Defects in Ceramic Processing Questions and Answers

Answer: A

A brittle material cannot redistribute stress by yielding, so failure begins at the single worst flaw and runs to completion. This is why one large pore matters far more than a great many small ones. Ceramic quality control targets the extremes rather than the mean.

Answer: B

Both are true and the reason gives the cause: a metal blunts a crack tip by plastic flow whereas a ceramic cannot, so the stress at a flaw rises until the crack propagates and the whole piece fails. There is no ductile mechanism to blunt the crack tip.

Answer: C

Failure begins at the worst flaw present, so an otherwise excellent body containing one large pore fails at a low stress. Removing the extremes matters far more than shifting the average. Quality effort is spent on the tail of the distribution.

Answer: D

Correct answer: how widely its strength varies between specimens

The Weibull modulus is a shape parameter describing the scatter of measured strengths, not their average. A high modulus means the strengths cluster tightly; a low one means they are widely spread. Ceramics need such a parameter because they fail from their largest flaw, and the size of that flaw varies from specimen to specimen - so a mean strength alone tells a designer very little.

Answer: A

Both are true and the reason gives the necessity: two materials of identical mean strength differ greatly in the stress that can be carried safely if one has a much wider distribution than the other. Design stress falls steeply as the modulus drops.

Answer: B

Correct answer: material B, whose strengths scatter less

Design is done at a low probability of failure, in the weak tail of the distribution, and it is the scatter that sets how far below the mean that tail extends. With the same mean, the material with the modulus of 20 has a much shorter tail than the one with 5, so a far higher stress may be allowed at the same risk. A high Weibull modulus is worth more to a designer than a high mean strength.

Answer: C

It indicates a narrower scatter and a more uniform flaw population. The convention runs opposite to the intuition, since a larger number means a tighter rather than a broader distribution. The sign of the effect catches people out regularly.

Answer: D

A larger volume is more likely to contain a severe flaw, so the expected strength falls with size. Extrapolating from small test bars to a large component without allowing for this overestimates what the component will bear.

Answer: A

Both are true and the reason gives the statistics: the strength is set by the worst flaw, and the chance of a very bad one rises with the volume under stress, which is what Weibull theory quantifies. Test bar results must be scaled before they apply to a component.

Answer: B

Characteristic markings on the fracture surface radiate from the origin, and the flaw at that origin can usually be identified. Knowing whether it was a pore, an inclusion or a surface scratch points directly at the process stage responsible.