Ask A Scientist Materials Science Archive

  
name         Jay
status       educator
grade        9-12
location     OR

Question -   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.
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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.
AFRL/PR
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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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