Ambient and elevated temperature fracture and cyclic-fatigue properties in a series of Al-containing silicon carbides Page: 50 of 86

                                
CHAPTER 3

A GRAIN BRIDGING MODEL FOR NON-TRANSFORMING CERAMICS
3.1 REVIEW OF GRAIN BRIDGING MODELS
Subcritical crack extension in ceramics has received much attention since the
1960s. Before grain bridging was identified as a principal mechanism associated with
such subcritical crack growth, microcracking was believed to be the main reason for
the behavior of rising resistance with crack size (R-curve) in non-transforming
ceramics. In 1977, Hubner and Jillek [1] first compared the R-curve behavior of
alumina (A1203) specimens with sharp cracks and blunt notches and found that
phenomena in the crack wake is mainly responsible for the increased toughness. Five
years later, Knehans and Steinbrech evaluated R-curve behaviors of A1203 at different
notch lengths and confirmed that this rising R-curve behavior is only associated with
crack wake phenomena [2]. However, the primary toughening mechanism for many
non-transforming ceramics remained somewhat unclear until Swanson et al [3] in
1987 directly observed the microstructual features in the crack wake of toughened
alumina by scanning electron microscopy (SEM), precluding the previously assumed
microcracking mechanism. Based on their results, Mai and Lawn [4] first proposed a
grain bridging toughening mechanism by introducing the concept of a compressive
bridging stress which arises from the zone of interlocking grains behind the crack tip.
This reduces the stress intensity factor experienced at the crack tip, thereby requiring
additional external work to be done for the crack to grow which in turn causes the
rising behavior of the resistance curve.
Since that time, numerous studies [3-28] have attempted to develop an
understanding of this mechanism from two different aspects, namely, quantification of

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