Physics investigation Essay
For this investigation I will be testing various materials to determine how much force is required for them to snap. Figure 1 below shows the forces that will be acting upon the test material.
The set-up above will increase the force applied to the test material until a crack, started in the surface in tension, propagates to cause the material to break. To experimentally determine the level of stress that is required to break an object, I need to be-able to exert a force onto an object and be able to quantitatively measure and record that force. This, I found, is not as simple as it sounds due not only to the limited equipment available to me, but also to what I could use practically and safely. I came up with many methods of measuring the force exerted as well as many ways of creating that force. A summary of the major methods and their associated problems is included below.
Method 1: Creating force using my own strength.
This has several obvious advantages including ease of use. My own strength requires no preparation to use, is easy to store when I’m not using it and can create a very broad range of force i.e. up to 800N. Unfortunately for all the benefits, there are several disadvantages that make this method untenable. The greatest concern is safety, namely my own safety. To apply the force needed to break the objects, I would have to lean heavily on the object. This placed me in danger of being impaled on the device I was using to concentrated force onto the object. As well as this, there was the problem of splinters from the object damaging my hand or eyes.
Method 2: Measuring the force using bathroom scales.
The main advantage of this method is that a large range of forces can be measured i.e. over 1,000N. Also, they are easy to set up and use, difficult to break and can be stored easily ready for repeating experiments. The disadvantages are fewer but still prevail over the advantages. The greatest downfall is the fact that the max force reached and therefore the point at which the material broke is difficult to determine accurately. This means that my results could be out by an unacceptably large margin of error.
Method 3: Creating the force using a vice.
On the surface, this method seemed pretty ideal. A large, steady force could be created safely. With the help of a metal tip attached to the vice, the force exerted could be carefully increased by turned the handle on the vice until the material broke. The force generated could be very powerful due to the lever effect of the handle on the vice. This makes it many times more powerful than any force I could create on my own. This time, the problem with the method lies not with the method itself but with the accompanying method of measuring the force created. Bathroom scales cannot be used because they are made inaccurate by concentrating force onto a small area of them. Smaller scales cannot be used because the ones available to me couldn’t be used to measure a large enough force i.e. 20N maximum. This method would have been used were it not for the failings of measure methods.
Method 4: Measuring the force using piezo electric crystals.
This was a bit of a non-starter really. Although they seemed like a good solution due to the quantitative data that could be produced and recorded; I didn’t realise that the crystals couldn’t be used to measure the magnitude of force that I required. They would have been crushed instantly.
Method 5: Creating the force using pneumatics.
The advantages of this system are similar to that of the vice. A large, steady and easily controlled force can be produced with safety. The disadvantage of the vice is also overcome; there is no need to measure the force directly as it can be inferred from the amount of power used by the pneumatic rams them selves. Unfortunately there was one insurmountable stumbling block. The equipment available to me was not strong enough or powerful enough. This method seemed ideal and is the one used in professional laboratory tests, unfortunately the facilities and equipment were not available to me to utilise it effectively.
Method 6: Creating the force using weights.
This method of producing force has the added benefit of measuring the force as it is produced. The force produced is quantitative, steady and the maximum level reached is easily found. There are disadvantages however; the maximum force that can be produced is dependant on the maximum strength of the string that attaches the masses to the test medium. The larger the area of contact, the less pressure is produced. This means that for the test to be fair, the string used has to be the same in each instance. This method is the one that I finally decided to use.
My choice of method means that a small alteration is required to my original force diagram; see figure 2.
In order to perform this experiment fairly I will need a means of supporting the material and ensuring the struts stay in the correct place because the force I exert will be laterally transferred to them. I considered several ways of supporting the material and how to prevent the supports shifting during testing. Figure 3 shows the forces involved.
Vertical force applied to material, causing it to flex and become compressed and under tension.
The flex of the material means that the edges of the material are closer the centre of the material and that the material is therefore shorter horizontally despite being slightly longer in length. Because the material is trying to shorten it’s horizontal length, the supports are pulled inwards.
The only thing stopping this from happening is the frictional force between the base and the tabletop. I have outlined below some of the methods I used to prevent movement of the stand; they all work on the principle of increasing the friction force by means of altering the parameters of ï¿½ or F from the equation:
R = ï¿½ F (see figure 4)
R = resultant force i.e. the level of friction.
ï¿½ = the co-efficient of friction i.e. the stickiness of the surface.
F = the vertical force produced by the mass of the clamp stand.
The material under stress will increase in flex and slightly in length until the elastic limit or yield point of the material is reached. At this limit, the material will start to crack on the side under tension. These cracks will propagate until the material finally snaps. The amount of force required for this to happen is known as the breaking stress. The higher the breaking stress the more tensile strength the material possesses. This is what I shall be measuring in my experiment.
Problems Using Clamp Stands
This was the first method that I thought of using because of their ease of storage and being the most ubiquitous support available to me. The disadvantages became very clear as soon as I tested them. The main problem I discovered was that of movement of the supports. I tried many ways of securing them to a surface so as to limit movement but most were unsuccessful or unsatisfactory. Firstly I tried placing masses on the footplates of the stands but could not place enough mass on them to prevent movement. This method works by increasing the parameter F in the above equation.
I also tried using sticky paper to increase the co-efficient of friction of the surface the stand was placed on but couldn’t increase it enough to change the outcome by any great amount. After that I placed stoppers on the base to stop it moving. This did stop it moving but instead caused the stand to topple over instead; obviously not ideal. The last method and by far the most successful I tried should have been more obvious. I attached a large G-clamp on the footplate of each stand and tightened them as far as I could. Due to the size and power of the clamps I used, movement was completely eliminated. This method works in the same manner as placing masses on the bases; it increases the F parameter. With the clamp I used I was able to generate huge vertical forces onto the footplate that created a resultant frictional force more than enough for my requirements.
The second problem I only discovered after solving the first. The bosses I used to attach the clamp arms to the stand were not strong enough to hold themselves up and not rotate towards each other as the material flexed. Figure 5 below shows the set-up and how it changed when masses were added.
To solve this problem I had to find a way of holding apart the arms. I tried tying them together with string but found it flexed too much when used in the length required. To solve the problem I needed a material that produced a low strain and could be applied in the way required.
After a quick search of the materials that were on offer I decided on carpet tape. I chose it because it is reinforced with many fibres that give it extra strength and it is sticky so it can hold together the clamp arms.
Through out the planning process I have been looking at my experiment and constantly analysing it for sources of potential danger I might encounter whilst performing it. I met with the following issues and adjusted the experiment thus.
The first thing that I thought would be a safety concern in the experiment was the force involved in breaking the materials. Creating large forces would mean large amounts of mass being used and this is a hazard. To minimise any risk involved; I selected materials that would not require huge forces to break and I used only small samples of them so as to further reduce the force required e.g. the wood samples I used were cut to a thickness of 2mm.
Another concern was that of what the material would do when the stress point was reached. Some of the materials I used could shatter or splinter e.g. plastic and wood. To cut the risk of personal injury to myself and those around I erected Perspex safety screens, wore protective goggles and performed the experiment in a closed classroom. I also used shatterproof plastic in tests instead to normal plastic as I considered this too dangerous.
Due to the method used in generating the force i.e. using suspended masses, I had to consider where the masses were going to land once the material gave way. I placed a carpet tile where the masses were going to fall so as to prevent damage to the floor and furthermore I suspended the masses as close to the floor as possible so as to reduce the velocity at which the impacted the floor.
With the preparation of the experiment sorted and a few preliminary test runs completed, I decided to start collating results from the various materials I had collected.