Crack initiation environments

Other factors which can affect impact behaviour are fabrication defects such as internal voids, inclusions and additives such as pigments, all of which can cause stress concentrations within the material. In addition, internal welds caused by the fusion of partially cooled melt fronts usually turn out to be areas of weakness. The environment may also affect impact behaviour. Plastics exposed to sunlight and weathering for prolonged periods tend to become embrittled due to degradation. Alternatively if the plastic is in the vicinity of a fluid which attacks it, then the crack initiation energy may be reduced. Some plastics are affected by very simple fluids e.g. domestic heating oils act as plasticisers for polyethylene. The effect which water can have on the impact behaviour of nylon is also spectacular as illustrated in Fig. 2.80. 

In neutral solutions the application of cathodic polarisation prevents crack initiation and this could be taken to indicate that hydrogen embrittlement is not the operative mechanism, since the discharge and entry of hydrogen might be expected to fracture the specimen more readily. The beneficial effect of cathodic polarisation has been interpreted , however, to result from more rapid film repair in the alkaline catholyte generated by the cathode reaction. The film serves as a barrier to rapid hydrogen entry. Consistent with this is the observation that in an environment of low pH (e.g. 10 N HCl) where film formation would not be expected, cathodic polarisation has no effect upon crack propagation. 

The growth or extension of a fatigue crack under cyclic loading is principally controlled by the maximum load and stress ratio (minimum/maximum stress). However, as in crack initiation, there are a number of additional factors that may exert a strong influence, especially in the presence of an aggressive environment. 

Mechanisms of SCC. Crack initiation of EAC is complex and not well understood till now. Most of the SCC systems exhibit short initiation times ranging from minutes to weeks and cracking often occurs due to the change in the environment rather than to a very long initiation time. Stress-corrosion crack growth rates are usually 10 11 and 10-6 m s In systems such as stainless steels in chloride solutions, localized corrosion may create the local conditions prone to crack development, but it is still difficult to explain the initiation of the crack in the absence of localized corrosion in environmental conditions different from that of the crack propagation.95 It should be mentioned that dealloyed surface layers such as certain copper alloys in ammonia-containing solutions are believed to cause SCC.54... 

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. 

Long-term exposure of composites to oxidative environments can have deleterious effects on short-term mechanical behavior, such as resistance to crack initiation. This is particularly true in the case where the composite oxidizes to form an oxide surface scale. Although such reactions can be beneficial in limiting oxidation reactions, when the composite is subsequently cooled to room temperature, the reaction product can be a source of flaws and increase the composite s susceptibility to crack initiation. The following example illustrates this point. 

ESC testing has been conducted at 50°C only, since at 70°C the detergent was immiscible in water even at very low concentrations. Fig. 8 shows crack initiation times obtained in a 10% detergent/water solution at varying applied stress intensity factor for the two materials examined (filled points). For comparison purposes, data obtained in air at the same temperature are also reported (empty points). For sufficiently high values of the applied stress intensity factor, a linear trend which is quite close to that obtained in the non-aggressive environment is observed. 

Comparison between the results obtained in air and in detergent shows that aggressive environment affects the crack resistance only below a certain "critical" value, K ic. This is probably to be attributed to a diffusion-controlled plasticization mechanism, which requires times larger than a certain "critical" time (for crack initiation) or crack speeds lower than a certain "critical" speed (for crack propagation) to be activated. This assumption is confirmed by literature data. 

Stress corrosion cracking is a form of localized corrosion, where the simultaneous presence of tensile stresses and a specific corrosive environment prodnces metal cracks [157, 168]. Stress corrosion cracking generally occnrs only in alloys (e.g., Cn-Zn, Cu-Al, Cu-Si, austenitic stainless steels, titaninm alloys, and zirconinm alloys) and only when the alloy is exposed to a specific environment (e.g., brass in ammonia or a titaninm alloy in chloride solutions). Removal of either the stress on the metal (which must have a surface tensile component) or the corrosive environment will prevent crack initiation or cause the arrest of cracks that have already propagated. Stress corrosion cracking often occurs where the protective passive film breaks down. The continual plastic deformation of the metal at the tip of the crack prevents repassivation of the metal surface and allows for continued localized metal corrosion. 

Material/Environment Variables Affecting Crack Initiation and Growth... 

The failure time, however, incorporates both the time required for crack initiation and a period of slow crack growth so that the separate effect of the environment on each of these stages cannot be ascertained. (Some of the difficulty stems from the lack of a precise definition for crack initiation.) This difficulty is underscored by the results of Brown and Beachem [1] on SCC of titanium alloys. They showed that certain of the alloys that appeared to be immune to stress corrosion cracking in the traditional (smooth specimen) tests are, in fact, highly susceptible to environment-enhanced crack growth. The apparent immunity was explained by the fact that these alloys were nearly immune to pitting corrosion, which was required for crack nucleation in the same environment [1]. 

Kondo, Y., and Wei, R. P., Approach On Quantitative Evaluation of Corrosion Fatigue Crack Initiation Condition, in International Conference on Evaluation of Materials Performance in Severe Environments, EVALMAT 89, Vol. 1, Kobe, Japan, November 20-23... 

Crack initiation is promoted by corrosion and, particularly, by pitting or crevice attack, even of low depth, which can cause local conditions of acidity and thus the development of hydrogen. The duration of this stage depends both on the characteristics of the steel and the environment (e. g. the surface finishing of the steel bars, the pH and chemical composition of the environment, etc.) and the time required to initiate the preliminary pitting or crevice attack. However, it does not depend on the stress applied to the steel. 

CF cracks are always initiated at the surface, unless there are near-surface defects that act as stress concentration sites and facilitate subsurface crack initiation. Crack initiation takes place independently of fatigue limit in air as it can be decreased or eliminated through the increase of dissolution rates at anodic sites. Localized corrosion such as pitting favors fatigue crack initiation through stress concentration and a local acidic environment. The two main mechanisms of CF are anodic slip dissolution and HE (80). 

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