Review Article

J. Pressure Vessel Technol. 2017;139(6):060801-060801-21. doi:10.1115/1.4035885.

The American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (Code) provides rules for the design of Class 1 components of nuclear power plants. However, the Code design curves do not address the effects of light water reactor (LWR) water environments. Existing fatigue strain-versus-life (ε–N) data illustrate significant effects of LWR water environments on the fatigue resistance of pressure vessel and piping steels. Extensive studies have been conducted at Argonne National Laboratory (Argonne) and elsewhere to investigate the effects of LWR environments on the fatigue life. This article summarizes the results of these studies. The existing fatigue ε–N data were evaluated to identify the various material, environmental, and loading conditions that influence the fatigue crack initiation; a methodology for estimating fatigue lives as a function of these parameters was developed. The effects were incorporated into the ASME Code Section III fatigue evaluations in terms of an environmental correction factor, Fen, which is the ratio of fatigue life in air at room temperature to the life in the LWR water environment at reactor operating temperatures. Available fatigue data were used to develop fatigue design curves for carbon and low-alloy steels, austenitic stainless steels (SSs), and nickel–chromium–iron (Ni–Cr–Fe) alloys and their weld metals. A review of the Code Section III fatigue adjustment factors of 2 and 20 is also presented, and the possible conservatism inherent in the choice is evaluated. A brief description of potential effects of neutron irradiation on fatigue crack initiation is presented.

Commentary by Dr. Valentin Fuster

Research Papers: Design and Analysis

J. Pressure Vessel Technol. 2017;139(6):061201-061201-12. doi:10.1115/1.4037564.

Bellows structure is used to absorb the thermal expansion maintaining the boundary of the inside to outside, and it is applied to constitute the containment vessel (CV) boundary of the nuclear power plant. In this study, in order to develop the evaluation method of the ultimate strength of the bellows structure subject to internal pressure beyond the specified limit, the failure test and finite element analysis (FEA) of the bellows structure were performed. Several types of the bellows structure made of SUS304 were tested using pressurized water. The failure modes were demonstrated through the test of five and six specimens with six and five convolutions, respectively. Water leakage was caused by contact of the expanded convolution and the neighbor structure in the specimens with the shipping rod mounts. On the other hand, local failure as leakage in the deformation concentrated location and ductile failure as burst in the expanded convolution were observed in the specimen without shipping rod mounts. The maximum pressures in the test observed local and ductile failure were over ten times larger than the estimated values of the limited design pressure for in-plane instability by the EJMA standard. To simulate the buckling and deformation behavior during the test, the implicit and explicit FEA were performed. Because the inversion of the convolution accompanied by convolution contact observed in the test was too difficult a problem for implicit analysis, the maximum pressures in the step of solution converged were compared to the maximum pressures in the tests. On the other hand, explicit analysis enabled to simulate the complex deformation during the test, and the results were evaluated considering ductile failure to compare the test results.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;139(6):061202-061202-19. doi:10.1115/1.4037042.

Linearized buckling analysis of functionally graded shells of revolution subjected to displacement-dependent pressure, which remains normal to the shell's middle surface throughout the deformation process, is described in this work. Material properties are assumed to be varied continuously in the thickness direction according to a simple power law distribution in terms of the volume fraction of a ceramic and a metal. The governing equations are derived based on the first-order shear deformation theory, which accounts for through the thickness shear flexibility with Sanders type of kinematic nonlinearity. Displacements and rotations in the shell's middle surface are approximated by combining polynomial functions in the meridian direction and truncated Fourier series with an appropriate number of harmonic terms in the circumferential direction. The load stiffness matrix, also known as the pressure stiffness matrix, which accounts for the variation of load direction, is derived for each strip and after assembling resulted in the global load stiffness matrix of the shell, which may be unsymmetric. The load stiffness matrix can be divided into two unsymmetric parts (i.e., load nonuniformity and unconstrained boundary effects) and a symmetric part. The main part of this research is to quantify the effects of these unsymmetries on the follower action of lateral pressure. A detailed numerical study is carried out to assess the influence of various parameters such as power law index of functionally graded material (FGM) and shell geometry interaction with load distribution, and shell boundary conditions on the follower buckling pressure reduction factor. The results indicate that, when applied individually, unconstrained boundary effect and longitudinal nonuniformity of lateral pressure have little effect on the follower buckling reduction factor, but when combined with each other and with circumferentially loading nonuniformity, intensify this effect.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;139(6):061203-061203-11. doi:10.1115/1.4037808.

Shell structures are built using a number of welded curved panel parts. Hence, some geometrical imperfections emerge. These imperfections have a direct impact on structural behavior of shells during the external compressive loading. In this research, a field study was accomplished on the implementation of the storage tanks in a refinery site, and then the resulted imperfections were identified and categorized. The survey of imperfections revealed that imperfection resulted from deviation with respect to the vertical direction has the highest number in tank bodies. This imperfection experimentally modeled, and the buckling behavior of these tanks was evaluated under uniform external pressure. The cylindrical tanks were examined using finite element analysis, and results obtained were compared with experimental results. Investigation of finding results demonstrated that such imperfection has a significant role in reducing the number of circumferential waves in body of the tanks under uniform external pressure. Comparing the results obtained by estimation, American Society of Mechanical Engineers (ASME) code, experimental research, and finite element method (FEM) represented a considerable difference in the amount of buckling load. Results show that tanks with oblique body imperfections exhibit high initial strength against buckling due to the uniform external pressure.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;139(6):061204-061204-6. doi:10.1115/1.4037634.

In recent years, the gas wells with high pressure, high temperature, and high H2S are increasing gradually, but the burst of casing and tubing in these wells will cause the gas channeling and overflow, and the gas with H2S flows up to surface, which causes huge damage. Although the API 5C3 and ISO 10400 standards have presented the prediction model of minimum internal pressure yield strength (IPYS) and burst strength for the casing and tubing in the process of strength design, the effect of temperature on the internal pressure strength is not considered completely. It is well known that it is extremely important to understand the failure mechanism of casing and tubing under the synergy of temperature and internal pressure. Hence, the full-scale internal pressure test is performed for N80 casing under temperature and internal pressure by adopting self-developed experimental equipment, by which the important mechanical parameters (such as minimum IPYS, burst strength, stress-hardening rate, and so on) of casing before and after hardening have been obtained. The impacts of temperature on the internal pressure strength are analyzed based on the comparison of test values with theoretical values given by API 5C3 and ISO 10400 standards. Finally, the failure mechanism and hardening characteristic of N80 casing have been clarified under the synergy of temperature and internal pressure. Research results can provide important references for internal pressure strength design of casing in deep well with high temperature.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;139(6):061205-061205-7. doi:10.1115/1.4037635.

The deflagration pressure analysis code (DPAC) has been upgraded for use in modeling hydrogen deflagration transients. The upgraded code is benchmarked using data from vented hydrogen deflagration tests conducted at the HYDRO-SC Test Facility at the University of Pisa. DPAC originally was written to calculate peak pressures for deflagrations in radioactive waste storage tanks and process facilities at the Savannah River Site. Upgrades include the addition of a laminar flame speed correlation for hydrogen deflagrations and a mechanistic model for turbulent flame propagation, incorporation of inertial effects during venting, and inclusion of the effect of water vapor condensation on vessel walls. In addition, DPAC has been coupled with chemical equilibrium with applications (CEA), a NASA combustion chemistry code. The deflagration tests are modeled as end-to-end deflagrations. The improved DPAC code successfully predicts both the peak pressures during the deflagration tests and the times at which the pressure peaks.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;139(6):061206-061206-9. doi:10.1115/1.4038013.

A containment of a nuclear power plant is a final barrier against the release of radioactive materials and withstands internal pressure due to an accident. Buckling is a critical failure mode of an ellipsoidal head of steel containment vessel under internal pressure. First, a vessel was designed to measure buckling pressure and shape of the ellipsoidal head. Second, an experiment was successfully performed on an ellipsoidal head which has a diameter of 4797 mm, a radius-to-height ratio of 1.728, and a thickness of 5.5 mm. The initial shape and deformations of the ellipsoidal head were measured by using three-dimensional (3D) laser scanners. The detailed buckling characteristics including shapes, deformations, strains of buckles, and buckling pressures were obtained. Finally, initial buckling pressures were predicted by nonlinear finite element analysis considering the initial measured and the initial perfect shapes of the ellipsoidal head, respectively. The agreement between the initial experimental buckling pressure and that predicted by the analysis considering the initial measured shape is good.

Commentary by Dr. Valentin Fuster

Research Papers: Fluid-Structure Interaction

J. Pressure Vessel Technol. 2017;139(6):061301-061301-10. doi:10.1115/1.4037716.

In this article, we simulate traveling liquid slugs in conduits, as they may occur in systems carrying high-pressure steam. We consider both horizontal and inclined pipes in which the slug is accelerated by a suddenly applied pressure gradient, while at the same time, gravity and friction work in the opposite direction. This causes a steep slug front and an extended slug tail. The shapes of front and tail are of interest since they determine the forces exerted on bends and other obstacles in the piping system. The study also aims at improving existing one-dimensional (1D) models. A hybrid model is proposed that enables us to leave out the larger inner part of the slug. It was found that the hybrid model speeds up the two-dimensional (2D) computations significantly, while having no adverse effects on the shapes of the slug's front and tail.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;139(6):061302-061302-7. doi:10.1115/1.4037717.

The simple tube and channel theoretical model for fluidelastic instability (FEI) in tube arrays, as developed by Hassan and Weaver, has been used to study the effects of pitch ratio and mass ratio on the critical velocity of parallel triangular tube arrays. Simulations were carried out considering fluidelastic forces in the lift and drag directions independently and acting together for cases of a single flexible tube in a rigid array and a fully flexible kernel of seven tubes. No new empirical data were required using this model. The direction of FEI as well as the relative importance of fluid coupling of tubes was studied, including how these are affected by tube pitch ratio and mass ratio. The simulation predictions agree reasonably well with available experimental data. It was found that parallel triangular tube arrays are more vulnerable to streamwise FEI when the pitch ratio is small and the mass-damping parameter (MDP) is large.

Commentary by Dr. Valentin Fuster

Research Papers: Materials and Fabrication

J. Pressure Vessel Technol. 2017;139(6):061401-061401-10. doi:10.1115/1.4036852.

Interaction of fundamental torsional ultrasonic pipe guided mode T(0, 1) from defects caused by induction pressure welding (IPW) process is studied using three-dimensional (3D) finite element (FE) analysis validated by experiments. Defects are assumed as cross-sectional notches along the weld bond-line, and both surface-breaking and embedded features are considered. Results show that T(0, 1) mode reflection from weld defects is strongly influenced by features of the weld itself. However, with supplementary results such as the mode-converted flexural F(1, 3) and F(1, 2) modes and circumferential variation of T(0, 1) reflection, there is potential for an effective screening solution.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;139(6):061402-061402-8. doi:10.1115/1.4037445.

Pressure vessels comprise critical plant equipment within industrial operations. The fact that the vessel operates under pressure, and may carry toxic, dangerous, or hazardous contents, necessitates that care is taken to ensure safety of humans operating it and the environment within which it operates. Residual stress developed during welding of pressure vessel structures can adversely affects fatigue life (mean stress effect) of such structure and lead to corrosion crack growth. The present study applies the neutron diffraction (ND) technique to formulate the stress field distribution of a nozzle-to-shell weld joint of a pressure vessel. A number of experiments are conducted using the submerged arc welding (SAW) process at various parametric combinations to develop a number of specimens with different stress profiles. It is shown that the hoop stresses close to the weld center line (WCL) are highly tensile and have values close to the yield strength of the material. The ideal parametric combination is also determined based on the results with lowest stresses. The results obtained in this study are congruent to the results of similar studies in the literature.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;139(6):061403-061403-8. doi:10.1115/1.4038012.

Commercial pure titanium has been widely used in the aerospace, chemical, and biomedical industries because of its light weight, high corrosion resistance, high strength, high heat resistance, and good biocompatibility. Pure titanium takes the form of a hexagonal closed-pack structure with anisotropic elasticity and plasticity, with most of its components being polycrystalline aggregates having different crystal orientations. Small mechanical loading under elastic conditions therefore always induces inhomogeneous microscopic deformation, and the resulting inhomogeneity brings about various defects such as inhomogeneous plastic deformation, microcracking, and necking. It is therefore important to investigate the microscopic inhomogeneous deformation under elastic and plastic conditions. In this study, a plate specimen of commercial pure titanium was subjected to a tensile test on the stage of a digital holographic microscope (DHM), and the microscopic deformation of grains in the specimen under elastic and plastic conditions were observed and measured. During the test, the grains’ height distribution was measured in a fixed area on the specimen’s surface at each tensile loading step, and the correlation between height distributions at different loads was examined. We found from the measurements that each grain shows a different height change even under elastic conditions with a small load. This inhomogeneous height change was enhanced as the load was increased to plastic conditions. A strong correlation between the height changes under elastic and plastic conditions was also found. This result suggests that the microscopic deformation experienced under plastic conditions is predictable from that observed under elastic conditions.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;139(6):061404-061404-6. doi:10.1115/1.4038018.

Most of the microstructural changes in weldment takes place during weld cooling from 800 ° C to 500 ° C (t8/5) specially in the heat-affected zone (HAZ). Weld strength and cracking tendency can also relate to t8/5. A generalized model using dimensional analysis has been proposed for estimation of the weld cooling time (t8/5) for variable plate thicknesses. The proposed model is based on rotatable central composite designed submerged arc welding (SAW) experiments. The model considers material properties, weld parameters, and environmental conditions for submerged arc welding. The model is validated with experimental data and cooling time observed by other researchers. The adequacy of the model was found to be 97% and able to predict cooling time for a plate thickness ranging from 8 to 41.5 mm thickness.

Commentary by Dr. Valentin Fuster

Research Papers: Seismic Engineering

J. Pressure Vessel Technol. 2017;139(6):061801-061801-15. doi:10.1115/1.4037809.

This article presents the experimental and numerical studies of fatigue-ratcheting in carbon steel piping systems under internal pressure and earthquake load. Shake table tests are carried out on two identical 6 in pressurized piping systems made of carbon steel of grade SA333 Gr 6. Tests are carried out using similar incremental seismic load till failure. Wavelet analysis is carried to evaluate frequency change during testing. The tested piping systems are analyzed using iterative response spectrum (IRS) method, which is based on fatigue-ratcheting and compared with test results. Effect of thickness variation in elbow on strain accumulation is studied. Excitation level for fatigue-ratcheting failure is also evaluated and the details are given in this paper.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2017;139(6):061802-061802-11. doi:10.1115/1.4037952.

The accident at the Fukushima Dai-ichi Nuclear Power Plant (NPP) resulting from the 2011 Great East Japan Earthquake raised awareness as to the importance of considering Beyond Design Basis Events (BDBE) when planning for safe management of NPPs. In considering BDBE, it is necessary to clarify the possible failure modes of structures under extreme loading. Because piping systems are one of the representative components of NPPs, an experimental investigation was conducted on the failure of a pipe assembly under simulated excessive seismic loads. The failure mode obtained by excitation tests was mainly fatigue failure. The reduction of the dominant frequency and the increase of hysteresis damping were clearly observed in high-level input acceleration due to plastic deformation, and they greatly affected the specimens’ vibration response. Based on the experimental results, a procedure is proposed for calculating experimental stress intensities based on excitation test so that they can be compared with design limitations.

Commentary by Dr. Valentin Fuster

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