Creep crack growth assessment by means of a crack tip/far field concept

A method has been developed to assess crack initiation and crack growth behaviour in the creep range. The proposed method is based on a two-criteria-diagram, which covers the crack tip/far field situation. The approach is similar to the application of the elastic stress concentration factor K, in the creep rupture range.     

Strain distribution around a crack tip in high temperature environment using picture processing

A new optical strain measuring method developed by the authors is described. Here, we employ a simple method which uses point recognition picture processing and the least square approximation method.The surface of the specimen for which we are going to obtain the strain distribution is marked with points using a punch or a paint marker beforehand. At each loading stage, the digital data of the black and white picture of the specimen's marked surface are taken with the television camera under the control of the computer to which the data are sent. From these data, the locations of the marked points at each loading stage are obtained by the picture processing technique. Using these locations decided, we obtain the least square approximation of the displacement distribution, and then the strain distribution.To show the effectiveness of the present technique, the method is applied to the strain measurement near the crack tip in the CT specimen of Type 304 stainless steel. Because of the high ductility of this material, the crack tip blunting is notable so that the crack tip opening displacement observed by this method is as large as 2.5 mm at the specimen surface before initiation.Next, the strain distributions of a center-notched panel made of Type 304 stainless steel under high temperature creep condition are obtained.It is found from the above two experiments that the present strain measuring technique is powerful for the measurement of high strain, say more than 1%, whereas it is unsuitable to apply to the case of small strain at the present developing stage.     

Modes of long crack growth under non-stationary temperature fields

The exploitation practice of structures under thermal loads evidences that the final length of a quasistatic crack can be considerably greater than the thermal tension zone, sometimes causing that the structure approaches complete fracture. This occurs in one or several cycles of a gradual crack growth due to the evolution of thermal field in time resulting in that fracture zone follows the moving tension zone. By the extreme example of quasistationary thermal stress field the set of quasistatic crack growth modes and their peculiarities for the case of moving thermal stresses are described here. These are modes developing both in the direction of the thermal stress field propagation and in the opposite direction. The critical condition of each mode is described, and the crack growth rates are estimated. The rational crack growth evaluation procedure is also proposed. The theoretical conclusions are supported by the experiment, which demonstrates the growth of long thermal cracks.     

Evaluation of the changes occurring in the crack resistance of casing materials of water-cooled power reactors using the results of impact tests

Translated from Atomnaya Énergiya, Vol. 69, No. 5, pp. 289–294, November, 1990.     

Investigation of axial power gradients near a control rod tip

Control rod withdrawal in BWRs induces large power steps in the adjacent fuel assemblies. This paper investigates how well a 2D/3D method, e.g., CASMO5/SIMULATE5 computes axial pin power gradients adjacent to an asymmetrical control-rod tip in a BWR. The ability to predict pin power gradients accurately is important for safety considerations whereas large powers steps induced by control rod withdrawal can cause Pellet Cladding Interaction. The computation of axial pin power gradients axially around a control rod tip is a challenging task for any nodal code. On top of that, asymmetrical control rod handles are present in some BWR designs. The lattice code CASMO requires diagonal symmetry of all control rod parts. This introduces an error in computed pin power gradients that has been evaluated by Monte Carlo calculations.     

Surface small crack growth behavior in low-cycle fatigue at elevated temperature and application limit of macroscopic crack growth law

Surface small crack growth behavior of Type 304 stainless steel in low cycle fatigue under fast-fast and slow-fast cyclings was investigated at a temperature of 873 K, by means of the smooth specimens with various grain sizes. It was shown that once the small cracks had grown up to a few grains size, they predominantly propagated with strain cycling, while most of small cracks stopped propagating when they grew up to one grain size. It was also shown that the small crack growth rate significantly slowed down where the crack length was integral multiple of the grain size. Above behavior resulted from the grain boundaries temporarily impeding the small crack growth. The crack length below which the grain boundaries affected the small crack growth rate was also given as function of the relative length to the grain size. Furthermore, the small crack growth rate was compared with the macroscopic crack growth one. In fast-fast cycling, the small crack growth rate was about ten times as large as as the macroscopic crack growth one where its length was comparable to the grain size. Based on the results thus obtained, the application limit of macroscopic crack growth law to the surface small crack growth was discussed. The macroscopic crack growth law was not applicable to the small crack growth, until the crack length was about ten times average grain size in fast-fast and slow-fast cyclings.     

Prediction of crack coalescence of steam generator tubes in nuclear power plants

Prediction of failure pressures of cracked steam generator tubes of nuclear power plants is an important ingredient in scheduling inspection and repair of tubes. Prediction is usually based on nondestructive evaluation (NDE) of cracks. NDE often reveals two neighboring cracks. If the cracks interact, the tube pressure under which the ligament between the two cracks fails could be much lower than the critical burst pressure of an individual equivalent crack. The ability to accurately predict the ligament failure pressure, called “coalescence pressure,” is important. The failure criterion was established by nonlinear finite element model (FEM) analyses of coalescence of two 100% through-wall collinear cracks. The ligament failure is precipitated by local instability of the ligament under plane strain conditions. As a result of this local instability, the ligament thickness in the radial direction decreases abruptly with pressure. Good correlation of FEM analysis results with experimental data obtained at Argonne National Laboratory’s Energy Technology Division demonstrated that nonlinear FEM analyses are capable of predicting the coalescence pressure accurately for 100% through-wall cracks. This failure criterion and FEA work have been extended to axial cracks of varying ligament width, crack length, and cases where cracks are offset by axial or circumferential ligaments.     

Economic benefits of advanced materials in nuclear power systems

A key obstacle to the commercial deployment of advanced fast reactors is the capital cost. There is a perception of higher capital cost for fast reactor systems than advanced light water reactors. However, cost estimates come with a large uncertainty since far fewer fast reactors have been built than light water reactor facilities. Furthermore, the large variability of industrial cost estimates complicates accurate comparisons. Reductions in capital cost can result from design simplifications, new technologies that allow reduced capital costs, and simulation techniques that help optimize system design. It is plausible that improved materials will provide opportunities for both simplified design and reduced capital cost. Advanced materials may also allow improved safety and longer component lifetimes. This work examines the potential impact of advanced materials on the capital investment cost of fast nuclear reactors.     

A TEM method for analyzing local strain fields in irradiated materials

Of great interest to the field of fracture mechanics is the strain field in front of a crack tip. In irradiated materials, cavities which naturally form as a result of radiation provide convenient internal markers. If a miniaturized irradiated tensile sample is pulled in situ in a transmission electron microscope (TEM), both the relative displacement of these cavities and their distortion in shape provide information on localized strain on a microscopic level. In addition, the TEM method allows direct correlation of active slip systems with crack propagation characteristics. To illustrate this method a strain field map was constructed about a crack propagating in a helium irradiated type 316 stainless steel sample containing large cavities.     

Irradiation damage studies of high power accelerator materials

High-performance production targets and other critical accelerator components intercepting intense, energetic proton beams are essential as the accelerator community envisions the next generation, multi-MW accelerators. Materials that have served the nuclear sector well may not be suitable to play such a role which demands that the material comprising the beam-intercepting element must, in addition to the long exposure which leads to accumulated irradiation damage, also endure short exposure that manifests itself as thermo-mechanical shock. The ability of materials to resist irradiation-induced degradation of its properties that control shock and fatigue is of primary interest. The need for such materials that extend beyond resistance to the neutron-driven irradiation damage of reactor components has led to an extensive search and experimentation with new alloys and composites. These new high-performance materials, which appear to possess the right combination of mechanical and physical properties, are explored through a multi-phased experimental study at Brookhaven National Laboratory (BNL). This study, which brings together the interest in accelerator targets of different facilities around the world, seeks to simulate conditions of both short and long exposure to proton beams to assess the survivability potential of these new alloys and composite materials. While thermo-mechanical shock effects have been studied in the early stages of this comprehensive effort, it is irradiation damage that is currently the focus of the study and results to-date are presented in this paper along with the status and objectives of on-going studies. Of special interest are results depicting damage reversal through post-irradiation annealing in some of the materials. High fluences of 200 and/or 117 MeV protons provided by the BNL Linac beam that serves the Isotope Production Facility were used to assess irradiation damage in these new composites and alloys.     

Research-reactor neutron fields for calibrating in-reactor detectors in nuclear power plants

The neutron spectra of research and power reactors are compared. The spectra were measured by the neutron-activation method and calculated using the KASKAD computer code. The a priori spectrum in the calculation was constructed as a superposition of physically validated spectra. A method of calibrating in-reactor detectors in nuclear power plants on the basis of the sensitivity to the ²³⁵U fission rate in 1 g of uranium using the neutron fields of research reactors is proposed. __________ Translated from Atomnaya énergiya, Vol. 100, No. 2, pp. 97–107, February, 2006.     

Textural changes at the tip of a moving crack during fracture of sheet Zr-1% Nb alloy

Moscow Engineering-Physics Institute. Translated from Atomnaya Énergiya, Vol. 73, No. 8, pp. 121-128, August, 1992.     

Investigation of the yield behaviour of the area close to the crack tip for a fine-grained structural steel

In the case of cracked structures with wall thicknesses such as are preferred in pipework construction, transition curves that are obtained by means of impact bending tests contribute to the improvement of knowledge on the notch-conditioned shift of the transition temperature and the modified conditions for cracks to start. The use of the grid etching technique provides possibilities of ascertaining the plastic deformation that takes place quantitatively at the start and during the course of cracking both in the near and distant fields. In the present case the investigations carried out provided no indications for the possibility of distortion-free, rittle cracks occuring (in the course of the propagation of already primary cracks). Since with the grid etching method the distortion behaviour of small structural areas can be examined by means of the passage of instably or stably propagating cracks, the technique opens up additional possibilities for the assessment of deformation characteristics of the types of structure occuring in welded joints. The cohesive resistance specific to the structure under the influence of material faults can be ascertained quantitatively. With suitable notches, separating processes in fault-free heat affected zones that are hard and frequently low ductile because of unfavourable structural conditions can be forced under static or dynamic strain and the deformation behaviour and indirectly the work capability can be analysed.     

Repair materials and techniques for concrete structures in nuclear power plants

The paper summaries portions of work of the Structural Aging Program, sponsored by the Nuclear Regulatory Commission (NRC). The paper addresses the assessment and repair of concrete structures in nuclear power plants. It presents the results of a survey of the the nuclear power plants in the United States to identify susceptible concrete components, rates of occurrence of deterioration, and to determine the durability of repairs. The paper describes deterioration mechanisms and discusses their effect. Repair techniques are described. Evaluation techniques and nondestructive test techniques are also discussed.     

On the dynamic fracture toughness and crack tip strain behavior of nuclear pressure vessel steel: Application of electromagnetic force

This paper is concerned with the application of the electromagnetic force to the determination of the dynamic fracture toughness of materials. Taken is an edge-cracked specimen which carries a transient electric current I and is simply supported in a steady magnetic field B. As a result of their interaction, the dynamic electromagnetic force occurs in the whole body of the specimen, which is then deformed to fracture in the opening mode of cracking.Using the electric potential and the J - R curve methods to determine the dynamic crack initiation point in the experiment, together with the finite element method to calculate the extended J-integral with the effects of the electromagnetic force and inertia, the dynamic fracture toughness values of nuclear pressure vessel steel A508 class 3 are evaluated over a wide temperature range from lower to upper shelves.The strain distribution near the crack tip in the dynamic process of fracture is also obtained by applying a computer picture processing.