Jamie Sawdon (12006813)

Contents An Analysis of Material Behavior When Processed Through Stages of Cold Working (Part A). Introduction:...............................................................................................................................................2

Results.........................................................................................................................................................3

Discussion....................................................................................................................................................4

Extended Analysis:.......................................................................................................................................5

Task 1.......................................................................................................................................................5

Task 2.......................................................................................................................................................6

Conclusion...................................................................................................................................................7

An Analysis of the Creep and Creep Recovery Behavior of Two Thermoplastics (Part B). Introduction:...............................................................................................................................................8

Results:........................................................................................................................................................9

Discussion:.................................................................................................................................................10

Discussion cont: ………………………………………………………………………………………………………………………………… 11Extended Analysis:.....................................................................................................................................12

Task 1.....................................................................................................................................................12

Task 2.....................................................................................................................................................13

Bibliography:.............................................................................................................................................14

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An Analysis of Material Behavior When Processed Through Stages of Cold Working (Part A).

Introduction: The process that we are investigating here is of the name ‘Cold Working’. (Asekland, Fulay and Wright, 2011)The name is derived from the low temperature at which the metal is plastically deformed, by exceeding the yield strength of the material. Without the plastic deformation, the material would revert back to its original shape and no change will have taken place. The deformation occurs below its Recrystallization temperature, which means that new-grains can’t emerge in the material. However the amount of dislocations that occurs, in the metal’s crystalline structure, increases due to this idea of ‘slip’. Slip is where planes of atoms in a crystal structure slide against each other. Which occurs when a sufficiently high enough shear stress is applied to the crystal structure, of which has a dislocation. The result being that the top half of the crystal structure has moved by a distance of 1 lattice parameter, relative, to the lower half. (Asekland, Fulay and Wright, 2011)The bonds across the slip plane are broken, and the atoms below the slip plane establish a new bond with the atoms of the dislocation.

This type of mechanical deformation is used in two main cold working processes called:

‘Cold Rolling’. The product being achieved is sheets of metal, through rolling a metal flat until its diameter has decreased. The metal that is produced by the rolling, is more compact due to the distortion of the grains, in the direction of the rolling process. The yield strength of the metal needs to be lower than that of the compressive force acting upon the metal.

‘Wire drawing’. A metal rod is pulled through a die, which is smaller at the end than the beginning, to reduce the diameter of the metal rod, as it passes through. The metal in this process is drawn, and therefore a tensile force is applied here. The force must be higher than the yield strength of the metal, but only enough to plastically deform the metal without leading to fracture, due to the increased brittleness of the metal at this stage.

A careful selection of force strength must be made for both processes to achieve the required product without causing it to fracture.

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Results.

Specimen. A B C D E F% Cold work

received.0 10 25 35 45 50

Test specimen cross sectional

areas (m2) – before cold

working.

1.5x10-5 15 x10-5 15 x10-5 15 x10-5 15 x10-5 15 x10-5

Test specimen cross sectional

areas (m2) – after cold working.

15.0 x10-5 1.35 x10-5 1.12 x10-5 9.75 x10-6 8.19 x10-6 7.41 x10-6

Tensile strengths (MPa)

702 919 1182 1201 1430 1470

Yield strengths (MPa)

320 524 820 1000 1160 1230

Ductility(% Elongation)

65 46 29 20 12 10

(Worked examples and graph summarising material properties, are displayed on the following pages).

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Discussion.

The samples were all from one material that were subjected to increasing yield stress until the material reduced in its thickness to the required amounts. Therefore the thickness was the aim achieved by the independent variable of yield load.

Trends noticed are therefore talked about as the thickness decreased:

Percentage Cold Working increased, because more work must be done to reduce the thickness of the material under load. As % cold work is a measure of area which decreased from 1.5x10-5

to the thinnest sample (sample F) 7.41 x10-6 the percentages of cold work starting at 0% and increasing to 50% were expected due to the increased difference in areas form sample A – F.

Tensile strength was seen to increase throughout specimens A – F which was expected as the process is one of a hardening process due to the dislocation hindering effect that is caused by increased amounts of new dislocations hindering other dislocations forming.

I expected the yield strength to trend the same way as the tensile strength which was indeed what was shown in the results. From specimens A – F increasing by about 900 MPa. This is usually the case as the yield strength must be below the tensile strength at all times, otherwise the material would fail instantly.

The percentage elongation shows a constant decrease which surprised me as I expected the specimen to elongate as its thickness decreased. But in fact due to the constant increase in dislocation created in cold working they eventually lead to limited ductility.

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Extended analysis: Task 1.

If one specimen had longer annealing time to the other than they would be expected to show different tensile strengths when tested. Annealing is the process of reheating a cold worked metal to restore the original mechanical properties, microstructure and other properties for example tensile and yield strengths plus ductility. When a metal is annealed it enters 3 main stages of the entire process:

1. Recovery = Reducing the Internal residual stress which has been created at the cold working process. Residual stress forms when not all of the energy put into deforming the metal has been used in that form but instead remains in the metal after cold working.

2. Recrystallisation = When the metal enters a high temperature of annealing new sets of strain free soft equiaxed grains replace the cold worked grain structure. This can only occur due to the high temperatures causing greater levels of energy meaning that the atoms in the crystal lattice are able to move producing the new grain structure. By the end of this stage the original microstructure has been restored.

3. Grain growth = if the metal is kept at elevated temperatures strain free grains continue to grow. This leads to a deterioration in the mechanical factors such as yield strength, because as larger more coarse grains develop at the expense of smaller grains, less grain boundaries are present creating a higher risk of slippage in planes, due to an increased amount of different angles created by multiple grains. Which is what defines the strengths of a metal.

Therefore the process of annealing is generally favored to be stopped at the end of the recrystallisation process (recognized at one hour) plus a temperature will have been calculated to ensure that it maintains the metal below the 3rd development stage but allowing the first 2 stages to occur. This temperature is called the recrystallisation temperature and is calculated using this calculation:

0.4×Tm (absolute melting temperature )=recrystallisation temperature .

For the requirements of task 1 the recrystallisation temperature is:

0.4×14200C=5680C

If one specimen was allowed to continue annealing, that specimen would enter the stage 3 and the mechanical properties that were restored will begin to deteriorate due to the overtaking of coarse grains from their smaller counter parts. Causing increased plane slippage and therefore reduced strengths such as the yield strengths shown in the graph below.

(Graph follows this page, showing expected stress-strain curves).

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Task 2.

From the graph that I created for the material properties that occurred due to cold working process in the results section, I am able to extrapolate the data needed for this task.

I started by drawing a horizontal line from 15% elongation across to where it meets the elongation data line, then I was able to draw a second line vertical from this point to where it meets the yield strength. Then a second horizontal line was drawn from 950 MPa to where it meets the yield strength data line, then a vertical line was drawn down to the percentage cold worked axis. These two vertical lines represent the maximum and minimum working area for the cold working process of the die. Therefore I found the average point, at 36.5 % cold worked, and draw a vertical line crossing the percentage elongation at 19%, yield strength at 1020 MPa, and continuing the line to the tensile strength gaged at 1240 MPa. These figures are correct to the required specifications of the customer.

36.5% CW 19% EL (above 15%) 1020MPa yield strength (above 950MPa) 1240MPa tensile strength

Then I began to work on the dimensions needed for the wire drawing die. I know that the material needs to be 0.46mm in diameter, therefore I can reverse the percentage cold worked equation to calculate what the opening dimensions of the die needs to be.

1. 0.46xπ = 1.45mm2

2. (1.44x36.5)/100 = 0.53 mm2

3. 0.53+1.44 = 1.97mm2

4. 1.97/π = 0.63mm2

Finally a force needs to be calculated so that it is great enough to plastically deform the metal so that the diameter can reduce, but not so much that breaks whilst being drawn under force. Therefore again I looked at the graph, previously drawn, between the lowest yield strength (320MPa) and lowest tensile strength (702). The average between the two values came to 521MPa and to then calculate a force required, the calculation for stress=force/area, can be used to find the force.

Force (N) = stress (MPa) x area (mm2) 1008 = 521 x 1.97

Therefore the required force needed to reduce the die diameter to 0.46mm is 1008N.

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Conclusion. Using the experimental data to evaluate the changes in mechanical properties was successful. With the aid of a graph to explain the yield and tensile strength changes and percentage elongation at values of percentage cold work. All the dependent variables helped me to explain what had occurred and assisted me to visualize the data, to aid my understanding of what changes occurred. With aid from textbooks and other sources, I was able to determine whether the changes that occurred were meant to occur or was unexpected for an experiment of this kind.

When it came to a more in depth look at the changes that had occurred, I found out about the microstructural changes that occurred due to cold working and the following processes such as annealing. Therefore I was able to make a more detailed conclusions to the changes that occurred by applying the learnt knowledge of the microscopy.

But what I found to be most fun and helpful was to be able to apply my data to practical situations, this helps to make a better picture of what is occurring, and to really understand the science that is taking place in the experiments.

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An Analysis of the Creep and Creep Recovery Behavior of Two Thermoplastics (Part B).

Introduction: The investigation here is based upon the subject of Creep and therefore to be able to delve further into the investigation and start to apply theories to numbers and gain valid results, one must understand what creep is. Creep is a plastic deformation that occurs in thermoplastic materials due to a constant stress or constant load applied, with high temperatures, over a sustained period of time. Even though the stress may be lower than the yield strength of the material, the material can fracture. Lastly creep recovery is another part we will be looking at in this investigation. Simply it states in the name what it means. But in slight more detail, it means that, the deformation occurred due to creep has been removed due to the removal of the stress or load that was applied. The way that we draw up conclusion about the creep and creep recovery of these two thermoplastics, is by using the data collected from the deflection of the materials when left hanging out of the vice. Where gravitational force and the weighted load put on the material under stress. Then the materials are left to recover by removing the weighted load. A useful way of demonstrating creep and creep recovery is to draw spring and dashpot components. These components were used due to the similarity between the polymer behavior and the spring and the liquid-like component being represented by the dashpot. These drawings are found in task 1.

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Results:

Time Deflection (mm)- Polypropylene

Deflection (mm)- GP Polystyrene

Values of true strain-Polypropylene

Values of true strain-GP Polystyrene

Creep moduli-Polypropylene

Creep moduli-GP Polystyrene

0.0 16.4 12.3 6.3X10-03 4.72X10-03 2.24X10+02 2.97X10+020.3 18.4 12.3 7.07X10-03 4.72X10-03 1.99X10+02 2.97X10+020.5 18.9 12.3 7.26X10-03 4.72X10-03 1.94X10+02 2.97X10+021.0 19.6 12.3 7.53X10-03 4.72X10-03 1.87X10+02 2.97X10+022.0 20.4 12.3 7.83X10-03 4.72X10-03 1.80X10+02 2.97X10+023.0 20.9 12.3 8.03X10-03 4.72X10-03 1.76X10+02 2.97X10+024.0 21.1 12.3 8.10X10-03 4.72X10-03 1.74X10+02 2.97X10+025.0 21.5 12.3 8.26X10-03 4.72X10-03 1.71X10+02 2.97X10+027.0 21.9 12.3 8.41X10-03 4.72X10-03 1.68X10+02 2.97X10+0210.0 22.3 12.3 8.56X10-03 4.72X10-03 1.65X10+02 2.97X10+0215.0 22.7 12.3 8.72X10-03 4.72X10-03 1.62X10+02 2.97X10+0220.0 23.2 12.3 8.91X10-03 4.72X10-03 1.58X10+02 2.97X10+0225.0 23.5 12.3 9.02X10-03 4.72X10-03 1.56X10+02 2.97X10+0230.0 23.6 12.3 9.06X10-03 4.72X10-03 1.56X10+02 2.97X10+0234.8 24.0 12.3 9.22X10-03 4.72X10-03 LOAD RELEASED35.0 7.6 0.5 2.92X10-03 1.92X10-0435.3 5.8 0.0 2.23X10-03 0.00X10+0035.5 5.0 0.0 1.92X10-03 0.00X10+0036.0 4.3 0.0 1.65X10-03 0.00X10+0037.0 3.6 0.0 1.38X10-03 0.00X10+0038.0 3.0 0.0 1.15X10-03 0.00X10+0039.0 2.8 0.0 1.08X10-03 0.00X10+0041.0 2.6 0.0 9.98X10-04 0.00X10+0045.0 1.8 0.0 6.91X10-04 0.00X10+0050.0 1.6 0.0 6.14X10-04 0.00X10+0060.0 1.0 0.0 3.84X10-04 0.00X10+0080.0 0.8 0.0 3.07X10-04 0.00X10+00

(Worked examples and graphs showing strain values and flexural creep modulus, found on following pages)

Discussion:

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As the graphs have been drawn based upon the data found, for creep and creep recovery. I am able to see clearer the differences in the two materials. It is easier then to see that GP polystyrene has the greater strength of the two materials, although it does deform to 12.3mm, the deflection that occurs is instant and doesn’t continue to increase, such like polypropylene. Polypropylenes delfection value rises up to 24mm after 34.8 seconds, plus polypropylenes measure of delfection starts at a higher value than that of GP polystyrene. The same trend, for polypropylene, is therefore shown in the true strain graph showing a constant increase after loaded to the point of load release. What occurs in the spring and dashpot components shows that the spring increases in length as well as the dashpot. When the load is released for both specimens, the deflection values decreases rapidly, showing elastic deformation occurred, as they are able to return to their original state. Polysytrene shows the best elasticity of both specimens. This is demostrated well in the dashpot and spring components, by means of the spring retracting and bringing the dashpot back up towards it original value. As soon as the load is released the delfection occurred reduces dramatically by 11.8mm in the first 0.2 seconds and by the time a second has past the material has retracted to its original length. This occurs due to the structure of the material. The material although they are not fully crystalline, they display some of the associated features, hence why these materials are named semi-crystalline. For example, as stated in Askelands engineering book, between the regions of crystalline lamellae and spherulties there are amorphous regions. These different regions are ‘tied’ together by polymer chains. It is when the load is applied that the crystalline regions begin to slide past one another. Eventually, if enough time is allowed and the load isn’t too great to break the material, the chains of crystalline regions become aligned with the direction of load. If the load is released sufficiently before these regions begin to entangle, and cause failure, they can slide past each other, returning to their original form. Whilst the amorphous regions that were stretched, allowing the crystalline regions to align, can also reform to their unstrecthed state. This explains what happens but why it happened like this is due to the difference between the glass transition temperatures between the materials tested. Polypropylene has a Tg of -25⁰C to -20⁰C, which means that throughout the whole process of testing the material, it is above the Tg by about 40⁰C, due to testing at room temperature(20⁰C). When a polymer is above or at their Tg the uncoiling of the chains can occur allowing the crystalline regions to align and eventually they are easily able to reform the randomly coiled form they naturally exsist in. Where as the GP polystyrene which has a Tg of 85⁰C to 125⁰C, which means that the temperature at which the testing is done is well below this range. This means that there isnt the recommened amount of energy for the uncoiling to be done succesfully, and the crystalline regions to slip past each other and then reform the original shape. The amorphous regions are hard and ‘glass’ like, hence why the crystalline regions find it difficult to move, as their once rubbery amorphous regions, which were used in the uncoiling process, are unable to move well as they are stiff. This we can determine from the graph and data as they show for GP polystyrene that the amount of deflection that occurs slowly increases, whilst load is on and slowly decreases returning the crystalline regions to their random state from the ordered state when the load is removed.

Flexural creep modulus is a measure of stress divided by strain. It shows that over time the ratio between stress:strain, shows the tendancy for the material to bend. Whilst the load is on, it causes deflection which decreases quite rapidly after 5 seconds. After that the rate begins to steady, whilst still decreasing for the next 25 seconds of load time. There is a constant decrease in the tendancy for the

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material to keep bending, due to the ratio between stress and strain increasing, as the stress is constant but true strain increases throughout. The true strain will continue to increase because the material is constanly being put underpressure, as the load drags it downwards. The use of the materials tested were for the correction of mis-placed teeth. Therefore the material that the orthodontic appliance is made out of must not deform with time. Which is why polypropylene would be a poor material to use, as not only does it deform with load but due to the Tg being close to that of the operating temperature the amorphous regions are more able to help the alignment of the crystalline regions which causes creep. Because of this the material will deform over time, plus as we have seen in from the experimental data, when polypropylene is deformed it takes a long time to reform back to the original shape if ever (at the end of the experimental time polypropylene had not reformed completely, showing possible plastic deformation). Which is why, polystyrene is a more suitable material because when load is placed on the material the material does indeed accomodate the load through deflection, but when the load is taken off it reverts to its original shape. Therefore this material is more acceptable as it is showing more elastic deformation, which is exactly what is needed for the appliance it will be used for.

Extended Analysis:Task 1.

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The graph, which follows on the next page, demonstrates the fall and rise of the dashpots due to the spring action, caused by the elasticity of the polypropylene. The spring, when forced to elongate due to the gravitational force applied with the 0.3kg weight to the polypropylene forcing it to deflect downwards, also forces the dashpot to elongate in length (from the bottom of the measured line in the dashpot to the bottom of the dashpot). Creep is allowed to occur as the temperature of the experiment is 40⁰C above its Tg, therefore the amorphous regions are rubbery and can stretch the crystalline lamellae forcing them to align, in the direction of the force. It is the covalent bonds of the monomer chains that are being stretched at this point. As multiple monomer molecules make up a polymer (hence why it is known as a macromolecule), these chains are what intertwine through the crystalline and amorphous regions, attaching them together. When the spring is allowed to recover, as the load is removed, it recoils upwards bringing the dashpot with it, demonstrating creep recovery as the material returns to its original form. This recovery occurs because the crystalline regions don’t like being lined up, they prefer to be in a randomly coiled state. Therefore that is why it takes energy (the load) to stretch them, but when the load is removed there is nothing to hold them in place, and they will recoil once again.

I used the Voigt element graph to demonstrate these changes to the materials curvature, as the spring is needed to be connected to the dashpot to show creep recovery, as a dashpot on its own can’t show recovery. Polypropylene does to a certain extent show creep recovery, but not fully, therefore suggesting an element of plastic deformation occurred.

(Graph and Voigt graph with dashpot and spring components follows on next page, explaining worded findings).

Task 2. Suitability for the orthodontic application. GP polystyrene would be a more effective and appropriate material of the two tested. Due to the length at which GP polystyrene stays when placed under weighted load conditions, causing deflection on the

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material. The deflection that occurs is minimal compared to that of polypropylene. When the load is released, almost instantly the material GP polystyrene returns to its natural randomly ordered semi-crystalline state. This shows that elastic deformation has occurred, and that the yield strength was not exceeded as permanent deformation has not occurred. Whereas polypropylene still hasn’t returned to its natural state by the time given, after the load was released. Therefore this material isn’t suitable as when load is applied, and this will happen in the real situation in the oral environment, the material takes too long if ever to return to the normal structure. Plus the material itself deforms more than GP polystyrene did. The material should be allowed to deform when put under load in the maths to accommodate pressure, but should revert to its constructed shape so that the correctional process of the teeth continues.

Issue regarding boisterous children. The constant harsh load placed on the appliance may result in its failure. GP polystyrene because of its high glass transition range, means that at lower temperatures, such as 36⁰C - average oral temperature - it is brittle due to the lack of fluidity of the amorphous regions. This means that the crystalline regions are less able to move around and align, allowing deflection and deformation to occur whilst in a solid state. Because of this the material is brittle and when excessive force is applied to the appliance it could break. If this excessive force is constantly applied to the material it could result in plastic deformation occurring, as the crystalline regions are unable to untangle and align when a high strain rate is applied, the material will snap.

As an alternative material to polystyrene.I would suggest that 6, 6-nylon and Polyethylene terephthalate were two possible materials in contention for being better due to Tg’s closer to that of the temperature used in the experiment and the temperature of the oral environment. The Tg may still be above that of the operating temperature but at least it is closer and therefore it will be less brittle than polystyrene, due to more ‘rubbery’ amorphous regions.

Temperature being a crucial factor in the testing done. I would increase the temperature closer to that of the oral environment, which is about 36⁰C although it is variable. When this increase of temperature was used in the same method of testing that has been done, I would expect the results to show a greater amount of deflection that results gained in the previous test. This is because when the temperature is increased the amout of energy in the material increases meaning that the amorphous regions are able to become more rubbery and therefore the crystalline lamellae and spherulties in the crysalline regions are able to slide more fluidly passed each other to become ordered.

Bibliography:

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Donald R. Askeland, Pradeep P. Fulay, Wendelin J. Wright (2011). The science and engineering of materials. Stamford: Cengage Learning.

John F. McCabe, Angus Walls (2008) Applied Dental Materials. Oxford: Blackwell Publishers.

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