Kirsty's dental materials Flashcards
What features would create the ideal dental restorative material?
Biocompatible Good bonding ability to tooth or bone Good aesthetics Hold properties similar to those it is reconstructing Capable of initiating tissue repair
What are the three categories of dental materials?
preventative restorative auxiliary
What features lead to obselence in a dental material?
Failure of the material to satisfy any of the features which render it an ideal restorative Replacement with an equal but cheaper product which performs all the same tasks. replacement with an equal product which is less technically sensitive
What is the diffusion coefficient?
The amount of a substance that diffuses across a given area through a unit thickness of the substance in one unit of time.
What happens to the diffusion coefficient of metals with a low melting point and why?
The diffusion coefficient increases as melting point decreases because as the temperature increases in a substance the atomic bond energy decreases allowing for more rapid diffusion
What is meant by wetting ?
The ability of a liquid to flow easily over a solid surface and adhere to the solid.
How does contact angle affect wetting?
The higher the contact angle the less wettable the solid. A contact angle of 0 is a perfectly wettable solid and one of 180 is unwettable
What is viscosity?
The resistance of a fluid to flow. It is defined as stress divided by strain. stress being the force divided by area over velocity divided by distance of deformity due to force.
What does thixotropic mean?
Repeated applications of pressure cause a fluid to become less viscous and subsequently more flowablee
What is creep?
Creep is the the time dependant plastic strain of a material under a static load or a constant stress.
What is hue?
the predominant colour of an object
What is value relating to colour?
It it is the relative lightness or darkness of a colour on the gray scale.
What is chroma?
iI is the degree of saturation of a hue.
What is metamerism?
Different lights will trait different spectrums and so sub subsequently a colour matched under one light may differ under another light this concept is known as metamerism.
What is thermal diffusivity important?
materials with a high thermal diffusivity will transmit heat rapidly.
What is the coefficient of thermal expansion?
The change in length per unit of the original length of the material when its temperature is raised by 1 degree.
What is the proportional limit ?
The point at which a material fractures or undergoes permanent deformation having reached the limit of elastic stress.
What happens to a material when a stress is applied which exceeds the proportional limit of that material?
It breaks or permanently deforms.
What is the difference between elastic and plastic strain?
elastic strain is completely revresible plastic strain results in permanent deformity
What are the three types of stress?
Tensile : elongation or stretch stress Compressive: shortening or compression stress Shear : where there is sliding or twisting stress.
What are the four reasons why shear failure is unlikely to occur in isolation
As the force to create shear failure must be applied parallel to the material interface the following features will make shear failure less likely 1. many of the brittle materials used in restoratives have rough, curved surfaces 2. The presence of chamfers, bevels etc change the curvature of a bonded surface and make shear failure less likely 3. The force must be applied immediate to the shear interface and this is difficult to achieve. 4.the tensile strength of brittle materials is usually below their shear strength and so this normally fails first
What does the straight line region represent in the stress strain curve?
The straight line represents the reversible elastic deformation which occurs before the proportional limit is reached.
What does P represent on the stress strain curve and what happens at this point?
P is the point at which the material begins to undergo platic deformation and the stress at this point is the proportional limit.
What is stiffness?
Stiffness is determined by the slope of the elastic portion of the stress/strain curve
What does UTS represent?
The point at which the material fractures in this case 1625 MPa
it stands for ultimate tensile strength.
What does E represent on this graph and how is it calculated?
E = the elastic modulus or youngs modulus and it is calculated as stress/strain from the proportional limt.
ie for dentin 176/.015 =11.7 GPa or 11700MPa
for enamel 235/.007= 33.6 GPa or 33600 MPa
The higher the elastic modulus the stiffer a material
looking at the graph aboove dentin is a tougher material more able to withstand flexion withut breaking
How is shear modulus calculated?
Shear modulus = elastic modulus divided by 2 x( 1 + Poisson’s ratio)
Poisson’s ratio is typically 0.25-0.3
subsequently Shear modulus is typically 38% of elastic modulus.
What is resilience? how is ist determined on a stress strain curve?
resilience is the sponginess of a material and is determined on a stress strain curve as the area under the curve in the period of elastic strain
it is the amount of energy absorbed within a unit volume of structure when it is stressed to its proprtional limit
Demarcated as the green area on this graph.
What is proportional limit?
The point at which stress is no longer proprtional to strain.
What is elastic limit?
The maximum stress a material can withstand before it becomes plastically deformed.
What is yield strength?
The amount of stress required to produce a given amount of plastic strain.
Ie if want 0.2% strain then straight line to curve and across to stress and have yield strength for that strain
What is ultimate tensile strength?
the amount of stress required to fracture the material.
