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Tubes of 2.5Nb zirconium alloy were fatigued in the tension-tension mode by cyclic internal pressures to cause axial crack growth and unstable fractures at room temperature. Pressure-cycle rates ranged from 400 to 3000 cph. Both the cold-worked and heat-treated conditions, before and after hydriding (200 to 300 ppm H2), were investigated. Exploratory tests were done to determine the effect of the axial length of the surface-stress-intensifying defect on fatigue-crack initiation, growth, and critical length at unstable fracture.
From short (≤tube-wall thickness) length defects, fatigue-crack initiation and growth will occur at nominal peak hoop stresses equal to or less than the estimated endurance limit (∼25,000 psi). For fatigue cracks initiated at the outer diameter surface, the shape of the crack front is semicircular. The number of stress cycles required to initiate fatigue cracking at the surface is inversely proportional to the square of the surface-defect length. Crack-growth rates, for non-hydrided tubing are proportional to (ΔK)5, and to (ΔK)4 after hydriding. There was a tendency for the critical crack length at unstable fracture to decrease with an increase in the number of fatigue cycles required to initiate fatigue cracking.
The fracture toughness of the heat-treated tubing before hydriding is less than for the cold-worked condition either before or after hydriding, and after hydriding the fracture toughness is further reduced.
zirconium, tubing, cold work, heat treatment, hydriding, fatigue, crack growth, unstable fracture
Pankaskie, P. J.
Senior Engineer, Pacific Northwest Laboratory, Battelle Memorial Institute, Richland, Wash.