The manner in which the spread of inelastic deformation of a softening cohesive zone affects the load capacity is examined. The analysis makes use of an elastic-plastic analogy to the strip yield model applied to pure bending. The example of pure bending is one case where inelastic deformation contributes to enhancing the load capacity. The analytical solution to the elastic-plastic case is developed for zero hardening (baseline for strip yield case for which analytical solution is known) as well as for a range of linear softening rates. Evaluation of the results shows that the maximu m bending load capacity is always reached before the stress at the surface becomes zero.

The core of a CANDU (CANada Deuterium Uranium) pressurized heavy water reactor includes several hundred horizontal fuel channels that pass through a calandria vessel containing the heavy water moderator. In each fuel channel, annulus spacers are used to maintain the gap between the cold calandria tube and the hot pressure tube, a pressurized vessel containing the nuclear fuel in contact with heavy water coolant. In order to carry the loads between the pressure tube and calandria tube, the annulus spacers are required to possess adequate structural strength throughout the operating life of the reactor. The Inconel X-750 spacers used in some reactor units are susceptible to irradiation induced degradation. As irradiation fluence increases with operating time, material embrittlement has been observed due to helium bubble formation in the X-750 spacer material. An engineering approach for assessing the structural strength of CANDU annulus spacers has been recently developed. When the ductility of the material is relatively low, the region susceptible to fracture under applied tensile stress may be adequately idealized as a strip-yield process zone surrounded by elastic material and associated with restraining stress. The engineering approach is based on applying the strip-yield process zone methodology to fracture at a nominally smooth surface. Finite element modeling was undertaken to simulate the strip-yield based fracture process zone. The finite element analyses and results are presented in this paper. The finite element results verify the engineering equations developed to assess the structural strength of annulus spacers.

The carbide precipitation of 2.25Cr-1Mo-0.25V steel is studied during the head-fabrication heat treatment process using gold replica technique, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and selected area electron diffraction (SAED). Shapes, structures and sizes of carbides before and after heat treatment are analyzed. The dissolution of strip-shaped carbides and the precipitation of granular carbides are confirmed. Amorphous films at the boundaries of carbides are observed by high-resolution transmission electron microscope (HRTEM), which is formed due to the electron irradiation under TEM.

The aim of the present work is to carry out the checking of the tube bundle of heat exchanger for the occurrence of tube-tube collision caused by cross-flow vibration with and without the use of impingement plate. This will be achieved using numerical 2D CFD (computational fluid dynamics) analysis. The 2D analysis is done using ANSYS Fluent software. Tube movement in the shell side is provided by UDF (user-defined function) DEFINE_SDOF_PROPERTIES. By determining the stiffness and weight of the tubes, two-way fluid and tube interaction can be achieved. Due to limitations of 2D CFD analysis, only the occurrence of the tube-tube or tube-shell collisions can be observed. Unfortunately, the first collision causes termination of the simulation due to negative volumes in dynamic mesh. Possible solutions to the issue are also discussed in presented paper. The analyzed geometry of the shell side is taken from the Heat Exchanger Tube Vibration Data Bank [2]. This publication collects heat exchanger data for which vibration phenomena have been reported. The above-mentioned geometry is a domain with tube bundle at the shell side under the inlet. In the same domain, both the tie rod and the seal strips and the 45° turn of the partitions are considered.

An evaluation of the fracture toughness of the heat-affected zone (HAZ), which is located under the weld overlay cladding of a reactor pressure vessel (RPV), was performed. Considering inhomogeneous microstructures of the HAZ, 0.4T-C(T) specimens were manufactured from the cladding strips locations, and Mini-C(T) specimens were fabricated from the distanced location as well as under the cladding. The reference temperature ( T o ) of specimens that were aligned with the middle section of a cladding strip (HAZ MCS ) was ∼12°C higher than that of specimens that were aligned with cladding strips at the overlap (HAZ OCS ). T o values of partial area in the HAZ were obtained using Mini-C(T) specimen. The T o values obtained near the side of the cladding were ∼13°C higher than those away from the cladding. T o values of HAZ for both 0.4T-C(T) and Mini-C(T) specimens were significantly lower than that of the base metal at a quarter thickness by 40°C–60°C. Compared to the literature data that indicated fracture toughness at the surface without overlay cladding and base metal of a quarter thickness in a pressure vessel plate, this study concluded that the welding thermal history showed no significant effect on the fracture toughness of the inner surface of RPV steel.

The core of a CANDU (1) (CANada Deuterium Uranium) pressurized heavy water reactor consists of a lattice of either 390 or 480 horizontal Zr-Nb pressure tubes, depending on the reactor design. These pressure tubes contain the fuel bundles. Each pressure tube is surrounded by a Zircaloy calandria tube that operates at a significantly lower temperature. Fuel channel annulus spacers maintain the annular gap between the pressure tube and calandria tube throughout the operating life. To meet this design requirement, annulus spacers must have adequate structural strength to carry the interaction loads imposed between the pressure tube and calandria tube. Crush tests that have been performed on specimens from as-received and ex-service Inconel X-750 alloy spacers have demonstrated that the structural strength of Inconel X-750 spacers has degraded with operating time due to irradiation damage. There was a need for an engineering model to predict the future maximum load carrying capacity of the spacer coils for use in Fitness-for-Service evaluations of spacer structural integrity. An engineering process-zone model has been developed and used to analyze the spacer crush test results, and provide predictions of the Inconel X-750 spacer coil future maximum load carrying capacities. The engineering process-zone model is described in this paper. The process-zone model is based on the strip-yield approach of a process zone with a uniform restraining stress representing the fracture region that is surrounded by elastic material.

Hydrophobic surfaces, enabling flow slip past a solid boundary, can be effective for suppressing flow unsteadiness, as well as for heat transfer enhancement; both are important for heat exchanger applications. In the present work, a computational investigation of forced convection heat transfer in cross-flow past a hydrophobic circular cylinder is performed at a Reynolds number value of 300, for which flow past a non-hydrophobic cylinder is three-dimensional. Here, the cylinder surface is maintained at a constant temperature, whereas a Prandtl number of unity is considered. Surface hydrophobicity is modelled based on the Navier model. In a first step, slip conditions are implemented on the entire cylinder surface (full slip), for a nondimensional slip length b* = b/D = 0.20, b being the slip length and D the cylinder diameter. This results in a suppression of flow unsteadiness, as well as in a simultaneous heat transfer enhancement; the latter is quantified by the increase of the mean Nusselt number. Next, in order to reduce the extent of the hydrophobic region, and thus the associated cost, a partial slip setup is considered. This setup consists of alternating hydrophobic and non-hydrophobic strips along the spanwise direction, the width of which is selected considering the spanwise wavelength, λ z , of three-dimensional flow. Further, following recent studies of the authors on two-dimensional flow, a non-hydrophobic region is considered around the average rear stagnation point (in the circumferential direction), for all hydrophobic strips. It is shown that the present setup can result in values of mean Nusselt number comparable to those attained with full slip. Overall, the present results illustrate that a proper implementation of partial hydrophobicity on the cylinder surface, along the circumferential and the spanwise direction, results in a suppression of wake unsteadiness and fluctuating forces, as well as in a simultaneous enhancement of heat transfer rates.

The strip yield model is widely used to describe crack tip plasticity in front of a crack. In the strip yield model the stress in the plastic zone is considered as known, and stress and deformation fields can be obtained from elastic solutions using the condition that the crack tip stress singularity vanishes. The strip yield model is generally regarded to be valid to describe small scale plasticity at a crack tip. The present paper examines the behavior of the strip yield model at the transition to large-scale plasticity and its relationship to net section plasticity descriptions. A bar in bending with a single edge crack is used as an illustrative example to derive solutions and compare with one-sided and two-sided plasticity solutions.

The core of a CANDU ®(1) pressurized heavy water reactor consists of a lattice of either 390 or 480 horizontal Zr-Nb pressure tubes, depending on the reactor design, which contain the nuclear fuel. Each pressure tube is surrounded by a Zircaloy calandria tube that operates at a significantly lower temperature. Fuel channel annulus spacers maintain the annular gap between the pressure tube and the calandria tube throughout the reactor operating life. To meet this design requirement, the annulus spacers must have adequate structural strength to carry the interaction loads between the pressure tube and the calandria tube. Crush tests performed on specimens from Inconel X-750 spacers, both non-irradiated and ex-service, have demonstrated that their structural strength had degraded with operating time due to irradiation damage. An engineering process-zone model was developed and used to analyze the spacer crush test results, and to predict the maximum load carrying capacities of the Inconel X-750 spacer coils, as described in the companion paper “Engineering Process-Zone Model for Evaluation of Structural Strength of Fuel Channel Annulus Spacers in CANDU Nuclear Reactors” presented at the PVP2017 Conference. The developed model is based on the strip-yield approach of a process zone with a uniform restraining stress that represents the fracture region surrounded by elastic material. This baseline process-zone model has been improved by allowing the restraining stress to evolve with the variation in the opening displacement in accordance with a traction-separation constitutive relation. The development of this improved engineering process-zone model incorporating a non-trivial traction-separation constitutive relation is described in this paper.

The J -integral is widely used as a fracture parameter for elastic-plastic materials. The J -integral describes the intensity of the stress field close to the crack tip in a power-law hardening material under a set of well-known restrictions. This study investigates what happens when one of these restrictions is broken, namely the requirement for no unloading to occur. In this work, a centre-cracked plate is subjected to a single cycle of load in which unloading occurs. A remote tensile stress is applied, then released, then applied again up to and beyond its initial magnitude. The J -integral at each step of the analysis is calculated using finite element analysis. Its validity as a fracture parameter at each step is discussed with the aid of results from a strip yield analysis of the same problem. The relevance of the results in the context of structural integrity assessment is discussed.

High responsibility components operating under cyclic loading can have their resistance against initiation and growth of fatigue cracks highly influenced by previous thermomechanical processing. Within the interest of the present work, different manufacturing processes and installation techniques incorporate cold plastic straining to engineering structures; two typical examples on the oil and gas fields are: i) the offshore pipelines installation method called reeling; ii) the fabrication of pipes using the UOE method and pressure vessels through calendering. Within this scenario, this work investigates the effects of plastic prestrain on the fatigue crack growth rates ( da/dN vs. ΔK ) of a hot-rolled ASTM A36 steel. Different from previous results from the literature, in which prestrains were applied directly to machined samples, in this work uniform prestraining was imposed to steel strips (1/2” thick) and specimens were then extracted to avoid (or minimize) residual stress effects. Prestrain levels were around 4, 8 and 14% and C(T) specimens were machined from original and prestrained materials according to ASTM E647 standard. Fatigue crack growth tests were carried out under load control in an MTS 810 (250 kN) equipment using R = 0.1. Results revealed that plastic prestraining considerably reduced crack growth rates for the studied material, which was expected based on the literature and hardening behavior of the studied material. However, results also revealed two interesting trends: i) the larger is the imposed prestrain, the greater is the growth rate reduction in a nonlinear asymptotic relationship; ii) the larger is imposed ΔK , the more pronounced is the effect of prestraining. Crack closure effects were also investigated, but revealed no influence on the obtained mechanical properties. Consequently, results could be critically discussed based on effective crack driving forces and elastic-plastic mechanical properties, in special those related to flow and hardening. The conclusions and success of the employed methods encourage further efforts to incorporate plastic prestrain effects on structural integrity assessments.

In this paper, analytical stress intensity factor solutions for welds in lap-shear specimens of different materials and thicknesses under plane strain conditions are presented in the normalized forms. The stress intensity factor solutions for welds are expressed in terms of the structural stresses based on a strip model. The analytical stress intensity factor solutions are selectively verified by the results of the two-dimensional finite element analyses. The interface crack parameters for the stress intensity factor solutions for welds in lap-shear specimens of dissimilar steel, aluminum, magnesium, and copper sheets are listed for different thickness ratios. The analytical stress intensity factor solutions are obtained and selectively presented in the normalized forms as functions of the specimen thickness ratio for the combination of aluminum and steel sheets for fracture and fatigue analyses.

An evaluation of a residual stress of hydraulically expanded tube-to-tubesheet joints is important in strength evaluation of certain devices such as heat exchangers, steam generators, etc. In this study, we carried out the analytical and experimental investigation in order to develop a model for the prediction of residual stress. The experiment of the tube expansion was carried out with nickel-base alloy tubes and a low-alloy steel tubesheet. The residual stress of each tube was evaluated by sectioning method where the released strain was measured after each tube was cut into thin strips. As a result, the tensile residual stress was detected at the transition zone between the expanded zone and the non-expanded zone on each tube. A finite element analysis was also performed with a three dimensional model for the tube-to-tubesheet joint. The accuracy of the analysis, however, needed to be improved. Therefore FEM models were developed where the following factors were considered: (i) work-hardened layer on the tube before expansion, (ii) friction between the inner surface of the tube and the mandrel used for hydraulic expansion. The accuracy improvement of the analysis was confirmed through the comparison of the numerical and experimental results.

The flow of fluid over the inner surface of rough bore flexible pipes may create vortex induced pulsations. For gas risers these pulsations can in some cases be the source of a high amplitude tonal sound. This phenomenon is in the industry referred to as “singing risers”. Under certain circumstances these pulsations can result in large structural vibrations with potential fatigue failure of the connected topside or subsea pipe systems. The singing behavior is dependent on the operating conditions such as product density and viscosity as well as the piping layout of the topside and subsea piping. However, the most important factor is the geometry of the corrugations. In traditional designs the carcass is made from a folded metallic strip. NKT Flexibles has developed a novel approach to carcass manufacturing originally intended for deep and ultra deepwater applications, wherein the carcass is made of helically wound wires rather than a folded strip. Although the main focus for this development was to devise a carcass of superior collapse strength compared to conventional systems, the new carcass design provides a very smooth inner surface, thus mitigating vortex formation which will reduce or eliminate the formation of flow induced pulsations. The susceptibility to singing of the new carcass design is compared to that of a traditional carcass design. This includes the onset of the singing and the flow pressure drop. Due to the much smaller cavities in the new design, the singing tendency is shown to be significantly reduced. In particular, the expected onset velocity is 5–9 times that of a classic rough bore design with 20%–50% of the amplitude. An additional benefit of the new design is that the pressure drop for the pipe is close to that of a smooth bore pipe. Therefore, it is expected that for many applications, the new design will not lead to the generation of high amplitude tonal noise within the desired operating envelope.

Savannah River Remediation, LLC (SRR) is implementing a statistical sampling strategy for in-service inspection (ISI) of liquid waste (LW) tanks at the United States Department of Energy’s Savannah River Site (SRS) in Aiken, South Carolina. As a component of SRS’s corrosion control program (CCP), the ISI program assesses tank wall structural integrity through the use of ultrasonic testing (UT). The statistical strategy for ISI is based on the random sampling of a number of vertically oriented unit areas, called “strips,” within each tank. The number of strips to inspect was determined so as to attain, over time, a high probability of observing at least one of the worst 5% in terms of pitting and corrosion across all tanks. The probability estimation to determine the number of strips to inspect was performed using the hypergeometric distribution. Statistical tolerance limits for pit depth and corrosion rates were calculated by fitting the lognormal distribution to the data. In addition to the strip sampling strategy, a single strip within each tank was identified to serve as the baseline for a longitudinal assessment of the tank safe operational life. The statistical sampling strategy enables the ISI program to develop individual profiles of LW tank wall structural integrity that collectively provide a high confidence in their safety and integrity over operational lifetimes.

As the design life of new nuclear power plant increases, the austenitic stainless cladding integrity of reactor vessel becomes one of the new concerns. Since 1970’s, there have been some specific recommendations on delta ferrite content of austenitic cladding of reactor vessels and welds. It has been known that the delta ferrite is beneficial for reducing micro-fissure in welds, though the high delta ferrite content increases the probability of embrittlment of welds. In this study, the mechanical and microstructural properties of austenitic weld metals with the limit values of the recommended range (5 ∼ 18 FN) of the delta ferrite control on low alloy steels were characterized by using bending test and scanning electron microscopy. The base metal was ASME Code Sec. II specification SA 508 Gr. 3 Cl. 1 plate and weld materials were EQ308L and EQ309L strips. Four kinds of cladding were deposited with submerged arc welding process on SA508 cl.3 plates. The bending tests were performed through ASME code Sec. IX and the microstructure of fractured surfaces was analyzed by scanning electron microscopy (SEM). In bending tests, there were no fractures except the highest delta ferrite content specimens (28FN). From the SEM observation of fractured surfaces, cracks initiated from the interface between austenite and ferrites phases in the cladding layer and propagated through the continuous interfaces between two phases. For specimens without continuous interfaces of two phases, though the cracks were observed in the interface of phases, the propagation of cracks was not observed. From the test results, continuous interfaces between austenite matrix and ferrite phase provide the path for crack propagation. And the delta ferrite content affects the integrity of cladding of reactor vessel.

The internals in the deaerators of a refinery plant were reported to have experienced a series of failures since their installation in 1985. These failures included development of cracks in the floor plates, damage of supports and breakage of fillet welds. Two possible root causes were initially identified; thermal stresses due to transient conditions and flow induced vibration. The former cause was classified as unlikely since the deaerators were always operating on steady-state conditions. No cyclic operating conditions were imposed on these deaerators. Vibrations however posed as the most likely root cause for the series of failures. The refinery plant inspectors reported that vibrations on the deaerators, although have not been measured, could be physically felt. These vibrations appear to be continuous and increase linearly with load. A finite element analysis was performed to determine the natural frequency of the deaerators. Mode shapes predicted from this calculation show that vibrations could have caused the failures of the internals. Furthermore, the lowest natural frequency of the deaerators appeared to fall within the actual vibration frequency on site (∼20 Hz). Although not confirmed, it is highly suspected that the vibration was excited by the flow (low pressure steam). Several repair options were explored to overcome this problem. These options were concentrated in increasing the stiffness of the steam inlet pipe and the deaerator floor. Finite element assessments demonstrated that the current flexible deaerator floor was the reason for the low natural frequency. An option of introducing reinforcement strips to the bottom side of the floor was identified as the best option to increase the natural frequency of the deaerator and this is expected to overcome the vibration problem. Only one vessel was assessed but the results apply to the other vessels since they are similar in design.

It is well known that the crack tip stress and strain fields for a crack in an elastic-plastic body depend on the crack tip contour integral J , the Q -stress, and the elastic-plastic properties of the material. This dependence is the fundamental basis of conventional two-parameter J-Q fracture mechanics assessments. It is normally assumed that the crack is created in an unstressed body, or else is inserted concurrently into an existing non-zero stress and strain field such that the crack tip fields build up monotonically and dominate at the crack tip. In such cases, the crack may be regarded as stationary and the J-Q procedure is valid provided that care is taken to calculate J and Q properly when initial stress and/or strains exist. When a crack is introduced progressively and quasi-statically into a component, the location of the crack tip will move along a distinct path. If the component contains residual stress and this is of a significant size along the crack tip path, a re-distribution of the residual stress will occur as the crack tip moves. Specifically, the stress field ahead of the crack tip will unload as the crack tip advances so that non-proportional loading will occur behind the advancing crack tip. In elastic-plastic materials, a wake of plasticity will usually be deposited in the material behind the moving or growing crack tip. Similar effects will also occur when a stationary crack extends due to critical or sub-critical processes. The presence of a plastic wake alters the stress and strain fields at the crack tip so that they do not generally match the fields of a stationary crack. Moreover, J and Q may not describe the stress and strain fields, invalidating the use of the fracture mechanics procedure for such cases. In this paper, a Finite Element analysis of J and Q is carried out for a quasi-statically extending crack inserted in a strip of elastic-plastic material containing an initial residual stress field. Care is taken to model the crack tip conditions appropriately as the crack extends and J is determined using the JEDI post-processing program which can allow for the effects of initial plastic strains and non-proportional loading. An assessment is made of the crack tip field and the likelihood of further extension or fracture is made using local approach models. The analysis considers both cleavage and ductile fracture. The extent of the relationship between J and Q and the crack tip fields is established and the validity of the J-Q procedure to such cases is discussed. The paper considers whether the procedure is conservative when J and Q are determined from an analysis of a stationary crack of the same size inserted into the same initial field.

This paper describes the construction of a new type pressure vessel using thin shell cross-helically wound interlocking steel strip, based on the technologies of interlocking strip-wound developed by Germany and flat steel ribbon wound by China which are code cases listed in ASME BPVC. Analysis and comparison between the new and the traditional structure types, and discussion on issues such as their stress characteristics, operation safety and manufacturing methods etc. are presented. It shows this new method of pressure vessel construction is feasible, cost effective, and well deserved of further studies.

Spacer grid springs support fuel rods so that the rods keep the position laterally and axially in pressurized water reactor fuel assemblies. The spring load-deflection characteristic, i.e. spring force and stiffness, is needed to evaluate the rod support conditions in the case of fuel assembly manufacturing, shipping and handling. In general, the load-deflection characteristic of grid spring is obtained by mechanical test, but it takes long time to get the new designed grid specimen because the grid manufacturing process comprises strip material manufacturing, stamping die and punch preparation, heat treatment and welding, etc. Therefore the analytic method such as finite element method (FEM) is tried to predict the nonlinear load-deflection characteristic of new designed grid. The spring characteristic mechanical test is simulated with unit cell model and analyzed by FEM tool. Comparing the results between test and analysis shows that more details are needed in the modeling because the boundary conditions of the spring are very complicated and the spring material thickness is changed by the stamping process. The analysis of modified model including expanded cells and thickness changed springs is performed. Using the analytic method of the work to obtain the load-deflection characteristic of spacer grid spring is expected to be useful in the prediction of the characteristic of new designed grids.