LABORATORY 2.1: TENSILE TEST
G ROUP ONE
D AVID B ROWNE 851594
OBJECTIVES The objective of this experiment is to investigate the behavior of two material specimens under a Tensile Test. The materials to be investigated are Copper and Steel. From performing the Tensile Test the following properties will be determined; young’s modulus, yield stress, ultimate tensile stress, percentage elongation at fracture, percentage reduction in cross-sectional area at fracture and fracture stress. This experiment is used to determine a material’s properties, and is used in a wide range of industries. One example of this could be to determine the Ultimate Tensile Stress of a material to be used for a shopping bag, to check it can hold enough weight.
EQUIPMENT AND METHOD The two specimens will take the form of a threaded rod with a length of 81.42mm and a diameter of 6.03mm. The test will be performed by placing the two specimens under a “Lloyd test machine”. The machine will provide a threaded attachment to connect the specimens. The machine will exert a tensile force on the specimen causing it to extend. The force exerted to create each increment of extension is displayed on the machine along with the total extension. For this test the force exerted for every 0.5mm increment of extension will be recorded.
RESULTS The Stress–Strain relationships of the two specimens are shown in fig.2. The values for the ultimate tensile stress for the Steel and Copper sample are 536MPa and 445MPa respectively. The Yield Stress for copper is not clearly represented by the graph as it shows the material yielding gradually, but it could be estimated to be at 200MPa by using a 0.1% proof stress. The Yield Stress for steel occurs at 500MPa. The Elastic Modulus for the Steel and Copper samples as calculated from using the Yield Stresses stated previously are both 14.5GPa. Fig.3 shows that after the experiment the Steel sample had elongated 3.5% and the cross sectional area at the point of fracture had decreased by 53.1%. The Copper elongated 6.5%, and the cross sectional area at the point of fracture decreased by 25.9% Fig.3: Dimensions of the sample before and after the test
DISCUSSON AND CONCLUSION
Material
Steel
Copper
The Stress-Strain graph shows that the Copper sample experienced more plastic deformation that the Steel sample, and this is reflected by the higher perentage elongation (fig.3).
Origional Length (mm)
200
200
Length after fracture (mm)
207
213
3.5
6.5
Origional Cross Sectional Area (mm^2)
28.6
28.6
After it had fractured, the surface of the Copper was rough and irregular. The 2 halfs of the fractured sample showed a “cup” and a “cone” shape with an inclination of approximately 45° on their fracture surfaces. In a uniaxial tensile test, this orientation represents the angle of principle shear stress and the surface demonstrates this principle Shear Stress caused the crystalline boundries to slip over each other before failure (3).
Cross Sectional Area after fracture (mm^2)
13.4
21.2
Perecentage Reduction in Area (%)
53.1
25.9
Percentage Elongation (%)
Fig.4: A diagram showing a cross section of the fractured copper sample
Both of these obeservations are characteristics of Ductile materials, which is a commonly stated property of Copper. The Copper sample also displayed a higher Toughness than the sample, which is represented by the larger area beneath the stress strain graph. Despite being smaller than the Copper sample, the plastic region of the Steel sample is significantly large enough to be considered to have some ductile properties. The surface of the fracture also possesed a cup and cone geometry at a lesser extent than the copper sample.
Fig.5: A diagram showing the necking on the a)Steel sample and b)Copper sample
(A)
(B)
he steel sample had a larger necking region than the Copper sample, which explains the greater reduction in cross sectional area at the point of fracture, but as shown in fig.5, the Steel sample showed a very rapid transition between the decreased area and the rest of its length, whereas the Copper showed a gradual transition. Necking is a property of a ductile material. Referring to Engineering Materials (2) the Yield Stress’s for Copper is 60MPa, compared to the 200MPa value that was obtained experimentally. The difference between these results suggest that; a) The yield stress for copper that was predicted using a proof stress may of given an inaccurate answer that is higher than the real value b) The stress-stain results that were read from the machine were inacurate. One innacuracy is that the experiment used the ‘nominal stress’ of the sample rather than the ‘true stress’. However, the difference between the two are very small, particuarly in the elastic region of the test, and could not cause such a large difference between the experimental and theoretial value of yield stress. This would mean the difference is more likely to be caused by (a) and that very little confidence can be placed on determining the yield stress with one run of an experiment and by detemining the yield stress using the graph. The experimental value for the Yield Stress of Steel is within the theoretical range of value which is between 260MPa and 1300MPa (2). Because this value was more clearly defined on the graph than it was for copper and it was not derived using a proof stress, it would be expected to be more accurate and could have a high confidence placed on it. The values for the Modulus of Elasticity obtained experimentally are around one order of magnitude smaller than values stated in Engineering Materials 1 (1) which quotes it to be 200GPa for mild Steel and 124GPa for Copper. Determining the Modulus of a material using a uni-axial tensile Stress experiment is generally regarded as being inaccurate and is instead commonly determined by measuring the natural frequency of a sample using an oscillation test (1). The reasons for this are; Recording small displacements of the sample is imprecise due to the measuring equipment (1). Factors such as creep can contribute to the strain (1). When exerting large forces the equipment can begin to flex, and the displacement of the machine is mistakenly read as a displacement of the sample. The ultimate tensile stresses recorded are very close to the theoretical values, which are 400Mpa and 500-1880MPa for copper and Carbon Steel Alloy (1), The difference between the experimental and theoretical values for the Modulus suggests that in this case, very little confidence could be made with the results. In conclusion, copper can be regarded as a more Ductile material than steel with a higher Toughness, and Steel can be considered to have a higher Yield and Tensile Strength with an equal elastic Modulus.
REFERENCES 1) Ashby, M. (2006). Engineering Materials 1: An Introduction to Properties, Applications and Design. 3rd ed. Butterworth-Heinemann 2) Hibbeler, R.C. (2004). Statics and Mechanics of Materials. Prentice Hall. 3) Tarr, M. (no date). Stress and its effect on Materials [online]. Available from http://www.ami.ac.uk/courses/topics/0124_seom/index.html. [Accessed 26/04/09].
