Fracture origins

Macro visual examination is done with the unaided eye or low power (up to 2 OX) magnification. An item may need to be cleaned prior to examination. Fracture origins and location of samples to be removed for further analysis are determined by macro visual examination. Nondestructive evaluation methods may be necessary for more accurate determinations (see Step 4). 

The most common parameters derived from the TPA curve are shown in Table H2.3.1 (Friedman et al., 1963 Bourne, 1968). The peak force during the first compression cycle is defined as hardness. Fracturability (originally called brittleness) is defined as the force at the first significant break in the curve during the first compression cycle. The ratio of the positive force area during the second compression cycle to that during the first compression... 

Lange, F.F. and Metcalf, M. Processing-related fracture origins II, Agglomerate motion and cracklike internal surfaces caused by differential sintering , J. Am. Ceram. Soc. 66 (1983) 398 106. 

From these results, Everage and Ballman (1974) concluded that melt fracture originates at a point where fluid elements are subjected primarily to extensional deformation. They could correlate the results of Ferrari with a critical extension rate of about 1000 s-1. 

Some typical features of fracture origin such as internal or surface crack are shown in Figure 2.32. The fracture mirror is a smooth surface formed when an internal crack travels radially outwards upon acceleration. Then it starts to deviate from the original plane upon reaching a critical velocity, and intersects with the microstructural features such as inclusion or precipitate. This may also occur as the direction of principal tensile... 

Figure 2.32 Schematic showing typical fracture features surrounding the fracture origin in a ceramic material (a) internal crack origin (b) surface crack initiation. (Reprinted with permission from ASM International. All rights reserved www.asminternational.org)...

The first and foremost step in failure analysis of ceramics consists of identifying the fracture origin and the type of cracking, which throws light on the type of failure such as failure due to impact, residual stress combined with load, thermal shock, improper machining, oxidation and corrosion. This is aided by micro- and macrofracto-graphy, examination of microstructure by SEM, chemical analysis and metallographic examination. 

The fracture surface of the sample fatigued at a higher frequency, 2 Hz, and at the same stress amplitude of 27.5 MPa, is shown in Fig. 48 b. In this sample, fracture originated from two separate surface sources and these are separated by a torn ridge of material in between. The size of the slow growth region, here in two parts, is somewhat larger than for the sample tested at 0.02 Hz. There is some evidence, on both portions of the smooth part of the fracture surface of Fig. 48b, of discontinuous... 

Fig. 19a, b. SEM scans of fatigue fracture surface of dry samples, tested at 41.4 MPa and 2 Hz, of HMW, C-PMMA a Low magnification scan, b High magnification scan near the fracture origin... 

Wei, R. P., Baker, A. J., Birkle, A. J., and Trozzo, P. S., Metallographic Examination of Fracture Origin Sites, included as Appendix A in Investigation of Hydrotest Failure of Thiokol Chemical Corporation 260-Inch-Diameter SL-1 Motor Case, by J. E. Srawley and J. B. Esgar, NASA TMX209 1194 (Jannary 1966). 

By fractographic inspection fracture [20, 21] origins within the volume (inclusions) were found in 3 specimens. In the other 27 specimens no clear evidence for the type of fracture origin was found. It is assumed that all flaws are small surface flaws, i,e. their geometric factor is F = 1.12. 

All fracture surfaces were analysed by ffactographic means in order to identify fracture origins. In any case the fracture origins are at or very close to the surface. In the case of set C, the surface flaws identified by the dye penetrant and positioned in the highest loaded area of the specimens were fracture origins in every case. An example is shown in Fig. 6.b. The fracture origin is a Hertzian contact crack (a part of a ring crack), which had previously been detected before fracture (Fig. 6.a). 

In the specimens of set A and B similar fracture origins were found in some, but not in all, cases. In general, the ffactographic analysis clearly demonstrates the strong influence of surface damage on... 

Figure8.76 Lenticular void that acted as a fracture origin in an alumina-zirconia composite scanning electron micrograph. (From Green et ai, 1989, reproduced courtesy of CRC Press, Boca Raton, FL.)...

Figure 8.80 Silicon carbide inclusion that acted as a fracture origin in silicon nitride scanning electron micrograph.

C1322-02 Practice for Fractography and Characterization of Fracture Origins in Advanced Ceramics... 

FIGURE 6. Typical examples of fracture surfaces showing fracture origins for (a) 0 mol% (10-YSZ) (b) 30mol% alumina particulate composite [2]. Bar = 500 p,m. Reprinted with permission of The American Ceramic Society, www.ceramics.org. Copyright [2002]. 

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