Effect of load ratio and hydrogen concentration on the crack growth rate in Zr-2.5Nb tubes

Crack growth rates (CGR's) were determined under sustained and cyclic loads using 17 mm compact tension and cantilever beam specimens taken from Zr-2.5Nb tubes charged to 6-100 ppm H. The cyclic load effect on the CGR was investigated at 250 deg. C where load ratios, R were varied from 0.13 to 1 with a constant Kmax. Under sustained loads, the CGR of the Zr-2.5Nb tube increased with supersaturation of hydrogen, ΔC and leveled off above 20-35 ppm H of the ΔC. Under cyclic loads with 1 cycle/min, the CGR at 250 deg. C decreased with decreasing R: 3.2 x 10-8 m/s at R = 1 and 4.8 x 10-9 m/s at R = 0.13. The striation spacing, corresponding to the critical hydride length, decreased with decreasing R, indicating easier cracking of the hydrides under cyclic loads. The decreased CGR under cyclic loads and its dependence on the ΔC are discussed using Kim's delayed hydride cracking model.