Indentation cracks

Flowever, in order to probe the fracture toughness of thin films or small volumes using ultra low load indentation, it is necessary to use special indenters with cracking thresholds lower than those observed with the Vickers or Berkovich indenters (for Vickers and Berkovich indenters, cracking... 

Indentation crack paths in alumina matrix composites containing 20 vol% of particles of (a) TiN and (b) Cr3C2. The alumina matrix is the darker phase in each case. 

Fig. 4.16 Cavity distribution around the tip of an indentation crack in KX01. Cavities form a cloud around the crack tip, which later link up as the crack propagates. Taken from Wiederhorn et al.36...

The Vickers hardness of the ceramic was measured in air on a polished section at indentation loads of 5 and 10 Newtons. Indentation crack length measurements were made in air and water on the same section. The crack lengths introduced in air were used to calculate KIC by Equation (1). 

Mechanical Properties. The hardness of the Ba2YCu307 x ceramic and its room temperature fracture toughness as determined in air by the indentation crack length procedure are given in Table I. The value of Young s modulus used in both this calculation as well as the... 

Bleise, D. and Steinbrech, R.W. (1994) Flat R-curve from stable propagation of indentation cracks in coarse-grained alumina. J. Am. Ceram. Soc., 77 (2), 315-322. 

Chuchai A, Kenneth JA, John JM, Jr. Residual stress in glass Indentation crack and fractography approaches. Dental Materials. 2009 25(ll) 1453-8. DOI 10.1016/j.dental. 

Morris DJ, Myers SB, Cook RF. Indentation crack initiation in ion-exchanged aluminosilicate glass. Journal of Materials Science 2004 39(7) 2399-2410. 

The indentation crack growth behavior is summarized in Fig. 8.45. For crack length and the crack will arrest when K= T, the fracture toughness, i.e.. 

Further crack growth will only occur if the material can undergo sub-critical crack growth (see Section 8.12). Now, consider the effect of the applied stress, as shown in Fig. 8.45. The additional term in Eq. (8.63) leads to a minimum in the total stress intensity factor. The indentation crack, therefore, undergoes stable... 

Figure 8.44 Schematic illustration of a semi-circular indentation crack under the action of residual and applied tensile stresses.

The development of indentation fracture mechanics has also allowed fracture toughness to be determined using indentation cracks. Indeed, the ease with which these cracks can be introduced and the simple specimen preparation involved has popularized this approach. Moreover, it has allowed crack behavior to be studied for cracks in a size range that is close to that found in practice. There are two main approaches for determining fracture toughness from indentation cracks. In the first approach, the size of the radial cracks that emanate from the hardness impression are measured. It is recognized that the parameter x depends on the elastic properties of the indented material. It has been proposed that x=P E H), where )8 is a constant that depends only on the indenter geometry and H is hardness. Equation (8.64) can, therefore, be written as... 

Figure 8.45 Influence of applied stress on growth of an arrested indentation crack, length c. The crack undergoes stable growth to a length c. ...

The second indentation approach to measure fracture toughness is to break indented specimens. The median-radial cracks are loaded in mode I and the indentation strength is determined. In some cases, more than one indentation crack is introduced. In this case, failure will occur from one indentation and the other indentations can be used to determine the extent of the stable growth. Equation (8.65) can be re-arranged to the form... 

R. F. Cook and G. M. Pharr, Direct observation and analysis of indentation cracking in glasses and ceramics, J. 

Figure 9.5 Crack front markings showing the position of an indentation crack as it undergoes sub-critical crack growth optical micrograph. (From Dwivedi and Green, 1995, reproduced courtesy of The American Ceramic Society, Westerville, OH.)...

Fracture toughness. determined using an indentation crack length approach, was found to be 3.6 MPa Vm. 

Fracture toughness can be measured from indentation cracks by measuring either their length or. ... 

Why do indentation cracks undergo stable growth prior to failure ... 

Cho Choi, S. R., Salem, J. A. Fracture toughness of PMMA as measured with indentation cracks. J. 

FIGURE 17. Indentation crack length and trajectories for MoSi2-50%, Si3N4. (a) MS-40, (b) MS-50, (c) MS 70 An Indent load of 98 N was apphed to initiate the cracks. 

FIGURE 12.5 Surface traces of indentation cracks in alumina-glass dental composite by backscattered scanning electron microscopy (SEM). Grain bridging sites are near the arrows in (a) and transgranular fracture sites in (ft). (From Ref. 21.)... 

Figure 8.8 R-curve behaviors of the self-reinforced (open symbols) and the seeded and tape-cast (closed symbols) materials, determined by as-indented crack lengths...

However, Ando et al. have investigated the healing of not the machining cracks but the indented crack in the oxide-based ceramics, i.e., mullite and alumina (based... 

The ratio of as- indented crack length to the characteristic dimensions of the plastic impression, da, was larger than 2 in the range of the indentation load investigated for all samples. 

T. Lube, Indentation Crack Profiles in Silicon Nitride, Eur. Ceram. Soc., 21,211-18 (2001). D. B. Marshall, Controlled Flaws in Ceramics A Comparison of Knoop and Vickers Indentation, / Am. Ceram. Soc., 66,127-31 (1983). 

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