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Research Papers: Design and Analysis

J. Pressure Vessel Technol. 2019;141(4):041201-041201-11. doi:10.1115/1.4043374.

The rigorous stress analysis of tube-to-tubesheet joints requires a particular attention to the transition zone of the expanded tube because of its impact on joint integrity. This zone is the weakest part of the joint due to the presence of high tensile residual stresses produced during the expansion process, which coupled to in-service loadings and harsh corrosive fluids results in joint failure. In fact, it is often subjected to stress corrosion cracking caused by intergranular attack leading to plant shutdown. Therefore, the evaluation of the residual stresses in this zone is of major interest during the design phase and its accurate assessment is necessary to achieve a reliable joint in service. In this study, an analytical model to evaluate the residual axial and hoop stresses in the transition zone of hydraulically expanded tubes based on an elastic perfectly plastic material behavior has been developed. The model is capable of predicting the stress state when maximum expansion pressure is applied and after its release. Three main regions are identified in the transition zone: the fully plastic region, the partially plastic region, and the elastic region. Therefore, various theories have been applied to analyze the stresses and deformations neglecting the elastoplastic region because of simplicity. The validation of analytical model is conducted by comparison of the results with those of 3D finite element models of two typical joints of different geometries and mechanical properties. The effect strain hardening and reverse yielding of the expansion zone are also investigated.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2019;141(4):041202-041202-8. doi:10.1115/1.4043296.

Radial gaps were found in multilayered cylindrical vessels which experience inner explosion accidents in chemical plants in the past few years worldwide. It is necessary to investigate the dynamic response of multilayered structures with radial gaps to ensure the vessel safety. This paper presented a numerical modeling of the dynamic response of a multilayered structure with radial gaps of cylindrical pressure vessel under plane strain conditions by using the ANSYS/ls-dyna package. The effects of the dynamic loading profile and the radial gap height are considered in the investigation. The stress spatial distribution, the stress and the plastic deformation variation curves with time are emphatically analyzed. The results show that the stress variation of the entire loading process can be divided into four stages: the oscillation stage, the yield stage, the fast increase stage, and the redistribution stage. The layer stress distributes discontinuously at the gaps between layers and distributes unevenly in any single layer. The inner layer stress is not always larger than the outer layers' during the whole loading process. The effect of loading profile on the dynamic response is not as obvious as the gap height. As the gap height increases, the stress oscillation stage is suppressed and becomes shorter. While the loading recovers to the operation pressure, the stress and the plastic deformation of inner layers increases and vice versa for the outer layers.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2019;141(4):041203-041203-9. doi:10.1115/1.4043546.

Cylindrical explosion containment vessels (ECVs) are widely applied in transportation, nuclear engineering, public security, and scientific research fields to ensure the safety of the staff and equipment. In this paper, a cylindrical ECV model under a nonuniformly explosive load was established. The nonuniformly explosive load is simplified as parabolic pressure acting on the internal wall of the ECV. And then, based on the stress function method and boundary conditions, an analytical solution of the ECV subjected to the parabolic load was obtained. Next, the dynamic burst pressure equation of the ECV under the explosive load was obtained. In the end, the accuracy of the dynamic burst pressure equation was evaluated by comparing with the finite element method (FEM) under different pulse duration. The results demonstrated that the equation can accurately predict the dynamic burst pressure of the ECV. In addition, our researches can provide a benchmark for approximate or numerical solutions. It is rewarding to analyze the failure problem and evaluate the safety and integrity of the pipe and vessels under a nonuniformly explosive load.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2019;141(4):041204-041204-14. doi:10.1115/1.4043594.

The energy approach is used to analyze the buckling stability of toroidal shells. A closed and an open toroidal shell, as well as a shell segment are considered. Linear strain energy and nonlinear strain energy due to a uniform external pressure are formulated. Variations of the in-surface and normal displacement components in the circumferential and meridional directions are assumed in the form of a double Fourier series. The eigenvalue problem for the determination of the critical pressure is formulated by the Rayleigh–Ritz method (RRM). The proposed procedure is evaluated by numerical examples: one for a closed and another one for a simply supported open toroidal shell. The obtained results are validated by a comparison with results obtained by the finite strip method (FSM) and the finite element method (FEM), which shows a very good agreement.

Topics: Shells , Buckling , Stiffness
Commentary by Dr. Valentin Fuster

Research Papers: Fluid-Structure Interaction

J. Pressure Vessel Technol. 2019;141(4):041301-041301-10. doi:10.1115/1.4043321.

The motion of liquid filling a pipeline is impeded when the gas ahead of it cannot escape. Entrapped gas will lead to a significant pressure build-up in front of the liquid column, which slows down the column and eventually bounces it back. The pressure and temperature in the gas may become dangerously high, and for example, lead to fires and explosions caused by auto-ignition. This paper considers the case where the trapped gas can escape through a vent. One new element is that the pressure head of the liquid supply reservoir is fluctuating instead of staying constant. The obtained analytical and numerical solutions are utilized in parameter variation studies that give deeper insight in the system's behavior.

Commentary by Dr. Valentin Fuster

Research Papers: Materials and Fabrication

J. Pressure Vessel Technol. 2019;141(4):041401-041401-10. doi:10.1115/1.4043372.

The bimetal composite pipe has found wide ranging applications in engineering owing to its excellent mechanical and physical performances. However, the interlaminar cracks which are usually invisible and inaccessible may occur in the bimetal composite pipe and are difficult to detect. The ultrasonic interface wave, which propagates along the interface with high displacement amplitudes and low dispersion at high frequencies, provides a promising nondestructive testing (NDT) method for detecting cracks in the bimetal composite pipe. In this study, the interlaminar crack detection method in the steel–titanium composite pipe is investigated analytically and experimentally by using interface wave. The interface wave mode in steel–titanium composite pipe is first identified and presented by theoretical analyses of dispersion curves and wave structures. The selection of suitable excitation frequency range for NDT is discussed as well. Then an experiment is conducted to measure the interface wave velocities, which are in good agreement with the corresponding numerical results. In addition, interlaminar cracks with different locations in steel–titanium composite pipe are effectively detected and located, both in the axial and circumferential directions. Finally, the relationship between the reflection coefficient and the crack depth is experimentally studied to predict the reflection behavior of interface wave with crack. The numerical and experimental results show the interface wave is sensitive to interfacial crack and has great potentials for nondestructive evaluation in the bimetal composite pipe.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2019;141(4):041402-041402-12. doi:10.1115/1.4043375.

High strength low alloy steels are extensively used in different applications like oil and gas transmission line pipes, pressure vessels and offshore oil drilling platforms. Submerged arc welding (SAW) is mainly used to weld high thickness steel plates. Flux composition and welding parameters play an important role in determining the adequate quality and mechanical properties of the weld. Agglomerated fluxes were formulated based on TiO2–SiO2–MgO and SiO2–MgO–Al2O3 flux system using constrained mixture design and extreme vertices design approach. The chemical compositions of the bead on a plate have been studied using formulated fluxes. Twenty-one beads on plates were applied using submerged arc welding process keeping the parameters: current, voltage, and welding speed constant. Regression models were developed for bead on plate content in terms of individual, binary, and ternary mixture flux constituents for submerged arc multipass bead on plate deposition for pipeline steel (API 5 L X70). In the present study, chemical composition, grain size, and microhardness properties of the multipass bead on a plate (for API 5 L X70 grade pipeline) were optimized using multi-objective optimization approach.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2019;141(4):041403-041403-9. doi:10.1115/1.4043384.

With the application of high-density polyethylene (HDPE) pipe with thick wall in nuclear power plant (NPP), great attention has been paid to the safety of the pipeline joints, which can be assessed by phased array ultrasonic testing (PAUT). PAUT creates constructive interference of acoustic waves to generate focused beams according to delay law based on time-of-flight. However, due to the existence of acoustic attenuation and dispersion, waveform distortion occurs when ultrasonic pulse propagates in HDPE, which will accumulate with the increase of propagation distance, and then results in imaging errors. In this paper, the relationship between acoustic attenuation and dispersion in HDPE was obtained by numerical simulation in Field II®, which can be verified by the experiment of our previous work. Then, the investigation of the waveform distortion revealed the linear relation between peak offset and propagation distance. Considering the relation, an improved delay law was proposed to increase the intensity of ultrasonic field. This improved delay law was compared with the conventional one by numerical simulation of ultrasonic field and PAUT experiments, which showed that the improved delay law could increase the image sensitivity.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2019;141(4):041404-041404-9. doi:10.1115/1.4043463.

To understand the residual stress distribution in the welded joints of high density polyethylene (HDPE) pipes is essential to the assessment of its structural integrity. However, limited knowledge of their residual stress was available in this regard. In this paper, the hole-drilling strain-gage method was used to measure the residual stress in the welded seam of HDPE pipes, which was produced by the butt fusion welding technique. The finite element modeling using viscoelastic constitutive model with Prony series was carried out to determine the temperature field and corresponding stress field in the welding stages. The measured residual stress near the surface shows good consistency with the numerical results. It is shown that the residual stress in the hoop direction is much larger than those in the radial and axial directions. The effect of the pipe thickness on the residual stress distribution was also investigated by numerical simulation. The positions of the maximum tensile stress in the welded joints were found within the normalized depth region (the radial depth to the thickness) of 0.2 to 0.8.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2019;141(4):041405-041405-9. doi:10.1115/1.4043544.

Analyses of the notched tension tests of carbon steel show that ductile failure is initiated when the sum of flow stress and mean stress reaches the limit for the material, regardless of stress triaxiality. Therefore, this explicit critical stress condition could be a candidate criterion for local failure. An equation expressing the relation between stress triaxiality and critical strain was derived from the critical stress condition, and it was found that the critical strain diagram obtained by the equation nearly overlapped with that obtained by the conventional empirical equation. This suggests that the critical stress condition can be approximately determined if the critical strain diagram was obtained for a particular steel. The critical stress condition was consistent with the classical void nucleation theory, and the theory was incorporated into the void nucleation term of stress control in the Gurson–Tvergaard (GT) model—a well-known damage mechanics model for ductile failure. Since only the strain-controlled term is used in the recent GT model, herein, a finite element method (FEM) code was newly developed to implement the GT model with the stress-controlled term. Notched tension tests were analyzed with the critical stress condition using the developed code, and the analyses reproduced the failure behaviors and critical strains of the tests considerably well. These results strongly support the practicality of the stress-based criterion and demonstrate that ductile failure could be appropriately predicted by combining the GT model using the void nucleation term of stress control with the critical stress condition.

Commentary by Dr. Valentin Fuster

Research Papers: Pipeline Systems

J. Pressure Vessel Technol. 2019;141(4):041701-041701-11. doi:10.1115/1.4043590.

During pipeline construction, the pipeline may be impacted by sharp rocks or excavators. To study the failure mechanism of the pipeline, the damage degree and springback rate of the pipelines with two typical dents (transverse and longitudinal) were analyzed in terms of various factors (indenter size, pipeline size and internal pressure, and dent depth). The results reveal the following: (1) when pipeline size and internal pressure are unchanged and indenter size is changed, the integral value I used to measure the damage degree of the dented pipeline increases with increasing dent depth. When the dent depth reaches a certain value, at the same dent depth, the smaller the indenter size, the larger the damage integral value; (2) when other parameters remain unchanged, the larger the pipeline size is, the larger is the damage integral value, and the larger the internal pressure is, the smaller is the damage integral value. (3) The curves for damage and springback for the two kinds of dents are basically similar. Generally, the maximum damage of the longitudinal dent is larger than that of the transverse dent. (4) By a combination of an orthogonal experimental design and a gray correlation degree calculation, for the damage integral value of the two typical dented pipelines, the order of importance of the influential factors was obtained. (5) Formulas for the damage integral value and influence factors were fit using a nonlinear regression method, which provides a reference for calculation of pipeline damage.

Topics: Pressure , Pipelines , Damage
Commentary by Dr. Valentin Fuster

Research Papers: Seismic Engineering

J. Pressure Vessel Technol. 2019;141(4):041801-041801-15. doi:10.1115/1.4043373.

The behavior of aboveground storage tanks subjected to seismic excitation was investigated using numerical methods by taking flexibility of foundation into account. The hydrostatic load due to stored liquid has an axisymmetric distribution on the tank shell and base. However, during seismic events, the hydrodynamic load originating from the seismic acceleration of liquid in the tank starts to act in the direction of the earthquake motion. This leads to a nonaxisymmetric loading distribution, which may result in buckling and uplifting of the tank structure. Finite element models were created having nonlinear material properties and large deformation capabilities. Three different tank geometries with liquid height to tank radius aspect ratios of 0.67, 1.0, and 3.0 were selected representing broad, nominal, and slender tanks. These tanks were subjected to two different hydrodynamic loading based on Housner's and Jacobsen–Veletsos' pressure distributions, which forms the basis of design provisions used in American Petroleum Institute API 650 and Eurocode 8, respectively. These pressure distributions were formulated under the assumption of rigid tank wall and base. Furthermore, each tank for a given geometry was subjected to two different foundations: (1) representing a rigid foundation and (2) representing a flexible foundation. The flexible foundation was created using a series of compression-only elastic springs attached to tank base having equivalent soil stiffness. Static analysis corresponding to maximum dynamic force was performed. The finite element results for circumferential and longitudinal stress in the shell were compared with the provisions of API 650. It was found that the effect of foundation flexibility from the practical design point of view may be neglected for broad tanks, but should be considered for nominal and slender tanks.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Pressure Vessel Technol. 2019;141(4):044501-044501-10. doi:10.1115/1.4043297.

A computational fluid dynamics (CFD) analysis was performed to investigate the hydraulic response of the flow field inside the pressurized water reactor (PWR) steam generator (SG) secondary side and the connected part of main feed water pipe to an abrupt main feed water line break (FWLB) accident. To realistically analyze the transient flow field situation, the flow field was assumed to be occupied initially by highly compressed subcooled water except that the upper part of the SG secondary side where steam occupied as in the practical case and the break was assumed to occur at the circumferential weld line between the feed water nozzle and the main feed water pipe. This would result in a subcooled water flashing flow from the SG through the short-broken pipe end to the surrounding atmosphere, which was numerically simulated in this study. Typical results of the prediction in terms of the fluid transient velocity and pressure were illustrated and discussed. To examine the physical validity of the present numerical simulation of the subcooled water flashing flow, the transient mass flow rates predicted in this study were compared with the other previous numerical predictions based on the subcooled water nonflashing (no phase change) flow or saturated water flashing flow assumptions and the prediction by a simple analysis method.

Commentary by Dr. Valentin Fuster

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