Biomedical Engineering Reference
In-Depth Information
cracks around a Vickers indentation after single and repeated quenching.
The critical thermal shock temperature difference, of the material can be
defined with reference to the number of propagating cracks and the amount
of crack extension (Maensiri and Roberts 2002).
1.8.6 Indentation fatigue after thermal shock tests
Indentation fatigue tests (Li and Reece 2000, Reece and Guiu 1991a, 1991b,
Takakura and Horibe 1992a, 1992b, Vaughan et al. 1987) (repeated
indentations on the same site) were conducted on thermally shocked
specimens of pure Al
2
O
3
and Al
2
O
3
/5 vol% SiC nanocomposite sintered at
1700
C using a Vickers profile indenter with applied loads of 5, 100, 200, 300
and 400N. The holding time of each indentation was 10 s, and the interval
between successive indentations was 30 s. The number of indentation cycles
needed to produce lateral chipping was noted in each case and the fracture
surfaces within the chipping area were characterized using SEM. The
observed superior thermal shock resistance of the nanocomposites could be
due to an increase in thermal conductivity and/or a simultaneous decrease in
the thermal expansion coefficient in the nanocomposite, caused by the
addition of SiC.
8
1.8.7 Non-destructive test (NDT) methods
Two acoustic NDT methods may be applicable to ceramic nanocomposites.
The first of these, acoustic emission (AE), is simply the monitoring of stress
waves generated by a dynamic process occurring within or on the surface of
a material by means of a sensitive transducer, usually of the piezoelectric
type. The dynamic process of most interest as far as ceramic nanocompo-
sites are concerned is the growth of microcracks and, indeed, AE has been
successfully used to monitor microcracking during mechanical testing
(Aeberli and Rawlings 1983, Dalgleish et al. 1980) and thermal shock
treatments (Thompson and Rawlings 1988). It should be noted that AE can
only detect active defects (i.e. growing microcracks) and is therefore suitable
for the continuous monitoring of damage occurring during production or
service but will not detect flaws in any post-production or post-service
assessment of a component that is not stressed.
On the other hand, the acousto-ultrasonic (AU) technique is capable of
detecting flaws in post-production and post-service ceramic nanocompo-
sites. In the AU method, stress waves are introduced into the component
under examination by means of a pulser and, after travelling through the
component, the waves are detected by another transducer. The signals from
the transducer are analyzed in a similar manner to that employed in AE.
Thus AU monitoring assesses the general condition of the volume of
