The development of the crack shape in a thick wall pressure vessel is important in fitness-for-service evaluations such as leak-before-break. In this work the crack tip stress field for surface cracks in a thick plate is investigated using 3-D finite element analyses. The mean stress around the crack from the deepest point to the free surface is investigated for a semi-circular surface breaking crack. A new empirical procedure is developed that allows ductile crack growth to be modelled on the basis of standard deep and shallow cracked fracture toughness test data. In conjunction with this procedure the results show that the crack growth will be suppressed at the deepest segments and the crack will preferentially grow in direction of 45°–70° measured from the deepest point in full plasticity.

Research is ongoing to validate and optimize the E.ON UK auto-reference creep management and control (ARCMAC) system for high temperature piping creep measurement. In conjunction with the ARCMAC system, research is also being undertaken to incorporate digital image correlation (DIC) into creep strain evaluation, via a rugged high temperature pattern. The aim is to have comprehensive monitoring of creep strain in high temperature steam pipes including bends, joints, welds and other pipe structural features. This paper firstly presents work done into the determination of the best method to capture and process ARCMAC images. It was found that a bi-cubic interpolation resizing function with a combined high light intensity image gives the best results with respect to accuracy and repeatability of image acquisition. Also presented was the testing of a modified (prototype) ARCMAC camera with modified (prototype) ARCMAC gauges. The prototype ARCMAC camera, based on a DSLR camera, is approaching the accuracy of the conventional ARCMAC but an improved lens system would be desirable. For the prototype gauge, it was found that the edge-finding technique using the knife-edge was just as accurate as using the target ball reflection for strain analysis. Both techniques can be used independently to evaluate creep strain. Finally, preliminary high temperature testing results for both the ARMCAC and DIC technique are presented. For the ARCMAC gauge and the DIC high temperature pattern, small changes in strains were observed over time that were not due to sample deformation. Reasons for these small changes in strain were identified and solutions arrived at. All results are very promising and it is foreseen that a combined ARCMAC/DIC tested technique will be applied to plant piping components soon.

An explosive welding technique which uses underwater shock wave to weld thin aluminum plate has been studied and the technical advantages were reported. In this research, we propose a method to produce a composite material using an underwater shock wave generated by detonation of explosive. In the production process, a metal plate (flyer plate) accelerates to a high speed by the underwater shock wave, and collided with diamond particles and penetrated the metal plate. Diamonds were used as the particles and aluminum plates (A1050) as the flyer plates. Diamond has high hardness and excellent thermal conductivity, therefore diamond should provide improvement in the thermal conductivity of the composite material. From recovered sample, the multilayer joined surface including diamond particles was observed using an optical microscope. The production of the pipe of composite materials was attempted using this technique as the application. Details of the experimental methods and results are reported in this paper.

During the last couple of decades, several design guidelines and codes relating to major nuclear components have been introduced and revised for risk-based design and evaluation. It is an important process to determine statistical parameters in practice, for instance, by using the traditional method of moments and Maximum Likelihood Method (MLM). Since appropriate estimation of the parameters is not easy due to mathematical complexity, a robust method adopting concepts of both Chi-Square test and genetic algorithm (GA) is proposed in this paper. The Chi-Square test is a useful technique to get the goodness-of-fit of distributions, which is represented in terms of error between observed frequencies and frequencies calculated by assumed probability density function (PDF) of certain statistical distribution. The GA is an efficient optimization algorithm to solve nonlinear optimum problems. Using the Chi-Square test, statistical parameters can be determined and transferred to an optimum problem, and then solved by the GA employing proper nonlinear objective function. Reliability of the proposed method is verified against fracture toughness test data sets of SA508 reactor pressure vessel material obtained from PCVN specimens at various temperatures. The large scatter of experimental data is examined in use of a distribution reported by Neville and Kennedy, Burr type III and XII distributions by Nadarajah and Kortz as well as well-known Weibull distribution. A systematic assessment is carried out by using the new method and its results are compared with corresponding ones derived from the traditional method. Pros and corns of the alternative distributions as well as technical findings from the statistical assessment are fully discussed to show applicability of them.

Dome structures are of architectural significance in many applications ranging from building decorations to fluid confinement usage such as pressure vessels and storage tanks in the petrochemical industry. Most domes are subjected to severe external loads caused by wind flow. Therefore, careful material selection and structural design of domes is imperative to avoid any unexpected failure. This paper presents the design of an experimental set-up to study the flow behavior around ABS dome models of hemispherical and elliptical shapes and their structural integrity under wind loads. The objective of this paper is to determine the dome’s wall thickness for various geometrical shapes. The domes were placed inside a wind tunnel where the wind speed was varied from 60 to 100 km/hr and pressure distribution on the surface of the dome roof was measured. Pressure measurements were carried out for various attack angles with respect to its centerline using a data acquisition system programmed in LabVIEW™. In addition, flow visualization of the air flow around the dome was carried out using a smoke generator. The experimental study was supplemented by a numerical simulation of the air flow around domes to mimic experiments using Computational Fluid Dynamics (CFD) techniques. The effect of wind on the dome structural integrity was studied using finite element analysis. The experimental results were used to validate the CFD models from which pressure distribution around domes were obtained. Results related to the pressure distribution around domes obtained from the CFD analysis were used as loading conditions to study the structural integrity of the domes using ANSYS™. Preliminary experimental results of wind speed effect on a hemispherical/elliptical dome revealed pressure variations for various angles of attack and height inclination along the dome roof surface.

The aim of this paper is a preliminary evaluation of the load bearing capability of a generation IV molten metal cooled reactor (LMFR), with reference, as an example, to the ELSY project in the event of a safe shutdown earthquake (SSE). Two relevant safety aspects were considered and analyzed by means finite element (FEM) codes: the fluid-structure interaction and the high thermal loads due to the residual heat levels of the possible high burnup fuel. The first issue depends on the presence of a liquid free surface that especially in seismic accident conditions, allows for fluid motion (“sloshing”) and may have a significant influence on the reactor structures stress levels. The second aspect deals with the irradiated core heat source thermal effects after the reactor shutdown. In order to evaluate the reactor and internal components structural response capability for both high temperature and hydrodynamic loads, appropriate 3-D models were set up. Moreover preliminary thermal analyses were carried out by means of MCNPX and Origen2.2 codes in order to determine the transient input decay power after shutdown. The obtained preliminary results allowed to individuate the structures mostly affected by the assumed loading conditions in order to make possible an upgrading of the design geometry, should any be necessary, for the considered reactor design. The mentioned activities was a part of a CE funded 6 th FWP.

An analytical model for the mechanical behavior of bolted joints subjected to transverse load has been theoretically formulated. It has been shown that transverse displacement of the cramped component can be separated into five factors: ( a ) bolt bending due to transverse force acting on the thread surface, ( b ) bolt bending due to thread surface reaction moment, ( c ) inclination of bolt head, ( d ) thread surface slip, and ( e ) bearing surface slip. In order to calculate the factors ( d ) and ( e ), contact force and slip displacement induced on the contact surfaces are modeled while factors ( a ) to ( c ) are formulated according to previous theories. In addition, it has been shown that variation in reaction moment induced on the contact surfaces, which affects ( b ) to ( e ), depends on contact state. Then, the relation between the reaction moment and transverse load is formulated. We have applied our analytical model to an M16 bolted joint and confirmed that the model shows good agreement on the load-displacement relation with FEM result.

Due to the prevalent leakage and rupture of a hydrocracking reactor effluent air cooler (REAC) pipes, the corrosion mechanism is studied based on the REAC operation mode and conditions. The mathematic model of imbalance degree is constructed according to the REAC structural property. The multiphase flow characteristics are obtained by a universal petrochemical software HYSYS; and the systemic imbalance of dynamic parameters, such as flux and volume fraction, is simulated using Computational Fluid Dynamics (CFD). Comparing with actual anatomical instances, failure analysis based on the imbalance criterion is achieved. Moreover, the optimum measures to enhance the running reliability are proposed.

Cylindrical shells pressurized from outside are required for several engineering applications, and a growing need of tubes with significant thickness has been recently experienced in the oil industry (very deep water pipelines) and in the frame of integrated primary system nuclear reactors (steam generators). Their collapse behaviour has been explored little if at all, both experimentally and numerically, as witnessed by the extremely conservative attitude that codes assume for very thick tubes. A numerical investigation has been performed in this context at the Politecnico di Milano, which was originally intended as a support for requesting a relaxation of ASME regulations. In fact, in 2007 Code Case N-759 [1] was approved, permitting significant thickness saving in the tube design. Nevertheless, the numerical investigation was pursued to assess the influence of different parameters, such as eccentricity, initial stresses and material hardening, on the collapse of tubes with diameter to thickness ratios D/t<20. Results are thought to be useful under at least two respects: first, they provide some understanding on the collapse behaviour in a thickness range so far unexplored; secondly, they give an indication on the assumptions on which computer codes ought to be based when numerical analyses are required.

Thin-walled steel cylinders surrounded by an elastic medium, when subjected to uniform external pressure may buckle. In the present paper, using a two dimensional model with nonlinear finite elements, which accounts for both geometric and material nonlinearities, the structural response of those cylinders is investigated, towards developing relevant design guidelines. Special emphasis is given on the response of the confined cylinders in terms of initial imperfections; those are considered in the form of initial out-of-roundness of the cylinder and as an initial gap between the cylinder and the medium. Furthermore, the effects of the deformability of the surrounding medium are examined. The results indicate significant imperfection sensitivity and a strong dependency on the medium stiffness. The numerical results are employed to develop a simple and efficient design methodology, which is compatible with the recent general provisions of European design recommendations for shell buckling, and could be used for design purposes.

This study was developed to provide an optimum method for designing metal alloys. Information on general corrosion of the most commonly used alloys, namely nickel-, iron-, aluminum-, stainless steel-, copper- and carbon-based alloys, was obtained from the National Institute of Standards and Technology (NIST). Parameters such as pH, temperature, conductivity, and composition of the electrolyte used in each experiment (alloy in contact with an environment), along with alloy composition (UNS) and corrosion rates, were collected. The parameters in the collected data were the electrolyte characteristics, the alloy composition, and the general corrosion rates. The data consisted of over 4000 samples. The next task was to cluster the data by similarities of the parameters. A web-based, publicly available Kohonen mapping software was used to perform the clustering analysis; the two dimensional Kohonen map was chosen. The map is composed of a number of cells on a plane of two dimensions and each cell stores a sample prototype representing that cell. Kohonen maps have the ability to preserve the topological properties of the data; i.e. samples with similar “high corrosion rates” will cluster together in a given cell of the map, while samples with “low corrosion rates” will cluster in a different cell, far apart from the cell storing the “high corrosion rates” samples. Once the Kohonen map is trained to cluster the samples by their corrosion rates, each one of the parameters of the samples (representing the electrolyte characteristics, the alloy composition and the general corrosion rates) is drawn as a function of the (X,Y) map position of the cell where the samples were stored. In this paper, we present the results obtained with nickel alloy data for which 1369 samples (or independent experiments) were collected.

The concept of risk evaluation exists for centuries. It is present everywhere, in the economy, in the industry and in our everyday life. In the decades past, the role of risk concept is increasing continuously, which is due to a lot of reasonable reasons. The paper shows, why the risk-based applications are extended in more and more branches of industry. This likes to be a “communication interface” between the “economists” and “engineers”. This new improvement in inpection planning involves numerous benefits, but raises some questions, e.g. from the regulatory point of view. What are needed to implement the risk-based strategy and for what can we expected as the result of it? The paper shows a solution of the implementation of the risk-based methodologies, which main principles can be applied in the most fields of industry. The main aim is to focus the maintenance and inspection efforts on the highest risk elements, but how can we establish an adequate order between them, on the basis of risk? Are there any standards or guidelines for risk-based strategies in the different industries? The paper tries to summarize the current state of risk-based approaches, considering the benefits and difficulties of it. The paper concentrates on the nuclear industry, which is one of the most controversial industries, since the whole society is divided on this issue. The concept of risk is unthinkable in this field for a lot of people, including some regulatory bodies. As opposed to it, in some countries there are an acceptable process is the risk-based managing of nuclear power plants. What is the cause of this sharp difference? The purpose of this article is to demonstrate the state of the art in this field and to show the trends in the development of risk-based approaches.

This paper discusses the ultimate tensile strength of girth-welded pipe joints with one or more soft interlayers, as determined by theoretical approaches and FE analysis. In FE analysis, the soft interlayer is assigned to be either the weld metal or heat-affected zone (HAZ). Based on the results of the FE analysis, an evaluation formula is proposed for the ultimate tensile strength of a welded joint including the soft interlayer.

The helium implantation has been successfully used for the obtaining of radiation damage in different Fe-Cr ferritic/martensitic steels. Implanted doses within the range 6.24×10 17 – 3.12×10 18 cm −2 corresponding to local damage up to 90 DPA (Displacement per Atom) were acquired in a thin (<1 μm) region. For observing the dependence of vacancy-type defects on the load and the chromium content, positron annihilation lifetime spectroscopy (PALS) has been used. Experiments showed that chromium had a significant effect on the radiation treated microstructures of the materials. In particular, chromium influences the size and density of the implantation induced defects and specific Cr content should prevent the formation of vacancy clusters.

Pipe and vessel structures made from fiber-reinforced polymer composites are know to commonly outperform metallic structures in terms of corrosion resistance and strength-to-weight ratio. However, composite pressure piping and vessels without internal lining are prone to leakage failure caused by matrix cracking. Microscopic fractures in the often brittle matrix phase grow and coalesce under loading, forming a network of matrix cracks that facilitates fluid to permeate the pipe or vessel wall. Hence, liners are often incorporated into composite pressure containment structures. Leakage failures usually occur considerably below pressures causing rupture of composite pipes and vessels. Hence, more efficient designs may be obtained if liners could be avoided altogether. To achieve this goal a thorough understanding of the damage mechanisms leading to leakage failure is required. Composite pressure piping and vessels are generally manufactured using filament winding or similar techniques. Resulting interwoven fiber architectures are generally considered to influence strain patterns and leakage behavior. Classical experimental methods are usually unable to verify this hypothesis, and therefore modeling techniques have largely been employed. In the present study, the effect of fiber architecture on surface strain patterns and the initiation of leakage were investigated experimentally using digital image correlation technique. Surface strain maps were produced for tubular filament-wound composite specimens subjected to combined internal pressure and axial traction. The findings of this study indicate that no distinct correlation exists between surface strain patterns and leakage initiation points.

In designing bolted flange connections under heat conduction conditions and internal pressure, the characteristics such as the contact gasket stress are important factors from reliable design standpoint. They must be examined analytically and experimentally. In this paper, in order to examine the above characteristics, thermal stress of bolted flange connections subjected to internal pressure and heat conduction conditions is analyzed using the theory of elasticity and FEM calculations. The effects of gasket properties and nominal diameter of flanges on the above characteristics are examined numerically. In the experiments, the amount of helium gas leakage in the connection was measured. Using the obtained gasket stress at elevated temperature, a method for estimating the amount of gas leakage is proposed. The analytical results of the above characteristics are compared with the experimental results, with reasonable agreements. The results reveal that the sealing performance increases due to the gasket properties as the temperature increases in this study.

The effects of load history on component fracture behaviour have been studied at length in terms of the generation and resulting influence of residual stresses. Despite this, the effect of plastic strain history, separate from the generation of residual stresses, is still not clearly defined. This work presents an investigation into the effect of accumulated strain on subsequent fracture behaviour. The effects of load history on low temperature cleavage fracture are modelled by means of a number of local approach methods, accounting for variations in stress and strain throughout the component’s load history. Prior strain was found to reduce the mean fracture load of 20mm thick CT specimens, irrespective of the level of room temperature strain applied. Local approach methods, calibrated to low and high constraint fracture data, were able to correctly predict a reduction in fracture load, although the exact magnitude of the reduced loads were not always correctly resolved. Further experimental data and further work on model formulation is needed to confirm the conclusions drawn here.

In this study, the finite element model of an API Line Pipe threaded pipe connection is presented. The non-linearities in material properties and contact behaviour are discussed. A series of modifications of the standard connection are simulated to gain a better understanding in the influence of geometrical and material parameters on the connection’s performance. Finally, test results obtained from a four-point bending fatigue experiment are presented and compared with numerical simulations.

In the case of a severe accident scenario of a pressurized water reactor which includes cracking of the vessel bottom head, it is crucial to predict the leak rate and hence the crack size for the ex-vessel accident management. We present an experimental framework to analyze the crack propagation under such severe conditions for different 16MND5 French nuclear steel grades. An original experimental setup has been designed in order to perform bi-axial tests (tensile load independent of internal pressure) at high temperatures (1180K – 1280K) on tubular test specimens. The temperature loading and the mechanical loading can be set to reproduce the stress distribution of the hemispherical vessel bottom head submitted to an internal pressure. Moreover, the test was designed to be easily transposable to the real structure in terms of crack propagation and depressurization thanks to an energy based scaling methodology. We observed the crack initiation and propagation with two high speed digital cameras. Force, internal pressure, displacement and temperature fields were also measured and synchronized with the optical measurements. The different creep stages are observed and characterized. The crack propagation and opening history have been measured. During crack initiation and propagation stages, the depressurization can be correlated with the crack geometry. Finally, the setup has been designed in order to validate future numerical analysis.

This paper presents a coupling technique for integrating the element–free Galerkin method (EFGM) with fractal two-level finite element method (F2LFEM) for analyzing homogeneous, isotropic, and two dimensional linear–elastic cracked structures subjected to mixed–mode (modes I and II) loading conditions. F2LFEM is adopted for discretization of domain close to the crack tip and EFGM is adopted in the rest of the domain. In the transition region interface elements are employed. The shape functions within interface elements which comprises both the element–free Galerkin and the finite element shape functions, satisfies the consistency condition thus ensuring convergence of the proposed method. The proposed method combines the best features of EFGM and F2LFEM, in the sense that no structured mesh or special enriched basis functions are necessary and no post–processing (employing any path independent integrals) is needed to determine fracture parameters such as stress–intensity factors (SIFs) and T –stress. The numerical results show that SIFs and T –stress obtained using the proposed method are in excellent agreement with the reference solutions for the structural and crack geometries considered in this study. Also a parametric study is carried out to examine the effects of the similarity ratio, and the number of transformation terms on the quality of the numerical solutions.