Subcritical cracking

Elastic energy release due to subcritical crack growth is one recognized source of structure-related AE within its acknowledged lunitations, AEBIL provides a viable means of early on-line deteetion and localization of stable crack propagation. 

The mechanism of subcritical crack growth is the reaction of the corrosive medium with highly stressed bonds at the crack tip. In siUca, in the absence of stressed bonds, the rate of the reaction between the bonds and corrosive media such as water is very low. The introduction of strain energy into crack tip bonds increases the activity of the bond. For siUca glass in water, attack and bond breakage occurs by the following reaction (47) ... 

Ceramic-matrix fiber composites, 26 775 Ceramics mechanical properties, 5 613-638 cyclic fatigue, 5 633-634 elastic behavior, 5 613-615 fracture analysis, 5 634-635 fracture toughness, 5 619-623 hardness, 5 626-628 impact and erosion, 5 630 plasticity, 5 623-626 strength, 5 615-619 subcritical crack growth, 5 628—630 thermal stress and thermal shock, 5 632-633... 

The reduction of strength after short time corrosion in acids (pit formation) cannot be correlated with the weight loss [511], whereas after intensive corrosion a correlation exists between thickness of the corroded layer and strength [507]. Acid corrosion appears to influence subcritical crack growth in Si3N4 ceramics [507, 523]. 

Ohji T (1994) Tensile Creep Rupture And Subcritical Crack Growth of Silicon Nitride. In Hoffmann MJ, Petzow G (eds) Tailoring of Mechanical Properties of Si3N4 Ceramics. Kluwer Academic Publishers, Netherlands, p 339... 

Hoffman, M., Rodel, J., Stemitzke, M. et al., Fracture toughness and subcritical crack growth in alumina/silicon carbide nanocomposites , Fracture Mechanics of Ceramics, 1996, 12 179. 

Structural failures in general occur by a combination of crack initiation followed by subcritical crack growth mechanism such as stress corrosion, fatigue, and creep until a critical crack size is reached. Thus, fracture mechanics is useful in evaluating ac that leads to failure by fracture. The application of fracture mechanics in failure analysis throws light on the progress from subcritical crack growth to fracture, the amount of load... 

Both AC and DC potential-drop methods are well-established techniques for monitoring subcritical crack growth. A combined AC/DC potential-drop measuring technique can, in some cases, help in obtaining more information from a single test, in particular for the onset of stable crack growth.172... 

Hancock, G.G. and Johnson, H.H., Hydrogen, oxygen, and subcritical crack growth in a high-strength steel. Transactions of the Metallurgical Society ofAIME, 236, 513-516, 1966. 

Very slow subcritical crack growth can be approximately characterized by an empirical law of the form... 

Fig. 16. Schematic of crack development in crazes I. stress direction and craze orientation in a CT-specimen, 2 Growth of individual crazes, 3. Nucleation of voids and their coalescence for crack formation (A), 4. Crack through craze acceleration (B), 5. Lateral coalescence of crazes with tendency to quasi-homo-geneous deformation and subcritical crack progress...

Fig. 17a—d. Details of the fracture surface on a CT-specimen of crystalline PETP a general view of notch (N), subcritical crack growth through crazes (A), transitional region (B), and brittle fracture part (C) b dimple pattern in the central part of the broken craze (region A) c patchwork structure in the transition zone to crack instability (region B) d brittle fracture part (region C)... 

In addition to their successful applications to fracture stability problems, the stress intensity factors have been widely used as correlation parameters in analyzing the subcritical crack growth rates da/dn (in fatigue) and dafdt (in corrosion), n and t referring to the number of... 

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