Dr. Omar S.M.J.Ali PhD Orthodontic Dental Material Dr. Omar S.M.J.Ali PhD Orthodontic

Forms of Matter Change of State   Matter exists in three forms (solid, liquid, and gas). The difference in form is mainly due to different in force that held the atoms together (bonds).

Interatomic bonds Atoms are held together by some forces. These interatomic bonding forces that hold atoms together are cohesive forces. Interatomic bonds may be classified as:   1- Primary bonds. 2- Secondary bonds.

Primary Bonds These are chemical in nature.   a- Ionic bonds: these are simple chemical bonds, resulting from mutual attraction of positive and negative charges. b- Covalent bonds: in many chemical compounds, two valence electrons are shared. The hydrogen molecule is an example of this bond.

c- Metallic bonds: The third type of primary atomic interaction is the metallic bond which results from the increased spatial extension of valence-electron wave functions when an aggregate of metal atoms is brought close together. This type of bonding can be understood best by studying a metallic crystal such as pure gold. Such a crystal consists only of gold atoms. Like all other metals, gold atoms can easily donate electrons from their outer shell and form a "cloud" of free electrons. The contribution of free electrons to this cloud results in the formation of positive ions that can be neutralized by acquiring new valence electrons from adjacent atoms.

Secondary Bond In contrast with primary bonds, secondary bonds weaker bonds may be said to be more physical than chemical, they do not share electrons. Instead, charge variations among molecules or atomic groups induce polar forces that attract the molecules. Since there are no primary bonds between water and glass, it is initially difficult to understand how water drops can bond to an automobile windshield when they freeze to ice crystals. However, the concepts of hydrogen bonding and Van Der Waals forces (two types of bonds that exist between water and glass) allow us to explain this adhesion phenomenon.

Van Der Waals forces: This is due to the formation of dipole. In the symmetric atoms (e.g. inert gas) a fluctuating dipole is formed, i.e. within an atom there is accumulation of electrons in one half leading to a negative polarity and on the other half a positive polarity. This attracts other similar dipoles. A permanent dipole is formed within asymmetrical molecules, e.g. water molecule.

Physical and Mechanical Properties of Dental Materials Stress When a force (external) acts on the body, tending to produce deformation, a resistance is developed within the body to this external force. Stress: it is the internal resistance of the body to the external force. Stress is equal in magnitude but opposite in direction to the external force applied. The external force is also known as load.

Stress = F/A F= A=

Types of Stresses 1- Tensile stress. 2- Compressive stress.   2- Compressive stress. 3- Shear stress.

Tensile Stress Tensile stress is a result in a body when it is subjected to two sets of forces that are directed away from each other in the same straight line. The load tends to stretch or elongate a body.  

Compressive Stress Compressive stress is a result in a body when it is subjected to two sets of forces in the same straight line but directed towards each other. The load tends to compress or shorten a body

Shear Stress Shear stress is a result of two forces directed parallel to each other. The load tends to twist or slide of one portion of a body over another

Strain   The application of an external force to a body results in a change in dimension (shape) of that body (deformation). For example, when a tensile force is applied the body undergoes an extension, the magnitude of which depends on the applied force and the properties of the material.

The numerical value of strain is given by the expression Strain =Change in the length/Orignial length

Complex Stress It is difficult to induce just a single type of stress in a body. Whenever force is applied over a body, complex or multiple stresses are produced. These may be a combination of tensile, compressive, or shear stresses.

Poisson's ratio If we take a cylinder and subject it to a tensile or compressive stress, there is simultaneous axial and lateral strain. Within the elastic range the ratio of the lateral to the axial strain is called Poisson's ratio.

Stress /Strain Curve A tensile load is applied to a wire in small increments until it break. If each stress is plotted on a vertical coordinate and the corresponding strain (change in length) is plotted on the horizontal coordinate a curve is obtained. This is known as stress strain curve. It is useful to study some of the mechanical properties.