Tensile Strength units?

[buccaneer]

Hello!

I have been completely stumped by the tensile strength unit N/mm2 ! I work in the pressure sensitive adhesives industry, and I normally measure the tensile strength of a film (Polypropylene, PVC, masking tape etc) following the AFERA method 5004, i.e. a strip of 25mm x 200 mm is stretched till breaking occurs, and I report the breaking strength (Newtons) and the elongation at break (%).

The results are commonly reported like this: Tensile Strength: 90 N/25mm (just to denote the methodology, because N/25mm is not a "valid" scientific unit). Now the questions:

1) The AFERA methodology proposes that one can use N/10mm unit also! But how can I go from one to the other? If I "calculate" proportionately... well... the numbers are quite off I believe...

2) The technical characteristics of MANY film manufacturers report tensile strength in N/mm2 !!! The "unit" is WIDELY used, but... HOW is it derived?? What does it mean? What methodology? Is it a REAL unit? Or descriptive (like N/25mm)? Surely here I CANNOT "calculate" and find the connection, because the numbers are... well let's just say unreal.

Can you help?

Thanks in advance!

 

[BobM3]

I'm not an adhesives guy but I'd say that the 90 N/mm number is for a particular thickness of the film. For example say the thickness is 1 mm. So the stress (force/area) is really Force/(width*thickness) or 90N/(25mm*1mm) = 90 N/mm^2.

1) 90N/25mm would be equivalent to 36N/10mm
2) Check the spec for a "standard" thickness. Then you can convert N/mm^2 to N/mm by multiplying by that thickness.

 

[MetalsInc]

N/mm2 = MPa (just for clarity)
"Force" / "cross sectional area"

Therefore, if every test is reported in MPa the thickness of your test specimen shouldn't matter - assuming your thickness does not affect the strength of the film. The larger the cross section, the larger the Force to break it.

Is that what you're asking?

 

[NickE]

Ok I guess my follow up to buccaneer didnt follow his new thread.....

His common testing is force to failure, commonly reported as strength. Since his typical testing is done on materials that will change their behavior when the original width of the sample under test changes.

Thus the result being given as N/25mm means that a sample of width 25mm breaks at N newtons of force.

A result in the unit N/mm^2 (or MPa) is one that assumed dimensionally independant. It is a force over unit area (pressure) and is a "true" unit.

To convert your results (which actually are in units of force alone, the 25mm is just refering to the specimen dimension) to force over unit area is simple.

First solve for the X-sectional area of the specimen. A 20um film 25mm wide and 200mm long that fails at 90N of force has a strength of 90N/25mm (his reporting format) and a tensile strenght of 180N/mm^2 (or 180MPa). (common tensile strength units...)



Nick
I love materials science!

 

[MintJulep]

a strip of 25mm x 200 mm is stretched till breaking occurs, and I report the breaking strength (Newtons) and the elongation at break (%).

The results are commonly reported like this: Tensile Strength: 90 N/25mm

Ok, so I can get that 90N is the breaking strength.

How is 25mm a percent elongation?

 

[buccaneer]

The 25mm is reported just to denote the methodology, i.e. the fact that a strip of 25mm wide was used. I did not give you the elongation because I did not ask about it. But if I have, let's say, a 40mm elongation at break, then that would be 40*100/200 = 20% El@B.

 

[buccaneer]

Oh, and to answer NickE... hmm can you please elaborate on the equation? I thought the AREA mattered, so I would get 0.018 N/mm^2 ? How the thickness fits in and gives you 180 N/mm^2 (it's perfectly logical, but then I would expect mm^3)?

 

[NickE]

The force of 90N is assumed to be equally distributed across a planar X-section
<<ahhh ha! the planar x-section is normal to the tension direction, and has nothing to do with the length.>>
equal to the area under tension. this area is equal to thickness times width. (for a 20um film 25mm wide the area is: 0.5mm^2) Thus this film has a tensile strength of 180N/mm^2 or 180MPa

Nick
I love materials science!

 

[pwtomlin]

The cross-sectional area is what's important, not the surface area of the film. If you are stretching the film along it's length (200 mm) then the relevent area for converting force to stress (Newtons to MPA) would be the width (25 mm) times the thickness (which I don't believe was mentioned). In other words, you divide the force by the area of your sample perpendicular to the direction of the force in order to get the stress.

 

[NickE]

what I said.......

 

[buccaneer]

Thanks everyone! What you all meant dawned on me right after my last post, but anyway everything now is crystal clear!