properties of dental materials; mahalaxmi Flashcards
list the attributes of thermal properties of materials
- thermal conductivity
- thermal diffusivity
- coefficient of thermal expansion
discuss thermal conductivity
- Thermal conductivity (κ) is the physical property that governs heat transfer through a material by conductive flow.
- The conduction of heat within a solid involves the transfer of thermal energy from one end of a material to another across a temperature gradient.
- Thermal conductivity is defined as the quantity of heat in calories per second passing through a material l cm thick with a cross section of 1 cm2 having a temperature difference of 1 kelvin (K) (=1 °C) and is measured under steady-state conditions in which
the temperature gradient does not change. - The International System (SI) unit or measure for thermal conductivity is watts per meter per kelvin (W⋅m−1 ⋅K−1).
- In general, thermal conductivities increase in the following order, although there are exceptions:
polymers < ceramics < metals. - Materials that have a high thermal conductivity are called conductors, whereas materials of low thermal conductivity are called insulators.
discuss thermal diffusivity
- Thermal diffusivity (h) is a measure of the speed with which a temperature change will spread through an object when one surface is heated.
- Thermal diffusivity is calculated from the thermal
conductivity divided by the product of density and heat capacity. - A material with a high density and high specific heat will likely have a low thermal diffusivity. Such a material changes temperature very slowly.
- Low heat capacity and high thermal conductivity lead to high diffusivity, and temperature changes transmit rapidly through the material.
coefficient of thermal expansion
- When materials undergo a temperature increase, the vibrational motion of atoms and mean interatomic (bond) distances increase. This results in an increase in volume or an expansion of the material.
- The increase is described by the coefficient of thermal expansion, α, which is defined as the change in length per unit of the original length of a material when the temperature of this material is raised 1 K.
- The units are typically expressed as either mm/m/K or ppm/K.
- This parameter is extremely important in dental applications as broad ranging as producing cast restorations that fit and maintaining the seal of a restoration margin.
- The influence of this property often
dictates the procedures that have been developed for using wax patterns, casting metal crowns, placing amalgam and composite resin restorations, and preparing metal-ceramic crowns and bridges.
discuss stress & strain
- Stress is the force per unit area.
- When a force is applied to a material, the material inherently resists the external force. The force is distributed over an area, and the ratio of the force to the area is called stress.
- Several types of stress may result when a force is applied to a material. These forces are referred to as compressive, tensile, shear, twisting moment, and bending moment (flexure).
- A material is subjected to compressive stress when the material is squeezed together, or compressed, and to tensile stress when pulled apart. Shear stress occurs when one portion (plane) of the material is forced to slide by another portion. These types of stresses are
considered to evaluate the properties of various materials. - Strain is the change in length per unit length of a material produced by stress.
- The change in length or deformation per unit length when a material is subjected to a force is defined as strain.
- Strain is easier to visualize than stress because it can be observed directly.
- The units of strain are dimensionless.
- Some dental substances, such as elastomeric impression materials, exhibit considerable strain when stress is applied; others, such as gold alloys or human enamel, show low strain under stress.
- There is elastic strain and plastic strains.
describe tensile stress
Tensile stress
Results 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.
describe compressive stress
Compressive stress
Results when the body is subjected to two sets of forces in the same straight line but directed
towards each other. The load tends to or shortens a body.
describe shear stress
Shear stress
Shear stress is a result of two forces directed parallel to each other. A stress that tends to resist a
twisting motion, or a sliding of one portion of a body over another is a shear or shearing stress.
discuss the stress-strain graph
Generally, for every material, the stress–strain graph can be prepared by gradually loading the material under standard testing machine and conditions.
The strain values, when the load is applied, are measured and these are used to calculate stress values.
These stress and strain values are then plotted on a graph; stress is plotted on the vertical y-axis and strain on the horizontal x-axis, producing the stress–strain graph for that material.
The graph can be explained in the following ways:
- As stress is applied to a material, strain is produced. Initially, this strain is proportional to the stress applied. This is represented by the straight line (green line) up to the point PL in the graph. This is the point of proportional limit (PL).
- The short line (blue line) from PL to EL represents the elastic strain/deformation where strain is not proportional to stress. EL is the elastic limit of the material.
- Up to EL, the deformation of the material is not permanent and can be recovered fully.
- The slope of the straight line up to PL represents the modulus of elasticity.
- The elastic range is the amount of stress that the material can withstand within the elastic limit. The line from point 0 to EL represents the elastic range.
- The area below the modulus within the elastic range is the resilience of the material.
- Beyond EL, the curved part of the graph (orange line) represents uniform plastic deformation of the material up to the ultimate tensile strength (UTS).
- The UTS is the point up to which the material undergoes plastic deformation without necking or fracture.
- The length of the curved part up to UTS represents ductility.
- In brittle materials, UTS and FS (fracture strength) coincide and is the point up to which the material can withstand stress before fracture. At UTS, the material just fractures.
- Materials that exhibit some elastic behavior do not fracture at the UTS. At this point, when further stress is applied, a cross-sectional area over a short length begins to reduce rapidly forming the “neck.”
- Since the cross-sectional area reduces, the stress required is also reduced and hence the curve bends downward (red curved line) up to FS, where the material can no longer withstand the stress and it fractures.
- The line from EL to FS represents the plastic range.
- The area under the entire length of the line up to FS represents the toughness of the material
What are the various hardness tests used for dental materials? Discuss the relevance of hardness to other properties.
Hardness is defined as the resistance of a material to plastic deformation typically measured under an indentation load.
The tests most frequently used in determining the hardness of dental materials are as follows:
Macro hardness tests:
- Brinell hardness test
- Rockwell hardness test
Micro hardness tests:
- Vickers hardness test
- Knoop hardness test
Brinell hardness test
It has been used extensively for determining the hardness of metals and metallic materials used in dentistry. In the Brinell test, a hardened steel ball is pressed under a specified load into the polished surface of a material. The load is divided by the area of the projected surface of the indentation, and the quotient is referred to as the Brinell hardness number (BHN).
Rockwell hardness test
It is somewhat similar to Brinell hardness test in which a steel ball or a conical diamond point is used. Here the depth of penetration is measured directly by a dial gauge on the instrument. Rockwell hardness number (RHN) is designated according to particular indenter and load employed.
Neither Brinell hardness test nor the Rockwell hardness test is suitable for brittle materials.
Vicker’s hardness test
It employs same principle as that used in Brinell test; instead of steel ball, a square based pyramid is used. Here the load is divided by the projected area of indentation. It is suitable for determining the hardness of brittle materials, e.g. tooth structure, casting gold alloys, etc.
Knoop hardness test
It employs a diamond-tipped tool rhombic in outline. The projected area is divided into the load to give the Knoop hardness number (KHN). The hardness values for exceedingly hard and soft materials can be obtained by this test.
proportional limit
The proportional limit is the maximum stress at which stress is proportional to strain and above which plastic deformation occurs.
The proportional limit is the greatest elastic stress possible in accordance with Hooke’s law.
poisson’s ratio
Provided the fact that the volume and density are fixed, a change in one axial dimension (axial strain) results in a change in the lateral direction.
This change in the lateral direction bears a fixed relationship to the axial strain.
This relationship or ratio within the elastic limit is called Poisson’s ratio after the name of its discoverer Siméon Poisson.
It is usually symbolized by n.
Poisson’s ratio indicates that the alteration in cross-section is proportional to the deformation within the elastic range.
modulus of elasticity
Modulus of elasticity is also known as elastic modulus or Young’s modulus.
It describes the relative stiffness or rigidity of a material.
Ratio of elastic stress to elastic strain or tensile stress/ tensile strain or compressive stress/compressive strain gives a proportionality constant known as elastic modulus or modulus of elasticity.
Modulus of elasticity is given in units of force per unit area, i.e. giganewtons per square metre (GN/m2 ) or gigapascals (GPa).
Correlation between elastic limit and yield strength
The elastic limit of a material is defined as the greatest stress to which a material can be subjected such that it returns to its original dimensions when the force is released.
Yield strength: The stress at which a test specimen exhibits a specific amount of plastic strain.
dimensional change
Dimensional or gross weight change A change to a physical dimension, on any axis, or gross weight over 20%, requires assignment of a new GTIN.
