Fig.4: A diagram showing a cross section of the

fractured copper sample

LABORATORY 2.1: TENSILE TEST GROUP ONE DAVID BROWNE 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%

DISCUSSON AND CONCLUSION

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).

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).

Fig.3: Dimensions of the sample before and after the test

Material Steel Copper

Origional Length (mm) 200 200

Length after fracture (mm) 207 213

Percentage Elongation (%) 3.5 6.5

Origional Cross Sectional Area (mm^2) 28.6 28.6

Cross Sectional Area after fracture (mm^2) 13.4 21.2

Perecentage Reduction in Area (%) 53.1 25.9

Fig.5: A diagram showing the necking on the a)Steel sample

and b)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.

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].

(A)

(B)