hardness & Impact

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EXPERIMENT: Hardness and Impact Toughness Correlation in Metals
Objectives:
To determine hardness, impact toughness and density of three specimen (4140 steel, Cold rolled steel, and brass)
To establish the possible correlation between physical and mechanical properties.
Theory
A number of tests exists in the field of material science, and one of the commonly used is the hardness test. Hardness is a compressive test whose outcome is influenced by the shape of the identer, and the applied effort. Rockwell and Brinnel tests are examples of hardness tests that can be carried out in the same way. Moreover, impact toughness test is also related to the two tests in the sense that designers use it to during application. Density is a physical property which influences impact toughness and hardness.
Procedure
The set up is arranged as shown below
Rockwell hardness test is done using a tester, five readings recorded, and the average calculated and recorded
Brinnel hardness tester is used on the opposite side, three readings recorded, and the average calculated and recorded

The dimensions are measured and recorded for each sample
Charpy impact impact test is done for each sample.
Results and Discussion
The necessary measurements and dimensions were measured and recorded as shown in the tables below:
Table 1: Measurements of density
Sample 4140 Steel Cold rolled Brass
Weight (g) 37.17 37.19 41.37
Length (inches) 2.0135 1.997 2.025
Width (inches) 0.

372 0.373 0.375
Thickness (inches) 0.375 0.375 0.376
The results here shows that brass had the largest length and cold rolled had the least length. Also, brass was the heaviest among the three, followed by cold rolled and then steel. The thickness and width of the three was relatively equal.
Table 2: Rockwell hardness measurements
Sample 4140 Steel Cold rolled Brass
Test 1 76.5 76.6 63.5
Test 2 76.3 79.4 63.5
Test 3 77.5 77.0 62.4
Test 4 77.7 76.7 63.1
Test 5 77.2 76.7 65.1
Average Test 77.04 77.28 63.52
Table 3: Brinnel hardness and Measurements
Sample 4140 Steel Cold rolled Brass
Test 1 0.113 0.105 0.150
Test 2 0.110 0.105 0.148
Test 3 0.111 0.105 0.149
Average Test 0.111 0.105 0.134
Impact Toughness 36 17.5 18
Density of each sample
Density=massvolumeThe results for each sample are as shown in the table below:
Sample Method for density (lengths converted to cm) Value of density
Steel 37.175.11302×0.94488×0.9595=8.019×1000=8019kg/m3
Cold Rowled 37.195.07238×0.94742×0.9595=8.065g/cm3
=8065kg/m3
Brass 41.375.1435×0.9595×0.955048.7773g/cm3
=8777kg/m3
The average readings for the Brinell and Rockwell hardness tests are as shown in tables 2 and 3 above. The impact toughness for the three samples are shown in table 3. Upon inspection, it was noticed that steel appeared to break in a ductile mode at normal temperatures, however, the cold rolled steel appeared to break in a brittle mode. On the other hand, brass could easily break in a ductile mode. From these observations, we notice that ductility or brittleness is determined by the temperature, while that of brass is greatly influenced by the characteristics of the component metals since it is an alloy

From the graphs above, the best correlation can be seen in the graph of density against the Brinnel test. The graph has a regression with positive gradient, besides having R2=1. The worst correlation is seen in density versus impact toughness. The two variables are negatively regressing as can be seen, besides having R2 =0.276 (a value less than 1).
Hardness tests were used in this experiment because they simple to carryout besides being cheaper as compared to the other possible tests. Also, the tests are neither destructive nor do they lead to the deformation to the specimen. The values obtained from this experiment may be slightly different from those that are available on the data from the manufacturers because it is not possible to get the exact values mathematically since various techniques are employed to arrive at the values on the manufacturer’s data. Also, the mechanical properties of the metals used as specimen are likely to influence the outcome.
Hardness tests are important in engineering practice since it is used in predicting plastic deformation of metals. The engineering ethics dictates that construction involving the use of metals be done with precision and exactness so that the quality standards are upheld.
Conclusion
From the results above, it is apparent that steel is harder than brass and cold rolled. Testing of metals before processing and use is imperative since, through this, the engineers are able to determine whether the specifications of the intended use are met or not. For metals, hardness is indeed the most crucial property hence it should be measured with certainty. The other properties like impact toughness and density also affect hardness and therefore, they should be done alongside hardness test. The two tests used here are the simplest, and therefore they should be learned perfectly.