Northridge
Experiment #4- Tensile Testing
Abstract:
This experiment puts emphasis on the tensile properties of various materials including two polymeric samples and two metals. Results based on this test help show how each is distinguished among the others and how they complement the basis for a safe design project. The tensile properties are gathered by exerting a tensional force that pulls a specimen by each end at a fixed rate until failure. Data for the test is obtained by measuring key factors such as amount of load, cross-sectional area and elongation.
Description of Work:
The four samples tested were steel, aluminum, low density polyethylene (LDPE), and high density polyethylene (HDPE). Each were measured for thickness and width, then placed into the United machine for testing of tensile properties. It had two grips, each one holding one end of the sample part. The machine, running in DOS, was then calibrated to ensure appropriate results for the respective sample material. At a set rate, the equipment piece would simultaneously pull each end and gather data to show elongation and load. A standard 2MB floppy disk was then used to record the data and transfer it into a digital spreadsheet to determine stress and strain.
Results and Discussion:
After obtaining the initial dimensional measurements for each sample, they were put through the tensile test. The machine gave two sets of values being, %strain and Load. With these values the engineering stress and strain were then calculated and graphed for all materials. The results were surprisingly off. For example the elastic modulus of steel is approximately referenced as 30Mpsi, while my experimental value came out to be 21.487Ksi. The elastic modulus of aluminum is referenced as 10.6Mpsi while my experimental value yielded 3.826Ksi. That for Low Density polyethylene was 30Ksi while experimentally 0.265Ksi, and for high density polyethylene 200Ksi was referenced while 0.601 Ksi was obtained experimentally. Being that these values were so off it would not be surprising to find the other values not to match with the referenced values. This could have to largely do with a human error factor in the experiment process. It could range form the initial dimensional measurements to the data input in the computer to the data manipulation and graphing process. There were many possibilities to make a mistake in this experiment unfortunately they seem to have been made in a large scale in this experiment.
The graphs on the other hand seem to have the correct forms. The true engineering stress and strain graphs showed much higher values for stress than did the engineering stress and strain graphs. this is due to the true engineering stress depending on instantaneous cross sectional area and not the the initial area. The reason for the decrease in instantaneous cross sectional area is due to the necking of the material that happens when the material transitions from the elastics region of the stress strain curve into the plastic region where it becomes permanently deformed. The values for the ultimate tensile strength are as expected with aluminum and high density polyethylene. They are high values as one would expect from such materials. The problem arises when looking at the ultimate tensile strength for steel, the value is lower than that of aluminum, which is incorrect. This mistake must be a byproduct of the human error factor that was mentioned earlier.
This lab’s purpose was to show and compare the various properties of a material at a specific temperature under tensile forces. Although human error played a large part in this experiment, the graphs still display the majority of the properties the materials exhibit under a tensile test.
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