The fracture and fatigue properties of cast irons used for trunk mains in the water industry
In order to assess the remaining life of cast iron assets in the water sector, an understanding of their fracture and fatigue characteristics is necessary. The present work is concerned with the toughness and Paris crack growth behaviour of cast iron materials with a range of micro-structures, taken from trunk mains currently in service. When considered with other data from the literature, the results from the present study enable the range of fatigue crack growth behaviour likely to be seen in service to be quantified. The role of microstructure in fracture and fatigue behaviour is discussed. Calculations of fatigue life based on integration of the Paris law are then carried out and compared with previously published data for samples from cast iron distribution mains. The results from these investigations support the development of asset management tools for use in the water industry.
Failure of small diameter cast iron pipes
The in-service strength degradation, as a result of corrosion, of cast iron water distribution pipes has been investigated. The strengths of 1 m lengths of pipe extracted from the ground have been measured in either 3- or 4-point bending and the size of the controlling defect has been estimated by visual examination of the fracture surface. The application of Weibull statistics to the bend test data demonstrates that there is bimodal behaviour which suggests that there are two populations of flaws present. It is postulated that the larger flaw size population is associated with corrosion pits that form during the process of graphitisation, while the smaller flaw size population is associated with the inherent flaws within the (brittle) cast iron pipe material. A critical pit depth is identified at the transition between the two competing flaw populations, where there is a change in slope on the Weibull plot. It is shown also that the residual strength/pit depth data are described equally well by either of the two conventional analyses, i.e. loss of section and fracture mechanics.
Martensitic transformation at a propagating crack
Abstract Localized transformation in the inhomogeneous stress field at a crack tip is found in alloys that are capable of strain-induced martensitic transformation (austenitic steels). Acceleration or deceleration of fatigue crack growth can occur, depending on stress intensity K and the difference of test temperature T and martensite temperature Ms. It is proposed in the present paper that internal compressive stresses as induced by a positive local volume change can retard crack growth. Measurements of fatigue crack growth rates of specially designed austenitic alloys with Ms little below room temperature were used to test this hypothesis.