Cold worked Zr-2.5Nb, heat treated Zr-2.5Nb and Zircaloy-2 pressure tubes were hydrided to the hydrogen concentration of 68 ppm, 49 ppm and 242-411 ppm, respectively, and compact tension specimens were machined from the hydrided materials. The crack growth rate by delayed hydride cracking was measured by potential drop method at various temperatures on the above mentioned three types of specimens. The activation energy obtained were 43 KJ/mol for cold worked Zr-2.5Nb and 37 KJ/mol for heat treated Zr-2.5Nb, which were in good agreements with that of Coleman (1977), while they were lower than the activation energy of 65.5 KJ/mol obtained by Simpson-puls (1979) and 71.5 KJ/mol by Ambler (1984). The DHC growth rate in Zircaloy-2 were about one fifth of that of Zr-2.5Nb, which is due to the texture and material strength effects. Striations which indicate stepwise DHC growth were observed at fracture surface by scanning electron microscope and unsymmetric crack tunnellings were also observed, which seems to be due to the difference in hydrogen diffusion rate caused by the difference in stress fields between inner and outer surface. The comparsion of test results with the DHC growth rate calculated by Simpson-puls model showed good agreement at high temperatures, whereas at the lower temperatures the crack growth rates were 2.5 times higher than the calculated values.
CANDU압력관에 사용되어 온 cold worked Zr-2.5Nb, heat treated Zr-2.5Nb 그리고 Zircaloy-2 압력관으로부터 취한 compact tension 시편에 수소를 주입, 각각 68 ppm, 49 ppm, 그리고 242~411ppm의 수소농도를 갖게 한후 전위강하측정법으로 여러 온도에서 Delayed Hydride Cracking (DHC)속도측정실험을 수행하였다. 실험 결과 각각의 시편의 DHC속도-온도 관계식(Arrhenius relationship)을 구하였으며 이로부터 얻어진 균열전파의 활성화에너지는 cold worked Zr-2.5Nb 시편의 경우 43,000 J/mol, heat treated Zr-2.5Nb 시편은 37,000 J/mol 이었다. Cold worked Zr-2.5Nb의 활성화에너지값은 Coleman의 실험결과 42,000 J/mol과는 잘 일치하였으나 Simpson-Puls의 65,000 J/mol, Ambler의 71,500 J/mol 과는 차이를 보였으며 Zircaloy-2의 DHC속도는 cold worked Zr-2.5Nb보다 1/5 이하로 낮게 나타났다. SEM을 이용한 DHC진전파면 관찰결과 DHC에 의한 균열진전을 확인할 수 있는 striation이 관측되었으며 시편 내측표면근처에서의 DHC진전량이 외측표면근처 보다 작은 불균형한 tunnelling현상이 관측되었다. Cold worked Zr-2.5Nb과 heat treated Zr-2.5Nb의 결정조직이 다른데도 DHC특성이 유사한 것은 heat treated Zr-2.5Nb의 집합조직완화에 의한 DHC속도감소요인과 heat treated Zr-2.5Nb의 높은 재료강도에 의한 DHC속도 증가 요인이 상쇄되었기 때문인 것으로 분석되었다. Simpson-Puls model에 의한 계산값과 실험결과를 비교한 결과 높은 실험 온도에서는 잘 일치하였으나 낮은 온도에서는 실험값이 계산값보다 약 2.5배 높게 나타났다.
