Version: 3.2_EN
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Date: 17. February 2015.
BUDAPEST UNIVERSITY OF TECHNOLOGY AND ECONOMICS FACULTY OF MECHANICAL ENGINEERING DEPARTMENT OF POLYMER ENGINEERING
Tensile Test of Polymers
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Version: 3.2_EN
Tensile Test of Polymers
Date: 17. February 2015.
TENSILE TEST OF POLYMERS 1. THE AIM OF THE MEASUREMENT The aim of the measurement for our students is to get to know the method of tensile testing through the example of an injection molded, thermoplastic, polymer specimen, the mechanical properties that can be determined in these tests as well as the effect of measurement conditions (tensile test speed, temperature, humidity) on the mechanical properties. The aim involves the presentation of the phenomena that occur in case of polymer materials (e.g. neck formation) during tensile testing and the differences compared to metals.
2. THEORETICAL BACKGROUND During the tensile test the specimen with the geometry according to the related standards is broken due to uniaxial tensile loading while clamped at its two ends. The tensile test is carried out under determined measurement conditions (tensile speed, temperature, moisture content); meanwhile the tensile force is measured and registered as a function of strain. When evaluating the results specific material properties are determined which makes possible for example the structural design of polymeric parts with complex geometry.
2.1. SPECIMEN At tensile tests the reproducibility of test is crucial therefore easy-to-manufacture specimens are required. If one would like to characterize a given material injection molded (B3 laboratory practice) specimens are to be used. Figure 1. shows the specimens that can be used for the examination of materials with large and normal elongation. In case of thermoplastic polymers type 1A (general) specimen is used in general. If a given part (or part of a construction or machine) needs to be characterized than the specimen can be machined out of it (1B, 1BA and 1BB types). In case of thermosetting matrix composites (for more details, see A5 laboratory practice) type 2 is used in common. The general feature of polymeric specimens that they have a dumbbel (aka. dogbone) shape. Their total length is l3≥150 mm, the gripping distance (the original/initial distance between the grippers) is 115 mm (Type 1A). The cross section of the specimens is 4 x 10 mm (d x b1), while they have a larger crossection at the ends. The dumbbel shapes makes available to avoid the break of the specimen too close to the gripping units (clamps). At polymer matrix composites (type 2) so-called tabs are applied at the two ends for achieving the same effect. 2
Version: 3.2_EN
Tensile Test of Polymers
Date: 17. February 2015.
Figure 1. Different standard tensile test specimens
2.2. MECHANICAL CHARACTERISTICS DETERMINED FORM TENSILE TESTS At tensile tests the elongation is increased until the specimen breaks and during the test the force is continuously registered. Break is indicated by a sudden drop in force that means the end of the test. One can also stop the measurement at a chosen elongation or when the force drops down by a chosen percent (e.g. 20% force drop). The elongation can be measured by measuring probe or optical ways where the distance change of two parallel indicators on the specimen is continuously determined and registered. Besides these ways, strain gauges can also be used. For less accurate elongation measurements the displacement values of the gripping unit (generated by the tensile testing machine) can also be used. As a result of tensile tests the force-strain (F-l) curve of the material is obtained for the given measurement conditions. This can be parameterized for a stress-relative strain (-) curve easily (Figure 2.): stress ( [MPa]) is obtained in the place of the force axis if force (F [N]) is divided by the initial cross section of the specimen (A0 [mm2]):
F [MPa], (1) A0 Relative strain () is the ratio of the strain of the specimen (l [mm]) and the initial measurement length (L0 [mm]): l 100 [%]. (2) L0
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Version: 3.2_EN
Tensile Test of Polymers
Date: 17. February 2015.
F
M FM B FB
M
Y F
B
Y
Y
P F
P
P
LP
L Y
P
LM
Y
L B L
M B
Figure 2. General tensile diagram
From the force-strain curve (or the stress-strain curve, which is equivalent to this) registered during the tensile test, the following mechanical quantities can be read and calculated:
p proportionality limit: until this value the strain is proportional to the stress, i.e. Hooke’s law is valid in this range:
E [MPa],
(3)
where E is the modulus of elasticity of the material. Y yield point: from this point the material elongates further without any further stress. In practice, it does not even occur in case of some polymers, but for other materials, several 100% yield strain can take place, preceded by neck formation on the specimen and accompanied by structural transformation. Yield can be revealed by so called stress whitening in case of some polymers. M tensile strength: ratio of the maximum force and the initial cross section. According to the newest ISO 527 standard tensile strength is calculated at the 1 st local maximum of the tensile curve. B breaking strength: ratio of the force measured at breakage and the initial cross section Elongation at the maximum force (M): L L0 M M 100 [%], (4) L0 where: L0 is the initial, unloaded length of the examined part of the specimen, LM is the elongated length of the examined part of the specimen at the maximum force. Breaking strain (B): L L0 B B 100 [%], (5) L0 4
Version: 3.2_EN
Tensile Test of Polymers
Date: 17. February 2015.
where: LB is the elongated length of the examined part of the specimen at breakage. Different elasticity moduli (E) can be determined for the material from the - curve. Two different types of elasticity moduli can be differentiated: Tangent modulus (Ee): the steepness of the tangent drawn to any point of the curve (Fig. 3.). Tensile modulus (E0): the steepness of the tangent drawn to zero point of the curve (Fig. 3.). It is the initial tangent modulus.
Figure 3. Determination of elasticity moduli
Tensile curves of some polymer materials can be seen in Figure 4. Small breaking strain indicates a rigid or brittle, while greater strains indicate a tough or ductile material. The small modulus is related to soft, while greater modulus is related to stiff materials.
Figure 4. Typical tensile curves of different synthetic polymers
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Version: 3.2_EN
Tensile Test of Polymers
Date: 17. February 2015.
2.3. MEASUREMENT CONDITIONS Test (tensile) speed: In case of larger tensile speeds the material seems stronger and stiffer (Figure 5. a). In case of thermoplastic polymers and elastomers that may have more than 100% relative strain, the range of applied tensile speeds is 2-20 mm/min.
a.) b.) Figure 5. Effect of (a) tensile speed and (b) temperature on the tensile curve
Temperature of the examination: in case of polymers, a small temperature change already influences strength and the character of the tensile process significantly. Under the glass transition point polymers behave rigidly, while in the largely flexible state they are tougher and have higher tensile strain, as well (Figure 5.b). Moisture content: there are some polymers that can absorb more or less moisture (e.g. PA, PAN, natural fibers). Moisture has a softening effect, hence it decreases strength and increases strain (Figure 6.).
Figure 6. PA tensile curves at different moisture contents
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Tensile Test of Polymers
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Date: 17. February 2015.
DEVICES
The measurements are going to be carried out on a computer controlled universal tensile tester type Zwick Z020 (Figure 7.FIGURE 7. ). The maximal measuring limit of the machine is 20 kN, the range of tensile testing speed is 0.001..750 mm/min.
Figure 7. Zwick Z020 type computer controlled universal tensile tester
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Version: 3.2_EN
Tensile Test of Polymers
Date: 17. February 2015.
TENSILE TEST Measurement report Polymer materials science and technology
Name: ........................................................................ Neptun code: .............................................................. Date: .......................................................................... Practice leader: ..........................................................
1. Task -
Tensile testing of injection molded (type 1A) polymer specimens
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Calculation of the mechanical properties based on the registered force-elongation curves.
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Drawing diagrams on the effect of test speed and humidity.
2. Basic data Environmental temperature, T: Relative humidity Gripping distance:
No.
Material
Thickness [mm]
[°C] [%] [mm]
:
Width [mm]
1 2 3 4 5 6 7
8
Test speed [mm/min]
Other factor
Version: 3.2_EN
Tensile Test of Polymers
Date: 17. February 2015.
3. Mechanical properties determined from the tensile test:
No.
A0
Fmax
M
M
E0
[mm2]
[N]
[MPa]
[%]
[GPa]
Material
1 2 3 4 5 6 7
4. Draw the tensile curves and indicate the yield stress, the tensile strength, the breaking strength, the initial modulus. How have the environmental factors affected the measured curves?
σ [MPa]
σ [MPa]
ε [%]
ε [%] Effect of test speed
Effect of humidity
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