Theme 2 is all about the properties of engineering materials. With most materials great care goes into their manufacture, though materials like sawn lumber are natural materials over which we have much less control. Nevertheless, it’s critically important that the methods of production of the materials we use as engineers are validated and cross checked by rigorous calibrated testing procedures and equipment. In this project you’re going to test the tensile strength of two samples of a material that will be supplied to you.
To complete this project, you are to undertake three tasks: prepare a testing rig; test your samples; and present your work in a report.
1. Prepare a testing rig – you can work with a fellow student to make one rig for both of you to use.
a.Clearly you need to be able to apply a force to each one of your test specimens separately (ie your two wires) that will cause the specimen to be stretched with a known force in tension.
b.There are many things around a typical household that can be used for this purpose – a bucket or kitchen container that’s gradually filled with water or dirt or sand can be a simple way of applying load, but you’ll need kitchen scales to measure at least the original weight of the container and the weight of material you fill it with. Of course if you use water you can determine its weight if you keep track of the volume of water you put in the container. The container will need a handle so you can lift and carry the container.
c. If you need to glue anything to the sample, make sure you use a rigid glue such as araldite or super-glue.
d.Before you do any serious testing, make sure your rig is stable under the largest load you’re going to apply, and that no one can get hurt by falling buckets or weights when the specimen fails.
e.You’ll also need to keep track of the deformation of the sample as the load on it increases. The safest way of doing this with your home-built test rig is to measure the distance between the two gauge marks that you’ve marked 100mm apart on the test specimen, until failure occurs.
f. You’ll need to be very careful that you fix the wire very securely to whatever you’re using to apply the load.
g.Take measurements of the components making up your test rig because you’ll need to produce a scaled drawing of your rig while it is testing a sample, using an A3 sheet and AS1100 as your basis for constructing the drawing.
2. Test the samples of materials given to you.
a.Before you test your specimens, conduct a number of high quality searches for information on the internet to help you make an estimate of how much load you think your test specimens will carry.
b.You will have received two samples for testing – if two of you are working together that will mean you have four samples to test. BUT you must only report on the performance of your own two samples. Use a similar piece of wire to prove the testing rig works successfully. Make sure you use the web to investigate what is the likely failure load of your wires before you start testing.
c.Make sure you carefully measure all the dimensions of your specimens before testing.
d.When you test your specimen, make sure you increase the load on the specimen in no less than 5 steps in which each step is about one-fifth of the expected failure load. Make sure you measure and record the load and the deformation of the specimen at each step until failure of the specimen. NB: deformation is determined by taking the readings of the distance between the two gauge marks at each load stage, and subtracting the original distance from each reading.
e.Take photos of your test rig, of your specimens before, during and after they’re tested. You’re urged to take a video of a sample being tested to be shown to the rest of the students later.
f. In Excel plot the load-deformation data for each sample tested. Calculate the stress in the specimens when carrying the maximum (ie failure) load. From these results determine the mean failure stress for your material.
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