Answer
$P$= 0.14 or 14 vol%
Work Step by Step
Required:
Compute the volume fraction porosity at which the flexural strength for MgO is 74 MPa
From 12.51a, $σ_{0} = 126.66 MPa $ and n =3.75.
Solution:
Using Equation 12.10 to compute value for $P$:
$ln σ_{fs} = ln σ_{0} - nP $
$P = \frac{ln σ_{0} - ln σ_{fs}}{n} = \frac{ln (126.66 MPa) - ln (74 MPa)}{3.75} $= 0.14 or 14 vol%