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Name: Jay
Status: Educator
Grade: 9-12
Location: OR
Country: USA
Date: January 2005


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.



Replies:
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


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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