Environmental Crack Growth Behavior Affected by Thickness/Geometry Constraint
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AN understanding of crack growth behavior under the application of mechanical stresses (cyclic and static) for different material/environment systems is a prerequisite for a reliable life prediction and dependable service maintenance. It is commonly assumed that, in environmentally assisted cracking, a crack extends mainly because of electro-chemical processes at the crack tip, such as adsorption, chemisorptions, dissolution, hydrogen diffusion, and embrittlement.[1–3] Hence, the associated research has been mostly concentrated on effects related to crack-tip electro-chemistry and transport issues of bulk solution into the crack-tip vicinity, with a lesser emphasis on the role of crack-tip stresses. This notion appears to be in accordance with the experimental observations, which indicate that in stress corrosion cracking (SCC) or corrosion fatigue (CF) at a given applied stress, the time required to nucleate a crack decreases with increasing aggressiveness of the environment. At the same time, an existence of the plateau region, in SCC in terms of da/dt vs K, seems to support the viewpoint that applied stress plays a little or limited role. In fact, using only the electro-chemical line of reasoning, it is difficult to explain in a self-consistent way, e.g., why a sharper notch require less time to nucleate an environmentally assisted crack compare to a blunt notch. Also, the rate of SCC in a thick specimen (plain strain) is usually higher than in a thin one (plain stress), whereas the transport of chemical spices from the bulk solution to the crack tip, seems to be easier, for thinner than for thicker samples. This indicates that better understanding of thickness and constraint could DANIEL KUJAWSKI, Professor, is with the Department of Mechanical and Aeronautical Engineering, Western Michigan University, Kalamazoo, MI 49008-5343. Contact e-mail: daniel.kujawski@ wmich.edu Manuscript submitted January 16, 2012. Article published online November 13, 2012 1340—VOLUME 44A, MARCH 2013
be important in crack growth modeling. It can be noted that existing studies on thickness/constraint effects on fatigue crack growth (FCG) behavior at different material/environment systems are self-contradictory, and they fall into three main categories: Small effects, FCG rate increases with thickness, or FCG rate decreases with thickness.
In this article, a short review on the effects of thickness/ constraint and environment on crack growth behavior is presented and discussed. This is done by contrasting different environments ranging from vacuum to air and NaCl solution, for a number of steels and aluminum alloys, to better understand the synergetic role of environment and mechanical stress on crack growth behavior. It is believed that this can also contribute to a better modeling of CF and FCG behavior.
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BACKGROUND
It is well recognized that constraint has important effects on the actual fracture behavior of cracked bodies. The effects of constraint have attracted a great deal of interest in the literature, in particular, wit
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