Date: January 2005
Where does metal get its strength? I know that edges
increase strength, and angles/folds help as well, but in terms of
overall tensile strength versus thickness or number of angles/bends.
I am not sure I am following your question, but I'll try to elaborate on
metal strength. Metals get their strength from their material make up
(i.e. their atomic makeup) and microstructure characteristics. Different
metals have different bits of material in them that bring out a certain
microstructure. The metal can also be manufactured a certain way to make
its microstructure different as well. These two things really define how
much stress a metal can withstand before it yields. This measurement of
stress (or strength of the metal) is in units of force per area, which in
English units is lbs/in^2. Metal handbooks will give you values of yield
strength and ultimate tensile strength for various metals.
Now if you know the force that will be placed on a metal, you can
determine the area over which you can disperse this load in order to never
reach the yield point of the metal. This is what I believe is your
question in regards to folds, angles or thickness. Different metal shapes
have different cross sectional areas that are the in^2 part of the
equation. The thicker the material, the higher the cross sectional area
and thus the lower the stress on the metal. However, putting holes or
sharp corners on a certain shape can reduce the cross sectional area and
thus raise the stress put onto a particular shape. So the shape plays a
part in the area of force that is placed on a piece of metal.
There are other things that change the strength of a material. Corrosion,
high or low temperatures, inherent flaws of manufacturing, and fatigue are
just of few things that can change the strength of a material. Material
engineering is not black and white when it comes to saying the strength of
a metal is always "x" under these conditions. There are many examples of
engineers making wrong assumptions about material strength that led to
failure of a component because they did not realize that the metal would
be subjected to fatigue or higher temperatures or higher loads, etc. When
determining an application for a metal, there are many variables that an
engineer must take into account before determining if a material is
suitable or not. That to me is what makes this job fun.
Hope this helped. Thanks for using NEWTON.
Christopher Murphy, P.E.
Molecules or atoms that form metallic bonds are inherently different from
other solids that form other types of crystal structures, or molecular
bonds. Take sodium, for example, when sodium forms a crystal, each sodium
atom comes into contact with 8 other sodium atoms and its outermost
electron in the 3s1 orbital is delocalized throughout the crystal. When we
go next to magnesium, we find that it has two electrons in the 3s orbital
and both of these electrons participate in the delocalization further
lowering the energy potential. As a result, magnesium has a higher melting
point then sodium and is a lot tougher. When we go to the transition
metals, something like iron for example, not only do the s-orbital
participate in the overlap, but the d-orbitals do so as well - and the
more electrons in the overlaps the tougher the metal. Finally, if we look
at the crystal packing constants of metals, we find that most of them are
found in "close-packed" lattices such as face-centered or body-centered
structures - such close packing gives greater strength.
Also important in this consideration is the fact that the metal crystal
structure is made up of one type of atom alone, or -in alloys- one type
plus another atom that is similar in properties and exchange for one or
more of the matrix atoms. This allows atoms to "slide" around from lattice
to lattice without necessarily disrupting the crystal structure. This is
not true for ionic crystals, such as table salt. While salt also forms
close-packed structures, it does not have the same "sliding atoms" ability.
We also need to take into account the existence of crystal grain
boundaries or dislocations. In ionic salts, such crystal grain boundaries
cause individual crystals to break from the mass and so we have individual
grains of salt. In metals, dislocations prevent the easy "sliding" of
atoms and so the metal becomes harder (less ductile or malleable). But the
grain boundaries are also were cracks may propagate. So increasing the
number of boundaries makes the metal brittle (like salt). -- For example,
banging a metal when it is cold produces more fissures and makes the metal
harder (and more brittle). Heating a metal allows the atoms to move around
and "fix" dislocations and grain boundaries, and returns it to a more
malleable and ductile state.
Greg (Roberto Gregorius)
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Update: June 2012