Role of Viscosity on Capillary Flow and Stress Corrosion Cracking Behavior
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STRESS corrosion cracking (SCC) is a common problem in a variety of industries including petroleum, offshore, fossil and nuclear power plants, paper plants, as well as bridges, ships, and aircrafts. It is believed that SCC results primarily from stress-enhanced chemical reaction at the crack vicinity. By plotting experimental data in terms of crack velocity, da/dt, vs applied stress intensity factor, K, a three-stage curve commonly observed in many materials is schematically depicted in Figure 1. It is a common practice to associate each of these three regions with different mechanisms that control/ limit the SCC process.[1–5] In region I, the crack velocity (da/dt) is highly sensitive to the applied stress intensity factor, K, and environmental concentration. Some data obtained in either hydrogen gas or aqueous environment suggest that the crack velocity may be reaction limited in region I. In these experiments, a threshold stress intensity KISCC can be defined below which the crack velocity is negligible. In region II, the crack growth rate also depends on the environmental concentration. However, in contrast to region I behavior, the crack velocity in region II does not depend strongly on the applied K. In general, in region II, there are two types of behavior observed. For type A behavior, the crack growth rate is independent of the applied K. On the other hand, for type B, the crack DANIEL KUJAWSKI, Professor, Department of Mechanical and Aeronautical Engineering, and SINDHOORA YERRAMILLI, Graduate Student, Department of Industrial and Manufacturing Engineering, are with Western Michigan University, Kalamazoo, MI 49008-4353. Contact e-mail: [email protected] A.K. VASUDEVAN, Program Director, is with the Office of Naval Research, Arlington, VA 22203. Manuscript submitted January 4, 2010. Article published online August 25, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
velocity has a smaller dependence on the applied K than in stage I. Region III of the crack growth behavior is characterized by a strong dependence on the applied K and an absence of dependency on the concentration or aggressiveness of the environment. It could be noted that the three stages of behavior shown in Figure 1 bear its relevance for most SCC systems (such as metals, ceramics, and polymers) in gaseous and aqueous environments as well as in liquid metal embrittlement.[1–5] The overall shape of the curve in Figure 1 can vary depending on the material/environment system and the apparent chemical reaction rate and its relation to loading/deformation rate. It has been inferred that crack growth behavior in region II is controlled by the rate limiting step such as transport of environment to the crack tip. This observation is supported by experiments where the plateau velocity decreases markedly with viscosity. As an example, Figure 2 shows classical Speidel’s[6] experimental data, which demonstrate the effect of viscosity on SCC behavior for 7079-T651 Al alloy. Different viscosities were obtained by changing the ratios of the mixture of
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