Fracture parameters for buried cast iron pipes subjected to internal and external corrosions and crackings

Akhi, Atika Hossain (2021) Fracture parameters for buried cast iron pipes subjected to internal and external corrosions and crackings. Masters thesis, Memorial University of Newfoundland.

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Abstract

Cast iron water main represents a significant portion of municipal infrastructure in North America and worldwide. The aging cast-iron pipes are subjected to deterioration due to corrosion, resulting in cracking and failure. The municipalities face problems with pipe breakage or leakage and associated socio-economic issues caused by water loss, service disruption, and damages to the nearby facilities. For a proper maintenance decision of the pipes, it requires evaluating the remaining strength of the deteriorating structure. Fracture mechanics is being preferred to assess the remaining strength of the deteriorating structure over the conventional strength-based method due to its ability to capture crack initiation and propagation. However, determining fracture parameters, such as Stress Intensity Factor (SIF), is a challenge in applying the fracture mechanics for evaluating the structure. This study presents an evaluation of the SIF for buried cast iron pipes subjected to internal and external corrosions and cracks. Semi-elliptical surface defects (crack only and crack with corrosion) are considered for a wide range of aspect ratios (crack-depth to crack-length ratio) and relative crack depths (crack-depth to pipe thickness ratio) to evaluate the SIFs. The SIFs are assessed for invert/crown and springline position cracks under internal pressure and vertical surface loads. The study revealed that the SIF for a crack due to internal pressure is not affected by the presence of surrounding soil and therefore can be calculated using the available solution for in-air pipes. The SIF due to surface load depends on its geometry and location of the crack. A design equation is proposed to calculate the SIFs due to the surface load using an influence coefficient. The influence coefficient is presented for internal and external semi-elliptical defects as a function of crack aspect ratios, depths and locations. A method is proposed to determine the SIFs for buried pipes as a sum of the SIFs due to the internal pressure and the surface loads.

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