The aim and objective of this test is to observe and investigate the way in which materials respond to stress. In the practical lesson we will be using three types of metal (Sample A; B; C) All three will be tested on their strength of tension and using the data that we recover we will be able to judge the name of the metal. The metal that breaks first will have a lower breaking point making it a brittle metal; however, the more withstanding metal will be more ductile.
The Tensile test involves stretching a material to a point where it fails, or breaks. This method of testing is essential to many engineering applications because it judges the ability of the metal to perform in various situations like buildings, ships or engines. Testing the tensile strength of a material involves placing it into a tensile machine that has special jaws that clamp the metal. After fixing the metal tightly, other procedures such as applying the extensometer can be done. Once everything is in place, you can start testing. With modern equipment, data can be analysed and observed using computers that tell you everything about the state of the metal whilst it undergoing the experiment. Using data retrieved from the computers, we use the tensile stress of the material to judge the specimen.
Theory – Tensile Test of Materials
The Tensile test is one of the most essential experiments that you can perform on a metal because it determines many factors such as stress, strain and various features that judge its strength. These tests are carried out to observe how the metal reacts to different loads and what sort of metals suit its application. During the testing you will get a set of results, but in the form of a graph you will obtain a curve showing the failing point of the particular metal.
Method & Materials
Before starting the practical we prepared ourselves by setting up the equipment that will be necessary for the tensile stress testing, we did so by collecting raw data that will be stored in the computer. For the cross-sectional area, we used electronic callipers that had an uncertainty of +-0.01
Next, we measured the length of the two inside lines using a compass with two needles, and then we find the length by aligning it with a ruler. After measuring the inner length, we find the outer length. These measurements will come in handy because the computer will automatically calculate them into cross-sectional area, which is needed for some formulae.
When all the important measurements have been carried out the test piece must be clamped into the tensile testing machine. The testing machine is a high… Once everything is in place, a small load is pressed against the specimen to keep it tightly in place. Whilst the material is tightly clamped, we fit in the extensometer. The extensometer is a mechanical device that is fitted onto the metal, parallel to its total length. Whilst a tensile stress testing is taking place the distance increase between the points that you clamped the meter, the increase is very accurate and gets stored on the computer, the machine does the same job as the extensometer however, it is less accurate. When the specimen starts behaving in a non-linear fashion, you stop the test and remove the extensometer. After that, resume the testing until the material fractures. During the last stages of the testing we note the maximum load applied and record the cross head extension.
Remove the test piece from the tensile stress testing machine and record the same measurements as before (Diameter of the test piece at the break point. Connect the two halves and measure the total length).
Then we analysed the surface of the break by sketching and making a brief summary about it.
Analysing these results we can determine the name of the type of metal, in this case
Sample A: Aluminium; Sample B: Steel; Sample C: Cast Iron
After analysing the results and plotting the graph we can evidently see that Sample A and B are ductile materials and have very high failing points. On the other hand, Sample C fails almost immediately after starting the tensile testing; this indicates that it is a very brittle metal. Throughout the testing I observed the behaviour of all 3 samples and concluded that Sample A had the most uneven break because it was more elastic then the other two, whereas Sample C has a very smooth break because it almost instantly broke resulting in a neat surface.
To summarise this eventful practical I concluded that Sample A;B; and C are Aluminium, Steel and Cast Iron, respectively by using a combination of raw data and formulae (Young’s Modulus) to finally achieve a set of results. Therefore I can confirm that Sample A is the most effective metal to use in real-world applications that require a lot of resistance towards stress. This can include agile structures, planes, ships. Sample B is ideal for typical structures such as multi-storey buildings. Lastly, Sample C is rarely used for high resistance demanding structures because iron is used to make steel, therefore it can be used for minor jobs.
Fig 1.1 Shows the diagram of the specimen and indicates the positions at which you must measure
Fig 1.2 Is a photograph of the two tools that we used to collect the raw data
Fig 1.3 Shows the tensile stress testing machine