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.
Service life of corrosion pitted pipes subject to fatigue loading and hydrogen embrittlement
Abstract The structural service life of brittle material pipes with exterior corrosion pits is likely to depend on crack initiation and crack development and this may be influenced by pressure loading fluctuations and the possibility of material hydrogen embrittlement. Recently developed methods are used to estimate the cracking pattern, the failure state of the crack development from external pits and the rate of Hydrogen-Assisted Cracking under fluctuating loadings. The effect of hydrogen from the surrounding environment on the cracking rate is formulated using a generalized form of Paris' law. The depth of cracks initiated from surface pits is estimated as a function of pipe age. A realistic example is presented and the results discussed.
Fatigue crack propagation and threshold in b.c.c. and f.c.c. metals at 77 and 293 K
Abstract Fatigue crack propagation was measured at 77 K in the low stress intensity range by means of 20 kHz ultrasonic stressing. Two f.c.c. materials (copper, austentic steel 304) and b.c.c. mild steel were used. Compared to 293 K tests the Δa/ΔN = f(Kmax) curves are shifted to 10–40% higher Kmax values at 77 K. This effect is explained by the raised tensile stresses at 77 K. The well known brittle fracture behaviour which is typical for b.c.c. metals is found in mild steel at 77 K only at cyclic stress intensities which are higher than about 20 MNm − 3 2 , below this value the fracture surfaces show ductile fracture modes with striations indicating different glide systems. These striations do not cross grain boundaries. This result shows that below a minimum stress and stress intensity of about 20 MN − 3 2 which is needed for cleavage fracture fatigue cracks yet propagate, but by a ductile mechanism.