• 대한기계학회논문집A
• /
• 제28권3호
• /
• pp.304-310
• /
• 2004

In this paper, cracking threshold for nanoindentation is analyzed by using 3D finited-element method. The analysis by maximum principal stress criterion can obtain the reliable results for determining to crack initiation location and load. Because the ratio of maximum principal stress to indentation depth for Victors indentation is smaller than flat-plane-column indentation and cracking for Victors indentation occurs from the inner part of specimen difficult to measure crack length, the nanoindentation facture test for flat-plane-column indentation is more effective.

• Nuclear Engineering and Technology
• /
• 제50권7호
• /
• pp.1138-1147
• /
• 2018

Delayed hydride cracking (DHC) is an important failure mechanism for Zircaloy tubes in the demanding environment of nuclear reactors. The threshold stress intensity factor, $K_{IH}$, and critical hydride length, $l_C$, are important parameters to evaluate DHC. Theoretical models of them are developed for Zircaloy tubes undergoing non-homogenous temperature loading, with new stress distributions ahead of the crack tip and thermal stresses involved. A new stress distribution in the plastic zone ahead of the crack tip is proposed according to the fracture mechanics theory of second-order estimate of plastic zone size. The developed models with fewer fitting parameters are validated with the experimental results for $K_{IH}$ and $l_C$. The research results for radial cracking cases indicate that a better agreement for $K_{IH}$ can be achieved; the negative axial thermal stresses can lessen $K_{IH}$ and enlarge the critical hydride length, so its effect should be considered in the safety evaluation and constraint design for fuel rods; the critical hydride length $l_C$ changes slightly in a certain range of stress intensity factors, which interprets the phenomenon that the DHC velocity varies slowly in the steady crack growth stage. Besides, the sensitivity analysis of model parameters demonstrates that an increase in yield strength of zircaloy will result in a decrease in the critical hydride length $l_C$, and $K_{IH}$ will firstly decrease and then have a trend to increase with the yield strength of Zircaloy; higher fracture strength of hydrided zircaloy will lead to very high values of threshold stress intensity factor and critical hydride length at higher temperatures, which might be the main mechanism of crack arrest for some Zircaloy materials.

• Nuclear Engineering and Technology
• /
• 제49권4호
• /
• pp.663-667
• /
• 2017

Delayed hydride cracking (DHC) was first observed in pressure tubes in Canadian CANDU reactors. In light water reactors, DHC was not observed until the late 1990s in high-burnup boiling water reactor (BWR) fuel cladding. In recent years, the focus on DHC has resurfaced in light of the increased interest in the cladding integrity during interim conditions. In principle, all spent fuel in the wet pools has sufficient hydrogen content for DHC to operate below $300^{\circ}C$. It is therefore of importance to establish the critical parameters for DHC to operate. This work studies the threshold stress intensity factor ($K_{IH}$) to initiate DHC as a function of temperature in Zry-4 for temperatures between $227^{\circ}C$ and $315^{\circ}C$. The experimental technique used in this study was the pin-loading testing technique. To determine the $K_{IH}$, an unloading method was used where the load was successively reduced in a stepwise manner until no cracking was observed during 24 hours. The results showed that there was moderate temperature behavior at lower temperatures. Around $300^{\circ}C$, there was a sharp increase in $K_{IH}$ indicating the upper temperature limit for DHC. The value for $K_{IH}$ at $227^{\circ}C$ was determined to be $2.6{\pm}0.3MPa$ ${\surd}$m.

• Journal of Welding and Joining
• /
• 제5권2호
• /
• pp.35-43
• /
• 1987

The effect of post weld heat treatment (P.W.H.T) on the propagation rate of stress corrosion cracking(S.C.C) and threshold stress intensity factor ($K_{IC}.c.c$) for stress corrosion cracking of 304 stainless steel has been investigated in boiling 45% $MgCl_2$ solutions with W.O.L specimens. Specimens were precracked by turning a pair of Cr-Mo steel bolts into a machined slot at the end of the specimen. The fracture surface was examined fractographically by Scanning Electron Microscope(S.E.M.)

• 한국원자력학회:학술대회논문집
• /
• 한국원자력학회 2001년도 춘계학술발표회요약집
• /
• pp.341-341
• /
• 2001

• 터널과지하공간
• /
• 제5권1호
• /
• pp.22-40
• /
• 1995

Thermomechanical characteristics of rocks such as thermal shock, thermal expansion, thermal cracking were experimentally investigaed using Iksan granite, Cheonan tonalite and Chung-ju dolomite to obtain the basic data for proper design and Chung-ju dolomite to obtain the basic data for proper design and stability analysis of underground structures subjected to temperature changes. The effect of thermal shock did not appear when the heating speed was under 3$^{\circ}C$/min. and there existed little difference between multi-staged cyclic heating and single-cycled heating. Thermal expansion of rocks was affected by mineral composition, crack porosity and the degree of thermal craking. In quartz-beraring multimineralic rocks such as Iksan granite and Cheonan tonalite, the thermal expansion coefficient increaseed continuously with temperature rise, but that of Chung-ju dolomite which was a monomineralic rock showed a constant value for the temperature above 250$^{\circ}C$, Chung-ju dolomite yielded the lowest critical threshold temperature(Tc) of 100$^{\circ}C$ and unstable thermal cracking was initiated above the new threshold temperature(Tc')of 300$^{\circ}C$. Above Tc' thermal cracks grew but they were not interconnected. Iksan granite showed closing of microcracks to the temperature of 100$^{\circ}C$, then expanded linearly to Tc of 200$^{\circ}C$. Above Tc, thermal cracking was initiated and progressed rapidly and almost all the grain boundaries were cracked at 600$^{\circ}C$. Cheonan tonalite also showed similar behavior to iksan granite except that Tc was 350$^{\circ}C$ and that thermal cracks propagated more rapidly. Thermal expansions calculated by Turner's equation were found to be valid in predicting the thermal expansion and cracking behavior of rocks.

• 한국방재학회 논문집
• /
• 제10권3호
• /
• pp.39-43
• /
• 2010

본 연구의 주목적은 아스팔트 혼합물의 피로균열에 대한 예측모델을 개발하는 것이다. 아스팔트 혼합물의 피로균열 시험을 위하여 응력제어 간접인장피로 시험이 수행되었다. 피로균열에 대한 예측모델 개발을 위하여 내적손상비 증가 개념이 도입되었다. 내적손상비증가 개념에서는 방출에너지 개념을 주로 사용하였으며 기준인장변형율 및 변형율 추이 요소 등이 추가로 사용되었다. 피로시험에서 나타난 방출에너지의 원인은 아스팔트 콘크리트 시료 내부의 손상증가와 재료 자체가 갖고 있는 고유의 점탄성 특성에 기인하는 것으로 판단된다. 방출에너지는 하중재하 횟수가 증가함에 따라 점차 증가함을 보였다.

• Corrosion Science and Technology
• /
• 제15권5호
• /
• pp.203-208
• /
• 2016

DC potential drop technique was employed during the slow strain rate tests to study the hot salt stress corrosion crack (HSSCC) initiation at 300 and $400^{\circ}C$. Threshold stresses for HSSCC initiation were found to about 88 % of the yield strength at both temperatures, but the time from crack initiation to final failure (${\Delta}t_{scc}$) decreased significantly with temperature, which reflects larger tendency for brittle fracture and secondary cracking. The brittle fracture features consisted of transgranular cracking through the primary ${\alpha}$ grain and discontinuous faceted cracking through the transformed ${\beta}$ grains.

• 한국방사성폐기물학회:학술대회논문집
• /
• 한국방사성폐기물학회 2019년도 춘계학술논문요약집
• /
• pp.74-75
• /
• 2019

• Corrosion Science and Technology
• /
• 제6권4호
• /
• pp.170-176
• /
• 2007

Iodine-induced stress corrosion cracking (ISCC) properties and the associated ISCC process of Zircaloy-4 and an Nb-containing advanced nuclear fuel cladding were evaluated. An internal pressurization test with a pre-cracked specimen was performed with a stress-relieved (SR) or recrystallized (RX) microstructure at $350^{\circ}C$, in an iodine environment. The results showed that the $K_{ISCC}$ of the SR and RX Zircaloy-4 claddings were 3.3 and 4.8MPa\;m^{0.5}, respectively. And the crack propagation rate of the RX Zircaloy-4 was 10 times lower than that of the SR one. The chemical effect of iodine on the crack propagation rate was very high, which was increased $10^4$ times by iodine addition. Main factor affecting on the micro-crack nucleation was a pitting formation and its agglomeration along the grain boundary. However, this pitting formation on the grain-boundary was suppressed in the case of an Nb addition, which resulted in an increase of the ISCC resistance when compared to Zircaloy-4. Crack initiation and propagation mechanisms of fuel claddings were proposed by a grain boundary pitting model and a pitting assisted slip cleavage model and they showed reasonable results.