After qualifying intermediate, we enthusiastically dream about our dental subjects but land up in the same boring physics of dental materials. Let us know why is it important to understand various properties…
Mechanical properties :
Stress – the force per unit area acting on millions of atoms or molecules in a given plane of material. Stress is the internal resistance of a material to an external load applied on that material.
- Residual stress is caused within the material during the manufacturing process. Eg – during welding
- Structural stress is produced in the structure during function. Eg – in abutments of fixed partial dentures
- Pressure stress is induced in vessels containing pressurized materials. Eg – in dentures during processing under pressure and heat
- Flow stress is produced when force of liquid strikes against the wall acting as load. Eg – molten metal alloy striking the walls of the mould during casting
- Thermal stress is produced by material which is subjected to internal stress due to different temperatures causing varying expansions in the material. Eg – materials that undergo thermal stress such as inlay wax, soldering and welding alloys.
- Fatigue stress is produced due to cyclic rotation of a material. Eg – rotatory instruments undergo rotational or cyclic fatigue.
Strain – it is defined as the change in length per unit original length and it may be elastic or plastic or a combination of both. Elastic strain is reversible i.e it disappears when force is removed. Plastic strain represents permanent deformation of the material which never recovers when the force is removed.
Young’s modulus : it is the stiffness of a material that is calculated as the ratio of the elastic stress to elastic strain i.e a stiff material will have a high modulus of elasticity while a flexible material will have a low modulus of elasticity.
Eg – principle of elastic recovery – burnishing of an open metal margin, where a dental abrasive stone is rotated against the metal margin to close the marginal gap as a result of elastic and plastic strain
Eg – impression material
The impression materials should have a low modulus of elasticity to enable it to be removed from the undercut areas in mouth. Modulus of elasticity should not be too low that the material cannot withstand tearing.
Hooke’s law : within the limits of elasticity the strain produced by a stress is proportional to the stress
Dentin is capable of sustainable significant plastic deformation under a compressive load before it fractures. Enamel – more stiffer and brittle than dentin. But dentin is more flexible and tougher.
Flexibility – defined as the flexural strain that occurs when the material is stressed to.its proportional limit. Materials used to fabricate dental appliances and restoratiots, a high value for the elastic limit is a necessary requirement. This is because the structure is expected to return to it’s origi al shape after it has been stressed and the force removed.
There are instances where a large strain or deformation may be needed with a moderate or slight stress such as in an orthodontic appliance. Here a spring is often bent a considerable distance under the influence of a small stress. In yhis case, the structure is said to possess the property of flexibility.
Resilience – the amount of energy absorbed within a unit volume of a structure when it is stressed to its proportional limit. When a dental restoration is deformed during mastication, it absorbs energy. If induced stress is not greater than proportional limit, the restoration is not permanently deformed i.e only elastic energy is stored in it. So restorative material should exhibit a moderately high elastic modulus and relatively low resilience.
Proportional limit – defined as the magnitude of elastic stress above which plastic deformation occurs. Below the proportional limit, there is no permanent deformation in a structure. Materials like cobalt/chromium alloy which has high proportional limit is widely used for the fabrication of connectors because they can withstand high stresses without permanent deformation.
Yield strength – defined as the stress at which a test specimen exhibits a specific amount of plastic strain. It is a property often used to describe the stress at which the material begins to function in a plastic manner. In the process of shaping an orthodontic appliance or adjusting the clasp of a removable partial denture it is necessary to apply a stress into the structure in excess of yield strength of the material is to be permanently bent or adapted.
Flexural strength – defined as the force per unit area at the instant of fracture in a test specimen subjected to flexural loading. Also known as modulus of rupture. Most prosthesis and restoration fractures develop progressively over many stress cycles after initiation of a crack from a critical flaw and subsequently by propagation of the crack until a sudden, unexpected fracture occurs.
Conclusion – while designing a dental appliance or a restorative material, it should have adequate mechanical properties to withstand the stress and strain caused by the forces of mastication. All the methods must be employed to minimize stress concentration so that the restorative material or the appliance is in harmony with the different types of forces occuring in the oral cavity.
Source : Phillip’s and Craig’s restorative dental materials