Water utility engineers have reported on large diameter water main failures that occurred suddenly without warning, no signs of prior leaks and no visual evidence of corrosion on the fracture surfaces. Often these failed mains had been operating without major problems for over 80 years. A possible explanation may be attributed to alternating or fluctuating stresses such as those caused by heavy traffic and cyclical operating water pressure with occasional occurrences of transients. These fluctuating stresses are accentuated if the pipe impinges on an object with sharp geometry and high stiffness like a rock or a stone. Fatigue of cast iron has been extensively studied in the context of cast iron bridges, structural elements such as columns, engine blocks, etc, but not in the context of buried, grey cast iron water pipes. Fatigue analysis methods developed over the years involve a lot of empiricism and combine engineering principles with experimental observations. Consequently, it is fair to say that fatigue analysis results should be taken more as a guide than as precise or accurate answers. A mechanistic approach to explain fatigue failures of buried cast iron pipes had not been previously explored. This paper explores the application of the fracture mechanics approach (LEFM) to explain some failures in cast iron pipes that occur through the fatigue mechanism. It endeavors to provide insight into the plausibility of fatigue failures in grey cast iron pipes when and if subjected to alternating (also often referred to as repeated or variable) stresses due to surface traffic loads, operating pressure variations and transient pressure occurrences. It is important to note that the proposed analysis refers to grey cast iron pipe type, with carbon in form of flake graphite, which is the predominant material of existing iron trunk mains in North America and Europe.
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