Lec # 3
Date: 23/2/2011
Dental Materials
In the last lecture we’ve talked about stress and strain, and we’ve discussed this chart….now let’s take an example:
If we get a 24 K piece of gold, 14 K gold, and a glass…match each one of these alloys with its appropriate graph in the following chart.
X:stiff, ductile,strong, and tough materials.
Y:flexible, and brittle.
Z:flexible, ductile, and weak.
So the 24 K gold is Graph Z, 18 K gold is X, And Glass is Y.
We’ve taken in the last lecture that if the gold is pure we can use it as a direct filling material (It’s actually been used in America), and to use it in this way we have to shape it the way we like. It won’t fracture easily and quickly and it actually has a long period of plastic deformation before fracturing. So we usually mix gold with other metals to gain hardness.
Sometimes adding or mixing metals reduce the hardness and the metal becomes less tough.
The hardest the material the highest the temperature we need in order to shape it the way we like such as Steel.
As For Glass, fracture may happen after a very short period of ending the elastic deformation period.
We all know that Metals have ionic bonds, and these bonds are moveable (can be shaped easily) compared to the covalent bonds which are hard to be shaped.
-A better way to measure strength is by testing rings cut from the material, and this test is called Diametral compression test in which a compressive load is applied by two pressure plates parallel to the ring access. For example, regarding brittle materials any notch from such material will fracture much easier.
Each term we’re going to take in this lecture is going to be taken and explained later….so you all just have to understand them generally.
Bond strength
It is the bond strength between two dental materials (*Adhesivetype of bond). They are either elastic or shear. Universally it’s known that the bond shouldn’t be less than 20 megapascal.
In order for the material to stay in the oral cavity dentists should take care of the: - Reliable Bonding: bonds’ strength should exceed 20 megapascal.
-Diablobond: because the material will face Saliva and temperature.
- Adhesion:in order to occur, an attraction between two materials is needed but there are conditions: -They must have the Same nature ex: both hydrophilic or both hydrophobic.
-Clean surface, with no grease.
-Rough irregular surface in order to have a larger surface area and a larger surface energy.
It’s written above that Bond strength is either tensile or shear, well why not compressive?
The reason is that most fractures occur because of tensile or shear stress….why?
We’ve taken before that compressive is stronger than tensile or shear because approaching atoms near each other is harder than distancing them, so the easier way to have a fracture is by tensile or shear not compressive.
-Fatigue Bond Strength is caused by repetitive force. Fractures don’t occur Because of just one impact load but by repetitive ones. So the more cycles (forces on the material itself) the more the fractures even if the force is low.
Bending and Torsion
Endodontic files and reamers, by this method we can pull debris form the root canal.
Transverse Strength
Materials might be supported between two ends, this is called Modulus of rupture or flexural strength. Another name for this test is the “3 points bending test”, two points at the side and one in the middle.
We’ve said that fatigue is a progressive fracture under repeated loading. You should know that cycles the more stress we need. For example: partial dentures are attached to the teeth by either stanless steel wires or copult wires, and these dentures should be removed daily at least 5 times per day….before eating etc….
So putting and removing it (more cycles) will decrease the stress to break down the material (Fracture is easier) and the endurance limit decrease too.
Endurance limit : after it, fractures happen.
Viscoelasticity
-Relates to load application rate
-Higher rate loads give higher elastic modulus
-Significance for impression materials
-Thixotropic impression material or behavior: it’s when the material is injected with a high force in order to have a runny material, and it soldifies when it appears around the tooth where the force is lesser.
Note: Viscosity is related to the liquid materials not solids but here in dental materials there is sometimes some viscosity due to the forces affecting the material.
The Homework
Viscosity (μ ) = shear of stress/ shear of strain
It’s unit = Pa.s = Kg/(s.m)
The word viscosity is derived from latin word “Viscum alba” for mistletoe.
The study of flowing matter is known as Rheology, which includes viscosity and related concepts.
-Newtonian type of viscous fluid behavior has a direct relation between shear stress and shear rate.
Which one of the following types is better for thixotropic behavior….Pseudoplastic, Newtonian, or Dilatant?!!
I’ve found that Pseudoplastic is the better one but I’m not sure about it, so please search for it.
Creep and stress relaxation
-Creep is the increase in strain in a material under pressure.
-Creep test is used for study of new amalgam materials.
The Homework
Impact strength and tear strength and their importance…..
Impact strength is the ability of a material to withstand shock loading.It is useful in predicting resistance of a material to breakage from dropping or other quick blows.
Tear strength is the force required to rip a sample of plastic film,sheeting, or rubber product.This test is very important for all films as well as for paper. High tear values may be needed for machine operations or for package strength.
Surface Mechanical Properties
- Hardness is the ability to resist scratches (surface properties)
-Toughness is related to elastic and plastic deformation.
-Sometimes Oxidation is beneficial such as in tetanium because it helps in osteoinduction, but in steel it is not.
Identation of Hardness:
-Brinell hardness test, Ball, (steel or tcarbide)
-Knoop hardness: Microindentation pyramid shape.
-Vickers:136 ° diamond pyramid
-Rockwell: Metal cone.
-Shore A hardness for rubber.
Stress Analysis
How much strain result from an applied stress.
Lab Based studies.
Photoelasticity.
Finite Element Analysis ( simulation) is a method.
“Learning without reflection is a waste, reflection without learning is dangerous.” ☺
Done By: Rawand Abu Abboud