Robbie Powell i (Youngs Modulus Lab Report) Page 1

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1 Abstract In this report I explain a recent experiment aimed at finding out the Youngs Modulus, Ultimate tensile strength and elastic limit, total elongation and total reduction section in cross section of two materials: - Steel and Aluminium. I discuss the atom structure of materials as well as the cause of the yield point phenomenon seen in steel. Finally I calculate values for the various properties discuss the results and explore applications for the different properties as well as the materials. Robbie Powell i (Youngs Modulus Lab Report) Page 1

2 Introduction: All products undergo stress and strain caused by forces on an object, whether big or small they must always be accounted for to avoid failure in a given design. In most cases, materials will have undergone rigorous testing in order to confirm whether they are fit for function. One such experiment aims to test the young s modulus of a given material. Young s modulus is a measure of the ability of a material to withstand changes in length when under lengthwise tension or compression (The Editors of Encyclopædia Britannica) Figure 1: Stress/Strain Equation A Young s modulus experiment is where a material s tensile strength is tested by being slowly stretched to find the elastic limit and finally see the breaking point. This information is then used later in accordance with other factors such as a cost and aesthetics to see if it suits the purpose. Apparatus and procedure: Equipment used - Hounsfield H20K-W* Two different materials underwent this experiment - 0.1% Carbon Steel normalised at 900 C - Pure Aluminium Diameter: 5mm Length: 27mm Test Speed: 5mm/min Experiments undertaken on 05/01/2015 Both materials were individually tested by clamping the test pieces into the machine* and beginning the stretching process. The machine* would slowly pull the test pieces apart until it found breaking point and then stop. Figure 2: Hounsfield Picture One Figure 3: Hounsfield Picture Two Robbie Powell i (Youngs Modulus Lab Report) Page 2

3 Results The experiment was only undertaken once so the results cannot be checked for precision. Measurements of materials after test: Steel - Cross section of break: 2.79mm - Longitudinal Extension: 10.88mm Aluminium - Cross section of break: 1.26mm - Longitudinal Extension: 11.23mm Figure 4: Steel Graph Figure 5: Steel Results Figure 6: Aluminium Graph Figure 7: Aluminium Results Robbie Powell i (Youngs Modulus Lab Report) Page 3

4 Figure 8: Steel Material Figure 9: Aluminium Material Results Analysis Figure 10: Load extension Graph Load: The force applied to a material during testing Yield point: Beyond the yield point, plastic deformation occurs. Ultimate tensile strength: This is the maximum load/force the specimen withstood. Failure (Breaking Point): This is the point where the specimen failed/fractured. Elastic deformation: Within this blue region the material will return to its original shape if the load is removed. Plastic deformation: Within the red region the material will show a permanent change if the load is removed. (State of New South Wales, Department of Education and Training 2009) Robbie Powell i (Youngs Modulus Lab Report) Page 4

5 Sress Stress Steel Strain Stress Figure 11: Steel Stress/Strain Graph Aluminium Strain stress Figure 12: Aluminium Stress/Strain Graph Results for steel Ultimate tensile strength (Maximum force/original Area) 7350(N)/19.63x10^-6(mm) = (MPa) Elastic Limit (Can be seen on graph) 6600 (N) Elastic Modulus (Using stress/strain graphs gradients Stress/Strain) ( )/(( x10^-6)-( x10^-6) = GPa Total Elongation (Extension/Original length x 100) 10.88(mm)/27(mm)x100=40.30% Total reduction in cross section (100-(Cross section at break/original cross section x100) 100-(2.79(mm)/5(mm) x100)=44.2% Robbie Powell i (Youngs Modulus Lab Report) Page 5

6 Results for Aluminium Ultimate tensile strength (Maximum force/original Area) 1440(N)/19.63x10^-6(mm) = (MPa) Elastic Limit (Can be seen on graph) 580 (N) Elastic Modulus (Using stress/strain graphs Stress/Strain) ( /( x10^-6)-( x10^-6) = 5.23 GPa Total Elongation (Extension/Original length x 100) 11.23(mm)/27(mm)x100=41.59% Total reduction in cross section (100-(Cross section at break/original cross section x100) 100-(1.26(mm)/5(mm)x100)=74.8% Material Ultimate Tensile Strength Elastic Limit Elastic Modulus Total Elongation Total Reduction in cross section Steel (MPa) 6600(N) (GPa) 40.30% 44.2% Aluminium (MPa) 580(N) 5.23 (GPa) 41.59% 74.8% Figure 13: Final Results Table Discussion As can be seen from figure 13 the two materials tested gave very different results. Steels Ultimate tensile strength and Elastic Modulus are both over 4 times of that of Aluminium s. This clearly suggests that Steel would be much better suited in load bearing scenarios than aluminium. These results amongst others aren t necessarily 100% accurate. Although the machine* used was calibrated previously, only one test was carried out for each material which could cause the precision of the results to be questioned. Other factors could include the technician s skill that carried out the test as well as fatigue and impurities in the material. Impurities can play a major part in testing and failure in a material and could refer to a few different things. Point defects are where an atom is missing or irregularly placed. Dislocations are where groups of atoms are arranged awkwardly weakening the structure. Figure 14: Dislocations Impurities weaken a material as gaps are left and bonds cannot connect atoms together. If there is an impurity in a certain section of a material it could cause that area to fail unexpectedly. As this test was only carried out once, the Robbie Powell i (Youngs Modulus Lab Report) Page 6

7 precision of results cannot be known. As can be seen from the information displayed on figure 13, Steel has a very high Elastic Modulus and Ultimate tensile strength making it very useful for high load structures such as bridges, vehicle frames and building supports. When looking at figure 11, we can see that after enough force is applied so that it passes the elastic limit, the stress drops and rises faltering as it goes. This is called the yield point phenomenon and is a result of strain aging as well as dislocations in the material. This is because as the metal is stretched before the elastic limit, the atom structure will not permanently change. However, after the elastic limit has been passed dislocations start to occur and while the molecules are rearranging; the stress drops. However once the bonds have attached once again, the stress will rise. The higher the dislocation density, the longer this will continue for until finally the graph should show a more fluid curve up to the Ultimate tensile strength. After this point is achieved, the cross sectional area will begin to decrease at a rapid rate, causing the stress to drop considerably until the breaking point. Compared to Steel, the aluminium can be seen to be weaker due to its low ultimate tensile strength and elastic modulus. As demonstrated in figure 12, the stress has a much gentle curve as the atoms are able to rearrange more effectively in the structure. This is why aluminium is more suited to less load bearing applications such as tin cans. The total elongation is roughly the same as steels however the reduction in cross section is significantly more, as is displayed in figure 13. This is ultimately due to the strength of the material in which materials with higher elastic modulus s can resist lateral stress more effectively. Aluminium, just like rubber (although an extreme example) will reduce in width as it s stretched, reducing the force and therefor the stress needed to pull it apart. Applications - Crane Wire An application in which the properties of this experiment would have needed to be measured could be a crane wire. A crane wire is made up of a huge amount of wires used together to create one - but for sake of this example, ill think of it as one large part. Cranes are capable of picking up extreme amounts of weight - often as much as 20 tons. The result of failure could be catastrophic so huge amounts of testing would have to be undergone before it could be cleared for use. Obviously a material with high, ultimate tensile strength, elastic modulus and elastic limit would be needed as the wire cannot be allowed to plastically deform and a factor of safety must be added in case the maximum limit is exceeded. For example of the maximum mass allowed is 20 tons ( kg) then we can work out the minimum elastic limit. Figure 15: Crane - Minimum elastic limit = x 9.81 = N This means that if any more than this weight is put on the wire then it will plastically deform. The ultimate tensile strength would have to be much higher than the elastic limit to avoid dramatic failure in the case of far too much weight being put on the wire. Robbie Powell i (Youngs Modulus Lab Report) Page 7

8 - Diving Board Another application would be a diving board. The most important properties for function in a diving board are the elastic modulus and the elastic limit, although ultimate tensile strength will ultimately control failure and fracture of a material. A diving board is an interesting one as the design of the board will also change the minimum properties required in the design. For instance, the Figure 16: Diving Board length of the board will effect the strain, and therefor the elastic modulus. Because of this we can presume the longer the board, the lower the resistance to bending it will hold. However, it still needs to hold its shape and be able to create an upwards force after bending so a middle ground must be found. In terms of the elastic limit, a high yielding material must be found as if to much force is applied to the board for it to pass this point, the board will not spring back. For this example let s presume the maximum mass allowed on the board for safety and function is 150kg. Let s also presume that a user will fall at a constant rate after jumping relative to gravity. - Minimum elastic limit = 150 x 9.81 = N Finally, the ultimate tensile strength would be important in terms of avoiding fracture so again; a factor of safety must be considered with this property. Conclusion In conclusion this experiment can be extremely important in testing materials so that dramatic failure is avoided once the product has been made. The two different materials tested both exhibited very different results in the tests showing their purpose for certain functions in industry, although steel proved to be the stronger material of the two with results showing as much as five times of that of aluminium. I ve found that dislocation densities can effect results from a material dramatically and must be tested for thoroughly. I ve also seen that the properties explored in this report generally rise together, although as shown in applications; particular properties of materials in given products will need to go against this theory in order to function. This would be achieved by a variety of different material altering techniques or by incorporating informed design of the product. References The Editors of Encyclopædia Britannica. Youngs Modulus. Available: Last accessed 17th February Figure 1: Anon. Stress/Strain Equation. Available: Last accessed 17th February Figure 2: Hounsfield Picture One Wood, Adam. Lab Report 1 and 2 - Photos. [Pictures] Bournemouth Robbie Powell i (Youngs Modulus Lab Report) Page 8

9 Figure 3: Hounsfield Picture Two Wood, Adam. Lab Report 1 and 2 - Photos. [Pictures] Bournemouth Figure 4: Steel Graph Bournemouth University. Lab Report Experiment (05/01/2015) Figure 5: Steel Results Bournemouth University. Lab Report Experiment (05/01/2015) Figure 6: Aluminium Graph Bournemouth University. Lab Report Experiment (05/01/2015) Figure 7: Aluminium Results Bournemouth University. Lab Report Experiment (05/01/2015) Figure 8: Wood, Adam. Lab Report 1 and 2 - Photos. [Pictures] Bournemouth Figure 9: Wood, Adam. Lab Report 1 and 2 - Photos. [Pictures] Bournemouth Figure 10: Load/extension Graph State of New South Wales, Department of Education and Training (2009). Load/extension Graph. Available: _it_mean/applets/highlighter_load_ext_text.htm. Last accessed 17th February State of New South Wales, Department of Education and Training 2009 Load/extension Graph labels. Available: _it_mean/applets/highlighter_load_ext_text.htm. Last accessed 17th February Figure 12: Aluminium Stress/Strain Graph Qi, Zang. Lab Report Graph. Bournemouth Figure 14: Spaceflight Editors. Dislocations. Available: 20.jpg. Last accessed 17th February Figure 15: Anon. Crane. Available: Last accessed 25 February Figure 16: SRS Smith. Diving Board. Available: 3_.jpg. SRS Smith. Last accessed 25 February Bibliography Robbie Powell i (Youngs Modulus Lab Report) Page 9