Crack initiation evolution

Ouyang, Z. and Li, G. (2009) Local damage evolution of DCB specimens during crack initiation process a natural boundary condition based method. ASME Journal of Applied Mechanics, 76, paper 051003. 

Crack initiation and propagation are accompanied by hydrogen evolution ... 

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... 

The incorporation of fly ash lowers the initial heat evolution during setting, thereby reducing the incidence of cracking and spalling. It is desirable to maintain the modified cement in monolithic form for optimum leach resistance. 

Directly from the in situ observations, the multiple film cracking is analyzed through the evolution of the crack density with the longitudinal strain applied to the system. Before any deformation, a representative zone of the sample is chosen. Its total length is 0.55 mm, extended parallel to the longitudinal axis of the sample. This initial length is denoted For any further deformation, it is denoted X(e) and corresponds to ... 

In Figure 13, one can observe suction as far as I meter from the wall whereas local measures show desaturated cracks 50 cm to 1 meter behind the wall. Along the tunnel wall, the simulation, which does not model the concrete wall, overestimates the damaged zone 2 meters instead of 50 cm observed. According to equation 20, a permeability evolution can be estimated. After excavation, the ration k/ko (current and initial permeabilities) reaches 10 and after 10 months of consolidation, its value is equal to lO". This second value is in good agreement with local measures. 

Evolution of atomic hydrogen on the steel surface may provoke the initiation and propagation of cracks starting from the metal surface, especially in the presence of notches or localised corrosion attack. Even in the absence of flaws on the surface, atomic hydrogen may penetrate the steel lattice, accumulate in the areas subjected to the highest tensile stress, above all at points corresponding to lattice defects, and lead to brittle failure beginning at one of these sites. 

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