Microcracking determination

It can be easily shown that compressive uniaxial or biaxial stress results in the upshift of the Raman band to higher wavenumbers, whereas tensile stress decreases the Raman frequency in these cases [47]. However, in the complex stress field under the indenter, further complicated by the volumetric changes during possible phase transformations and the breakdown of constitutive equations due to macro- and microcracking, determination of the strain tensor components becomes a challenging task and the simplifying analytical models already discussed here need to be used. 

In cement-based brittle materials, cracks spread out from a system of initial microcracks and multiply under relatively low loads (cf. Chapter 9). The opening and propagation of microcracks determine the material s behaviour under sustained and cyclic loads. 

Figure 5 shows interspherulite destruction initiated by a small microcrack appearing just at the starting point of the deformation as a result of lateral and local ribbon separation. The subsequent evolution of these two processes—deformation and destruction—is determined by the increase in the number and size of the cracks. Crack formation results... 

Usually the radius of curvature p at the sharp notch of the crack is determined by the atomic sizes and is very small. It is immediately evident that the stress concentration at the sharp notches of the microcracks can become extremely large due to the above stress intensity factor, and the fracture should start propagating from there. Although this analysis indicates clearly where the instabilities should occur, it is not sufficient to tell us when the instability does occur and the fracture propagation starts. This requires a detailed energy balance consideration. 

If one assumes that the measured crack length is likely to be only approximate in the presence of such extensive microcracking, the use of beam theory expressions requiring this parameter will also be subject to this uncertainty. However, corrected beam theory can be used to determine an effective crack length, a, via the measured compliance value and eqn (3) may be reananged to give an effective crack length thus fa)... 

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