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Note:
the following analysis and its write-up are property of The Madison Group
and cannot be copied and/or distributed in anyway without prior permission
from The Madison Group. This analysis in no way suggests that any or all
plastic pipe failures occur in the manner described. Each plastic failure
is unique and should be treated as such.
Plastic pipe, tubing and other profiles are an extremely popular alternative
to copper, steel, aluminum and other materials. In fact, by 2003 it is
predicted that 33% of all US pipe production will be made with plastic.
Plastic pipes, tubing and profiles are used in a wide variety of industries
including, building & construction, automotive, consumer goods, lawn
& garden, windows & doors, furniture, plumbing and electrical.
One of the most widely used materials for these products is polyvinylchloride
or commonly known as PVC. This material is popular in these industries
because of the wide range of properties that can be obtain depending on
the additives that are mixed with it. PVC can be made to have high strength,
rigidity and hardness; good electrical properties; high chemical resistance
and be self extinguishing - all this at a relatively inexpensive
price. However, depending if a plasticizer additive is used with
the PVC, along with what kind and how much, the characteristics of the
final part can be dramatically altered to have high impact strength with
relatively low hardness and rigidity.
An unplasticized PVC pipe, shown above, is quite rigid with high strength and
good chemical resistance. These properties make it attractive
for use in above or below ground plumbing applications. However,
a very important change in property occurs as the temperature
gets colder - the impact strength of PVC drastically changes for
the worse. This means that at low temperatures the ability of
PVC to dissipate the energy from a sudden blow is limited and
may result in part failure. The best way to describe this phenomenon,
apart from demonstrating the impact of several PVC pipes at different
temperatures, is to graph the impact strength of PVC as a function
of temperature, shown in the figure below.
The most interesting part of this graph, the area that can explain many
plastic failures, is boxed out in gray. Here, you will see a dramatic decrease
in impact strength as the temperature gets colder - the part is becoming
increasingly more brittle. The impact strength is 4x less at -10oC
than at 20oC - a temperature range that is easily experienced
in many regions of the US. This phenomenon is one that is not seen with
every day metals and is commonly overlooked when designing with plastics.
One can improve this situation by using additives, in this case a plasticizer,
that ultimately moves the graph to the left and gives the part a high impact
strength at a much lower temperature. However, the gain in one property
usually means the loss of other properties, in this case, the loss of stiffness.
The figure below shows the modulus (stiffness) of PVC as a function of
temperature (solid line). The dashed line indicates the temperature at
which the modulus will decrease dramatically, approximately 50oC
for this PVC. For many uses, 50oC is a temperature that the
product would never experience, however, if an additive is used to increase
the impact strength (as described above) then this graph will also move
to the left lowering the temperature that the stiffness is lost.
Thus, a comprise must be made for how much, if any, additive is to be
used for the application and environment that the product will be used.
In the case of PVC pipe, high mechanical strength, rigidity, hardness and
high chemical resistance is required at the lowest cost. Plasticizing additives
typically add to the cost of a product and are not use in pipe production.
Other additives can reduce costs, such as, calcium carbonate. Unfortunately,
these cost reducing additives typically make the product even more brittle
causing the Impact Strength graph shown in the first graph to move to the
right, making the product more brittle and more susceptible to failure.
An example of a failed PVC pipe is shown in the figure below.
To determine the cause of failure a variety of techniques can be used
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Visual Inspection of the Failed Part
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Structural Finite Element Analysis
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Dynamic Finite Element Analysis
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Material Evaluation
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Process Evaluation
A visual inspection of the part indicates that this was a brittle failure
as opposed to a ductile failure. Many brittle failures occur very quickly,
whereas, ductile failures will typically occur over a longer period of
time. It was revealed that the pipe was in a cold condition of approximately
-5.0oC. The pipe was in an enviornment where the temperature
was low enough that it became very brittle. A force, which could be caused
by an external blow or from internal pipe pressure, became to great and
the part failed catastrophically. The cause of failure may not be because
the engineer specified the wrong pipe for the job or enviornment, but because
of the formulation of the material or the processing conditions at the
production plant were wrong.
To determine if the pipe had the correct formulation a wide variety
of material tests can be performed. One such test is the thermogravimetric
analyzer (TGA). This device is often used to identify the components of
a plastic part. It works by gradually heating a small sample of the plastic
to a very high temperature. At different temperatures the compounds of
the plastic will decompose. The TGA accurately records the change of weight
with respect to the temperature. The figure below shows an example of a
TGA test on PVC sample. Here, the decomposition of the different compounds
can be seen along with the percent weight lose. Using data from an extensive
library, the decomposition peaks are matched with known materials to decompose
at the same exact temperature.
To establish the mode and forces of failure, along with providing confidence
that a failure took place in the manner that was determined, a
finite element analysis (FEA) can be made. This type of analysis
allows the engineer to place the part in a realistic environment
under normal to extreme conditions and observe what happens to
the part - if failure occurs. The animation below shows the predict
failure of a pipe caused by an extreme internal pressure using
FEA.
(click on figure to view FEA failure of animation - 340kB)
(click on figure to view FEA failure animation - 510kB)
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