Materials - The Effect of Strain Rate on Ductility of Polypropylene PDF

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THE EFFECT OF STRAIN RATE ON DUCTILITY OF POLYPROPYLENE MATERIALS LABORATORY REPORT

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Summary The materials laboratory consisted of an experiment which had the main aim of determining the relationship between strain rate and ductility of a material. The material used was a thermoplastic; polypropylene. Three samples of the polymer were dimensioned and placed inside the Tensometer to be drawn at three specific strain rates. Results show a significant effect on ductility when strain rate is varied.

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Table of Contents Introduction...........................................................................................................................................3

Theory...................................................................................................................................................4

Apparatus..............................................................................................................................................5

Method..................................................................................................................................................5

Results...................................................................................................................................................6

Discussion..............................................................................................................................................7

Conclusion.............................................................................................................................................8

References.............................................................................................................................................9

Figures.................................................................................................................................................10

Tables...................................................................................................................................................11

Graphs.................................................................................................................................................13

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Introduction A polymer is defined as a chemical structure in which the molecules (monomers) are bonded in chains which repeat themselves. Thermoplastics belong in the group of polymers which soften when heat is applied and harden when cooled again. The weak intermolecular bonds called Van der Waal forces and hydrogen bonds are responsible for this as they break easily when the thermoplastic is heated. The chains are then untangled and can easily slide over each layer, increasing the ductility. The temperature at which the intermolecular bonds break is called the glass transition temperature, where the thermoplastic turns into a viscous liquid. The materials lab was used to conduct an experiment which studied the ductility of polypropylene, which is a specific thermoplastic. The glass transition temperature for polypropylene is 253K. Polypropylene has a monomer which comprises of the structure C3H6. As well as being used in the clothing industry, it is also a part of the medical, and automotive industry. Polypropylene was heated at roughly 463K and moulded into three samples. The main objective was to investigate how varying the strain rate affects the fracture of the three samples of polypropylene used. The strain rate is the rate at which the sample was pulled in the tensile testing machine and for this experiment three different values of strain rates were tested. A load/extension graph was produced by the machine which assisted in calculating parameters which would be useful when analysing the experiment. The results of this experiment could be useful for manufacturers who create products which consist of polypropylene. This is because it provides information on how the product can handle various rate of strain.

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Theory True Fracture Stress=

Fracture Load Cross sectional Area at Fracture

Tensile Ductility=

Nominal Yield=

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Youn g s Modulus=

Extension at Break Original Gauge Length

Load at High Yield Initial Cross Sectional Area

Load at High Hield /Cross Sectional Area Extension at High Yield /Gauge Length

Ductility is the ability for a material to deform under tensile stress and Young’s Modulus is a measure of elasticity and is determined by the ratio of applied stress to produced strain. The gauge length is the distance between the grip sections of the specimens, which was used to measure the elongation.

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Apparatus 

JJ Lloyd Tensometer – This machine was used to pull the specimens and was set at a specific strain rate. (Figure 4)

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Micrometre – Was used to measure the thickness and width of each specimen at three different places to determine an average.

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Specimens (33 mm gauge length) – Three samples made from polypropylene were used to test in the Tensometer. A sketch of the samples can be seen in Figure 1.

Method 1) The thickness and width of each of the three samples was measured in three different places. This gave a value for the average cross-sectional area. 2) The machine was set to draw the samples at different strain rates (12.5 mm/min, 50 mm/min and 100 mm/min). 3) The samples were tested until they fractured and a force/extension graph was produced by the machine which had relevant values for certain parameters such as maximum load, extension at maximum load and extension at break.

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Results Table 1 shows the dimensions of each of the samples, including average thickness, width and cross-sectional area. The product of thickness and width gave the cross-sectional area. The parameters which were given by the graph produced by the machine are given in Table 2. Parameters of each sample are given in Table 3 which includes the following:        

Gauge length Initial cross-sectional area Final cross-sectional area Fracture Load Fracture Stress Tensile Ductility Nominal Yield Young’s Modulus

Below is the set of calculations for sample one after the experiment had taken place. The calculations of samples two and three were carried out in the same method, and were inputted into the table of results (See Table 3).

True Fracture Stress=

Fracture Load Cross Sectional Area at Fracture

True Fracture Stress=

288.01 N =150 N /m m2 2 1.92m m

Tensile Ductility=

Extension at Break Original Gauge Length

Tensile Ductility=

250.5 mm =7.59 33 mm

Nominal Yield=

Load at High Yield Initial Cross Sectional Area

Nominal Yield=

251.5 N =24.7 N /m m2 2 10.2 mm

Youn g' s Modulus=

Load at High Hield /Initial Cross Sectional Area Extension at High Yield /Gauge Length

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Youn g s Modulus=

251.5 N /10.2 mm 2 2 =104.3 N /m m 7.813 mm /33 mm

Discussion Polymers can be split into two main groups; Thermoplastics and Thermosets. The major difference between these is the fact that thermosets can only be moulded and shaped once into a product, whereas thermoplastics can be heated and reshaped as many times as needed. This is due to the molecular structures of both polymers (See Figure 2). Thermoplastics have a linear chain structure whereas thermosets contain a much more tangled, and complex structures with links in between the chains. Figure 3 illustrates the typical Force/Extension graph for a thermoplastic. The results which were obtained from the experiment coincide with the curve in Figure 3 The results show the higher the strain rate, the less ductile the polypropylene became. Sample one was the most ductile as the strain rate was set to 12.5 mm/min. Graph 1 shows the results of sample one and will be described below:    

From points A to B, the sample acts like a linear elastic until it reaches the yield point (Point B). From points B to C, the sample undergoes plastic deformation and a sudden decrease in cross-sectional area occurs. From C to D, there is an increase in strain due to a constant stress being applied. The necking of the sample spreads across the complete length of the sample. From D to E, additional stress is needed to further increase the strain of the sample.

The chains within the polypropylene are tangled before any necking begins. As stress is applied, the chains begin to untangle and straighten, in the same direction as the sample is being drawn. This arrangement of chains makes the polypropylene crystalline and the process is called cold drawing. Overall the strength is greater than before due to the new chain alignment. Graph two and Graph three are similar to graph one, however the plastic deformation phase increases as strain rate is increases (points C to D). It is bigger on the graph two than graph one and larger on graph three than graph two. This means the amount of necking decreases as strain rate increases. A high strain rate means the polypropylene does not have enough time to completely orientate. The digital micrometre which was used provided measurements to two decimal points. Therefore to improve the accuracy of the results, a micrometre with a higher degree of precision could have been used to measure the specimens. Repeat readings of thickness and width were taken to increase reliability of the measurements and reduce the magnitude of any errors.

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Conclusion The experiment was successful in determining the effect of changing the strain rate on the ductility of polypropylene. The main objective was achieved and a concluding statement can be made; the higher the strain rate, the more brittle the material. The graphs produced clearly show the effects of changing the strain rate and give an indication that the results obtained are similar to already published data.

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References Chavan, A. (2014). Difference between Thermoplastics and Thermosets. [Image] Buzzle. Available at: http://www.buzzle.com/articles/difference-between-thermoplastics-andthermosets.html [Accessed 24 Feb. 2015]. Lloyd-instruments.co.uk, (2012). LR150KPlus twin column floor-standing testing machine. [Image] Available at: http://www.lloyd-instruments.co.uk/Products/Twin-columnfloor/LR150KPlus.aspx [Accessed 5 Mar. 2015].

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Figures

Figure 1-Sketch of specimen shapes

Figure 2- Difference between structures of Thermoplastic and Thermoset (Chavan, 2014)

Figure 3 – Typical Force/Extension Graph for polymer

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Figure 4 – (Lloyd-instruments.co.uk, 2012)

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Tables

Specimen 1 (12.5 mm/min) Specimen 2 (50 mm/min) Specimen 3 (100 mm/min)

T W T W T W

Reading 1 (mm) 2.18 4.69 2.21 4.69 2.23 4.77

Reading 2 (mm) 2.17 4.75 2.19 4.75 2.21 4.73

Reading 3 (mm) 2.17 4.73 2.19 4.73 2.20 4.80

Average (mm) 2.17 4.72 2.20 4.72 2.21 4.76

Average C.S.A (mm2) 10.2 10.4 10.5

Table 1 - Dimensions of Specimens

Specimen 1 Specimen 2 Specimen 3

Maximum Load (N) 293.0 275.9 266.1

Extension at Max Load (mm) 245.1 7.324 7.080

Extension at Break (mm) 250.5 195.8 48.10

Load at High Yield (N) 251.5 275.9 266.1

Extension at High Yield (mm) 7.813 7.324 7.080

Table 2 - Parameters given By Graph

Specimen 1 Specimen 2 Specimen 3

True Fracture Stress (N/mm2) 150 78.4 67.1

Tensile Ductility 7.59 5.93 1.46 Table 3 – Calculations

Nominal Yield (N/mm2) 24.7 26.5 25.3

Young’s Modulus (N/mm2) 104.3 119.8 117.9

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Graphs

Graph 1 - Load/ Extension graph for specimen 1

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Graph 2 - Load/ Extension graph for specimen 2

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Graph 3 - Load/ Extension graph for specimen 3...