Abstract

The main objective of conducting this experiment was to show that the properties of the materials differ from one material to another. This experiment was conducted using the MTS Insight Tensile Testing machine, which applies load to the material being tested and gives us numbers such as the amount of load being applied and the elongation, and by using these data we can then calculate the stress and strain of the material and obtain a stress-strain graph for each sample being tested and designate important points on the curve, such as the elastic region, yield point, ultimate strength, and the breaking point. We used the numerical values to calculate also the margin of errors in our experiment, which will be shown in the sample calculations and error analysis page. These errors may be caused because of the materials not being handled well or because of the margin error within the devices being used in the experiment.

Introduction and Theory

There is no doubt that the properties of the materials are the main focus in studying the mechanics of deformable bodies. The only way that we can determine those properties is by conducting experiments on these materials. On our first lab experiment, we conducted the tension test, which means that a tension load will be performed on the material itself and notice its behavior.

When a steel bar is subjected to a load, it either extends or stretches, and if it goes back to its original length after the load is removed, it is known to be ‘elastic’. This behavior only occurs till a certain or limited range of loads. The stretching can also occur in a linear proportion to the load, then which it will satisfy Hooke’s Law and is called ‘Linearly Elastic’. However, when the load increases than the certain limit, it will cause a permanent deformation to the material itself, and this behavior is called ‘plastic’

This lab experiment teaches that to determine those behaviors, we need to define stress and strain that are dependent on the amount of loads applied and elongations that are in unit basis. In order to determine the elastic and plastic properties of the material, we need to be able to calculate the stress and strain by using the following equations:

Stress: (I-1)

Strain: (I-2)

Hooke’s Law: (I-3)

Where:

= The stress ( psi )

P = The applied load on the bad (lb , kN)

A= The cross-sectional area of the bar ( in2, m2 )

δ= Elongation (in, mm)

L = original length of the bar

E= Young’s modulus

Procedures

The following steps are the procedures we did in order to conduct this experiment. When we first started, the computers were already logged in and the program was already opened. Our instructor made sure that the Emergency Stop switch on the MTS frame was off by twisting it to pop up. He then proceeded to identify the samples we were going to test, which were coded Blue: 1018 Steel. Red: 1045 Steel. Aluminum: 6061 Steel. The first step we had to do was to measure the top, middle, and bottom diameter of each sample we were going to test. One condition was that the middle diameter has to be smaller than the top and bottom diameter. After checking that it’s smaller, we measured the middle diameter three times then took the average number of our measurements in order to be as accurate as possible. Our next step was to begin securing the sample bars in the jaws of the testing machine. There was a handset that controlled the machine; it helped us to jog the crosshead down and up. After inserting the specimen into the upper jaw, approximately a quarter inch should be left between the jaw frame and the end of the specimen. After that you close the jaw by turning the T-handle. One mistake we did during this experiment was not leaving an extra quarter inch between the jaw frame and the specimen, which led to damaging the specimen and getting a new one from the instructor.

After locking the specimen, we attach the extensometer. The blades of the extensometer must be placed against the sample and located as close as the center of the sample. After that we secure the extensometer to the specimen using the wire clips, the we remove the safety pin, which keeps the extensometer blades at the 2-insh spacing.

Zero all the numerical values about the load, crosshead, and extensometer by right clicking on each one of them and selection “zero channel”. Then click on the green arrow and enter the sample’s name and the average diameter measured in the beginning of the experiment, then click “ok”. We then observe the change in the load and the extensometer reading, and see how it is affecting the sample test. We collect the data till the sample breaks. After it breaks, we follow the instructions that are given on the computer, which were removing the extensometer and re-inserting the pin in the extensometer, and then removing the broken sample test. We then measure the new cross-sectional diameter after the break. The on-screen instructions will take the crosshead back to its normal position.

We then have to store the data and report the maximum load that the machine applied on the tested sample. To store the data, we give the file and appropriate name and select save. After that we click on File < Export Preview < Specimen. We save the data on the desktop and on a USB flash drive. The file can be imported into Excel by opening the same file from within Excel. Make sure after you import the file into Excel to save it again as an Excel file. Not saving is as an Excel file may cause of losing the graphs.

After finishing these steps, we then do all these steps again to the other next samples by clicking on File < New Sample.

Sample Calculations and Error Analysis

% Area Reduction=100x) (I-4)

Specimen Diameter % Area Reduction

1018 Steel in 100x()=39.9%

1045 Steel 100x()=27.9%

6061 Aluminum 100x()=32.2%

1018 Steel

+= 0.00579 0.579%

1045 Steel

+= 0.0071 0.71%

6061 Aluminum

+= 0.0029 0.29%

Calculating Stress:

σ= = 101107.64 psig

Calculating Strain:

Calculating the modulus of elasticity:

Summary of Important Results

Figure I-1. Stress vs. Strain Over the Elastic Range for 6061 Steel

Figure I-2. Stress vs. Strain Over the Entire Range for 6061 Steel

Figure I-3. Stress vs. Strain Over the Entire Range for 1018 Steel

Figure I-4. Stress vs. Strain Over the Elastic Range for 1018 Steel

Figure I-5. Stress vs. Strain Over the Elastic Range for 1045 Steel

Figure I-6. Stress vs. Strain Over the Entire Range for 1045 Steel

In every graph, by convention, strain is always plotted on the horizontal axis and stress on the vertical axis. For each sample, we plotted the stress-strain curve over both the elastic range and the plastic range. On the graph we can notice the elastic region, which is the linear equation “the straight line at the beginning of the curve”. And then the graph starts to curve, which indicates that the behavior of the material changed into the plastic behavior. The point that separates between the elastic region and plastic region is called “the yield point”. We also define on the graph both the ultimate strength and the breaking point. The ultimate strength is the highest point on the graph that has the highest stress value. The breaking point is the last point on the graph, which indicates that after this point the sample got fractured. The slope of this curve is known as Young’s Modulus or the modulus of elasticity. The error result for 1018 Steel was only 0.579% which means that this could have been caused by the material itself or by the margin of error within the devices used in this experiment. Moreover, for 1045 Steel the percent error was 0.71% percent which was a bit higher than the percent error for 1018 Steel. One explanation for this could be the fact that the material should have been better developed or that I should have handled the devices with greater efficiency. In addition, the percent error for 6061 Aluminum happened to be 0.29% which is lower than both the previous steel specimen. Another possibility for this could be that the material itself was better developed or that the software itself reduced its margin of error. Not to forget mentioning that human errors or calculation errors are hard to avoid.

Discussion and Conclusion

After successfully conducting the experiment and retrieving the data, we were able to calculate the stress, strain, and the modulus of elasticity and put them on a table form and graphs. Using these tables and graphs, it made it easier for us to find the percentage error for each specimen that was tested. We noticed that each material has its own unique stress and strain curve. These curves showed us a lot of properties including data to determine Young’s Modulus. After calculating the percentage error, we notice that it was minimal that could have been caused by the devices used or by the specimen since we used the same devices for all the samples and still got different values for the percentage error of each sample. Another reason can be the accuracy or the precision of the tools used as all tools have margin errors that eventually add up.

According to outer sources, the published Young’s modulus value for the Aluminum T6 6061 Steel is 68.9 (GPa), which equals to 9,993,256 (psi). Our calculated value that is shown in (Figure I-1) is 9,743,212 (psi), which shows the minimal difference between them. Moreover, for the blue bar we were able to find the published Young’s Modulus and determine which type of steel the sample is. The published modulus of elasticity is 205 (Gpa), which equals to 2.9733x107 (psi) compared to our calculated Young’s Modulus that equals to 2.80x107 (psi). We were able to determine that it is AISI 1018 Mild/Low Carbon Steel. In addition, for the third sample, which is the Red Bar 1045 Steel, we determined that it is AISI 1045 Medium Carbon Steel. Its published modulus of elasticity is 80 (Gpa) that equals 2.90x107 (psi) compared to our calculated 2.81x107 (psi).

References