Stress Field Around a Large Opening in a Pressure Vessel During a Major Repair

It is a regular practice in the oil industry to modify mechanical equipment to incorporate new technologies and to optimize production. In the case of pressure vessels, it is occasionally required to cut large openings in their walls in order to have access to the interior part of the equipment for executing modifications. This cutting process produces temporary loads, which were obviously not considered in the original mechanical design. Up to now, there is not a general purpose specification for approaching the assessments of stress levels once a large opening in a vertical pressure vessel has been made. Therefore stress distributions around large openings are analyzed on a case-by-case basis without a reference scheme. This work studies the distribution of the von Mises equivalent stresses around a large opening in FCC Regenerators during internal cyclone replacement, which is a frequently required practice for this kind of equipment. A finite element parametric model was developed in ANSYS, and both numerical results and illustrating figures are presented.

Effects of Surface Finish and Loading Conditions on the Low Cycle Fatigue Behavior of Austenitic Stainless Steel in PWR Environment for Various Strain Amplitude Levels

In February/March 2007, The NRC issued Regulatory Guide “RG1.207” and Argonne National Laboratory issued NUREG/CR-6909 that is now applicable in the US for evaluations of PWR environmental effects in fatigue analyses of new reactor components. In order to assess the conservativeness of the application of this NUREG report, Low Cycle Fatigue (LCF) tests were performed by AREVA NP on austenitic stainless steel specimens in a PWR environment. The selected material exhibits in air environment a fatigue behavior consistent with the ANL reference “air” mean curve, as published in NUREG/CR-6909. LCF tests in a PWR environment were performed at various strain amplitude levels (± 0.6% or ± 0.3%) for two loading conditions corresponding to a simple or to a complex strain rate history. The simple loading condition is a fully reverse triangle signal (for comparison purposes with tests performed by other laboratories with the same loading conditions) and the complex signal simulates the strain variation for an actual typical PWR thermal transient. In addition, two various surface finish conditions were tested: polished and ground. This paper presents the comparisons of penalty factors, as observed experimentally, with penalty factors evaluated using ANL formulations (considering the strain integral method for complex loading), and on the other, the comparison of the actual fatigue life of the specimen with the fatigue life predicted through the NUREG report application. For the two strain amplitudes of ± 0.6% and ± 0.3%, LCF tests results obtained on austenitic stainless steel specimens in PWR environment with triangle waveforms at constant low strain rates give “Fen” penalty factors close to those estimated using the ANL formulation (NUREG/6909). However, for the lower strain amplitude level and a triangle loading signal, the ANL formulation is pessimistic compared to the AREVA NP test results obtained for polished specimens. Finally, it was observed that constant amplitude LCF test results obtained on ground specimens under complex loading simulating an actual sequence of a cold and hot thermal shock exhibits lower combined environmental and surface finish effects when compared to the penalty factors estimated on the basis of the ANL formulations. It appears that the application of the NUREG/CR-6909 in conjunction with the Fen model proposed by ANL for austenitic stainless steel provides excessive margins, whereas the current ASME approach seems sufficient to cover significant environmental effects for representative loadings and surface finish conditions of reactor components.

Thermo-Mechanical Design of Brazed Plate-Fins Heat Exchanger Based on Finite Element Modeling Using Homogenization Techniques

Using classical Finite Element (FE) tools to model heat exchangers emphasizes the need to elaborate specific methods to reduce the size of the numerical problem. Among these methods, homogenization techniques can be adapted and used for Brazed Aluminum Plate-Fins Heat Exchangers (BAHX) including layers of periodic structures. Actually the core is formed by stacking single layers consisting of periodic corrugated fins, side-bar and parting sheets which are all made of aluminum base metals, and brazed in a furnace. So in this paper a global methodology of BAHX modeling and design is presented. It integrates homogenization techniques to perform FE calculation and localization techniques to allow applying the appropriate design criteria. Finally, to validate this methodology, results are then compared on a basic heat exchanger modeled both by classical FE tools and a dedicated software tool encapsulating both homogenization and localization techniques.

Meshless Simulation of Fracture in Thin-Walled Pipe Structures

A meshless shell method for dynamic fracture problems based on normalized Smoothed Particle Hydrodynamics (SPH) is presented. The SPH method is corrected by a normalization in order to fulfill completeness requirement. Instability are controlled by stress-point integration. The method is modified for Mindlin-Reissner shell analysis. Stress based fracture criterion is incorporated based on the visibility method. The method is applied to two dynamic fracture problems in thin-walled pipes including fluid-structure interaction. The results are compared to experimental data and they are very promising.

Effects of Water Jet Peening and Hardening Treatment on Fatigue Properties of SCr420

An experimental study of high pressure water jet peening treatment on chromium steal SCr420 H3V2L2 is conducted to study the effects of cavitation impacts of high-speed water on fatigue crack initiation and propagation of notched specimens. There are six different kinds of specimens. First three kinds are treated with; only annealing, only water quenching, and only oil quenching. Other three kinds are treated with above heat treatment and water jet peening, respectively. An axial tensile fatigue tests’ condition is 260MPa maximum stress amplitude, 0 stress ratio and 10Hz frequency, while in-situ observation by SEM is employed. Although fatigue life of the specimens with annealing and water jet peening is shorter than that of only annealing, fatigue life of water and oil quenching with water jet peening specimens is obviously longer than those without water jet peening treatment. Water jet peening has increased residual stress inside the specimens on the latter case and raised their fatigue strength. In-situ observation on the crack tips approves above analysis.

Determination of Ductile Tearing Resistance Curve in Weld Joints

Steels present in the ductile domain a tearing resistance which increase with the crack propagation up to the failure. This ductile tearing resistance is in general characterised with curves giving the variation of a global parameter (opening displacement at the crack tip δ, integral J) versus the crack extension Δa. These global approaches depend more or less on the specimen geometry and on the type of the imposed loading. Local approaches based on the description of the ductile tearing mechanisms provide reliable solution to the transferability problem (from the lab specimen to the component) but are complex and costly to use and are not codified. These problems get worse in the case of a weld joint where no standard is available for the measurement of their ductile tearing resistance. But the welded joints are often the weak point of the structure because of greater risk of defects, the heterogeneity of the microstructure of the weld, deformation along the interface between two materials with different yield stress (mismatch).... After briefly recalling the problems of transferability of the ductile tearing resistance curves obtained on lab specimen to the case of components, this article identifies the factors complicating the determination of the toughness in the welded joints and gives recommendations for the experimental determination of ductile tearing resistance curves of welded joints.

Nonlinear Stability Analysis of the Vaulted Roofs With Corrosion Defects of Oil Storage Tank

The vaulted roofs of oil storage tank are usually designed as the shallow spherical shells subjecting to a uniform external pressure, which have been widely observed that these shallow spherical shells undergo various levels of corrosion in their employing conditions. It is important to assess the stability of these local weaken shallow spherical roofs due to corrosion for preventing them from occurring unexpected buckling failure. In this paper, the uniform eroded part of a shallow spherical oil tank vaulted roof is simplified as a shallow spherical shell with elastic supports. Based on the simplification, a general pathway to calculate the critical pressure of eroded shallow spherical shell is proposed. The modified iteration method considering large deflection of the shell is applied to solve the problem of nonlinear stability of the shallow spherical shells, and then the second-order approximate analytical solution is obtained. The critical pressure calculated by this method is consistent with the classical numerical results and nonlinear finite element method, and the calculation errors are less than 10%. It shows that it is feasible to apply the method proposed here.

About Relation Between Irradiation Hardening of Ferritic Steels and Ductile Fracture Toughness Decrease

We showed recently that temperature dependence of the ductile fracture toughness can be predicted on the base of two assumptions: 1) assumption of constant characteristic length, 2) assumption of proportionality between J-R curve slope and deformation work in unit volume, evaluated from zero to critical strain for initiation of deformation bands determined in plane strain geometry for material modeled by deformation theory of plasticity. Temperature dependence of ductile fracture toughness results simply from temperature dependence of the stress-strain curve. Irradiation hardening changes stress-strain behavior in a qualitatively different way: It is observed that irradiation hardening to certain yield stress level changes the stress-strain curve of the material in the same way as prestraining of the unirradiated material to the same flow stress level does. Equivalence of irradiation and prestraining concerns all key properties of deformation theory; namely the secant modulus should be taken from the stress-strain curve of unirradiated material. With exception of this specific feature, the task of finding relative fracture toughness decrease by irradiation is the same as prediction of relative decrease of fracture toughness by temperature change. In the frame of the corresponding theory, relative decrease of ductile fracture toughness expressed by J-R curve slope can be obtained from the stress-strain curve of unirradiated material and irradiation hardening level. Quantitative results are presented for the weld metals 72W and 73W, studied in the Fifth Irradiation Series in the Heavy-Section Steel Irradiation Program, and compared with experimental data.

Through Wall Cracking in High Temperature Rotary Cooler

After operating for a number of years, a high temperature rotary ore cooler suffered cracking. The cracks grew through the shell wall resulting in leakage of water from the water bath into the ore. Under the extreme temperature, the risk of water dissociation into hydrogen and subsequent explosion was of substantial concern and instigated the investigation in to the root cause of the cracking which was deduced to be driven by high thermally induced stresses. The root cause for the thermally induced stressing was found to be related to a design flaw that was not immediately obvious. The investigation outcome was a recommendation to change the design to eliminate the high localized stresses which were believed to be the driving force behind the corrosion fatigue crack propagation. This paper presents the investigation approach which included advanced thermal and stress analysis and reports on the general design principle that should be adopted to avoid thermal stress induced corrosion fatigue cracking under high temperature operation.

Effects of Gaseous Hydrogen on Fatigue Crack Growth Behavior of Low Carbon Steel

In order to investigate the effects of hydrogen on the fatigue crack growth behavior of low carbon steel JIS S10C, bending fatigue tests were carried out using a specimen with a small blind artificial hole in a low pressure pure hydrogen gas atmosphere. The results show that the fatigue crack growth rate in hydrogen gas is higher than that in nitrogen gas, moreover, the degree of acceleration is greater in the high strain range. In fractography, intergranular facets mixed with ductile fracture and quasi-cleavage fracture with brittle striations appear in a hydrogen gas environment, while only ductile fracture mainly appears in nitrogen gas. In the low growth rate range, many intergranular facets are seen on the ductile fracture surface, and in the higher growth rate range, quasi-cleavage facets increase as the growth rate increases. The growth rate of a small crack in nitrogen gas can be expressed by dl/dN ∝ Δεpnl in the wide range of applied total strain range Δεt. The same type equation is also satisfied in hydrogen gas, but in the narrow range roughly from Δεt = 0.25% to Δεt = 0.37%. The fracture surface in this range shows only intergranular facets and a ductile morphology, but no quasi-cleavage fracture. Although the crack growth mechanism in hydrogen is different from that in nitrogen, observation of the mechanism of intergranular facet formation shows a similarity to the mechanism in nitrogen in which the slip-off mechanism of crack growth is valid. The formation of intergranular facets is also closely related to the slip behavior influenced by hydrogen. This means that there exists a high possibility for the application of the small crack growth law inhydrogen to not only S10C, but also to other carbon steels in which the intergranular facet appears.