DM.4 Flashcards
What chemical properties are their of the set material?
solubility , leaching and corosion
1) What coefficient do you need to know of the set material that links to its thermal properties
2) why?
3) What other properties do you need to be aware of ?
4) Can you link this to a clinical scenario?
1) Coefficient of expansion
2) contraction is major issue as it causes margins, increasing chance of secondary caries
3) Conductivity and diffusivity
4) high diffusivity = a good conductor, we would like to have insulators with low diffusivity, but the opposite is true for dentures
What do we need to be aware of in the set material that links to mechanical properties?
stresses (simple and complex)
Define stress
Internal forces are set up inside a body to oppose an externally applied force
What is the magnitude of stress?
the function of applied force and dimensions of the object to which force is applied
How do you calculate simple stress?
2) What are the units of stress?
3) convert 1 unit of stress to :
a) Pa
b) MPa
d) GPa
1) force/area
2) Nm-2
3) a) 1
b) 1* 10^-6
c) 1* 10^-9
What is strength?
The maximum stress which can be withstood before breaking
What does a filling material need to be stronger than?
the biting force
How do you calculate the change in dimension caused by stress/ stress?
2) What are the units?
ratio (change in) length / length
2) no units, may see it as a percent,
What are the 3 different types of deformation?
b) What does this relate to clinically?
1) fully recovers (elastic)
2) doesn’t recover at all (plastic)
3) Partially recovers (viscoelastic)
B)1) biting force may cause strain (change in dimension) but deformation may be recovered
When pushing filling material into cavity we are applying ____a_____, we want to material to _____B_____ to the cavity thus we want the material to be ____C____ at this point.
a) strain
b) permantly deform
c) plastic
When we pull something with a ___a___ there is a ___b___ associated with it.
a) stress b) strain
When we squahs something with a ___a___ there is a ___b___ associated with it.
a) stress b) strain
What is stiffness?
how easy it is to deform something (does not necessarily mean it is permantly deformed)
When we apply flexural stress to a beam what factors affect how much strain (change in shape) occurs?
- Strength of beam material
- Stiffness of beam material
- Dimensions of beam
- Particularly thickness (just because there is a lot of bulk in the way)
- amount of force applied
What’s another way of saying strain?
2) what is another way of saying
change in shape (this could be wrong check, talk board thing)
2) applying force to an object
What is yield stress?
Yield stress (stress required to permanently deform material)
What is proof stress?
Proof stress (easier to measure then yield stress), so its what we use instead of yield stress which is the stress required to permanently deform a material
What value was the biting force of the patient be bellow?
the yield stress (proof stress) of the material, its actually yield stress for the exam but we will use proof stress
What is ultimate tensile stress?
The maximum stress a material can stand before it breaks is called the breaking stress or ultimate tensile stress.
How do you work out the modulus of elasticity?
2) Whats another word for it?
the initial rate of change of stress over change in strain (measure the gradient up to yield stress)
2) young’s modulus
What is young’s modulus?
a measure of the ability of a material to withstand changes in length when under lengthwise tension or compression
If young’s modulus/ modulus of elasticity is high , describe the graph of y=stress and x=strain
the graph will be steep / gradient high up to yield stress
What is ductility?
how much something can be pulled
what is malleability?
how much something can be compressed
what is resilience?
how much energy can something take before it deforms
What is toughness?
how much energy something takes before it breaks
1) What is the proportional limit?
2) What else
1) Up to this amount of stress, stress is proportional to strain (Hooke’s law),
you are pulling to materials apart what property are you measuring?
2) What occurs after (ultimate) tensile strength is reached?
ductility as that is how much you can pull something apart
2) onset of necking then final fracture
What is happening when necking occurs?
2) When have you seen this happen?
1) When the neck forms bonds a braking and the material is becoming thinner in one point,
2) think about stretching out blue tac., or glass blowing when you pull the materials apart
What property does a material have if beyond (ultimate) tensile strength …
a) does necking occur
b) brakes but no necking
2) Give an example of a and b?
a) ductile material
b) brittle materials
2) a) hot glass/ blue tac
b) cold glass
What decides if it is ductile or brittle?
temperature dependence (e.g. hot glass vs cold glass) and strain rate dependence (e.g. polypropylene tested at slow vs fast strain rate)
What’s an easier way to find out the strength of the material than drawing a graph of y=stress and x=strain
measuring impact stength (i.e.hit it with a hammer lol)
What is greater when carrying out the impact strength test, energy crack initiation or energy crack propagation?
2) Why?
3) What is the clinical significance of this?
energy of crack initiation is greater
2) notch sensitivity, notchs reduce impact strength of a material.
3) brushing dentures= cracks, materials may have notches in anyway if so and we know the material is notch sensitive effecting the advice we give to patients
In the impact strength test, what sort of energy doe the pendulum have and what is this transferred to?
pendulum has potential energy converted to kinetic energy = fractures specimen
What is fatigue life?
2) What is fatigue
3) What is the clinical significance
Fatigue life: cycles (or time/age) survived at a value of stress
Fatigue limit: Stress below which material survives indinitely
3) a material which fatigues rapidly means that although a material may start of being the strongest it may be the weakest after many cycles, this needs to be taken into account when choosing amterials for restorations
What is hardness?
2) This is a factor in determining ___a___
3) What is hardness related to ?
Resistance to indentation under load
2) a) durability (possible if material is sensitive to notches)
3) scratch and wear
What is the purpose of the knoop and Vicker’s test, whats the difference between them?
The tests do pretty much the same thing. A diamond it pressed into the material and we measure the indent size. The bigger the more soft the material is. The difference between the tests is the shape of the diamond.
the hardness value of enamel is…
b) the hardness value of dentine is…
c) the hardness value of acrylic resin is…
d) the hardness value of amalgam is…
e) the hardness value of porcelain is…
a) 350
b) 60
c) 20
d) 100
e) porcelain
What is toughness?
2) What is stiffness?
toughness is the amount of energy per unit volume that a material can absorb before rupturing.
2 Stiffness is the extent to which an object resists deformation in response to an applied force
Does the hardness of dentine ever change?
yes with erosion, abrasion is more likely
1) What is the purpose of Knoop test KHN or Vicker’s test BHN?
2) What’s the difference between the tests?
measure hardness
2) the diamond head is different shapes,
