Anisotropic threshold stress intensity factor, KIH and crack growth rate in delayed hydride cracking of Zr-2.5Nb pressure tubes

Abstract

The objectives of this study are to systematically investigate the delayed hydride cracking (DHC) velocity and the threshold-stress intensity factor, K IH , of a Zr-2.5Nb pressure tube as a function of orientation and elucidate the cause of this anistropic DHC behavior. The DHC velocity as a function of orientation was determined using flattened cantilever beam specimens with 60 ppm H while the threshold-stress intensity factor K IH , was evaluated as a function of orientation on the curved compacttension (CT) and cantilever-beam (CB) specimens charged with hydrogen to 200 ppm H. To infer a difference in a stress gradient ahead of the crack tip as a function of orientation, tensile tests were conducted at temperatures ranging from room temperature (RT) to 560 ЊC using small tensile specimens of 2-mm-gage length taken from three directions of the tube. A textural change was investigated by comparing the inverse pole figures before and after DHC while the {1017} pole figures were constructed to find out the growth pattern of the DHC crack as a function of orientation. Faster DHC velocity and lower K IH were obtained over temperatures of 170 ЊC to 270 ЊC, when the DHC crack grew in the longitudinal direction of the Zr-2.5Nb pressure tube. The strain hardening after yielding and the extent of the textural change accompanied by DHC were higher in the longitudinal direction of the tube, suggesting a higher stress gradient ahead of the crack tip. Thus, the anisotropic DHC behavior of a Zr-2.5Nb pressure tube is discussed based on the stress gradient ahead of the crack tip governed by strain-hardening rate after yielding and a change in texture accompanied by DHC, and the distribution of the {1017} hydride habit planes.

Anisotropic delayed hydride cracking velocity of CANDU Zr–2.5Nb pressure tubes

“…4) even in the same cracking plane with the same stress being applied. Since it was demonstrated that the yield strengths of the CANDU Zr-2.5Nb tube are almost the same in both directions [5], Eq. (2) shows that the anisotropic DHCV depends only upon hydrogen diffusion and K IH with the orientation.…”

“…By demonstrating a larger extent of the textural change and higher strain hardening after yielding in the axial direction of a CANDU Zr-2.5Nb pressure tube when compared to that in the radial direction, Kim suggested that a steeper stress gradient ahead of the crack tip in the axial direction is the cause of an anisotropic DHCV with orientation [5]. However, his hypothesis has some defects because it was suggested based on the previous DHC models.…”

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

“…17 mm compact tension (CT) and cantilever beam (CB) specimens were used to measure the CGR in the axial and radial directions of the Zr-2.5Nb tube [17]. These specimens were pre-charged to 6-100 ppm of hydrogen using an electrolytic method followed by homogenization treatments and then pre-fatigued to introduce a 1.7 mm fatigue crack.…”

The Effect of Testing Direction on DHC Growth Rate Using Zr-2.5Nb Plate

“…A study by Kim et al (2002) attributed the difference between longitudinal and throughthickness crack velocities to anisotropy of the stress gradient ahead of the crack tip.…”

“…Stage 2) Stable crack growth, having little rate change with increase in K I . Stage 3) The onset of unstable cracking when K IC is reached ....................................................................................................................................... 25 Figure 17 -Schematic diagram of the stress field at a flaw .. ........................................................................ 26 Figure 18 -Use of Tresca criterion to predict plastic zone size and stress gradient [Bell 2001] ................. 27 Figure 19 -Schematic diagram of cantilever beam specimens from flattened pressure tube [Kim 2002] .... 29 Figure 20 -DHC velocity of flattened pressure tube cantilever beam specimens with cracking from 170 °C to 270 °C ....................................................................................................................................................... 30 Figure 21 -Strain hardening in the radial and longitudinal directions ( =(UTS-YS)/YS ) as a function of 1/T 31 Figure 22 -Steeper stress gradient ahead of the crack tip in the longitudinal direction of the pressure tube than the radial direction [Kim 2001] ...................................................................................... 31 Figure 23 -The effect of temperature on DHC velocity in the axial (a) and radial (b) directions [Jovanovic 2001] ............................................................................................................................................................. 35 Figure 24 -Ratio of the DHC velocities as a function of temperature for specimens annealed at 1*, 24, and 1000 hours at 400 °C. *For β-phase decomposition, 36 h at 300 °C equates to 1 h at 400 °C [Jovanovic 2001] ............................................................................................................................................................. 35 Figure 25 -Effect of grain size on time to fracture at several stress intensities [Coleman 1979] ................ 38 Figure 26 -Axial and radial crack velocities at 130 °C in quenched and aged 'KANUPP' pressure tubes compared to axial and radial velocity confidence limits in cold-worked pressure tubes [Cheadle 1996] .... 39 Figure 27 -A through-wall crack at two rolled joints in and Pickering 4 (b).…”