YIELD STRENGTH OF PLASTICS
§ This article is very illustrative of
the typical properties that can be calculated from a standard stress-strain
curve including the yield strength and elastic modulus.
§ We'll discuss a table showing the
differences between various plastics and other materials that are typically
used to design new parts. Read on if you want to learn some more.
Yield strength is one of the
most important material properties to be aware of. It’s something
that every mechanical engineer should know about.
Why do some plastics break cleanly
and others deform and elongate prior to break? This is a common question
and an important phenomenon to understand. The yield strength of plastics holds
the key.
Many plastics are used to replace
metals, and what is very important is to understand the strength to density
ratio. Plastics are typically much less dense than metals which help with
mechanical efficiencies and the reduction in energy load, but one must
understand the strength of the plastic. The most common way to measure the
yield strength of plastics is with a tensile test. A tensile test is typically
governed via standards, and the two most common in the plastics industry are A
The tensile test
The test measures the force as a
function of the strain being applied to the plastic sample. An example of the
test setup is shown below:
From this test, many important
mechanical properties can be derived. A graph showing a common stress-strain
curve for a plastic material is shown below:
Important terms
§ Young’s
Modulus –
This is the slope of the linear portion of the curve. Another term is elastic
modulus; it is a quantitative way to describe the linear elastic behavior of
the material. Units are typically in Pascals (Pa)
or pounds/inch (psi).
§ Yield
Strength –
The yield strength of the plastic is the where the material begins to deform in
a plastic fashion. Prior to the yield strength, the material will act
elastically meaning that if the strain were halted at any point in the elastic
portion, the material would return to its original length. Once the yield
strength of the plastic is attained, the material will not return to its
original length and will yield. Units are typically in Pascals (Pa) or pounds/inch
(psi).
§ Ultimate
Strength –
The ultimate strength is the maximum amount of stress that can be applied.
Units are typically in Pascals (Pa) or pounds/inch (psi).
§ Fracture – The point at which the
material snaps.
§ Strain
hardening –
This is the region where the material is experiencing some deformation but can
receive additional stress without weakening.
§ Necking – This is the region where the
material has passed the ultimate strength and is not deforming significantly
and is visually observed with a decrease in the cross-sectional area of the
specimen.
The yield strength of plastics in
mechanical design
A key property for mechanical
designers using plastics is the yield strength, but there is a
key caveat to mention. It would seem that the yield strength would be
exactly where the plastic becomes inelastic. In reality, due to molecular
bonding, the material can sometimes return to its original length after some
portion of inelastic deformation. Therefore, it is very common to note
the yield strength at a specific strain rate where 0.2% is the most standard.
So, an understanding of the
mechanical yield strength is paramount in designing with plastics or other
materials because of the predictability of the material. Prior to the yield
point, the properties are quite predictable but after that yield point, the
properties become more variable, and confidence in the predictability
significantly decreases. Another key point is that some plastics do NOT have a
yield point. Some plastics only deform in a brittle fashion, meaning their
deformation is linearly elastic and once the maximum strength is attained, the
material “fractures.” This is observed in some compounded
(fillers/additives/inorganic materials) products and some materials with a very
high elastic modulus. This type of failure is observed in composites as
well.
Yield strength of plastics table
The table below gives a comparison
of various plastics and other materials of interest:
Material
|
Yield Strength (MPa)
|
Polypropylene
|
12-43
|
Nylon 6,6
|
45
|
High density
polyethylene
|
26-33
|
Polyvinyl
chloride
|
55
|
Polyvinylidene
fluoride
|
48
|
ASTM A36
Steel
|
250
|
Human skin
|
15
|
Bone
|
104-121
|
Diamond
|
1600
|
Copper
|
70
|
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