition.
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11. EXPERIMENTAL
3.6.1 Applications
Differential scanning calorimetry can be used to measure a number of characteristic
properties of a sample. Using this technique it is possible to observe fusion and
crystallization events as well as glass transition temperatures (Tg).
Figure 3.6.1 a schematic DSC curve demonstrating the appearance of several
common features
DSC can also be used to study oxidation, as well as other chemical reactions. Glass
transitions may occur as the temperature of an amorphous solid is increased. These
transitions appear as a step in the baseline of the recorded DSC signal. This is due to the
sample undergoing a change in heat capacity; no formal phase change occurs. As the
temperature increases, an amorphous solid will become less viscous. At some point the
molecules may obtain enough freedom of motion to spontaneously arrange themselves
into a crystalline form. This is known as the crystallization temperature (Tc). This
transition from amorphous solid to crystalline solid is an exothermic process, and results
in a peak in the DSC signal.
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