Mechanical properties of polymers
If you ve been reading much of The Macrogalleria you ll notice that we talk a lot about polymers as being "strong" and "tough" or maybe even "ductile". Strength, toughness, and ductility are all mechanical properties. But what do these words really mean? How do we measure how "strong" a polymer is? What is the difference between a "strong" polymer and a "tough" polymer? This page is dedicated to sorting out all these matters.
Strength
Strength is a mechanical property that you should be able to relate to, but you might not know exactly what we mean by the word "strong" when we re talking about polymers. First, there is more than one kind of strength. There is tensile strength. A polymer has tensile strength if it is strong when one pulls on it like this:
Tensile strength is important for a material that is going to be stretched or under tension. Fibers need good tensile strength.
Then there is compressional strength. A polymer sample has compressional strength if it is strong when one tries to compress it, like this:
Concrete is an example of a material with good compressional strength. Anything that has to support weight from underneath has to have good compressional strength.
There is also flexural strength. A polymer sample has flexural strength if it is strong when one tries to bend it, like this:
There are other kinds of strength we could talk about. A sample torsional strength if it is strong when one tries to twist it. Then there is impact strength. A sample has impact strength if it is strong when one hits it sharply and suddenly, as with a hammer.
What is Strength?
But what does it mean to be strong? We have a very precise definition. Let s use tensile strength to illustrate. To measure the tensile strength of a polymer sample, we take the sample and we try to stretch it just like in the picture above. We usually stretch it with a machine such as an Instron. This machine simply clamps each end of the sample, then, when you turn it on it stretches the sample. While it is stretching the sample, it measures the amount of force (F) that it is exerting. When we know the force being exerted on the sample, we then divide that number by the cross-sectional area (A) of our sample. The answer is the stress that our sample is experiencing.
Then, using our machine, we continue to increase the amount of force, and stress naturally, on the sample until it breaks. The stress needed to break the sample is the tensile strength of the material.
Likewise, one can imagine similar tests for compressional or flexural strength. In all cases, the strength is the stress needed to break the sample.
Since tensile stress is the force placed on the sample divided by the cross-sectional area of the sample, tensile stress, and tensile strength as well, are both measured in units of force divided by units of area, usually N/cm2. Stress and strength can also be measured in megapascals (MPa) or gigapascals (GPa). It s easy to convert between the different units, because 1 MPa = 100 N/cm2, 1 GPa = 100,000 N/cm2, and of course 1 GPa = 1,000 MPa.
Other times, stress and strength are measured in the old English units of pounds per square inch, or psi. If you ever have to convert psi to N/cm2, the conversion factor is 1 N/cm2 = 1.45 psi.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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