Incubation, cracking

Figure 17.7 The length of the incubation crack as a function of the stress amplitude B - an Al-based alloy C - a Ti-based alloy D - a low-alloy steel E - a superalloy F - a stainless steel. See Table 17.1 for details.

The incubation period varies widely depending on such factors as crack morphology, water chemistry, and temperature. However, experience in a wide variety of cooling water environments has shown that many stainless alloys develop noticeable attack within 6 months of first being exposed to water. It is rare to see attack initiating many years after equipment commissioning unless service conditions change in the interim. 

Any test (several such tests are used) in which time to failure of smooth specimens is determined is an overall measure of the incubation period to initiate a crack, the ability to resist the propagation of a stress corrosion crack and the ability to resist final mechanical fracture. Since this test does not indicate the relative merits of an alloy in each individual aspect of the... 

S.C.C. has received a share of the potentiostatic approach to corrosion. Barnartt and van Rooyen reported that potentiostatically controlled corrosion in a potential range 50-100 mV above the corrosion potential provided an accelerated test for the s.c.c. of stainless steels. The elevation of the potential by means of a potentiostat eliminated the incubation period, and also increased the density of cracks. Booth and Tucker used potentiostatic methods in the s.c.c. of Al-Mg alloys. 

Annealed samples (0 percent strain) had an incubation period followed by a decrease in specific gravity. Steels with 5 percent strain had shorter incubation periods, and specific gravity decreased at a more rapid rate. Steels with 39 percent strain showed no incubation period at any test temperature, indicating that fissuring and cracking started immediately upon exposure to hydrogen. 

The ubiquity of this power-law behaviour in SCG tests on PE has been the subject of considerable discussion, usually based on the assumption of a fibril creep failure mechanism [43, 45, 46, 47, 76, 79]. At high and intermediate K, after a certain induction period, steady-state crack advance is generally observed to occur by a stick-slip mechanism all or part of the fibrillar zone breaks down rapidly after an incubation time during which fibril creep takes place. The crack-tip then advances rapidly over a short distance and a new fibrillar zone stabilises, as sketched in Fig. 12. 

Note that the stress intensity factors do not involve a characteristic dimension nor a characteristic time where t represents the time varying boundary condition. If the crack starts to propagate rapidly after an incubation time of t, then the above stress intensity factors should be modified with a scalar function of the crack velocity. 

Yet other methods have been proposed (Fig. 9.6) whereby the CAM vascular networks can be displayed in greater detail, except that the embryo with the extra-embryonic membranes and yolk must be transferred to an in vitro system during the early stages of development (day 3 or 4 of incubation). The system consists of a Petri dish (Auerbach et al., 1974), or an inert plastic container, previously equipped with a parafilm ring (4—5 cm inside depth) to provide a support for the embryo and adnexa (Dugan et al., 1991). The embryo in the petri dish or in the container is then incubated in an humidified CO2 atmosphere at 37°C. This CAM in petri dish, or cracked eggs method is among the most frequently used CAM assays. 

During the incubation period of a crack, when 5 increases, the length of the craze also increases. In unnotched specimens, there may be effects due to the nucleation of sites from which the crazes grow, but if an initial flaw is assumed, then from Eq. (31) we may write ... 

Crack-free layers can be obtained with a layer thickness of 1-2 pm and particle sizes as low as 100 pm at a minimum temperature of 95°C. The growth of the MFI layer can be divided into three periods (1) the incubation period (no observable layer), (2) the layer growth period, and (3) the stationary period (no further increase of thickness with time). Incubation and layer growth period increase with decreasing synthesis temperature and are about 12.5 h (incubation period) and about 50 h (end of layer growth period) at 95°C compared with less than 1 h and 10-15 h respectively at 120°C. 

The investigation of incubation effects, i.e., that the optical, chemical, and mechanical properties of a (transparent) material can be changed during repetitive illumination of the same spot without ablation, is rather difficult because of the complicated surface structure of a biological composite material. It can be stated that one femtosecond laser pulse at F0=2.0 J cm-2 (roughly three times Fth) applied to healthy human enamel led to minor ablation (Fig. 32a). Five pulses of the same fluence resulted in a crater with a depth of 2-3 pm (Fig. 32b). The shape of the excision is well defined, and no cracks could be observed. 

The overall SCC response is illustrated diagrammatically in Fig. 7.6, with the rate-limited stage of crack growth represented by stage II (left-hand figure) and a schematic representation of the influence of incubation (on the right). From a... 

Figure 7.3. Typical sustained-load crack growth response, showing incubation, transient (non-steady-state) and steady-state crack growth, under constant load (where K remained constant, with crack growth, through specimen contouring) [3].

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