Module 3.2 - Thermal Behavior

Question 1

For ceramics, residual thermal stresses can be induced by what 2 factors?

Answer 1

1. Different CTEs in multiphase materials
2. Anisotropy in single-phases, where CTE is dependent on the orientation

Question 2

If the CTE of the matrix and the inclusion is the same, this means ∆α=0. What does this imply?

Answer 2

No stress will develop upon heating

Question 3

If the CTE (α) of the inclusion is greater than the CTE of the matrix, what type of stresses are generated in the matrix and inclusion?

Answer 3

Compressive stresses (negative in value)

Question 4

If upon heating, the inclusions is totally constrained from moving, what does this mean for the CTE of the matrix and Em?

Answer 4

Em = infinite
CTE of the matrix = 0

Question 5

What causes thermal shock? How?

Answer 5

Rapid heating or cooling of a ceramic. Since there is a very high thermal gradient, there are going to be high thermal stresses, these thermal stresses exceed the part’s strength.

Question 6

What are the three steps in measuring the thermal shock resistance of a material?

Answer 6

1. Samples are heated to Tmax
2. Samples are quenched in a different media
3. Post-quench retained strengths are measured and plotted vs the severity of the quench (∆T)

Question 7

In predicting ∆Tc, what is assumed about flaws in terms of its geometry?

Answer 7

They are circular with radii (ci)

Question 8

What is assumed about the general flaws of the system in predicting ∆Tc?

Answer 8

The system has N identical, uniformly distributed, Griffith flaws per unit volume

Question 9

In predicting ∆Tc, what is assumed about crack propagation?

Answer 9

It occurs by simultaneous propagation of the N cracks

Question 10

In predicting ∆Tc, what is assumed about the cooling of the body?

Answer 10

It is uniformly cooled with external surfaces being rigidly constrained.

Question 11

If ∆T > ∆Tc in a material, what happens?

Answer 11

Cracks will grow.

Question 12

In thermal shock propagation, the driving force for crack propagation is finite. What does this entail?

Answer 12

Cracks only extend up to a certain length

Question 13

Compare the effect on the pre-existing cracks of the material if ∆T is small or large during thermal shock?

Answer 13

∆T (small) = pre-existing cracks won’t grow
∆T (large) = pre-existing cracks will grow

Question 14

Upon thermal shock, if only a FEW small cracks are present, what happens to crack extension and degradation of strength?

Answer 14

Long crack extension, high degradation of strength

Question 15

Upon thermal shock, if NUMEROUS small cracks are present, what happens to crack extension and degradation of strength?

Answer 15

Degradation of strength is low, crack extension is short

Question 16

Insulating fire bricks are porous and contain many flaws, how come they are still strong and can withstand thermal cycles?

Answer 16

This is because the number of defects is numerous.

Question 17

Spontaneous microcracking can occur in ceramics, and can’t be avoided by slow heating. What causes it?

Answer 17

Residual stress buildup

Question 18

What are the three reasons by which residual stresses are built up in ceramics materials?

Answer 18

1. Thermal expansion anisotropy (single-phase materials)
2. Difference in CTEs (multiphase materials)
3. Phase transformation

Question 19

For ceramics, how does transfer of heat occur?

Answer 19

Through phonons (vibrational motion) in solid

Question 20

Waves can be scattered by what structures in ceramics?

Answer 20

Imperfections, grain boundaries, pores

Question 21

Recall: what is the mean free path of the wave?

Answer 21

Average distance a wave travels before it scatters

Question 22

If kth (thermal conductivity) is high, what does this imply about the structures and atoms/ions?

Answer 22

1. they are highly ordered and open structure-wise
2. atoms and ions are similar size and mass