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Stretching Materials
Name: Tara, Brayden, Tim
Status: student
Grade: 6-8
Country: Australia
Date: Fall 2010
Question:
How dose stretching effect materials other than
rubber? Materials like plastic, metal, glass, etc.?
Replies:
Hello Tara, Brayden, and Tim,
This is a good question that will allow us to learn a couple of
technical terms: viscosity, elasticity, viscoelasticity, and tensile strength.
Viscosity is essentially a measure of how easily something can flow.
Water has a low viscosity because it flows easily, while honey has a
high viscosity because it doesn't flow very well. Viscosity is often
applied to fluids because they readily flow.
Elasticity is a measure of how readily something snaps back into its
original shape when it is hit. Steel has a high elasticity because
it doesn't change shape very much when it is hit, but chewing gum
has a low elasticity because it changes shape readily and does not
regain its shape on its own after being hit.
Many things, such as the rubber you mentioned, are said to be
viscoelastic (notice that this term combines the two previous terms:
viscosity and elasticity into one term). This means they flow
somewhat and also snap back somewhat. So rubber flows -because you
can stretch it (it is viscous), and it is also elastic (because it
snaps back after you stop stretching it). Many things are somewhere
in between being completely flowing or being completely elastic -
and so we call these materials viscoelastic.
Finally, there is tensile strength. Tensile strength is a measure of
much a material will resist being pulled in one direction. So rubber
and chewing gum, which can be pulled easily, have low tensile
strength, but steel or glass have high tensile strength.
So now you can see how I intend to answer your question. The effect
of stretching a material will largely depend on whether the material
is viscous, elastic, or viscoelastic. It will also depend on what
that material's tensile strength is. Materials like glass and metals
are more elastic than fluid, and they have high tensile strengths -
so these materials are hard to pull or stretch. Comparatively,
plastics are more viscoelastic so they can be pulled and sometimes
they don't completely snap back.
As to what controls whether a material will be elastic or fluid -
really depends on how well the bits of the material hang together,
or how the molecules of the material attract and hold on to each
other. If they hold on tightly, the material may be elastic, if they
allow some slipping but have some kind of limit as to how far they
can slip (kind of like springs that can be uncoiled and stretched
but then eventually snaps back), then the material may be
viscoelastic ... and so on.
Keep asking those questions!
Greg (Roberto Gregorius)
Canisius College
Hi Tara, Braden, and Tim,
When you bend or flex or stretch materials, scientists call this
'deformation' (the root word is 'deform'). Sometimes when you deform a
material, it bounces back into its original shape. Scientists call
this 'elastic' deformation. Other times, when you deform a material,
it stays 'bent'. This is known as 'plastic' deformation (do not confuse
the word plastic here with polymers).
For most materials, if you bend them just a little, they will spring
back into their original shape. Materials like rubber can be deformed
a lot and still bound back into shape, but brittle materials like a
ceramic vase will break quickly (but they still can elastically deform
a little).
All materials are connected by some kind of chemical bond (there are
lots of types), and when you deform them, the bonds shift or stretch,
and sometimes break. If you stretch the material "enough", you will
break enough bonds for the material to break. (what is "enough"? it
depends on the material). However, even when a material seems to go
back to its original form, it still may have microscopic cracks in it
that you can't see. This is known as "fatigue", and it's a big
problem. Airline companies spend a lot of time and money trying to
detect metal fatigue in airplanes because fatigue causes metal to
weaken, and it can break under weaker-than-expected loads.
You can look up these terms for more reading: plastic deformation,
elastic deformation, fatigue (or metal fatigue).
Hope this helps,
Burr Zimmerman
Hi Tara, Braydon and Tim,
This is a very insightful question. In fact, the results of stretching
various materials differs dramatically, depending on the material in
question.
Sheet metals such as sheet steel, sheet copper, and so on are already
stretched by passing large ingots through rollers in order to make thin
sheets, with little change in properties. Stretching copper can cause
"work hardening"; that is, an increase in stiffness due to stretching or
bending.
Glass and some plastics such as clear acrylics cannot be stretched at
all without breakage.
Stretching of many plastics is a key part of the manufacturing process.
Nylon fishing line, for example, is stretched during production,
causing the long plastic molecules to be oriented linearly, resulting in a
dramatic increase in strength. Many other types plastics (but by no
means all types) that are made into thin sheets have beneficial
increases in strength as a result of careful stretching.
Regards,
Bob Wilson
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Update: June 2012
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