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# 急! elastic limit 點搵

http://img189.imageshack.us/img189/9918/nylon6.jpg

假如我有晒上面D info...

點樣搵elastic limit, yielding stress 同Young's modulus(今次就有,冇的話點搵? 我知係stress/strain, 不過用邊個point?)

thanks^^

可唔可以睇睇上面幾個tim ar..thanks><

Young’s modulus, elastic limit, yielding stress, maximum stress, and elongation at failure, 最後2個係given, 我要搵頭3個..

nylon同pmma都係elastic limit= yielding stress= maximum stress?

Al同steel又係點..? 唔該晒!!>

i dont really get it..

i need the elastic limit in order to find young's modulus

but to draw the slope, i need to use E?

so in the end, how do i find the elastic limit?

the only thing im sure of now is that PMMA has elastic limit= yielding stress= maximum stress..

i want a more precise elastic limit was actually coz i got the whole data file of the tensile test i thought i didnt have the use the graph that i plotted to find elastic limit, but instead, i could somehow "see" it from the sea of values.

@sze chai: i dont know abt the machines, 2electromechanical testing machines were used, it seems we used steel on the more advance one. not sure. the lab demonstrator screw up a few time like slipping us a different garde of aluminium. so if u see anything strange it might be the specimen.

thank you so much for ur answers*=) i'll put it for everyone to vote, ok!?

### 2 Answers

- 10 years agoFavorite Answer
First of all, by definition:

Elastic limit = Yield stress

The lowest point in the stress-strain curve at which permanent plastic deformation begins to occur.

Nylon:

Nylon is a polymer which does not exhibit a clear yield point, proof stress is used to find its yield stress.Young's Modulus is the slope from the origin to the point of yield stress.Ultimate Tensile Stress (UTS) is simply the maximum point of the curve ~ 50MPa

Aluminium:

Aluminium is a ductile metal (which can also exhibit brittle manner) which does not exhibit a clear yield point, proof stress is used to find its yield stress. Young's Modulus is the slope from the origin to the point of yield stress.UTS is simply the maximum point of the curve ~215MPa.

PMMA:

PMMA is NOT a composite material. It is a hard rigid glassy, but brittle synthetic polymer.It is unusual to calculate the elastic limit of PMMA simply because it is not elastic.It is also very difficult for you to obtain a precise value of the Young's modulus for PMMA from your graph.However, you can tell that the UTS of PMMA is about 73MPa which is fairly accurate.Steel:

Steel is a metal alloy with a clear yield point. The yield stress is about 300MPaUTS is simply the maximum point of the curve ~450MPa

5 Steps to obtain Proof Stress:

Calculate the Young's modulus of your material from the graph, let it be EMeasure the total strain of your material, let it be XMark on the strain axis of your graph X*0.2% = 0.002XDraw a line from this point (0.002X) with a slope E until it hits your curveThe Proof stress is at the intersection of the line and the curve

Hope it helps~

2011-05-05 06:20:15 補充：

In theory, you need the elastic limit to find Young's Modulus (i.e. it is the definition). However, the concept of proof stress is a rather practical approach, and by that it means you should obtain the slope E from the graph, not by definition.

2011-05-05 06:27:32 補充：

So in the end, using a ruler you draw a straight line of the slope, and calculate the gradient of the slope (using dy/dx). You can then draw a slope with the same gradient from the point 0.002X and find the intersection accordingly.

Source(s): Myself - 10 years ago
If I have not remember wrongly,

the elastic point of rubber is the maximum point in the curve.

that is the point where stress = 50MPa

the rubber will return to it's original shape along the curve if u release the force.

And for the modulus,

just the slop of the graph.

rubber is NON-LINEAR material.

Non linear means the modulus is not constant and not obey hooke's law

2011-05-03 00:30:01 補充：

For steel and aluminium, the elastic point is quite obvious.

For steel, i will take the point when stress = 300MPa

For aluminium, i will take the point when stress = 170MPa

The modulus is just the slope of the from origin to elastic point.

yield stress = stress at elastic point.

2011-05-03 00:33:38 補充：

For PMMA, as this is composite material

it wont break until the plastic in it break.

The elastic point i take will be around 30-40MPa

as u can see from the graph, the slope changes when stress over 40MPa

2011-05-03 00:35:55 補充：

btw, for aluminium and PMMA,

as the elastic point is not quite obvious, so the yield stress as well

so sometimes u will see 0.2% proof-stress.

this is just the stress of the intersection point if i draw a line

the slop is = modulus and start from strain = 0.2%

2011-05-03 00:36:08 補充：

this is a reasonable approximation of the yield stress