Material Science &
Engineering
Material Science and Engineering is the study of how the composition
and internal structure of a material can be manipulated and altered
to create a given set of mechanical properties. It is important
not just to engineers (or students) but also to those consumers
that want to understand what makes a $300-700K carbon/Aramid Handlebar
better than the $15.00 2014-T6 aluminum bicycle handle bar.
As science gets more sophisticated it becomes increasingly more
complicated for apples-to-apples comparison shopping. Here are a
few material characteristics that are commonly identified on product
data sheets and some elements of interpretation.
It is important to appreciate how one feature can affect other
characteristics involved. For instance both alloying and annealing
can greatly increase tensile strength of a material that was initially
pretty weak.
Density: Often measured in grams per cubic centimeter. Usually
less weight per given volume is the preferred characteristic, but
this often implies forfeiting other mechanical properties like strength
or rigidity so weight can rarely be used solely as a deciding factor.
Often other elements are added to a mixture in small amounts that
can affect its mechanical properties, but because the amount is
very small it does not change significantly its density. As an example
a standard aluminum alloy may have a density of 2.7 g/cm3 whereas
airplane grade 7000 series may have a density of 2.8 (only incrementally
different). This process is referred to as alloying...
Alloying: Rarely are metals and or composites formulated
out of just one element. Metals like your basic carbon steels are
predominately iron with trace amounts of things like carbon, copper,
vanadium, nickel and molybdenum. Each impurity imparts characteristics
that make the original product more suitable for the intended application.
All alloys are identified by a numerical code that identifies the
type of impurities, their percentages by weight and the heat or
mechanical treatments applied using standard
AISI/SAE designations. It is important to know not
only that you are looking at aluminum, but that it is 7075-T6
aluminum. The first set of numbers reflects its composition and
the second set of numbers ("T6") typically refers to heat
and/or mechanical treatments applied.
Here's a great link for
looking up a data sheet for numerous materials and their
"Alloyed Hybrids."
Tensile Strength:Typically this is measured in MPa
or PSI where 1 MPa= 145 psi. It is an indication of how much stress
a material can withstand before fracture. It is significantly affected
by both alloying and annealing so these specifics must be understood
if one is going to get a correct value for a material's tensile
strength. It is an indication of how much abuse a particular material
can take, before it gets thrown out.
Modulus of Elasticity:This is a measure of a material
'stiffness', or resistance to stretch. It is rarely affected to
any significant degree by alloying. The higher the number the more
rigid the material will be. Like many other material-property indicators
it can not be used in isolation.
In the case of aluminum where it has a relatively low modulus of
elasticity, fabricators with play with their physical design to
compensate. Many cycling enthusiasts talk about the an aluminum
bike frame as being mushy, but all high end fabricators increase
the tube diameter and wall thickness to compensate for this material's
lack of stiffness bringing it in line with the rigidity of a standard
steel frame.
This gives them the ability to use aluminum's other more positive
characteristics without losing rigidity. This is why you don't always
see an aluminum frame as light as you would expect. The metal is
1/3 the weight of steel but the tube dimensions are oversized.
Applied Science:
Bicycle Handlebars
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