DM Finals Revision

Question 1

Fracture strength..

Answer 1

Max stress material can take at point of breaking, depends on shape of material

Question 2

Fracture toughness..

Answer 2

Total energy absorbed by structure b4 material fractures, does not depend on shape of material
- is the total area under the stress-strain curve
- ability to resist crack propagation

Question 3

Fracture toughness vs fracture strength

Answer 3

Strength
- depends on shape of material
- stress at the pt of fracture

Toughness
- does not depend on shape of material
- cumulative energy absorbed before material fractures
- total area under curve
- ability to resist crack propagation

Question 4

What is responsible for delayed expansion in amalgam on moisture contamination?

Answer 4

Zinc

• acts as a plasticiser => makes amalgam easier to carve
• w/o zinc, amalgam will be brittle
• acts an oxide scavenger, minimising formation of other oxides

Question 5

Components of stainless steel alloy? What does 18-8 stainless steel alloy contain?

Answer 5

18% Cr, 8% Ni (+60% Fe)

Question 6

Stainless steel alloy used for what in DM?

Answer 6

• metallic implants
• orthodontic wires
• crowns
• denture clasps
• root canal reamers
• surgical instruments

Question 7

[T/F] CoCr dentures cannot be used w denture cleaners.

Answer 7

T.

• denture cleaners contain hypochlorites/chlorines => corrosion

Question 8

What is C factor?

Answer 8

• ratio of bonded/unbonded surfaces
• increase in bonded surfaces => increase in C factor => increase in contraction stress of adhesive bond due to volumetric shrinkage

Question 9

ZOE impression material is used for? Properties?

Answer 9

Used for edentulous impression taking in pxs w little undercuts

• good detail reproduction
• low viscosity
• requires rigid tray
• good dimensional stability

Question 10

What is the relationship btw resilience and proportional limit?

Answer 10

Resilience:
- energy absorbed by structure when stressed elastically
- area under the graph

Proportional limit:
- max stress material can take w/o permanent deformation
- its a point

Question 11

Modulus of elasticity..

Answer 11

• stress/strain within elastic region
• measures stiffness of material within elastic region
• stronger attraction btw atoms = increased EM
• higher EM = steeper curve

Question 12

[T/F] MTA sets on hydration & has higher initial pH due to formation of CaOH2

Answer 12

True
- sets upon hydration, long ST (3H)
- after setting, pH decreases to neutral

Question 13

Titanium fractures are uncommon, but if they do fracture, its due to..

Answer 13

Metal fatigue from high cyclic occlusal loading
- not high impact

Question 14

Why do implant materials undergo passivation?

Answer 14

• enhance oxide layer => minimise release of metal ions => reduce corrosion
• done by immersion in 40% nitric acid/anodisation

Question 15

How is the hybrid layer formed?

Answer 15

• micromechanical bonding formed when adhesive molecules penetrate layer of collagen

Question 16

Creep is..

Answer 16

The permanent deformation of a material due to sustained stress/mild, continuous & cyclical forces

Question 17

Creep vs dimensional change

Answer 17

Creep has a ‘sustained stress’ component
Dimensional change: change of material density or dimensions of body

Question 18

What is dimensional stability?

Answer 18

Measurement of the linear dimensional change resulting from exposure to temperature

Question 19

Dimensional stability vs dimensional change

Answer 19

Dimensional stability has ‘temp change’ component
Dimensional change is just dimensional change regardless of what factor