physical and mechanical properties e lec Flashcards
we will discuss strength - different modes stress fatigue ware and hardness fracture toughness impact strength aesthetics
which two modes are there to establish strength
tension and compression
what does tension do
stretches the length
what does compression do
shrinks the length
what is Poisson’s ratio
ratio of transverse contraction strain to longitudinal extension strain in the direction of stretching force. Tensile deformation is considered positive and compressive deformation is considered negative.
what differences can teeth have
size
composition
history
how are properties of materials determined
by using a machine called the universal testing machine
how does the universal testing machine work
by deforming the sample in a known and controlled manner whilst monitoring the force
what can the data be used for
to plot a stress strain curve
what is a stress strain curve
where stress is the force/area
what is the strain
change in dimension/original length
what should we try to do when carrying out a test to the specimen
standardise it
what does anisotropic mean
having a physical property which has a different value when measured in different directions.
what does enamel consist of
parallel long hexagonal prisms
what do we also need to consider
the history of the tooth eg any trauma
what else makes a good experiment
a large enough sample size so it can be representative
what practicalities do we consider
eg if we need to grip the sample how do we not damage it
the stress strain curve starts to
increase proportionally and then levels off
what is on the x axis
strain
what is on the y axis
stress
what is the point called where the stress and strain meet
elastic limit, proportional limit or yield point
what is the highest value for stress called
strength
what is the slope called
the modulus of elasticity or youngs modulus
what does the PDL add
toughness and resilience
what can materials fail
at stresses well below the ultimate strength; this is usually due to cyclical loading
stresses well below the ultimate strength can be seen in teeth by a condition called
abfraction
abfraction
leads to the enamel pooling at the base of the tooth
fatigue is a major problem with
posts
why does the post fail in a crown
poorly designed post and core leads to a failure of the cement which allows the post to flex resulting in the post becoming fatigued.
what is the study of ware
tribology
what experiments are conducted in tribology
our limbs are joined and conducted with low friction collagen fibres which regenerate
also have high resistance collagen free enamel coatings on our teeth
what has changed in recent decades
diet
what should be of high importance when considering restorative materials
ware
what can happen to the opposing dentition
it can be wared out
how does LDI test their materials
by rubbing the material against standard abrasive material such as silicon carbide paper
what is the paper used in LDI to test the material
silicon carbide paper
hardness
tends to be indicative on how a material might ware
what is the effective field test and hardness test
mohs hardness scale
what is the scale of the mohs hardness scale
ten points on the scale with talc at 1 and diamond at 10
how does it work
we scratch the material together and the less hard one becomes scratched
what technique can be problematic to test the hardness of enamel
indentation techniques as enamel is very thin
what test is used to test the hardness of thin films
the Knop test
what us the fracture toughness equation
defined by the Griffith equation
Sigma is now joined by a subscript f indicating stress at failure. [a] is for crack size, all materials have flaws or cracks in them and when a material fails it is due to the growth of this crack [a].
do teeth have a different fracture structure
due to anisotropic
is there a single way of determining single fracture toughness
no as now there is a branch of linear fracture mechanics
what test do we use for the hardness of teeth
it is a version of the knops test called the vickers test
what are the disadvantages of the vickers test
it is difficult to do
what is the issue with teeth
they are a laminated composite structure and very small
as we measure the height the pendulum travels
we can calculate impact energy
how high is the instrument
0.5m
what is the energy of the instrument
2 joule
what is colour created by
by rods and cones cells which absorb different wavelengths
what is the colour wheel extended into
value
what is value
measure of how light and dark a colour is
what other colour space schemes are made
l,a,b scheme
when taking shades what should we consider
the quality of the light source- natural north light
what is north light
a flat even north light
