Etkinlik

Cast iron pipes can fail in many modes which in general can be summarized into two categories: loss of strength due to the reduction of wall thickness of the pipes, and loss of toughness due to the stress concentration at the tips of cracks or defects. Even in one category there can be many mechanisms that cause failure. The strength failure can be caused by hoop stress or axial stress in the pipes. A review of recent research literature (Sadiq et al., 2004; Moglia et al., 2008; Yamini, 2009; Clair and Sinha, 2012) suggests that current research on pipe failures focuses more on loss of strength than loss of toughness. As was mentioned in Section 3.3.7(b), the literature review also revealed that in most reliability analyses for buried pipes, multifailure modes are rarely considered although in practice this is the reality. Therefore the aim of this section is to consider multifailure modes in reliability analysis and service life prediction for ductile iron pipe. Both loss of strength and toughness of the pipe are considered. A system reliability method is employed in calculating the probability of pipe failure over time, based on which the service life of the pipe can be estimated. Sensitivity analysis is also carried out to identify those factors that affect the pipe behavior most.

Buried pipes are not only subjected to mechanical actions (loads) but also environmental actions that cause the corrosion of pipes. Corrosion related defects would subsequently cause fracture of cast iron pipes. In the presence of corrosion pit, failure of a pipe can be attributed to two mechanisms: (i) the stresses in the pipe exceed the corresponding strength; or (ii) the stress intensity exceeds fracture toughness of the pipe. Based on these two failure modes, two limit state functions can be established as follows.