Page 398 - T. Anderson-Fracture Mechanics - Fundamentals and Applns.-CRC (2005)

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378 Fracture Mechanics: Fundamentals and Applications

8.3 CERAMICS

Fracture toughness is usually the limiting property in ceramic materials. Ceramics tend to have
excellent creep properties and wear resistance, but are excluded from many load-bearing applica-
tions because they are relatively brittle. The latest generation of ceramics have enhanced toughness
(Section 6.2), but brittle fracture is still a primary area of concern in these materials.
Because toughness is a crucial property for ceramic materials, rational fracture toughness mea-
surements are absolutely essential. Unfortunately, fracture toughness tests on ceramics can be very
difficult and expensive. Specimen fabrication, for example, requires special grinding tools, since
ordinary machining tools are inadequate. Precracking by fatigue is extremely time consuming; some
investigators have reported precracking times in excess of one week per specimen [24]. During testing,
it is difficult to achieve stable crack growth with most specimen configurations and testing machines.
Several test methods have been developed to overcome some of the difficulties associated with
fracture toughness measurements in ceramics. The chevron-notched specimen [25–28] eliminates
the need for precracking, while the bridge indentation approach [24, 29–33] is a novel method for
introducing a crack without resorting to a lengthy fatigue-precracking process.

8.3.1 CHEVRON-NOTCHED SPECIMENS

A chevron notch has a V-shaped ligament, such that the notch depth varies through the thickness,
with the minimum notch depth at the center. Figure 8.21 shows two common configurations of
chevron-notched specimens: the short bar and the short rod. In addition, single-edge-notched bend
(SENB) and compact specimens (Figure 7.1) are sometimes fabricated with chevron notches. The
chevron notch is often utilized in conventional fracture toughness tests on metals because this shape
facilitates the initiation of the fatigue precrack. For fracture toughness tests on brittle materials,
the unique properties of the chevron notch can eliminate the need for precracking altogether.
Figure 8.22 schematically compares the stress-intensity factor vs. the crack length for chevron
and straight notch configurations. When the crack length = a , the stress-intensity factor in the
o
chevron-notched specimen is very high, because a finite load is applied over a very small net
thickness. When a ≥ a , the K the values for the two notch configurations are identical, since the
I
1
chevron notch no longer has an effect. The K for the chevron-notched specimen exhibits a minimum
I
at a particular crack length a , which is between a and a .
o
m
1

FIGURE 8.21 Two common designs of chevron-notched specimens: (a) short bar and (b) short rod. Taken
from E 1304-97, “Standard Test Method for Plane-Strain (Chevron Notch) Fracture Toughness of Metallic
Materials.” American Society for Testing and Materials, Philadelphia, PA, 1997 (Reapproved 2002).