Cracks macro

Examination of the crack profiles after cyclic fatigue revealed extensive branching at the crack tip and a substantial damage zone in the vicinity of the (macro)crack tip. Increases in test temperature caused an increase in the size of the zone. Close examination revealed bifurcation of cracks and ligaments extended across the crack faces. An example of a typical fatigue crack profile appears in Chapter 7 by Suresh. 

Coal is a kind of uneven double porous media made up of macro-cracks and micro-pores. Gas in coal seam remains in the macro-cracks and micropores in free state and adsorbed state. Once getting pressure relief, gas in coal seam begins to flow along the pores and fractures, while the absorbed gas is desorbed. Gas flow in Coal seam is actually a very complex process affected by many factors. In order to simplify the process, the coal seam gas flow model has the following assumptions—(Wang Xiaoliang, et. al, 2003). 

The aim of the AE measurement is to investigate the micro- and macro-cracking processes which are important for the evaluation of the stability of cavities and the hydraulic integrity of the rock, which is of special interest in the case of an underground disposal of hazardous waste in salt rock. 

The two modes of macro-cracking that occur in bending tests in case of large specimen deformation with attached TBC are shown in Fig. 2. With the TBC under tension almost equidistant segmentation cracks develop (Fig. 2a). The ratio of crack distance to TBC thickness is about E k, 2.25 which is in fair agreement with... 

Fig. 9 displays the side-face of a notched stand-alone TBC specimen before loading and after growth of the segmentation crack. Obviously the macro-crack selects and combines essentially the suitable segments from the pre-existing pattern of inter- and intra-splat cracks and thus the segmentation crack path comprises of segments of these micro-defects . ... 

Figure 9. Tortuous segmentation crack path. Macro crack follows pre existing pattern of intra and inter splat micro cracks.

Figure 7. Micrographs of the sample 2H aged at 250 °C surface of the disc (a) micro and macro cracks formed due to t m phase transformation (b) and fracture surface showing micro and macro cracks formed near the surface.

Figure 3. Normalised stiffness properties of a [0/90], SiC/CAS cross ply damaged by transverse macro cracks as function of transverse cracks density C (no longitudinal cracks).

Figure (1) represents examples of V-Ki curves in a polycristalline zirconia ceramic, which exhibit the typical pattern of SCG due to stress corrosion at the crack tip, including the presence of thresholds under vacuum, water and air conditions. The threshold, Kio, often observed on macro-cracks was questioned because of the differences observed between the behavior of micro- and macro-cracks in polycrystalline ceramics [15]. The results obtained in figure (2) for the same polycrystalline zirconia show that the threshold obtained for macro- (i.e. by Double Torsion) and micro- cracks (indentations) are in agreement. It is of course possible that reinforcement mechanisms, discussed below, can change significantly the value of the threshold if they lead to a R-Curve behavior. 

Light State crack initiation it is characterized by the growth and coalescence of the sub-microcracks, which transform into micro-cracks these start increasing under the cyclic loads, and form macro-cracks, which are typically detectable by measurement instrumentation. 

Austenitization temperature was selected on a basis of series of compression test, where it was selected as the highest temperature at which after deforming the samples to strain of 0.9 with a strain rate of 1 s" no macro cracks were present on the surface of the specimens [11-15]. 

Fig. 1.18 Time distribution in terms of cycles JVy (i = 1, 2, 3, 4) of the four fatigue phases (1) cyclic hardening or softening (2) slips with MSC formation (3) macro crack formation (4) growth of macro crack to failure, at four selected stress amplitude Sj...

Fatigue crack nuclei that precede macro crack formation originate in persistent slip bands well before the final failure. It has been said in 1.4.1 that they start to appear on the surface of the material as soon as the hardening/softening process saturate and, in fact, it has been also said that if the cyclic hardening were periodically removed by annealing the specimen subjected to a fatigue test, the life of the specimen would increase enormously to almost become infinite. Saturation... 

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