Crack arrest in pressure vessel steels in a K-gradient and at low KIa levels

A simple theoretical model is used to examine the effect of the gradient of the crack tip stress intensity K on crack arrest in a nuclear reactor pressure vessel which is subject to a hypothetical thermal transient. Attention is focussed on the case where arrest occurs at the lower end of the transition temperature regime, when crack propagation and arrest are not accompanied by the formation of ductile ligaments. The analysis shows that the arrest K values depend on the gradient of K, and this leads to variability in the arrest values. In particular the arrest K value should be lower when the K gradient is positive than when it is negative; this prediction is reconciled with recent experimental results on crack arrest in model vessel tests.     

The effect of ligaments on the reinitiation of fracture at the tip of a crack arrested during a hypothetical thermal shock event in a water-cooled reactor pressure vessel

During a hypothetical thermal shock event involving a water-cooled nuclear reactor pressure vessel, a crack can propagate deep into the reactor vessel thickness by a series of run-arrest-reinitiation events. Within the transition temperature regime, crack propagation and arrest in pressure vessel steels is associated with a combination of cleavage and dimpled rupture processes, the dimpled rupture regions being contained within ligaments that are normal to the crack plane and parallel to the direction of crack propagation. The present paper models the effect of ligaments on the reinitiation of fracture at the tip of an arrested crack, and the results of a theoretical analysis define the conditions under which ligaments might increase the reinitiation value above $\text{k}{}_{\mathrm{IC}}$, assuming that they fracture by a ductile rupture process. By comparing the predictions with experimental results for model vessels subject to thermal shock, it is shown that the ligaments, which are present at arrest, are unlikely to fail entirely by ductile rupture prior to the reinitiation of fracture at an arrested crack tip. Instead it is suggested that the ligaments fail by cleavage, whereupon they do not markedly affect the reinitiation $\text{K}$ value, which thus correlates with $\text{K}{}_{\mathrm{IC}}$.     

The influence of hydride blisters on the fracture of Zircaloy-4

The fracture behavior under near plane-strain deformation conditions of Zircaloy-4 sheet containing solid hydride blisters of various depths has been examined at 25 and 300 °C. The study was based on material with either model ‘blisters’ having diameters of 2 and 3 mm or a continuous layer of hydride; in all cases, the substrate material contained discrete hydride precipitates. The fracture strains decrease rapidly with increasing hydride blister/layer depth to levels of about 100 μm deep, and then remain roughly constant. For a given blister depth, the material is significantly more ductile at 300 °C than at room temperature although measurable ductility is retained even at 25 °C and for large blister depths. The material is somewhat more ductile if the hydride is in the form of a blister than in the form of a continuous layer (rim). The hydride blisters/layers are brittle at all temperatures, and crack shortly after yielding of the ductile substrate. Consequently, both experimental evidence and analytical modeling indicate that fracture of the sheet is controlled by the crack growth resistance of the substrate at 25 °C. At elevated temperatures, the hydride particles within the substrate are quite ductile, inhibit crack growth, and failure eventually occurs due to a shear instability.     

In situ crack growth observation and fracture toughness measurement of hydrogen charged Zircaloy-4

The effect of hydrogen on the fracture behaviour of a Zircaloy-4 alloy was analysed performing simultaneous fracture mechanics tests of small SE(B) specimens and in situ observation of crack initiation and propagation inside the chamber of a scanning electron microscope. Load and displacement were continuously measured and J_IC, J-R curves and CTOD determinations were obtained. Detailed images of the zone close to the crack tip were taken and the resistance to crack growth was correlated with hydrogen content and hydride morphology. The size and orientation of hydride precipitates showed an important influence on the fracture process. A good agreement with results obtained using standard CT specimens was met.     

The viablility of the KIa procedure for predicting the arrest of a crack in a nuclear reactor steel pressure vessel

The current ASME Code procedure for predicting crack arrest in a nuclear reactor steel pressure vessel is based on a static linear elastic fracture mechanics analysis: a crack is presumed to arrest when the crack tip stress intensity factor K_I^ST falls below K_Ia, which is assumed to be a material property and is referred to as the arrest toughness. The viability of this procedure has been questioned since the theoretical justification, in the strictest sense, for this very simple K_Ia approach is based on the behaviour of a semi-infinite crack propagating in an unbounded solid due to the application of time-independent loads. Against this background, the present paper examines the effects of initial crack size and crack jump length on the viability of the K_Ia procedure. A theoretical analysis shows that the procedure should give accurate predictions of the crack length at arrest certainly if the crack jump length is less than twice the initial crack size.     

Microstructure-mechanical property relations in HT-9

In this study we have investigated the possible relationships between microstructural features and cleavage fracture behavior of HT-9 specimens heat treated to twenty-five conditions. Fractographic analyses, including direct observation of fracture surfaces as well as subsurface examination, suggest that cleavage crack initiation is associated with the fracture of large carbides, primarily at prior austenite grain (PAG) boundaries. These microcracks are inhibited from propagating across lath packet or PAG boundaries, where ductile tearing occurs. Carbide size and the extent of ductile tearing can be used to rationalize the observed trends in cleavage fracture parameters.     

Micromechanisms and toughness for cleavage fracture of steel

A complete understanding of the fracture mechanisms of steel in the ductile/brittle transition region requires analysis not only of crack initiation, but also of crack propagation. This paper reviews micrographic and fractographic experiments that give insight into both phenomena, and suggests a frame-work through which both may be related.Unstable cleavage crack initiation can occur after some blunting of the original fatigue precrack or after some stable crack growth. In either event, instability appears to be triggered by the fracture of a brittle micro-constituent ahead of the precrack. The large scatter in reported K_Ic values within the transition region reflects the size distribution and relative scarcity of these “trigger” particles.While a large number of models have attempted to correlate toughness in the ductile/brittle transition regime to events occurring ahead of the crack tip, surprisingly little attention has been paid to events occurring behind the crack front. Fractographic evidence as well as metallographic sectioning of arrested cracks show that the mechanism of rapid crack propagation by cleavage is affected strongly by partial crack-plane deflection which leaves unbroken ligaments in its wake. The tearing of these ligaments by dimple-rupture is the dominant energy-absorbing mechanism. Etch-pit experiments using an Fe-Si alloy show that the crack-tip stress intensity based on plastic zone size is extremely low. It is suggested that the mechanism of crack arrest should be modeled using a sharp crack which is restrained by a distribution of discrete pinching forces along its faces. The same model is applied to crack initiation.     

A combined ligament rupture-cleavage model for crack arrest in thick-section steel plate

In the transition regime, plane strain crack propagation in ferritic steels proceeds by a combination of cleavage and ductile rupture processes, the latter being confined to ligaments that are parallel to the direction of macroscopic crack propagation. The paper models crack propagation, and particularly the limiting case of crack arrest, when fracture proceeds via these two modes. An important theoretical result is that, because of the unfractured ligaments which remain behind the crack tip, the plastic zone size is much smaller than when it is predicted for the operative $\text{K}$ values and assuming that there are no ligaments. Linear elastic fracture mechanics procedures may therefore be used to describe the arrest phenomenon at $\text{K}$ values that exceed the normally accepted limits for their validity. The theoretical results are also used to speculate upon the effect of neutron irradiation on the arrest toughness.     

Fatigue testing of metallurgically-bonded EBR-II superheater tubes

Fatigue crack growth tests were performed on 2¼Cr–1Mo steel specimens machined from ex-service experimental breeder reactor-II (EBR-II) superheater duplex tubes. The tubes had been metallurgically-bonded with a 100 μm thick Ni layer; the specimens incorporated this bond layer. Fatigue crack growth tests were performed at room temperature in air and at 400 °C in air and humid Ar; cracks were grown at varied levels of constant ΔK. In all conditions the presence of the Ni bond layer was found to result in a net retardation of growth as the crack passed through the layer. The mechanism of retardation was identified as a disruption of crack planarity and uniformity after passing through the porous bond layer. Full crack arrest was only observed in a single test performed at near-threshold ΔK level (12 MPa√m) at 400 °C. In this case the crack tip was blunted by oxidation of the base steel at the steel–nickel interface.     

Secondary cracking in hydrided Zr-2.5 Wt% Nb alloys

The occurrence of secondary cracking in cold-worked, hydrided Zr-2.5 wt% Nb pressure tube material has been studied. The results indicate that the secondary cracking can occur during three different modes of propagation of the primary crack, i.e. dynamic fatigue, hydrogen-induced delayed cracking, and ductile fracture. Metallographic studies revealed that the secondary and primary crack planes are approximately perpendicular, and, moreover, the secondary crack path lies either through the brittle zirconium hydride precipitates or the hydride-matrix interface. Evidence is presented to show that the secondary cracks nucleate and propagate within the plastic zone of the primary crack as a result of the transverse (through thickness) component of the triaxial stress at the crack tip. During hydrogen-induced delayed cracking, the secondary cracks either propagated with the primary stable crack, causing pronounced cusping of the crack front, or more typically growth of the secondary cracks terminated at an early stage, and the main crack broke away from the secondary cracks. This difference in behaviour is thought to be related to the degree of hydride continuity and alignment in the matrix.     

The development of the crack growth model in zirconium claddings in iodine environment

The theoretical model of iodine induced stress corrosion cracking of zirconium claddings which takes into account the cladding texture has been developed. The process of quasistationary crack growth is considered. In the model the crack is described with the set of brittle and ductile regions which are alternating along the crack front. The cladding material has polycrystalline structure and mechanical behavior of the grains depends on their orientation relative to the applied stresses. Chemical potential of dissolved iodine has minimum in the places with maximum tensile stresses which occur in the brittle region of the crack tip. Iodine weakens interatomic bonds in the crack tip and leads to growth of the crack. The stresses in the brittle region relax and the redistribution of the stresses causes the crack growth in the ductile region. In the model the stresses in both regions are calculated self-consistently. Critical iodine concentration in the gaseous phase near the tip determines the condition of crack growth This criterion can be presented in the form that the stress intensity factor exceeds some threshold value. The value of iodine flux to the crack tip has been calculated. The comparison of calculated rate of crack growth with the experimental results has demonstrated a satisfactory agreement. The model predicts the decrease of the threshold SIF for the increase of the tangential component of texture factor that is in a good agreement with the experimental observations.     

Implications of recent developments in the plastic fracture mechanics field to the pci stress corrosion problem

Fractographic observations on irradiated Zircaloy cladding stress corrosion fracture surfaces are considered against the background of recent developments in the plastic fracture mechanics field. Dimples have been observed on the fracture surfaces of failed cladding, even though the cracks in metallogiaphic sections are tight, i.e., crack propagation is associated with a low crack tip opening angle. This result is interpreted as providing evidence for an environmentally assisted ductile mode of fracture. The presence of this fracture mode forms the basis of an argument, which adds further support for the view that power ramp stress corrosion cladding failures are caused by stress concentrations that produce stress gradients in the cladding.     

The first step for delayed hydride cracking in zirconium alloys

Two models for delayed hydride cracking (DHC) in zirconium alloys are distinguished by their first step:

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The loading of a crack induces hydride precipitation. The hydride is postulated to create a hydrogen concentration gradient, where the bulk concentration is greater than that at the crack tip. This concentration gradient is taken as the driving force for diffusion of hydrogen to the crack tip, and subsequent hydride growth. This model is called the precipitate first model (PFM).

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The tensile stress at the crack tip induces a gradient in chemical potential that promotes the diffusion of hydrogen to the crack tip. Hydrides form if the hydrogen concentration reaches the solubility limit for hydride precipitation. The mechanism is postulated to create a hydrogen concentration gradient, where the bulk concentration is lower than that at the crack tip. The gradient in chemical potential is taken as the driving force for diffusion of hydrogen to the crack tip, and subsequent hydride growth. This model is called the diffusion first model (DFM).

The second model, DFM, is developed. This model is shown to describe the main features of the experimental observations of DHC, without invoking new phenomena, such as reduction in the solubility limit for precipitation of hydride, as required by the PFM.     

Effect of prior crack growth on cleavage fracture of low toughness precracked Charpy specimens

Cleavage fracture of reactor pressure vessel steels in the upper ductile to brittle transition region generally occurs with prior significant ductile crack growth. For low upper shelf materials and using PreCracked Charpy v-notch (PCCv) specimens that can be obtained from conventional surveillance programs, the effect of prior crack growth could be particularly important. In practice, the shape of the Master Curve and the failure distribution could be affected by ductile crack growth. To quantify the effect in practical applications, the effect of prior ductile on cleavage is evaluated on PCCv specimen.The methodology use finite element calculations to grow a ductile crack and infer the brittle failure probability using the local approach to fracture. It is found that for very low upper shelf toughness materials, ductile crack growth enhances the failure probability, induces a steeper failure distribution and affects the shape of the Master Curve. However, for low toughness materials, the enhanced failure probability due to crack growth is compensated by loss of constraint.     

Metallographic studies of fatigue in 20% cold-worked zircaloy-2 containing zirconium hydride

The fatigue behaviour of unhydrided and hydrided 20% cold-worked zircaloy-2 reactor pressure-vessel tubing has been studied for fluctuating tension at room-temperature and 300 °C and for reversed torsion at room-temperature. At room-temperature, high concentrations of zirconium hydride markedly reduce the critical crack length. This effect is attributed to a lowering of the fracture toughness, since hydrides fracture in the path of crack propagation and promote matrix cleavage. At 300 °C, hydride particles constrained by the matrix are plastically deformed without fracturing; the toughness is probably unaffected by the presence of hydride.     

Crack growth pattern and threshold stress intensity factor, KIH, of Zr–2.5Nb alloy with the notch direction

The objective of this study is to obtain a better understanding of the threshold stress intensity factor for an initiation of delayed hydride cracking (DHC) in a Zr–2.5Nb pressure tube. By changing the crack propagation from the longitudinal direction to the circumferential direction, the threshold stress intensity factor, K_IH, and the crack growth pattern were investigated in the Zr–2.5Nb pressure tube with a strong circumferential texture. The threshold stress intensity factor, K_IH, was discussed phenomenologically based on the crack growth pattern and analytically as a function of the tilting angle of hydride habit planes to the cracking plane. A supplementary experiment was conducted to demonstrate a linear decrease of K_IH with an increase in the basal pole component in the cracking plane. Thus, it is concluded that the DHC is controlled by the nucleation and growth of the hydride precipitates on the habit plane.     

Application of micromechanical models for predicting fracture toughness of sulphide controlled Fe 510 steels

As cleavage fracture follows a tensile stress criterion, it is possible to predict fracture toughness K_Ic as a function of temperature when small scale yielding stress distribution in combination with the Ritchie, Knott and Rice criterion is used. With increasing temperature and thus pronounced plasticity the crack tip blunts and the experimental values are underestimated. The Schmidtmann and Nierhoff proposal for modifying the cleavage stress criterion and introducing the effect of crack tip blunting is compared with the RKR-model and with experimentally determined K_Ic-values for four different qualities of Fe 510 steels. The steels mainly differ in sulphur content and sulphur shape control. Thus the effect of sulphur on cleavage fracture will be discussed.     

CANDU及RBMK压力管锆合金的氢致延迟断裂研究

采用紧凑拉伸试样(CT），在恒定载荷、不同氢含量、不同温度条件下，测量了CANDU堆和RBMK堆Zr-2．5Nb压力管材料氢致延迟开裂速率。用金相显微镜和扫描电镜观察断口及氢化物形貌，并测量临界应力场强度因子及开裂速率，对材料的微结构及氢化物分布进行分析。结果表明，氢致延迟断裂(DHC)生长呈阶梯状。与CANDU压力管比较，RBMK压力管的DHC开裂速率将近低一个数量级。其原因是RBMK压力管的屈服强度比CANDU压力管低得多。     

含缺陷韧性金属材料的失效评估研究

以韧性较好的核电站用结构钢S355为研究对象,进行了紧凑拉伸试样的断裂韧性试验,试验中发现随着拉力的增大,裂尖塑性垮塌代替了裂纹扩展,且失效载荷对应有限的塑性屈服范围。随后采用Gurson模型模拟了该试样受拉伸载荷时的失效过程,计算结果表明,该模型能较好地模拟裂纹尖端的塑性垮塌现象。最后结合试验结果和有限元模拟结果,建立了1条适用于S355钢的通用失效评估曲线,并确定了塑性失效载荷与裂纹长度之间的对应关系。     

Experimental and numerical investigations of stable crack growth of axial surface flaws in a pressure vessel

Stable crack growth of a surface flaw in a pressure vessel has been investigated experimentally and numerically. The results show that a purely J-based concept of ductile fracture is not able to predict the local crack extension of the surface flaw correctly. To explain the canoe shape of the grown crack, the local crack tip constraint has to be taken into account.