Tensile Strength and Hardness Testing

Introduction

In this lab, we explored the relationship between Tensile Strength and Hardness of metals. There are three types of tensile strength: Ultimate, Yield and Breaking strength. Ultimate strength is the maximum stress the material can withstand, Yield strength is the amount of stress the material can withstand without permanently altering the main shape of the material and breaking stress is the point where a material cannot withstand any more stress and would break. Tensile testing is one of the most widely used mechanical tests. By measuring the elongation that a force cause on certain materials can help designers to understand how materials and products would behave in their intended use. The benefits to this test are that it provides data on the properties of materials and products to ensure they are manufactured to the highest quality and fit for purpose. Hardness is the resistance of material to deformation cause by an indentation, sometimes this can also be used to describe the resistance against becoming scratched or marked. The hardness test compared to other types of material tests is simple and quick to complete and is nearly completely non-destructive. Depending on the force being applied and the indentation shape, hardness can be defined as macro-, micro- or Nano-hardness. Macro-hardness tests, Vickers, are the most used methods worldwide due to the ability the carry them out fast and precisely. The main aim of the lab was to see if there are any correlations between the same materials when used in two separate tests. Since both experiments tested different sections of the materials overall strength then it will be easy to see if the agree or disagree with each other. For both experiments we used the same set of metals: 0.1% carbon steel as drawn, 0.4% carbon steel as drawn, 0.55% carbon steel as drawn and 60/40 brass as drawn. We hypothesized that as the tensile strength of a material increased then the hardness would increase too.

Experimental Procedures

Tensile Testing

To measure the tensile strength of the metals chosen we used the Zwick/Roell Z050 (50KN) Tensile Tester with computer control micrometer. There was no preload set and the machine moves at a speed of 5mm/min. A metal dumbbell specimen was measured for its initial length and diameter before the specimen was mounted into the chuck grips of the machine. The load was reset to zero along with the extension scales, the test was then started. The load was observed as well as the displacement curve. When the specimen broke both parts of the specimen were removed from the machine as again the length and diameter of the dumbbell was measured again. This allowed the initial and final cross-sectional area to be calculated and compared to find the percentage reduction. Both halves were then placed into the extension gauge to measure the extension the specimen had underwent. The rest of the results were then read off the computer, these were: Young’s modulus, force at break, maximum force, word done and elongation at break. With all these results recorded the tensile strength, true stress at break, strain at break and elongation at break can be calculated. This process was then repeated for the rest of the samples of metals.

Vickers Pyramid Hardness Test

To measure Hardness a Wilson VH1202 Micro hardness tester was used. For the hardness test a work load has to be selected depending on which material is being used, the softer materials for example; 60/40 brass should be set at a low test load of 0.2kg while the other materials, carbon steels, should have a test load of around 1kg. The Metal specimens were cut from standard dumbbell and encased inside a resin; these were then polished to create a flat smooth surface to ensure for the best possible results. Once the specimen is placed under the turret the platform was adjusted to an appropriate height to start. The platform was now adjusted slightly so the image displayed on the eye piece was in sharp focus at 50X magnification and then a zero calibration was performed by moving the base filar line and the filar line together so that they are touching but without overlapping then it is reset. The standard dwell time for the Vickers hardness test 10-15 seconds, for the test the dwell time was set to 10 seconds to save time as there was a time restraint on the experiment. After everything is set the test can be ran, press the indent icon and the hardness tester will automatically go through the testing cycle, loading, dwelling and unloading. When the test in finished the filar lines are used to measure the lengths of the indentation, the hardness tester then will calculate hardness in the scale selected.

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Results and Findings

Metal Tensile Test

The results from the Tensile test provided data which can be used to calculate other useful engineering data by a series of different calculations. All results from tensile test can be found in Table 1. The tensile strength of each of the metals was calculated by: $\mathit{Tensile\; Strength}=\raisebox{1ex}{$F$}\!\left/ \!\raisebox{-1ex}{$A_i$}\right.$

Where F is the maximum force in Newtons and Ai is initial cross-sectional area of the specimen.

The strain at break of each of the metals can be calculated with the results given through the tensile test, the strain, ε, is calculated by:

$\epsilon =\raisebox{1ex}{$∆l$}\!\left/ \!\raisebox{-1ex}{$l_0$}\right.$

Where Δl is the relative change in length of the specimen and l₀ is the initial length. (See Appendix A for sample calculations)

The results from table 1 allowed a Force against Extension graph to be plotted, this shows the force needed to break the metal dumbbell also the extension at break. This is shown in Fig.1.

Table 1 – Tensile Test Full Set of Results

Figure 1 – Graph of Force against Extension

Red line = 0.1% Carbon Steel as drawn

Green line = 0.4% Carbon steel as drawn

Blue line = 0.55% Carbon Steel as drawn

Orange line = 1% Carbon Steel Annealed

Purple line = Aluminium Alloy

Turquoise line = 60/40 Brass

Vickers Hardness Test

The Vickers Hardness number can also be calculated theoretically by using the following:

$\mathit{HV}=1.854\times \raisebox{1ex}{$F$}\!\left/ \!\raisebox{-1ex}{$D^2$}\right.$

F – Applied Load (Kg)

D – Length of indentation diagonal (mm)

(See appendix A for sample calculations)

The results from the Hardness test can be found in table 2, this shows the material, test load, two occular no. readings and the average VHN.

These results were then used to plot a bar chart so that comparison between each metal specimen can occur easily. This can be found in Fig.2

Table 2 – Hardness Test Results

Figure 2 – Hardness Test Bar chart

Discussion of Results

The tensile test results can tell us a lot about the tested materials properties and how useful it would be in certain products. From Fig.1 it is very clear to see that as carbon content increases in steel the more force that is required to break the specimen, this results in a higher tensile strength. This is backed up by table 1 which shows that the carbon content clearly strengthens the steel specimen. However, adding carbon to steel strengthens and toughens steel to a point, at this point adding more carbon will strengthen the steel but will reduce toughness. The way carbon strengthens the steel is by distorting its crystalline lattice. The effectiveness of adding carbon to metals depends on the lattice spacing, crystal structure and possible chemical effects between the metal and the carbon. If we were to look at fig.1 we see for every material it reaches its maximum force, after this point the force decreases. “This phenomenon is termed necking, and fracture ultimately occurs at the neck. The fracture strength corresponds to the stress at fracture” (Callister, 2006). If we compare the neck sizes of all the metals, we see that all the carbon steels have a similar sized neck, this is due to the overall extension of the is small compared to say the aluminium alloy.

As previously mentioned, it is clear to see carbon content increases the strength of steel, with this being the case it would be expected that the 1% carbon steel would clearly be the strongest. The tensile strength of the 1% carbon steel is 810.26Mpa while the 0.55% carbon steel is 1097.70Mpa. To understand why this doesn’t follow the standard trend we have to investigate the manufacturing process of each of the metals. The 0.1, 0.4, 0.55% carbon steels are all as drawn. However the 1% carbon steel is annealed, this is a completely different manufacturing process which in turn alters the physical properties of the material. “The term annealing refers to a heat treatment in which a material is exposed to an elevated temperature for an extended time period and then slowly cooled. Ordinarily, annealing is carried out to (1) relieve stresses; (2) increase softness, ductility, and toughness; and/or (3) produce a specific” (Callister, 2010). This process is most commonly used to soften metals to allow them to be used in cold working procedures and to increase its overall machinability. Annealing also brings back ductility. During cold working, a metal can become hardened to a point where anymore work has a high risk of cracking. By annealing the metal before these cold working processes can occur without/minimal risk of cracking. This differs from as drawn metals as they’re un-heat-treated therefore physical properties follow the expected trends.

From both fig.1 and 2 we can see that the hardness of a material follows the same trend as tensile strength. This is due to many of the same reasons as tensile strength. If we compare the results for the 60/40 brass and the aluminium alloy in both the hardness and tensile tests we learn about they’re properties and why they are used in specific products. Brass is an alloy containing copper and zinc, where varying the proportions of each creates a range of brasses with different properties. From fig.2 we see brass has a VHN of 123.9 this compared to the 0.4% carbon steel which has a VHN of 281 shows how brass is considerably less hard then he steels which is expected, because of this brass is ideal to be used in industry for decorative and mechanical purposes.

Figure 3 – Properties of 60/40 Brass (RCI LTD)

From fig.3 we can compare our results to their properties for 60/40 brass, our tensile strength for the brass alloy was 538.50. If we study the table we can see that it’s most likely a He brass as the number is closest. This is then partially contradicted by our hardness number reading. Our hardness reading was 123.9 and when compared to the table this shows a greater difference in values. This is highly likely to be because of human error on our side. This alloy is known to have high machinability also being able to retain high strength after formation therefore applications for brass include door locks, keys and hinges for doors.

Conclusion

From both the tests that were done we can clearly see certain trends and we also have learned a lot more about the physical properties of the materials tested. As expected the carbon steels have the highest tensile strength and Vickers hardness number, while brass and the aluminium alloy have considerably less values. This proves our hypothesis to be correct. If we were to complete these experiments there a few things we would do differently to make the reliability and accuracy of our results to be greater. For the hardness test the machine that we were using had previously been damaged by another student which meant finding the indentation in the metal took time and it could be easy to mistake a previous indentation for the on you just created. Also with doing majority of tests human error plays a huge part in accuracy of results. From focussing the microscope to adjusting the filar lines to measure the indentation these are all things that could change from each run through of the tests. However to be able to fix these problems we encountered the tests would have to be run on a new fixed machine, this is however a very expensive fix to the problem. Completing these tests help engineers and companies to learn about the material properties and from that they will be able to asses if it would be suitable for their selected application.

References

• Callister, William D., (2006) pg. 165 Materials Science and Engineering, 7^th Edition. Dawson Publishing, United Kingdom
• Callister, William D., (2010) pg. 422 Materials Science and Engineering, 8^th Edition. John Wiley and Sons, United Kingdom

Websites

• https://www.rciind.com/portfolio-item/leaded-brass-60-40-cu-zn/

Appendix A

Example calculations for 0.55% carbon steel

Ts = F/A_i

= 21900/ 19.95

= 1097.70 Mpa

$\epsilon =\raisebox{1ex}{$∆l$}\!\left/ \!\raisebox{-1ex}{$l_0$}\right.$

= 3.97/25.25

= 0.15

HV= 1.854 x F/D²

= 1.854 x 1/1.69²

= 303.8