Microcrack nucleation

When the craze thickness attains the critical value A , craze fibrils break down and a microcrack nucleates with a related vanishing normal stress. 

Under the application of a tensile stress, 3-dimen ional flaws (e.g., pores and inclusions) cannot grow. Only cracks can grow under tensile stress. Sometimes one speaks of the growth of a flaw (not a crack), implying the growth of a microcrack nucleated at or near that flaw. It is clear that, when a material does not have a pre-existing crack, a crack must nucleate at some moment of time prior to fracture. 

STEP 4. Microcracks nucleated in Step 3 develop into a penny-shaped crack below the indenter and shallow Palmqvist cracks in the surface region. These are the median and radial cracks. STEP 5. In the surface the indenter causes compression, not tension ... 

The traditional microindentation of the surface of ionic and covalent crystals allows one to study the effect of adsorption on the movement of the screw components of the dislocation half-loops formed, but only outside the contact zone. The capabilities are broadened with the use of the micro-sclerometric and ultramicrosclerometric (scratching) methods developed by Savenko and coworkers [46,68,70]. A step-by-step increase in the load applied to the indenter allows one to observe a transition from the reversible elastic contact to the appearance of the very first damage, that is, nearsurface dislocations, and further to the development of plastic deformations, and then to microcrack nucleation (Figure 7.42). The adsorption taking place from the active medium can both facilitate damageability and retard it. 

Incubation phase During this phase, only microstructural changes, microcracks nucleation, and microcracking can be observed. It is to be noted that the study of this phase is more cumbersome to perform since microstructural changes and fatigue damage cannot be easily separated. 

Stolkarts, V. Keer, L.M. Fine, M.E. Damage Evolution Governed by Microcrack Nucleation with Application to the Fatigue of 63Sn-37Pb, PhD Thesis, Northwestern University, J. Mech. Phys. Solids 1999, 47, 2451-2468. 

The calculation above is vahd for a spherical nucleus forming in bulk solution or on an electrode surface completely wetted by the hquid electrolyte, where the wetting angle a — 0 (Fig. 14.8a). The work of nucleation decreases markedly when wetting is incomplete (Fig. 14.8fc), since the electrode-electrolyte contact area is smaller. The work also decreases when asperities, microcracks, and the like are present on the surface. Thus, Eq. (14.33) states merely the highest possible value of work... 

Fe impurities may cause snow flakes , because of the nucleation and crystallisation of YAG (Y3Al50i2) [532]. The volume change during crystallisation of the grain boundary phase leads to internal stresses which can be cause micropores, or microcracks, or relax by other mechanisms [534, 535]. Such microcracks have only been detected in ceramics with crystallised / -Y2Si207 as grain boundary phase [533]. 

It can be demonstrated that grain boundaries or microcracks in sediments are preferred environments of cement nucleation. Consider the environment between two grains of carbonate separated by a void area of radius r (Figure 7.17 ) the energy (AFcav) required for the formation of calcite cement domains in such a cavity of height h and radius r is ... 

The relationship of brittle fracture to plastic deformation has, of course, been elaborated in various ways with the aid of dislocation theory, e.g. nucleation of microcracks has been discussed in terms of piling-up of dislocations [124]. Davies [145] has shown that embrittlement requires the presence of islands of martensite (about 1 pm in size) and has suggested that cracks are initiated in the martensite or at the martensite-ferrite interface. 

Because atoms of LMPM are known to cause an embrittlement of GB, i.e., to reduce their cohesive strength erCH, this process will constantly weaken the GB. Once the cohesive strength at the triple joint (Figure 7.94) becomes equal to the local stress concentration, nucleation of a microcrack occurs. The microcrack will then propagate backward and coalesce with the main crack. 

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