(-Planning Exercise - Strain gauge Coursework Help -)

[Jack_Legacy]

(--Planning Exercise - Strain gauge Coursework Help!--)

Hi I've been stuck for several days in the early stages of my coursework i would have asked my tutor but I have been forced to stay home due to a very bad chest infection

I will first provide a full transcript of the question (please view figure 1.1 and 1.2 they are attacthed to this post) before i tell you where i am stuck

Fig1.1 illustrates a crane beign used to move a load. As the distance from the load to the control cabin increases, the force in the supporting struts of the metal frame work or the crane increases. It is important to engineers to know how these forces change as the load moves.

Please view fig 1.1

One way of investigating the forces in a beam is to attach to it a device known as a strain gauge. A Strain gauge is illustrated in Fig 1.2

Please view Fig 1.2.

If the length of the strain gauge is increased, wires in the gauge become longer and thinner, and therefore the electrical resistance of the gauge increases.

You are required to design a laboratory experiment to investigate how the electrical resistance of a strain gauge attached to a flat surface depends on the tensile force applied to the surface.

You should draw a diagram of the arrangement of your apparatus, and in your account you should pay particular attention to:

(a) the procedure followed
(b) the material on which the strain gauge is to be mounted and the dimensions of this material *
(c) the method of attaching the strain gauge to the material *
(d) how the force would be applied
(e) the approximate range of forces which you would use
(f) how the resistance of the strain gauge would be measured, including the range of any meters used
(g) any safety precautions you would take when carrying out the investigation
(h) particular features of the desing that would ensure accuracy and reliability of your results.

I have put a star next to the ones where i am completely lost

(b) the material on which the strain gauge is to be mounted and the dimensions of this material * - I think is reffering to the plastic backing sheet if so what would be the dimensions used for the plastic backing sheet would'nt a larger backing sheet exert more force on the strain gauge.
(c) the method of attaching the strain gauge to the material * - would i have to use some form of adhesive/blu tac maybe to attatch the the strain gauge to the sheet.

I have come up with a diagram see apparatus.bmp

My procedure is simply to move the strain gauge plus weight/load across the ruler at 0.1m intervals and measure what the resistance is at each of them. Do i need a weight or should i just use the strain gauge and the backing sheet? I plan to measure resistance with a multimeter is that correct?
if my diagram is wrong could you please provide another (doesnt have to look good )

Also I've never used a strain gauge in my life could you post a link to a site where i can see what one looks like and how it is used.

 

[Vance]

http://search.yahoo.com/search?fr=fp-pull-web-t&p=Strain+gauge

I am sure my brother whose English name is also Jack never changes his major...lol...
or he doesnot dare to do so...

 

[Jack_Legacy]

thanks... atleast i know what a strain gauge looks like now

 

[km_michelle]

i also need some help.. i m going yo do da same experiment as you..m please help

 

[leeez]

I'm doing this as well.

I think your sposed to just use the plastic backing to stick it onto the surface. I think something like double sided sticky tape. Nothing hot, or you could damage the wires, and I don't think blu-tack is strong enough.

Regarding material, something like a metre rule, with a hole drilled at one end to put weight on?

 

[titmouse]

Regarding material, something like a metre rule, with a hole drilled at one end to put weight on?

The apparatus diagram by Jack Legacy is completely flawed. All it would be measuring is the strain on the wire due to the mass on the end of it. Moving it along the meter rule will change nothing, and the fact that it looks remotely like a crane means nothing. You are supposed to test the strain in the STRUTS of the crane, not the cable attaching the mass to to the crane. I would therefore suggest something like the diagram I have attached.
But I'm pretty sure that's not the right thing to do either.

 

[titmouse]

The pictures not terribly clear; the diagonal is acting as the strut, with the strain gauge on it. The mass can then be moved along at set intervals, taking readings from the gauge along the way.

 

[aimi]

Im so stuck, i don't know how to work this web site, n i haven't a clue what a strain gauge is! help!

 

[Tosh]

well the idea is that the strain gauge has to be glued so securely that the entire strain on the material beneath it is equally strained. The change in resistance in the strain gauge is relative to the strain being put on it... I am sure of a lot of the other stuff

 

[Tosh]

i meant that I am unsure

 

[titmouse]

But what do you stick it to?

It would be very helpful if someone had an idea of the size of the strain gauge, and wether it shows horizontal or vertical strain

 

[titmouse]

And also on what part of the crane the strain is being measured

Come one, someone must have done this experiement, or have an idea how to do it

 

[djdunko]

I am doing the design tomorrow and i have no idea what to do! But i do know that the experiment dosn't have to have anything to do with a crane that's just there to give you a reson to be working out the strain. It needs to be a experiment that can be done in a lab so u can't use a model of a crane! My teacher told me! I just can't work out how to attach the material to a piece of string or something to attach the load to to strech the material!

 

[Tosh]

Thanks for that ... I thout the crane was important.

Does anyone know any safety precautions?

There is an adhesive called cyanoacrylate that is commonly used. I think that it is suitable to assume that this can be made available in a school lab but I am unsure.

Well your not working out the strain either ( sorry if you didn't literally mean that), you are actually workin out the increases in resistance in the strain gauge caused by the strain on the material it is attached to. The resistance increases because of the factors included in the equation [resistance= (resistivityxcross-sectional area)/length].

I'm rather stuck and mine is due in 2moro ...eeeeeeeekkkkkkkkkkkkkkkkkkkkk

 

[littlerich]

HIya me too, i am completely stuck with this whole plan thing, it is absolutely stupid, me too in 4 2moro, please help

 

[titmouse]

Well the day is upon us, and I havnt slept at all (damn DT coursework..grumble grumble)
Anywhoo good luck, and remember its only 5% of your total As.

And if it helps, use GLUE and a low voltage stable power supply, and GOGGLES ;)

(oh and ignore my apparatus I posted, utter nonsense it was)

Let us know how you all got on!

 

[aimi]

Damn this physics!

I feel so bad 4 you! mines due in thurs, so i got a while to think about it! I sort of know that the whole gauge stretches, even theplastic its mounted on, but no use now if youve already done your work! ope you did ok!

 

[moh51n]

I am doing the same exercise and its in 4 2moro can anyone help?

 

[Byj]

My Physics plan is supposed to be in tommorrow, if I don't do it, I am in deep ****.

All I know is that a strain gauge has a resistance of approximately 100 ohms. It is 15mm long and inside it is a looped copper/nickel foil. Newton weights must be hanged off the end of the wire the strain gauge is attached to, we would need to plot a graph of force against resistance with various forces applied on the wire e.g fron 5-15 Newtons. The actual value of resisivity of a strain gauge is 2.

For the resistance to vary, the strain gauge must be really stuck to the wire so that any change in the length of the wire, will strech the copper/nickel foil inside the strain goauge, making the foil longer, therefoer it's resistance increases. But, the change in resistance is too small to be measured with a standard classroom ohmeter, an alternative needs o be used, and I don't know what! Something about a wheatstone bridge, anyone heard of that??

IF anyone has any details, plllleeeeeeeeeeeaaaaaaassssssseee fill me in!

 

[sbenj]

you have to use something called a wheatstone bridge circuit, to measure the resistance accurately. look it up, it is too complicated to explain. look in http://zone.ni.com/devzone/conceptd.nsf/webmain/C83E9B93DE714DB08625686600704DB1?OpenDocument and u need to attach the gauge to foil, i think. what i am worried about is how u apply the force to the foil in measurable amounts.

 

[asloversgo]

You guys are lucky! For some reason, my head of physics decided it would be a great idea for me to resit my waves/practical paper even though I got a B, so not only do I have to do this one, but I have to do the A2 one as well! And you know what? The A2 one looks so much nicer than this one...

Mine has to be done in class tomorrow or something, I duno, I wasn't listening, but there are some great sites on strain gauges if you just search "strain gauges" on google.com, within the first 6 or so pages.

Good luck guys n gals.

 

[taurinne]

im totally stuck. i haven't got a clue how strain gauge work. i only know about resistivity something to do with this formula R = pL/A.

 

[djdunko]

Hey!
The day is over and i have put a completely wrong answer down on the paper and handed it in and felt stupid! I just hope that you guys and gals have sum good look and learn how to do it by your deadlines and do well!1 I have no hope now but i hope you get really good grades! Thanks for all the help and next year when the A2 one comes out we will all probably meet again

Thanks again
See you next year

 

[aimi]

Well thanks! i now think i have a bit of a clue as to what to do, which is good! i am at college know doing the whole thing in rough! due in tomorrow!

 

[moh51n]

i don't have a clue what to do? can anyone explain a bit.

 

[aimi]

well the strain gauge is exactly like it is in the paper! I am sure all you have to do is apply a load by using a wire and pulley. when you know what you're doing its not that bad!

 

[aimi]

but you need to make sure it won't move, just stretch. 1 thing I am not sure of is the dimensions of the material, or the actual material, but as far as i can see it doesn't seem to have that much effect!

 

[moh51n]

do you have to use a wheatstone bridge

 

[aimi]

.

oh, I've just found out maybe all you have to do is put the force on top of the gauge, but maybe this won't measure the strain. i don't know what dyu think? oh no, it says tensile stress, this is not right then is it?

i don't really know what a wheatstone bridge is and so i don't think I am using 1, 4 that fact!

 

[moh51n]

could you not use an ammeter and voltmeter to work out the resistance using R=V/I

 

[moh51n]

if you apply the force on top of the gauge then how will the gauge strech

 

[aimi]

yeah! that would work, i think. so then what about the size of the voltage in the circuit? would it make a difference if it was low or high?

 

[zxltony]

what about the ranges of different measurements used?

 

[aimi]

the gauge would be compressed! i realized this wouldn't b a tensile force then! god this stuff is stressin me! bug style!

 

[aimi]

which measurements/ the voltage and current? i think by adding a range of loads, the measurements can be made each time another is added.