Tensile Testing of Materials

Lab script

Tensile testing is an important means of discovering some of the mechanical properties of given materials. With this information engineers are able to select the most effective materials for any given use. For the purposes of this experiment the materials that we tested were all metals, furthermore, the metals we tested were; Mild Steel, Brass and Aluminium.

The three metallic strips were first measured using a ruler for the length and a micrometer-screw-gage for both the width and thickness of the strips. We realised that although thickness and width varied a little, all three strips were 50mm long. Using this information we were able to outline five segments with a length of 10mm so that after the bar had been extended and fractured it would be possible to recover the extension by accumulating the extension of each segment. Our next step was to work out the cross sectional area of each strip of metal, an easy task as both the thickness and width had already been measured.

Following this, the strips were placed into a Hounsfield machine; this boasted of a mercury reader that we could use to see how much force we were applying to the metallic strips. In addition we used a Lindley to measure the extension that was taking place as the force was increased. By taking note of the Lindley’s readings at regular and appropriate Force intervals and taking into account all original dimensions of the strips it was possible to draw up a Stress Vs Strain graph for each of our metals.

The above process was carried out until all materials were extended to roughly 200x10-6m. Then the Lindley was removed and the metal strip (still kept in tension by the Hounsfield machine) stretched to failure. Once this is done, the new length and therefore extension can be calculated, as well as the length, the new width and thickness were measured too. Using all this information it was now quite easy to work out the percentage elongation and the percentage reduction in area.

Results

Using the data collected, we could calculate the stress and strain of each material tested. The equation we used is as follows:

Equation:

(it is important to note that Strain has no

units and they cancel out

as you divide them)

Example: Here is how I calculated the first stress and strain values for Figure 1

Force=0.4kN Length= 50mm

Original Area= 19.03mm² Extension= 6µm

Stress= (0.4E-3)÷(14.49E-6) = 27.6

Strain= (6E-6)÷(50E-3) = 0.12E-3

Using the new calculated values, we drew up Stress Vs Strain graphs that we could use to easily compare the tensile properties of the three. Other values other than Stress and Strain could also be obtained, like measuring the gradient of the graphs for example, to obtain the elasticity modulus.

Graphs and Tables:

Mild Steel

(1)

Original Length (mm) 50Final Length (mm) 66Original Area (mm2) 19.03 Final Area (mm2)  14.49Percentage elongation 32%Percentage reduction in area 23.86%

Table: 1.0.

Figure: 1; A Stress Vs Strain graph to show how both stress and strain change as the force applied to Mild Steel strip is varied.

As I mentioned earlier, by working out the gradient I can work out the Modulus of Elasticity, however this is not all. By finding the maximum point on the graph I can distinguish the Tensile strength. I could also find the Upper and lower Yield stresses, however this graph has not been drawn to enough accuracy as I cannot find any local maxima.

Modulus Of Elasticity (GPa) 161.54Upper Yield Stress (MPa) 210Lower Yield Stress (MPa)Tensile Strength (MPa) 273.25Brass

Original Length (mm) 50Final Length (mm) 66

MILD STEEL

0

50

100

150

200

250

300

0 500 1000 1500 2000 2500 3000 3500

STRAIN [10-6]

STRE

SS [M

Pa]

MILD STEEL

Table: 1.1.

Original Area (mm2) 19.03 Final Area (mm2)  14.49Percentage elongation 32%

Percentage reduction in area23.86

%Table: 2.0.

Figure: 2; A Stress Vs Strain graph to show how both stress and strain change as the force applied to Brass strip is varied.

Once again we are able to work out the Modulus of Elasticity by Dividing the change in Stress by the Change in Strain i.e. the gradient. However this time there is no upper or lower yield stress as that is a unique property steel has. I am though, enabled to notate the Limit of Proportionality, which is the point where the stress and the strain are no longer directly proportional, hence where the line begins to curve.

Aluminium

Original Length (mm) 50Final Length (mm) 54

BRASS

0

50

100

150

200

250

0 1000 2000 3000 4000

STRAIN [10-6]

STRE

SS [M

Pa]

BRASS

Table: 2.1.

Modulus of Elasticity (GPa) 100Limit of Proportionality (MPa)

Tensile Strength (MPa)234.7

4

Original Area (mm2) 24.81 Final Area (mm2)  23.33Percentage elongation 10Percentage reduction in area 5.97

Figure: 3.0.

Figure: 3; A Stress Vs Strain graph to show how both stress and strain change as the force applied to Aluminium strip is varied.

As before The Modulus of Elasticity, Limit of proportionality and Tensile Strength can be worked out.

ALUMINIUM

0

20

40

60

80

100

120

0 500 1000 1500 2000 2500 3000

STRAIN [10-6]

STRE

SS [M

Pa]

ALUMINIUM

Modulus of Elasticity (GPa) 52Limit of Proportionality (MPa)Tensile Strength (MPa) 96.7

4Table: 3.1.

Tables of Results for all three metals.

MILD STEEL

LOAD [KN] STRESS [Mpa]EXTENSION [10-

6m] STRAIN [10-6m]0 0 0 0

0.4 21.02 6 1200.8 42.04 12 2401.2 63.06 18 3601.6 84.06 25 500

2 105.1 31 6202.4 126.12 37 7402.8 147.14 43 8603.2 168.16 51 10203.6 189.42 58 1160

4 210.19 65 13004.4 231.21 74 14804.8 252.23 92 18405.2 273.25 152 3040

Figure: 1.2.

ALUMINIUM

LOAD [KN] STRESS [Mpa] EXTENSION [10-6m] STRAIN [10-6m]0 0 0 0

0.2 8.06 3 600.4 16.12 11 2200.6 24.18 16 3200.8 32.25 23 4601 40.31 30 600

1.2 48.37 39 7801.4 56.43 46 9201.6 64.19 57 11401.8 72.55 70 14002 80.61 85 1700

2.2 88.61 106 21202.4 96.74 135 2700

Figure: 2.2.

Table: 1.1.Table: 1.1.Table: 1.1.

BRASS

LOAD [KN] STRESS [Mpa]EXTENSION [10-

6m] STRAIN [10-6m]0 0 0 0

0.3 15.65 4 800.6 31.3 14 2800.9 46.95 21.5 4401.2 62.6 30 6001.5 78.25 37.5 7501.8 93.9 45.5 9102.1 109.55 53.5 10702.4 125.2 61.5 12302.7 140.85 70 1400

3 156.5 80 16003.3 172.14 89.9 17883.6 187.8 102 20403.9 203.44 119 23804.2 219.09 145 29004.5 234.74 189 3780

Figure: 3.2.

Comparison

Figure: 4; A Stress Vs Strain graph representing three different strips of metal in a tensile test to compare the similarities and differences between each material.

It is immediately obvious to the eye that all three graphs follow the same pattern, that is, a straight-line start that later on turns into a curved finish, they all pass through the origin and all experience a decrease in gradient relative to its start. On the other hand, there are differences, although some more subtle than others. It is clear in Figure 4 that the straight-line gradient is the largest for Mild Steel and so, even without knowing the values it is easy to deduce that Mild Steel will have the largest Modulus of Elasticity and Aluminium the smallest. We can also appreciate that Mild steel has the highest tensile strength as its graph reaches the highest Stress point. The Area under the graph is equal to the energy needed to deform the material, and so we can logically see that it harder to deform the Steel while Aluminium is the easiest to deform.

0

50

100

150

200

250

300

0 500 1000 1500 2000 2500 3000 3500 4000

STRAIN [10-6]

STRE

SS [M

Pa]

ALUMINIUM

BRASS

MILD STEEL

Further Comparison

Having the Highest tensile strength meant that Mild Steel required more stress applied than Aluminium and Brass to fracture it as its value is about 162MPa which is about 62MPa above the Brass, the material that was the closest to its values out of the two. The tensile strength of brass was then followed by Aluminium at around 52MPa making it the weakest out of our three materials. We can now also work out that the tensile strength of Mild Steel is about three times that of Aluminium and about 1.6 times that of Brass.

Conclusion

On the surface, I believe that our results are quite satisfactory; however I know they are not. The Hounsfield is a manual tool and was hard to operate smoothly at times. Its because of this inconsistent force I believe a few errors may have occurred as only with constant force can you measure the true fracture point. Knowing this, we decided to test one of our materials on a modern strain machine, more precisely Mild steel.

This machine was able to constantly increase the strain, but not only this, it was also able to record data more accurately and eliminated a lot of human error also. A quick comparison with our first Mild steel graph shows the scale of error involved, e.g. here the modulus is 220MPa where as our results were around the 162MPa mark. I would also like to point out how defined the upper and lower yield stress’ are compared to

Figure: 5

our original values. I put this down to the fact that the spacing between our results was simply to large, results would need to be taken down a lot more often than every 0.4kN to achieve a graph like this. This machinery is very expensive, on the other hand, and so not the most economical way to test the tensile strength of three metals.

As I brought up in the previous page, Human error comes into play quite a lot. I believe this also had something to do with the deviation of our results. For example, our mercury force reader has large surface tension and so created a dome-like end, this meant that it was hard to get the exact readings for force. Also as there were two of us needed to record the results, as one person notified the other, reaction times came into play and obscured the results further. Unfortunately Human error is not quantifiable and cannot be accounted for; hence if something does go wrong it is very hard to tell.

Having said all this, I am still quite confident that my results are not altered enough to change the order of strength, and so using this information I can conclude that If I were to build a bridge out of any of the three materials tested, it would without doubt be the Steel.