Civil Engineering Reference
In-Depth Information
an energy state higher than what the atom can withstand and will force the atom to
break its bonds and reform new bonds and share valence electrons.
In essence, the classification of the bond type is dependent on how the valence
electrons are utilized by the material. In ionic bonds of NaCl, the valence electrons
are permanently transferred from a metallic element to the nonmetallic element.
In doing so, the two elements now will have equal and opposing charges. This is
a strong type of bonding because the valence electrons are not shared, but actually
transferred, and each atom exclusively owns their electrons.
Materials with ionic bonding are ceramics. These materials can also withstand
high temperatures, since their bonding strength is high and it would take a lot of
heat to increase the energy of the atoms to cause the bonds to break.
In covalent bonds, the valence electrons are shared between multiple atoms.
Methane (CH
4
) is an example of a covalently bonded material, since electrons are
shared between the carbon element and the four hydrogen elements. In this case,
each element needs the shared valence electrons to stay bonded. This type of bond
is typically not as strong as the ionic bond because the valence electrons are being
shared, rather than actually being transferred from one atom to another, so several
atoms own the electrons and each can have an effect on what happens to the total
electron count in the bond. In the case of CH
4
, if one out-of-the four total hydro-
gen elements (per CH
4
molecule) breaks and reforms with another set of elements,
then the CH
4
molecule is now left at a higher energy state and desires another
hydrogen element to share electrons.
1.4.2 Dislocations
Dislocation motion is required for plastic deformation by slip. A dislocation is sim-
ply a misalignment of the atomic structure in the lattice of a metal. There are three
types of dislocations (edge, screw, and mixed dislocations). In the edge dislocation
shown in Fig.
1.6
, there is an extra set of atoms within the top half of the lattice. The
location of where the string of atoms is un-bonded at its end is the dislocation in
the lattice structure, and it is at a higher energy state as compared with the bonded
atoms. As a force is applied to the metal, the string of atoms that were previously
un-bonded at its end will now bond with the neighboring string of atoms in the lower
half plane. This will then leave the neighboring string of atoms in the top plane,
which are one atomic unit in the direction of the force, and un-bonded. Thus, the
dislocation or the bonding defect in the lattice moved one atomic unit. As the bonds
continue to break and reform, which is caused by the external force exerted on the
metal leading to the stress exerted on the dislocation, the bonding defect (disloca-
tion) will migrate through the metal's lattice. This overall shifting of the dislocations
is termed plastic deformation. To clarify the difference between elastic and plastic
deformation, in elastic deformation, the bonds are only stressed, and the lattice goes
back to its original spacing once the metal is unloaded. Conversely, bonds must be
broken for plastic deformation to take place. Once bonds are broken, the metal can-
not go back to its original shape without re-breaking and reforming of the bonds.
