# Young's/elastic modulas of perspex/ Forces in truss membersWatch

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#1
Hi everyone!,

i have recently completed this experiment:

on a model perspex truss i took the zero load Demec readings on all the members. then loaded two of the trusses nodes with 20lb loads and measured the Demec readings again. from this i worked out the strain in each member and could use the youngs modulus to work out the forces in each member.

then i did a structural analysis on the truss to find the theoretical forces. these were different to the actual forces. i need to explain why this is but am unsure. any help? am aware of the 3 key assumptions used when doing the analysis and would be grateful for any help.
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5 years ago
#2
(Original post by kevlid)
Hi everyone!,

i have recently completed this experiment:

on a model perspex truss i took the zero load Demec readings on all the members. then loaded two of the trusses nodes with 20lb loads and measured the Demec readings again. from this i worked out the strain in each member and could use the youngs modulus to work out the forces in each member.

then i did a structural analysis on the truss to find the theoretical forces. these were different to the actual forces. i need to explain why this is but am unsure. any help? am aware of the 3 key assumptions used when doing the analysis and would be grateful for any help.
They are different because in real life the stress distributions in the members are spread out differently and depend on a number of other factors i.e. cross sectional areas etc. This is why assumptions are made to make the problem solvable by analytical means otherwise you wouldn't be able to solve it (one of the assumptions is that the forces are considered to act at the pin joints but in reality this is not true).
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#3
(Original post by a10)
They are different because in real life the stress distributions in the members are spread out differently and depend on a number of other factors i.e. cross sectional areas etc. This is why assumptions are made to make the problem solvable by analytical means otherwise you wouldn't be able to solve it (one of the assumptions is that the forces are considered to act at the pin joints but in reality this is not true).

hi, how would the strain/time variation on the member forces affect the two values then? i had a feeling it was to do with the stress/strain.

would the members that had their strains measured later have a greater difference between the real and theoretical forces because of this strain/time variation?

Thanks.
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5 years ago
#4
(Original post by kevlid)
hi, how would the strain/time variation on the member forces affect the two values then? i had a feeling it was to do with the stress/strain.

would the members that had their strains measured later have a greater difference between the real and theoretical forces because of this strain/time variation?

Thanks.
I can't remember the relationship between strain and the time(it's been a little while since i touched materials so maybe someone with more fresh knowledge can help)...but the stress is proportional to the time if I'm not mistaken hence the longer you have the members under some kind of load the greater stress they will have to endure. The theoretical values don't take time into account but what they do tell you is whether each member is under compression or tension.
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#5
(Original post by a10)
I can't remember the relationship between strain and the time(it's been a little while since i touched materials so maybe someone with more fresh knowledge can help)...but the stress is proportional to the time if I'm not mistaken hence the longer you have the members under some kind of load the greater stress they will have to endure. The theoretical values don't take time into account but what they do tell you is whether each member is under compression or tension.
since i have done the experiment i can tell you that the strain tends to increase with time. do you have any idea what the possible reasons for this behaviour is?
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5 years ago
#6
(Original post by kevlid)
since i have done the experiment i can tell you that the strain tends to increase with time. do you have any idea what the possible reasons for this behaviour is?
yes that makes sense...the bonds in the material are stretching (which means strain will increase). I'm assuming you kept loading the member after a certain time period in which case you're essentially putting the material under some kind of cyclic loading.
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#7
(Original post by a10)
yes that makes sense...the bonds in the material are stretching (which means strain will increase). I'm assuming you kept loading the member after a certain time period in which case you're essentially putting the material under some kind of cyclic loading.
i actual put a 40lb load on a tensile test specimen and measured the strain immediately after and then every 20 seconds fro a 5 min period. my results of strain are:

as you can see it tends to increase but for some time it stays constant. do you have any idea why it increases and sometimes stays constant?

me and my group can't think of any reasons why.

thanks
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5 years ago
#8
(Original post by kevlid)

as you can see it tends to increase but for some time it stays constant. do you have any idea why it increases and sometimes stays constant?

me and my group can't think of any reasons why.

thanks
when you put a material under any load the bonds in the material will begin to stretch...when the bonds stretch and you wait some seconds before putting it under load again the molecules in the material re-adjust and the material will actually re-strengthen for a little bit requiring you to put even more force to overcome this until you break it. Look up strain hardening.
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#9
(Original post by a10)
when you put a material under any load the bonds in the material will begin to stretch...when the bonds stretch and you wait some seconds before putting it under load again the molecules in the material re-adjust and the material will actually re-strengthen for a little bit requiring you to put even more force to overcome this until you break it. Look up strain hardening.
i didnt add any more loads. i just applied the 40 lb load to it and then measured the strain every 20 seconds. no more loads were applied in that period. thanks
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#10
(Original post by a10)
when you put a material under any load the bonds in the material will begin to stretch...when the bonds stretch and you wait some seconds before putting it under load again the molecules in the material re-adjust and the material will actually re-strengthen for a little bit requiring you to put even more force to overcome this until you break it. Look up strain hardening.
hi mate,

was wondering if you could here.
the strain in the perspex tensile specimen increases with time after a single load is applied to it (i didnt add or take any more weight) but sometimes the strain stays constant for a while.

can you help explain the reason for this whole behaviour.

thanks
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