Crack initiation process

Duquette, D.J., Corrosion Fatigue Crack Initiation Processes A State-of-The-Art Review, in Environment-Induced Cracking of Metals, R.P. Gangloff, M.B. Yves (eds.), NACE-10, Houston, TX, p. 45, 1990. 

For elastic materials, the contact problem is usually solved as a unilateral contact problem obeying Coulomb s friction law. The algorithms used here are based on those pioneered by Kalker [66]. The contact area, the stick and slip regions, the pressure and traction distributions are numerically determined first and then the stress and displacement distributions within the elastic bodies can be established at the various stages of the tangential cyclic loading. On the basis of these calculations, the occurrence of crack initiation processes can subsequently be analysed in the meridian plane of the contact, y = 0 (Fig. 12), where the cracks first initiate. As a first approach, parameters based on the amplitude of the shear stress, rm, along a particular direction and the amplitude of the tensile stress, [Pg.174]

Surface films appear to play a major role in the initiation of SCC and may also contribute to HE effects. The main role of the surface film is to localize the damage inflicted on the material by the environment. The damage can be caused by the mechanical breakdown of the passive film by slip step or electrochemical breakdown of the passive film (73). SCC may be related to the nature of the surface film. The SCC of carbon steels is related to the presence of magnetite in environments at 90 °C except when pitting is involved in the crack initiation process, as in nitrate medium or in high-temperature water (115, 116). 

Fatigue failure in metals is known to consist of crack initiation and growth. Crack initiation starts with dislocation movement. Submicrocracks are then formed at slip bands. They subsequently grow and coalesce to form a crack of detectable size (short crack) to complete the crack initiation process [12]. This is then followed by the growth of a single crack until final rupture. The period of crack initiation and submicrocrack growth covers most of the fatigue life. 

Inamura, T. et al.. Renormalized molecular d5mamics simulation of crack initiation process in machining defectless monocrystal silicon. Bull JSPE, 63.1 (1998) 86. 

Ouyang, Z., Li, G., Ibekwe, S.I., Stubblefield, M.A., and Pang, S.S. (2010) Crack initiation process of DCB specimens based on first order shear deformation theory. Journal of Reinforced Plastics and Composites, 29, 651-663. 

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. 

M. Elboujdaini, Y.-Z. Wang, R. W. Revie, R. N. Parkins, and M. T. Shehata, Stress corrosion crack initiation processes Pitting and microcrack coalescence. Paper No. 00379, CORROSION/2000, NACE International, Houston, TX, 2000. 

Fig. 5.7. Schematic representation of a craze which emerges at the crack tip in the stress crack initiation process and which precedes the actual crack growth. The representation is based onR. P. Kambonr s work. Source (Menges 1973a)...

After the specimen is loaded in tension at intermediate stress levels, an incubation period is required to initiate a crack. During this time, lattice-contained or weakly-trapped hydrogen diffuses ahead of the notches and local stress raisers, so as to equilibrate under the influence of any triaxial stress created there-this equilibration involves hydrogen redistribution associated with the equilibration of its chemical potential, under the combined influences of all factors that affect it. The first cracks will be initiated in this region after a critical hydrogen concentration has accumulated. The kinetics of the crack-initiation process are therefore dependent on the temperature and the stress gradient as they relate to the diffusion and concentration of hydrogen at the site of maximum triaxial stress in the steel. 

Water environment promotes crack initiation in silica, Tempax, and soda-lime glasses (Table 1). This is because the crack initiation process is assisted by the adsorption of water molecule. In the conceptual framework of this idea, " a water molecule is adsorbed at the strained bonds formed by a scratch process. After that, the attacked Si-O-Si bond and the water molecule break one Si-0 bond to leave new two Si-OH bonds. West and Hench reported that the energy barrier of hydrolysis of strained 3-fold rings is 97% smaller than that of fracture by water-free dilation. This water-assisted bond-breakage process is the origin of lower crack initiation load obtained in water. 

Well-documented studies have been performed to compare the fatigue behavior in air and under vacuum at low or moderate temperature of copper (Wang et al., 1984 Bayerlein and Mughrabi, 1992) and austenitic stainless steels (Gerland et al., 1988 Mendez et al., 1993). As an example. Fig. 5-12 shows the marked effect of an air environment at room temperature, even for a corrosion-resistant alloy. High cumulative plastic strain amplitudes can be reached under vacuum. The oxygen partial pressure controls the nature of the surface oxide and localization of the crack initiation process in persistent slip bands formed by cyclic straining. 

The electrochemical approach presented here has many limitations. First of all, the kinetics of the electrochemical reactions are closely dependent on the cyclic plasticity and the number of cycles [9,10]. Thus, predictive laws are very complex. Moreover, these laws use the local current densities, which are very difficult to model. Finally, this approach does not really take into account the local corrosion-deformation interactions (GDIs) and the effects of the corrosive solution on the deformation mode. Indeed, such synergetic effects between corrosion and deformation can be of prime importance the following examples emphasize the role of GDI in crack initiation processes. 

Observations of die crack initiation sites by scanning electron microscopy showed that at low plastic strain amplitudes (A p/2 < 10 ) for which the fatigue resistance of die a-y alloy is close to that of the 7 alloy, cracks nucleate only in the austenitic phase (Fig. 11a), but at higher strain amplitudes (Aep/2 < 10 ), the first cracks nucleate principally in the ferritic phase (Fig. 11b). The excellent CF resistance of duplex stainless steels (for Aey2 < 10 ) can then be understood through the electrochemical and mechanical coupling effects on crack initiation processes. 

The macroscopic cycling softening effect observed in H2SO4 solution at room temperature (which is not due to microcracking) is very relevant to take into accoimt quantitatively the local dissolution-deformation interactions that will lead to the fatigue crack initiation process. 

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