Molecular stability (rheology) of a plastic carrier bag through stress strain tests Essay Example

Molecular stability (rheology) of a plastic carrier bag through stress strain tests Essay I am going to study the molecular stability (rheology) of a plastic carrier bag through stress strain tests. I will do this through a simple viscoelastic experiment of where I will be able to calculate the Young Modulus and assess Hookes Law. Plastic carrier bags are made from polyethylene or polyethene, which is a homopolymer. Polyethylene comes from ethylene. Ethylene is an alkane made up of a series of saturated hydrocarbons. The alkane series are known as homologous series as they share the properties and general formula: (CnH2n+2) 1.1 Origin of plastic Essentially plastics are materials that can be heated and moulded and maintains this moulded shape once it cools. We will write a custom essay sample on Molecular stability (rheology) of a plastic carrier bag through stress strain tests specifically for you for only $16.38 $13.9/page Order now We will write a custom essay sample on Molecular stability (rheology) of a plastic carrier bag through stress strain tests specifically for you FOR ONLY $16.38 $13.9/page Hire Writer We will write a custom essay sample on Molecular stability (rheology) of a plastic carrier bag through stress strain tests specifically for you FOR ONLY $16.38 $13.9/page Hire Writer Plastics have existed since the beginning of time. Plastic contains natural elements such as carbon(C), hydrogen (H), nitrogen (N), chlorine (Cl) and sulphur (S). These elements can be originated from naturally grown organic materials such as wood, horn and rosin. Animal horn and amber are examples of natural plastics. Already renown for his work in the rubber industry, Alexander Parkes invented a material that was based on cellulose nitrate at the Great Exposition of 1862 in London. He called this material Parkesine, which was the first synthetic polymer. His invention was due to a new scientific movement to utilise by-products of natural gas production. We are now in the Age of plastics where plastic dominates our industrial world. Plastic is a force that will shape the twenty-first century, bringing to fruition new wonders in tomorrows world. Carin Glaser 1.2 Polymer Polymer comes from the Greek word, poly meaning many and mer meaning part. Polymerisation is the chemical process of forming polymers from their components of monomers. Polymerisation is often an intricate process that may be initiated or sustained by pressure, heat or with catalysts. Monomers generally contain carbon and hydrogen including, sometimes, chlorine, nitrogen, oxygen or fluorine. A polymer is a chemical compound with a high molecular mass constructed of chains of monomers linked together through bonds. Polymers can be categorised into two forms of polymers, addition polymers and condensation polymers. Condensation polymers consist of repeats of units that are bonded, which contain fewer atoms than the original monomer/s because of the loss of substances such as water. Addition polymers consist of the same monomer units that attach one at a time i.e. they have the same structural unit. A resultant of additional polymerisation, of monomer to polymer, with polyethylene is shown below. Ethylene; Polyethylene; 1.3 Cracking in the plastics industry Cracking is a process in industry where large chains of aliphatic hydrocarbons that make up crude oil/petrochemicals are broken down into smaller, more useful fractions. Factors such as high temperature, high pressure and catalysts are needed to break these long chains into shorter chains. Petrochemical such as oil or natural gas contain hydrocarbons. These hydrocarbons are processed through a reactor into a procedure known as cracking. In the plastic industry a natural gas derivative known as resin is produced. Resin pellets are then moulded or formed to produce several different kinds of plastic products with application in many major markets. The fickleness of the resin pellets proves a complex resin to be specifically designed for the consumers requirements. The different resins produced in the industry is the reason why certain plastics are better for different applications while others are best suited for entirely different applications. The production of a plastic bag is done through a process known as blown film extrusion. This process involves the resin being blown up and extruded to produce tubes of film. 1.4 Polymerisation of ethylene Branched polyethylene is produced through a free radical addition reaction. As shown above polyethylene are ethylene molecules bonded together. There are three stages to this process, these are: 1.4.1 Initiation Oxygen is the initiator of this process. The oxygen reacts with the ethylene to produce organic peroxide. Oxygen peroxide consists of double oxygen single bonds that are extremely reactive and break easily to give free radicals. This process can be avoided by adding other organic peroxides directly to the ethylene. The free radicals vary depending on the source, the basic formula for these free radicals is Ra*. These free radicals then react with the monomer to find its pair. This then pushes the monomers molecule leading the reaction back to the free radical stage. 1.4.2 Propagation The adding of more and more monomers to the growing chains is called propagation. The conductor of the experiment can add extra ethylene molecules to construct a long chain of them. Self-perpetuating reactions like this one are called chain reactions. The ethylene molecule consists of a double bond. One pair is held loosely on an orbital, this is known as a pi bond. The other pair is called a sigma bond and is held sturdily onto the nuclei. The free radical breaks the pi bond to form with another monomer. This is efficient because the previously weak pi bond is broken and the new carbon bond is stronger. This concluding that the more energy that is produced results in an additional stability to the system. This propagation results in the radical becoming larger and lengthened. Therefore each longer, larger radical can react with its other radicals to produce a polymer that extends further. 1.4.3 Termination This chain reaction must eventually end. The instability of the radicals means that eventually they will collide and form a pair without making a new radical. This then becomes one of the final molecules of the polyethylene chain. This termination process is called coupling. 1.5 Metal and plastic As we move closer to the future we can see that plastics are slowly taking over the function of metals on our society. The reasons for this are pretty simple, plastics are cheaper to make in industry than metals. Forms of iron are renown for its strength in the field of life but iron oxidises and slowly degrades. Plastics are non-biodegradable, meaning that they are suitable for long term uses unlike metals that are prone to disintegrate. Plastics do not need a great deal of heat to be moulded into shapes but metals tend to have a high melting point, this is cheaper for industries because less energy is used to make their products. The strengths of plastics are shown in industry through fire resistant material, bulletproof vests and puncture resistant tyres, buildings etc. Plastics are also known now to be able to conduct electricity (although not to a degree of a metal like copper) by modifying polyacetylene by blasting the material with iodine vapour. Thus, eradicate an electron giving the material a positive charge, allowing the material to conduct electricity. This may eventually resulting in plastics replacing metals in the electricity components, making modern technological equipment, such as DVD players, computers, TVs etc., cheaper to produce. Plastics are currently manufactures from crude oil through the cracking process but now scientists have discovered producing biodegradable plastics using waste food. The process is considerably cheap and could revolutionise issues such as protecting our ozone (as waste food release methane gas) and also keeping landfills clear of pollution. 1.6 The forces between the molecules The physical properties of polyethylene are judged by its intermolecular forces. The intermolecular force is the force attracting one molecule to another, Van der Waals attractions or hydrogen bonds. The melting and boiling point of any molecule would depend on the potency of the intermolecular forces. Van der Waals attractions are more likely to occur with larger molecules and these attractions need more energy to split. Presence of hydrogen bonds will lift the melting and boiling points. When covalent molecular substances are heated, they usually melt at low temperatures. The molecules do not decompose but remain as separate groups of atoms. This implies that the forces within the molecules are strong but the forces between molecules are not very strong. The forces holding the molecules together are not the strong forces of attraction resulting from the sharing of electrons. Source http://www.nelson.com.au/chemistry/guide/unit1chemistry/s1b.htm Polymers are normally long chains of hydrocarbons, the longer the chain of molecules, the higher the melting and boiling points are. This is due to the increase in dispersion forces as the size of the molecule increases. Hydrogen and carbon are non-metals and in general covalent bonds are associated with two (or more) non-metals bonding. In this bond the electrons are shared between the atoms. Electronegativity is the power of an atom in a molecule to attract electrons to itself. Carbon has an electronegativity of 2.5 and hydrogen 2.2. Therefore the higher electronegativity of the carbon atom signifies that the bonding electrons are attracted to the carbon atom rather than the hydrogen atoms. As the carbon atom contains more electrons it becomes vaguely negative and the hydrogen becomes slightly positive. This negative and positive bond is known as polarised and each molecule is a polar molecule or dipole. The hydrocarbons can organise themselves to result in the negative end of one molecule to be supported with the positive end of the next molecule. This type of intermolecular bonding is called dipole-dipole attraction. Molecules that are not polar have the weakest forces of intermolecular bonding between them. These types of substances are observed to have the lowest melting and boiling points. These molecules are held together by dispersion forces, which result from instantaneous dipoles caused by the constant movement of electrons within the molecules. The strength of the dispersion forces increases as the number of electrons in the molecule increases. Source http://www.nelson.com.au/chemistry/guide/unit1chemistry/s1b.htm 1.7 HDPE ; LDPE High and low density polyethylene are created in conditions under that ethylene is polymerised, the molecules become attracted towards each other in the solids by Van Der Waals dispersion forces. 1.7.1 High-density polyethylene (HDPE) A linear polymer is the most basic form of polymer structures (HDPE) HDPE has generally no branched chains. This allows molecules to attach nearer to the molecule in a normal way, which is almost crystalline. This signifies that the dispersion forces are more effectual therefore the material is moderately stronger and has a higher melting and boiling point compared to LDPE. 1.7.2 Low Density polyethylene (LDPE) LDPE has many short branches besides the chain. The extra chains stop the other chains from lying closely together in a neat arrangement, dispersion forces are therefore lower compared to HDPE and the material is fragile and has a lower boiling and melting point. The density of the material is lower since the wasted space within the irregularly packed formation. 1.8 Thermosets Thermoplasts There are two distinguished features of plastic. These two are thermosetting and thermoplastic: * Thermoplastic (thermoplast) is a polymer that is cross-linked. When heated the cross links are softened and then the polymer has the ability to be moulded into different shapes. Once cooled the polymer returns to its original stability. This process can be repeated many times. Its is of great use in the plastics industry because materials that do not need great strength but personalised structure such as plastic bags can be made with ease. * Thermosetting polymers are significant for their strength and durability. They are vastly stronger but brittle compared to thermoplasts. They are, in general, linearly structured such as HDPE. When heated a thermoset is made substantially infusible or insoluble, therefore the polymer cannot return to its original shape, or the polymer experiences plastic deformation. 1.9 Youngs Modulus The Youngs Modulus is stress/strain at the elastic limit. It is basically a measurement of the strength of a material or how much the material yields for each pound of force. Deformation relates to the stress strain behaviour of a material. Stress can be seen as the pressure exerted onto a material. To calculate stress the formula stress = load/area of cross section is used. Strain is the deformation of a material in relation with stress, or the deflection of a material due to force. For this experiment I need to calculate these three. 2.0 Prelimary work Preliminary work is done to pre determine the needs for the final experiment to be a success. I shall cover issues such as safety and dimensions. 2.1 Dimensions of plastic strip To calculate the length and width of the plastic strip of bag that I would use for the experiment I had to test the maximum strength of different lengths and widths of bags. From pre-knowledge of structures of molecules, the wider strip should be able to withhold more weight and the longer bag should extend further. I will choose various lengths of the plastic strips and summarise the maximum load limit of the strips. To measure this limit I shall use the method chosen for the experiment. N.B: Throughout the experiments I shall be taking Gravity (g) as 10ms-1 2.1.1 Results Prelimary Experiment Max Load (N) Length (mm) Width (mm) 1 2 3 Average 100 10 8 8 9 8.3 150 10 10 9 9 9.3 200 15 12 14 12 12.7 200 20 16 15 16 15.7 250 20 17 17 17 17 2.1.2 Summary I conclude that the wider the strip of plastic, in general, the heavier it holds before breaking. The weight holders carry around 15 Newtons each therefore I shall use 200mm by 20mm for my experiment. I noticed that when I added more weights the width of the bag shortened as the extension became greater, this may affect the eventual outcome of the result. To indicate this later in the experiment I shall identify any anomalies. I also need to calculate the impact that I did when I added the weights on directly, later in the experiment I need to add the weights on Newton by Newton simultaneously. Calculating stress To calculate the stress I need to use the formula: Stress = Load Area of cross section First I need to calculate the area of the cross section. To do this I shall use the formula: Area = width ? thickness The cross section of the paper strip is: The thickness of the bag varied in each strip so I took the average of the thickness of the plastic strips from 6 samples using a micrometer. Average (mean) = ?x ?f Samples thickness: 1. 0.019mm 2. 0.022mm 3. 0.02mm 4. 0.018mm 5. 0.019mm 6. 0.02mm These results are fairly close together therefore I shall use these results, as they seem accurate. ? Mean = 0.019 + 0.022 + 0.02 + 0.018 + 0.019 + 0.02 6 Thickness = 0.0197mm If area = width x thickness Thickness = 0.0197mm Width = 20mm ? Area = 20 ? 10-3 (m) ? 0.0197 ? 10-3 (m) Area = 3.94 ? 10-7 m2 From these results I can calculate the stress. I shall go to the force of 17N because from previous knowledge I can say that the strip will be likely to break at this point or below. Load Applied (N) Stress = load/area of cross section (Pa) 0 0 1 2.5 x 106 2 5.1 x 106 3 7.6 x 106 4 1.0 x 107 5 1.3 ? 107 6 1.5 ? 107 7 1.8 ? 107 8 2.0 ? 107 9 2.3 ? 107 10 2.5 ? 107 11 2.8 ? 107 12 3.0 ? 107 13 3.3 ? 107 14 3.6 ? 107 15 3.8 ? 107 16 4.1 ? 107 17 (max) 4.3 ? 107 2.3 Prediction I predict that to the elastic limit the force will be proportional to length. Beyond the elastic limit the extension of the plastic strip will increase excessively. 2.3.1 Deformation This is because of deformation. Deformation is the modification of a material in response to force. The two types of deformation are elastic and inelastic (plastic). Elastic deformation is when a material returns to its original shape after the force has been applied. Inelastic deformation is the change of the original shape after a force has been removed. The reason of deformation is related to the bonds in the material. In deformation the bonds are being re-arranged. For example, when stretching a polymer the connecting covalent bonds are given energy and the bonds are stretched. The reason of inelastic deformation is that when the energy is applied to the bonds the cross links are moved then the object therefore changes its original shape. With a large enough force the bonds will break and the polymer breaks. 2.4 Reasons for testing The reasons for testing are: 1. Safety measurements. 2. Provide a basis for reliability. 3. Quality control. 4. Establish design ideas. 5. Meet the standards and specifications set by producers and standard agencies. 6. Verify manufacturing process. 7. Evaluate and compare against competitors products. 8. Establish history for new materials. 2.4.1 Costs of testing I need to research the cost of testing polymers to understand the importance of testing in the plastic industry. From Dr. Shastri I have found that the costs of testing polymers in industry are: Property (Single-point data) Cost Range Per Grade Average Cost Mechanical properties $780 $3120 $1500 Thermal properties $1030 $3270 $2000 Rheological properties $ 370 $ 650 $ 500 Electrical properties $1020 $1860 $1500 Other properties $ 170 $ 540 $300 $5800 Property (Multi-point data) [viz, creep, stress-strain] Average Cost $14,484 $93,140 This concluded that the testing of polymers at an industry level is out of my budget and time range. Therefore I must keep the experiment at, to a degree, a simplistic level. The large cost of the testing of polymers in the industry justifies the importance of the material to society. This relates with why that safety is needed when testing the material, I shall later resolve safety procedures. 2.5 Possible solutions to conduct this experiment in a laboratory are: 1. This method of conducting the stress relaxation test is needed with a pc to measure the length and force applied; the computer can then create a diagram of force/extension. I cannot execute this method because I have not got the right equipment and this experiment does not allow me to calculate the effects manually, which is what I wish to accomplish. 2. This experiment is capable of doing but I think that the material stretched along the table at such a length would mean I would have to use a long plastic bag which are hard to get hold of; apart from that this method would be acceptable for wire dimensions materials. 3. The ruler held into a fixed position means that there are less human errors in the measurements. The fixed position with the clamps and retort stand shows the stability and rigid structure of this method. This experiment allows me to manually draw up diagrams to calculate the Youngs Modulus. I am going to use experiment 3 to perform my experiment. Method to perform experiment 3 2.5 Apparatus Apparatus that I will need to conduct this experiment from the method I have chosen are: * Ballpoint pen. * Paper. * Metal wire. * Meter rule. * Micrometer. * Plastic carrier bag. * Retort stand. * Scalpel. * Safety (metal) ruler. * Scissors. * Sticky tape. * Wire clippers. * Cushion. * Weights. * Stop watch. A strip of plastic bag needs to be measured using a ruler accurately onto the bag with a pen. The marked out strip then needs to be cut using a sharp tool such as a scalpel and a safety ruler is needed for precision cutting and safety. The device I have chosen to hold the material to the weights and clamp is a metal wire that has been twisted. The wire will be 100mm long and then twisted into a circle like shape as shown in the diagram (left). The wire will be firstly looped into a ring of plastic from the bag; each side of the bag will be a fixed length for the wire to loop into. 2.6 Method 1. Set up the experiment as shown in the diagram. 2. Measure the length of the plastic bag between the two marked points using the ruler that is attached to the retort stand. 3. Add one Newton to the plastic bag. 4. Begin the stopwatch. 5. Once 5 seconds have passed measure the new length using the ruler. 6. Continue steps 3-5 adding one extra Newton each time until the plastic bag snaps. 7. Repeat the experiment 5 times for a fair test. The most important factor is always making accurate measurements. By checking the results of measurements more than once using a ruler and then obtaining an average will provide me with suitable evidence of recurring measurements therefore showing that they have been accurate. 2.7 Safety procedures Safety is one of the major factors of doing any experiment, or anything. The safety factors that I will take into hand in this experiment is: * Work in a large open area that will be sufficient to be a good distance away from people. * Keeping my body away from the weights and putting a cushion underneath the weights to soften the blow when the plastic strip breaks. * Take care when handling scalpels. * Use a safety metal ruler when cutting with the scalpel. * Using wire clippers to cut wire. * Use retort stand so weights are sturdily held. * Use weight holders so weights are held tightly together in an order. * Always be aware. 2.8 Fair test For the result of the experiments to be valid and contain fewer errors I need to suggest factors that will affect the result and then resolve them with solutions. The fair test factors that I have taken into practice in this experiment are: * Use the same plastic bag because of specimen variations. * The molecule variation in the bag, the bags handles might be made stronger than the bag, although this may not be true. * Do the experiment in room temperature, as the heat affects the atom bonds. * Keep the rate of loading the weights simultaneously because of impact strength. * Keep the ruler attached to the same place on the retort stand. * Repetitive. * Conduct the experiment five times to devise anomalies. * Use the same dimensions of plastic strips and wires. * Accurately measure with a ruler and avoid parallax errors. * Accurately cut the plastic strips with a sturdy ruler. Other factors that I have taken into place are the weight of the metal wire, but this was under a gram so it would not have affected my result by a lot and if I used the same wire consistently I could justify with the errors. 2.9 Creep Creep is a major factor when using heavier weights because the strip will gradually deform to relieve stress. This is why I need a fixed time to measure the extension when doing the experiment. 3.2 Conclusion My prediction was: I predict that to the elastic limit the force will be proportional to length. Beyond the elastic limit the extension of the plastic strip will increase excessively. My prediction in accordance to my results states clearly that this statement is true. Therefore my results are a success. The strips broke at different forces. This means that some of the strips may have been stronger than the other ones. This may have affected my results before the material reached its elastic limit because the bonds might have been stronger, also maybe because of human errors. The bonds in the different parts of the bags may have been engineered differently to suit its purposes i.e. the handle of the bag may have been made stronger than the body because of the force applied to it. I found that after the material reached its elastic limit the results became a little random. This may have been due to creep and the difficulty to measure the extension when the material is constantly going through deformation and the bonds are weakening making the material extend at a faster and easier rate. I have not included results after the elastic limit in my calculations because of this. The bag may also have contained additives such as colorants to produce colours on the bag; this may affect the bonds in the bag slightly and therefore affecting my results. I found the strip went through plastic deformation at around 10 N, which is equal to 1 kg. Bags are made to carry around 7 kg and the strip I cut was 1/10th of the bag, meaning that the strip cannot be justified as the strength of the bag as the shape of the bag differs from the strip and is stronger than the strip because of the way it is built. To summarise the results fairly I have created an averages of my results. I shall use the average results to do my calculations of the Youngs Modulus and strain energy. 3.3 Evaluating Graphs 3.3.1 Calculating Strain Energy When the strip is stretched we can say that energy has been applied to it, i.e. it extends (goes through deformation). The force extension graph can calculate the work done on the strip. Energy is measured in joules and is the area below the elastic limit on the graph. This energy is called work done and is the elastic potential energy, or the strain energy in the strip. Strain energy is the area underneath the force extension graph up to the elastic limit. ?This area must be a triangular shape because the line up to the elastic limit is straight. The area of a triangle = 1/2 base x height ? Strain energy = 1/2 f.x Where f is the force to produce the extension, x. Strain energy = 1/2 x 9 x 8 x 10-3 = 0.036 J 3.3.2 Calculating Youngs Modulus (E) Youngs Modulus is a measurement of the stiffness material. It is similar to calculating the gradient of the line up to the elastic limit. Youngs Modulus = Stress Strain = 2.5 x 107 0.096 = 2.604166667 x 108 Pa = 2.60 x 108 Pa (to 3 s.f) 3.4 Calculating the % errors for Strain Energy and E All experimental measurements have some degree of error associated with them. Physics Dept. New College Durham. There are systematic and random errors. A systematic error is when the method of measuring is always to large or to small. Such as a non-parallax error or when instruments such as the micrometer are not zeroed correctly. Random errors are when repeated results have different outcomes. To avoid this the experiment can be repeated and then take an average from the results. The formula used to calculate error = ? Smallest division Height gained Error in 100g masses = ? 0.1g ? ? 1.0 x 10-4kg Error in metre rule reading = ? half the smallest division of the rule. = ? 0.5mm Since a measurement is the difference between 2 readings error = ? 1mm ? ? 1.0 x 10-3m Error in vernier reading = ? half the smallest division = ? 0.0005mm Therefore the measurement error (2 readings) = ? 0.0001mm ? ? 1.0 x 10-6m 3.4.1 Strain Energy Strain Energy = 1/2 x load x extension Elastic Limit = 0.9 kg and 8 x 10-3 m % Error in mass (9 x 100g) = 9 x 1.0 x 10-4 kg x 100 9 = ? 0.1% % Error in extension (metre rule) = 1.0 x 10-3m x 100 8 x 10-3 m = ? 12.5% To obtain the total error for strain energy, add the % errors = ? (0.1% + 12.5%) = ? 12.6% Strain energy = 1/2 f.x = 1/2 x 9 x 8 x 10-3 = 0.036 J ?Strain energy = 0.036 J ? 12.6% = 0.036 J ? 0.00454 J 3.4.2 Youngs Modulus Youngs Modulus = Stress Strain = (Force/area of cross section) (Extension/original length) = Force/area of cross section x original length/extension = (Mass x gravity/thickness x width) x (original length/extension) Mass % error = ? 0.1% Material thickness = 0.0197mm ? 0.0197 x 10-3m Material width = 20mm ? 20 x 10-3m Original length (between lines) = 100mm ? 100 x 10-3m Extension = 8mm ? 8 x 10-3 m at elastic limit Material thickness % error = 1.0 x 10-6m x 100 (Vernier) 0.0197 x 10-3m = ? 5.076142132 ? ? 5.08 (to 3 s.f) Width % error = 1.0 x 10-3m x 100 (Rule) 20 x 10-3m = ? 5% Original length % error = 1.0 x 10-3m x 100 20 x 10-3m = ? 0.5% Extension % error = 1.0 x 10-3m x 100 8 x 10-3m = ? 12.5% For the total error, add the % errors = (mass + thick + width + ori length + extension) = ? (0.1 + 5.08 + 5 + 0.5 + 12.5) = ? 23.18% ?Youngs Modulus = 2.60 x 108 Pa ? 23.18% = 2.60 x 108 Pa ? 60268000 Pa = 2.60 x 108 Pa ? 6.03 x 107 Pa (to 3 s.f) 3.5 Evaluation The experiment conducted allowed me to calculate the Youngs Modulus using a carrier bag, the method I used was suitable to the objective of measuring force against extension (the investigation). From my results and graphs I can see that there were no spurious readings in the investigation. Therefore my experiment was a success. The problems of this experiment was the cutting of the plastic bag as due to the lack of friction the plastic bag was not held properly onto the work surface. I tried to resolve this by embracing the carrier bag onto the work surface whilst I cut it but this was still difficult as the bag still moved. When the strip had a force applied its width reduced and the strip became compact into a wire dimensional shape. I would have preferred f I was able to hold the original shape intact when adding the weights. I did try other methods, such as replace the wire with a pencil and hold the weights on each side but this meant that the weight addition was going up by 2N instead of 1N simultaneously. Also putting on the 2 weights at the same time was difficult because of balancing the pencil, this meant that creep came into affect and I it was hard to record measurements at a fixed time. I would have preferred to use a PC to help me record my results, as this would have reduced the human errors and recorded the results more accurately than I could have. But the costs of using a computer and setting it up was time consuming and would not have correlated with the time I was given to conduct this experiment. The strain energy and Youngs Modulus % errors were both above 10% and this is quite a high figure when trying to measure accurately. If I used a computer this would have reduced my errors and made my measurements accurate. If I were going to extend this testing into measuring hardness, viscosity or ductility then I would have re-done the experiment using a PC. At a more advanced level plastic deformation is caused by the motion of dislocations, which involves the breaking and reforming of bonds. This can be seen as a random procedure because the force applied would have to be done on the same material with the exact structure and the force applied in the exact place. As carrier bags are made in masses I doubt that the structures are exactly the same. Therefore it would be pointless for me to have measured beyond the elastic limit.