Materials

Question 1

1. What is the modulus of elasticity and what does it reflect regarding bonds?
2. Which part of the stress-strain curve does it represent?
3. What is the formula?
4. On the diagram- what is it related to?
5. What is it not related to?
6. What is the modulus of elasticity unaffected by?

Answer 1

1. The rigidity or stiffness of the material; reflects the strength of the interatomic or intermolecular bonds.
2. The slope.
3. Stress/strain- modulus of elasticity
4. Only related to elastic portion of diagram
5. Modulus of elasticity is NOT related to strength or proportional limit.
6. Unaffected by age hardening, heat tx and cold working.

Question 2

1. Define malleability.

2. How does temperature affect malleability? Why?

Answer 2

1. The ability to undergo permanent COMPRESSIVE deformation without permanent fracture or rupture.
2. Increase T = increase malleability b/c malleability is dependent on dislocation movement and dislocations occur more readily with higher temperatures.

Question 3

1. What factors influence sensitivity of brittle materials? (x5)
2. Which forces positively influence brittle materials? Why?

Answer 3

1. Internal cracks; flaws; voids; and TENSILE and FLEXURAL bending forces as they promote propagation of cracks
2. Compressive forces b/c they tend to close cracks.

Question 4

1. Define brittleness.

2. What is the general atomic configuaration of brittle materials and why do they easily fracture?

Answer 4

1. The ability to fracture or rupture with little or no prior permanent deformation.
2. brittle materials have a very ordered atomic structure that does not permit easy movement of dislocations.

Question 5

1. Define resilience?
2. Aka?
3. Which part of the stress strain curve diagram represents resilience?
4. What are some dental examples of resilient materials?

Answer 5

1. The resistance of a material to permanent deformation under SUDDEN IMPACT.
2. Aka: ‘spring-back potential’;
   
   • the amount of energy absorbed just before the proportional limit.
3. The AREA under the linear part (slope) of stress-strain diagram
4. Ortho wires- store energy that is delivered over an extended time (beta-Titanium wires and stainless steel)

Question 6

Which is more resilient: Beta-titanium wires or stainless steel? (Ortho)
Why?

Answer 6

Beta-titanium.
Event though both have roughly the same proportional limit, beta-titanium has a lower modulus of elasticity= lower slope angle
- because of the lower modulus of elasticity (stiffness), beta -Ti reaches the same proportional limit (stress) at a higher amount of strain

Question 7

1. Define toughness.
2. Where is it represented on stress/strain diagram?
3. What other 2 properties have the most profound effect on toughness?
4. What are two types of tests for toughness?

Answer 7

1. The resistance to fracture under SUDDEN IMPACT.- it is the energy absorbed just shy of the fracture point.
2. It is the area under the entire stress/strain diagram.
3. Ductility and brittleness have the most profound effect on toughness.
   Increase ductility = increase in toughness; increase in brittleness = decrease in toughness.
4. Two tests: pendulum impact (Chary- horizontal positioning of material) or the Izod test (vertical positioning of material)