Docstoc

Selection of Materials

Document Sample
Selection of Materials Powered By Docstoc
					                                  Selection of Materials
                                         By Michael J Gunner



A Study on the Environmental Impact of the Use of Materials


Materials on the whole have a huge environmental impact. From deforestation to toxic waste, the
way we obtain, use and dispose of our materials is incredibly important for the future sustainability
of the environment. It is therefore vitally important when designing a new product that these issues
are taken into consideration. A particular material may be the best for the application, but its
environmental impact on the whole may render it a wholly unsuitable and possibly irresponsible
choice.
A good case study of selection of materials and their environmental impact is shopping bags for
supermarkets and retail outlets. The use of plastic bags has been huge over the last few decades
however as they do not degrade underground for thousands of years, therefore creating a huge
landfill problem, alternatives are being suggested. Plastics are made from the waste products of Oil
Refining, and therefore the impact of bags on the environment depends on how efficiently they
were produced. The general view on plastic bags is that they are environmentally unfriendly and a
cause of large amounts of waste that does not degrade.

Life Cycle Analysis
However, whether the much vouched alternative paper is a better option is arguable. It is often
overlooked how environmentally unfriendly paper can be. By thinking in terms of life cycle
analysis, where the material is looked at from being obtained to being disposed, the selection of
materials can be looked at. Depending on the source of the paper, in obtaining the material
deforestation may have occurred. Deforestation is highly environmentally damaging, causing soil
erosion, the loss of wildlife habitats and a loss of wildlife. Therefore the extraction of wood is a
highly debatable issue. However, if the material is extracted from sustainable sources then wood is
more friendly then plastic. The transportation of wood is also highly damaging as wood takes up a
lot of space. The result of this is that it is inefficient to transport, and much more inefficient than
plastics. Manufacturing wood into paper is also a problem. Many toxic chemicals are injected into
the paper and the process creates a lot of waste.




Life Cycle Analysis – By looking at the LCA of paper and plastics, which one is actually the most
environmentally friendly can bet determined.
Source: http://techalive.mtu.edu/meec/module14/title.htm
One issue for paper bags is longevity. The issue of waste is also highly debated. Many argue that
the refusal of plastic to degrade means that we must use paper because it does degrade. However,
they are misinformed. Because of the way landfill sites are designed and operated, nothing
degrades. It makes almost no difference at all whether you are land filling paper or plastic – neither
will degrade because the waste is isolated from air and water in order to prevent groundwater
contamination and air pollution. Due to the much less space occupied by land filled plastics over
paper, it is therefore reasonable to conclude that if a shopping bag is disposed of through landfill,
plastic is actually the more environmentally friendly material choice. However, the debate between
the two is too vast to conclude.
Recycling is an option, and enables a material to be almost completely re-used. Many materials can
be recycled, including paper, plastics and some metals. The benefits are clear, there is a highly
reduced amount of waste and the need for obtaining raw materials is significantly reduced. The end
result is therefore a much lower impact on the environment. Selecting a material that can be
recycled is a good decision and helps ensure the product has low environmental footprint. The
manufacturing process can also be adjusted to encourage recycling – leftover material is put back
into the process rather than just being discarded.



Domestic Product Material Failure

iPod Nano Screen Failure

Apple have re-invented the portable music player with their range of iPods. In 2005, they added to
their range with a completely different take. The Nano was just the thickness of a pencil, yet packed
a full colour screen and hard drive storage. But when the Nano first came out, it became plagued by
bad media about problems with its screen. Cracking, scratching too easily and screen failure became
common complaints from Nano customers.




"I don't really care if the case on my Nano gets scratched but my screen has scratched up so badly
that all the images are starting to become distorted," iPod Nano owner Brian Cason says, echoing
the sentiment of many others in the discussion. "I have only carried it in my small pocket in my
shorts and nothing is in there to scratch it. I still can't figure how the screen looks like it has been
rubbed with sandpaper when the entire time it has been safe in my pocket (with absolutely no
items)." Source: c|net News.com

The iPod Nanos screen is made from Polycarbonate. The plastic has a very high hardness rating in
CES of 150Mpa. Although it has a Poor rating for wear, so do all thermoplastics. So the issue here
doesn’t seem to be so much to do with material selection. However, numerous reports on the
Internet suggest polycarbonate is not very resistant to scratching and that the use of it for the Nano
is unsuitable.

It was revealed later on by Apple that the issue was a problem with the quality of the material used
on a small batch of Nanos. Therefore there may have been a problem with the manufacturing
process for the screens.

However my friend up until recently owned a Nano, a whole 2 years on from its release. The screen
broke. The player was only 7 months old and wasn’t put under any unusual stresses or strains.
When applying pressure, the screen felt very thin. I think the issue here may be that the small design
of the Nano means the thickness of the plastic is less than desirable. On the actual iPods, the
polycarbonate is very thick. This means it can resist cracking and breaking more easily. With the
Nano, although the whole player is covered with polycarbonate, there is a gap between the material
and the screen and when pressure is repeatedly applied, the screen suffered from fatigue and broke.
The original material, polycarbonate, was selected because it had previously been used for the
bigger iPods. A short while back, Apple re-designed the Nano with a wraparound anodized
aluminum shell and a separate thicker polycarbonate screen. Apple could have avoided the original
failures and poor design by using aluminum for the body shell from the start, and using a thicker
polycarbonate screen.


Large Scale Engineering Materials Failure

deHavilland Comet Airliner

The deHavilland Comet Airliner was Britains first foray into the world of Jet Liners. In fact, it was
the worlds first commercial Jet Airliner. However, after several disastrous crashes, it became clear
there were problems with the craft.




On the 10th of January 1954, G-ALYP crashed off the Italian island of Elba killing everyone on
board. The Abell Committee which investiagetd the crash looked at metal fatigue, engine failure
and other areas but concluded that fire had caused the crash. Comet flgihts resumed on the 24th of
March. However, on the 8th of April, G-ALYU crashed near Naples. It was then that a fully fledged
investigation began.

After two investigatons by the Minister of Transport and the Royal Navy, it was concluded that the
craft was suffering from metal fatigue. The Comet was made from fairly thin aluminium, which
only has a fracture toughness of 32Mpa m1/2 meaning it was susceptible to cracking as a result of
fatigue. The fatigue caused the airliner to break up in mid-air. Engineers subjected a Comet
airframe to repeated repressurisation and overpressurisation and after 3057 flight cycles the Comet
airframe failed due to metal fatigue near the front side hatch. Further research found that stress
levels around the corners of the windows were extremely high. The original Comet had square
windows, and the cracks were forming from the corners. The stress was said to be around 40,000
Psi (Source-Wikipedia). The principal investigator concluded, "In the light of known properties of
the aluminium alloy D.T.D. 546 or 746 of which the skin was made and in accordance with the
advice I received from my Assessors, I accept the conclusion of R.A.E. that this is a sufficient
explanation of the failure of the cabin skin of Yoke Uncle by fatigue after a small number, namely,
3.060 cycles of pressurisation.". Stress naturally is higher on the corners of a material, and it was
concluded the square-shaped windows were the major cause of the crashes. Subsequent Comets
were built with oval windows but the damage was done, and Comet never regained their superiority
in the air.

Sources: Wikipedia,
http://www.tech.plym.ac.uk/sme/Interactive_Resources/tutorials/FailureCases/index.html

				
DOCUMENT INFO