Failure assessment of cast iron water mains using fracture mechanics

Debnath, Suborno (2019) Failure assessment of cast iron water mains using fracture mechanics. Masters thesis, Memorial University of Newfoundland.

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Abstract

Cast iron was one of the most dominated materials for municipal water mains in the last century. Cast iron pipes are being gradually replaced by ductile iron, steel or plastic pipe due to their poor performance. However, a significant portion of cast iron pipelines is still in service for the municipal water distribution system. Many of these pipelines are deteriorated due to corrosion and often fail. The remaining strength assessment of these pipelines is required for maintaining the integrity of municipal water distribution systems. Researchers employed conventional continuum mechanics approach for the assessment of cast iron water mains. However, the continuum mechanics based modelling was found to be unsuccessful explaining some failure mechanisms observed in the field. The fracture mechanics approach could be used to investigate the failure mechanism of the pipelines. The major challenges in the application of fracture mechanics include i) availability of a tool for calculating the fracture parameters and ii) availability of material parameter for fracture mechanics based strength assessment. In this study, mechanical properties for cast iron pipe materials are explored for fracture mechanics based strength assessment. Uniaxial tensile tests are conducted to understand the stress–strain response for stress–deformation analysis to calculate the fracture parameter. The influence of the rate of loading on stress–strain behavior and loading-unloading responses are investigated. Fracture toughness is also determined by using a simplified chevron notch method. The parameters obtained from the test are used in finite element analysis to determine the fracture parameters of cast iron pipes. Numerical techniques for finite element modelling are developed for the assessment of fracture parameter (i.e., stress intensity factor). The stress intensity factors for different shapes of corrosion defects are examined. The employed fracture mechanics approach is found to successfully explain the failure mechanism of cast iron pipes observed in the field.

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