Observation and Modeling of Stress Corrosion Cracking in High Pressure Gas Pipe Steel
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INTRODUCTION
STRESS corrosion cracking (SCC) is a phenomenon of brittle fracture of normally ductile material resulting from a combined action of mechanical stresses and chemically aggressive environment. SCC has been observed in many different engineering materials such as various metals, polymers, concrete, ceramics, etc. under stresses in the presence of a certain aggressive environment specific for each material. Usually, SCC appears as a colony of small cracks. Figure 1(a) illustrates a part of the SCC colony in X-60 steel pipe developed in service in a near neutral pH environment. The section of pipe with SCC colony was removed from one of the North American natural gas transmission pipelines. The colony consists of a large number of closely spaced small cracks with preferentially axial orientation. A number of cracks that appear to be linked together form a cluster. Detailed analysis reveals that the cracks in a well-developed cluster are separated by small plastically deformed ligaments.[1–3] Premature failure by SCC of polybutylene (PB) tubing (one of engineering thermoplastics) employed in the water distribution system is shown in Figure 1(b). Similarly to the preceding steel example, SCC in PB results from a combination of oxidative degradation of PB and mechanical stresses. In this particular case, the SCC BYOUNG-HO CHOI, Associate Professor, is with the School of Mechanical Engineering, Korea University, Seoul, 136-701, Republic of Korea. ALEXANDER CHUDNOVSKY, Distinguished Professor, is with the Department of Civil and Materials Engineering, University of Illinois at Chicago, Chicago, IL 60607. Contact e-mail: achudnov@ uic.edu Manuscript submitted January 28, 2010. Article published online August 13, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
colony consists of a network of random rectangular cells. A number of open clusters of cracks with size noticeably larger than a typical cell size and oriented along the axial direction (perpendicularly to the hoop stress) are well visible in Figure 1(b). Figure 2 illustrates the general process of cluster formation on an example of numerical simulation of two-crack cluster formed in a system of four cracks. The upper left frame (1) presents initial four small cracks loaded by vertical stress (hoop stress in the tubing). The upper right frame (2) shows individual crack extension in the axial direction. A specific mechanism of the ICG is not important for this example and is not discussed here (Section III is dedicated to kinetics of stress corrosion (SC) crack growth). It is assumed in the numerical modeling that the crack growth rate is proportional to some power of stress intensity factor (SIF) or energy release rate (ERR), and the directions of crack extension are determined by the maximum ERR direction. The interaction between the neighboring cracks increases as the distance between the individual cracks diminishes. As a result, the crack trajectories start to curve, as can be seen in the lower left frame (3) of Figure 2.[4] Finally, the two middle cracks
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