Review Article

J. Pressure Vessel Technol. 2018;140(5):050801-050801-8. doi:10.1115/1.4039695.

It is generally accepted that the presence of imperfections in pressure vessel components can significantly reduce their buckling strength. In fact, the discrepancies between theoretical predictions and experimental results have been attributed to various kinds of existing and unavoidable imperfections. This is not a new problem but despite of substantial research effort in this area over the recent decades, it is far from being satisfactorily resolved. This review provides insight into the past findings and current activities related to the role of different types of imperfections on the buckling strength. It aims to contribute to a better understanding of the influence of imperfections on the structural stability of cones, cylinders, and domes when these are subjected to external loading conditions. The review concentrates not only on the prominent role of initial geometric imperfections of the shell's generator but also on less known defects. This includes uneven axial length of cylinders, eccentricities, and nonuniformities of applied load, inaccurately modeled boundary conditions, corrosion of the wall, influence of material discontinuity or crack, and effect of prebuckling deformation. The study examines: (i) how the data were obtained (analytically, experimentally, and/or numerically), (ii) the type of material from which the shell structures were made, and (iii) the importance of findings of the previous works. Metallic and composite components are considered.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(5):050802-050802-15. doi:10.1115/1.4039206.

Autofrettage is a metal forming technique widely incorporated for strengthening the thick-walled cylindrical and spherical pressure vessels. The technique is based on the principle of initially subjecting the cylindrical or spherical vessel to partial plastic deformation and then unloading it; as a result of which compressive residual stresses are set up. On the basis of the type of the forming load, autofrettage can be classified into hydraulic, swage, explosive, thermal, and rotational. Considerable research studies have been carried out on autofrettage with a variety of theoretical models and experimental methods. This paper presents an extensive review of various types of autofrettage processes. A wide range of theoretical models and experimental studies are described. Optimization of an autofrettage process is also discussed. Based on the review, some challenging issues and key areas for future research are identified.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(5):050803-050803-15. doi:10.1115/1.4039068.

This paper provides a critical review of the advancements made in the application of the variable material properties (VMP) method over the past two decades. The VMP method was originally proposed in 1997 (Jahed and Dubey, 1997, ASME J. Pressure Vessel Technol., 119(3), pp. 264–273; Jahed et al., 1997, Int. J. Pressure Vessels Piping, 71(3), pp. 285–291) and further developed in 2001 (Parker, 2001, ASME J. Pressure Vessel Technol., 123(3), p. 271) as an elastoplastic method for the analysis of axisymmetric problems. The model was originally developed as a boundary value problem to predict the spatial distribution of stress. However, since 1997, it has been extended to include thermal effects to solve thermomechanical residual stresses; time domain to solve creep of disks and cylinders; finite deformation to solve cylinders under large strains; numerical solutions to make them more efficient; and asymmetric hardening behavior to accommodate nonslip deformation modes. These advancements, made over the past 20 years, are reviewed in this paper, and future trends and frontiers are discussed.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(5):050804-050804-13. doi:10.1115/1.4041058.

This paper reviews the literature on variational method in limit load analysis and presents both analytical and numerical approaches. One of the most successful applications of variational method in theory of plasticity is limit load analysis. The main objective of the limit load analysis is to estimate the load at the impending plastic limit state of a body. However, for complicated problems it may be very difficult to find the exact limit load. Therefore, based on the extremum principles of limit load analysis, the lower-bound theorem or the upper bound theorem is employed to estimate the limit load directly without considering the entire loading history. In general, limit load analysis plays an important role in design and fitness-for-service assessment of pressurized vessels and piping.

Topics: Stress
Commentary by Dr. Valentin Fuster

Research Papers: Design and Analysis

J. Pressure Vessel Technol. 2018;140(5):051201-051201-7. doi:10.1115/1.4040891.

When subjected to external forces, bolted joints behave in a complex manner especially in the case of the joints being clamped with multiple bolts. Friction type joints are widely used for the joints subjected to shear loads. Bearing type joints, which support the shear loads on the bolt cylindrical surface, are used less frequently, since its mechanical behavior is too complicated to accurately estimate the load capacity. In this study, mechanical behavior of the bearing type multibolted joints subjected to shear loads is analyzed by three-dimensional (3D) FEM. As a result of comprehensive calculations, it has been found that the shear load applied to bearing type joints distributes with a concave shape along the load direction, and a fair amount of the shear load is supported by friction forces as in the case of friction type joints. In addition, a simple method that calculates the shear load distribution using elementary theory of solid mechanics is proposed, which can estimate the shear load distribution with sufficient accuracy especially for the case of small friction coefficient.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(5):051202-051202-8. doi:10.1115/1.4040892.

Ratcheting, collapse, and fatigue are the probable failure modes which can occur under alternate dynamic loading like seismic loading. The objective of this study is to propose a failure mode map for rectangular beams by determining the conditions of occurrence of the ratcheting and collapse failure modes. The paper considers the analogy between thermal ratcheting and dynamic ratcheting. The nonlinear dynamic finite element method was used to analyze a rectangular beam model for different loading conditions. The results were plotted on a nondimensional primary and secondary stress parameter graph similar to the Bree diagram for thermal ratcheting. The similarity between thermal load and dynamic load was observed. The main difference between thermal and dynamic loading is the effect of the frequency of dynamic loading on the occurrence of ratcheting and collapse. Experimental observations of ratcheting have been obtained and are used for comparison to validate the analytical predictions. From the above results, a failure mode map has been proposed which can evaluate the occurrence conditions of ratcheting and collapse under dynamic loadings.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(5):051203-051203-10. doi:10.1115/1.4040894.

The transmissivity of metal-metal sealing joints is investigated experimentally and compared to predictions obtained by modeling. The focus is laid upon a wavy surface contacting a flat rigid part, representative of a seat-to-plug contact in an internal sealing valve encountered in nuclear power plants for instance. Experimental transmissivities are obtained from water leak-rate and pressure drop measurements carried out on a model ring-shape sample seat holding a controlled wavy defect and pressed against a rigid flat plug with a controlled normal load. The sample seat surface is manufactured by face turning a tubular part under radial stress and waviness is obtained after elastic relaxation. Modeling is performed on a three-dimensional finite element model of the assembly, composed of the plug, the sample seat, and its holder. The upper sample seat surface, in which topography is recorded by confocal microscopy, is reconstructed using a modal decomposition on the basis of vibrational eigenmodes. Its lower surface, in contact with the holder, is considered as perfectly flat or with its own defects. The contact aperture field between the seat and the plug is computed for a given normal load and is used to solve the incompressible Reynolds equation with a boundary element method, yielding the transmissivity. Predicted transmissivities reveal to be in good agreement with experimental data at low clamping loads and are overestimated for larger ones. Defects on the lower surface of the seat are shown to have a significant impact on the seat-to plug contact transmissivity.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(5):051204-051204-7. doi:10.1115/1.4040640.

Subsurface flaws are sometimes found as blowholes near free surfaces of structural components. Net-section stress at the ligament between the free component surface and the subsurface flaw increases when the ligament size is short. It can be easily expected that the stress intensity factor at the tip of the subsurface flaw increases with decreasing the ligament size. Fitness-for-service (FFS) codes provide flaw-to-surface proximity rules, which are transformation from subsurface to surface flaw. Although the concepts of the proximity rules of the FFS codes are the same, the specific criteria for the rules on transforming subsurface flaws to surface flaws are significantly different among FFS codes. This study demonstrates the proximity criteria provided by the FFS codes and indicates that the increment of the stress intensity factors before and after the transformation depends on the flaw aspect ratio and the ligament size at the transformation from subsurface to surface flaws. In addition, it is shown that remaining fatigue lives for pipes with flaws are strongly affected by the ligament size at the transformation from subsurface to surface flaws.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(5):051205-051205-9. doi:10.1115/1.4040641.

The drop weight tear test (DWTT) has been widely used to evaluate the resistance of linepipe steels against brittle fracture propagation. Although there is an ambiguity in the evaluation of DWTT results if inverse fracture appears on the fracture surfaces, the cause of inverse fracture is not yet fully understood. In the present work, DWTTs were performed with X65, X70, and X80 steel linepipes. In addition to the conventional DWTT specimen with a pressed notch (PN), PN specimens with a back slot (BS) and specimens with a chevron notch (CN) or static precrack (SPC) were also examined, and the fracture appearances in different strengths and different initial notch types were compared. Although the frequency of inverse fracture in these DWTTs was different with each material and each specimen type, there was no material or specimen type that was entirely free from inverse fracture. The purpose of the DWTT is to evaluate the brittle crack arrestability of the material in a pressurized linepipe. Therefore, the DWTT results should be examined with a running brittle crack arrest (BCA) test. A large-scale BCA test with temperature gradient was also performed with the X65 mother plate, and the shear area fraction measured in the DWTT fracture surface was compared with the local shear lip thickness fraction in the BCA test. Based on the results, the count of inverse fracture in the DWTT was discussed in comparison with the long BCA behavior in the BCA test.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(5):051206-051206-4. doi:10.1115/1.4041059.

Based on the unified analytical (UA) method and the unified and refined analytical (URA) method of stress analysis for fixed tubesheet (TS) heat exchangers (HEXs), floating head, and U-tube HEXs, the applicable configuration of HEX which depends on the combination of the TS edge conditions is discussed in this paper. Comparison shows that the UA and the URA methods cover a wide range of HEX configurations well beyond established ASME methods.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(5):051207-051207-7. doi:10.1115/1.4041198.

In service pipelines exhibit bending loads in a variety of in-field situation. These bending loads can induce large longitudinal strains, which may trigger local buckling on the pipe's compressive side and/or lead to rupture of the pipe's tensile side. In this article, the post-buckling failure modes of pressurized X65 steel pipelines under monotonic bending loading conditions are studied via both experimental and numerical investigations. Through the performed full-scale bending test, it is shown that the post-buckling rupture is only plausible to occur in the pipe wall on the tensile side of the wrinkled cross section under the increased bending. Based on the experimental results, a finite element (FE)-based numerical model with a calibrated cumulative fracture criterion was proposed to conduct a parametric analysis on the effects of the internal pressure on the pipe's failure modes. The results show that the internal pressure is the most crucial variable that controls the ultimate failure mode of a wrinkled pipeline under monotonic bending load. And the post-buckling rupture of the tensile wall can only be reached in highly pressurized pipes (hoop stress no less than 70% SMYS for the investigated X65 pipe). That is, no postwrinkling rupture is likely to happen below a certain critical internal pressure even after an abrupt distortion of the wrinkled wall on the compressive side of the cross section.

Commentary by Dr. Valentin Fuster

Research Papers: Fluid-Structure Interaction

J. Pressure Vessel Technol. 2018;140(5):051301-051301-11. doi:10.1115/1.4040549.

This paper presents an experimental investigation of the near-wake flow characteristics for isolated crimped spirally finned cylinders in cross-flow and its influence on the generated sound pressure during flow-excited acoustic resonance. Four crimped spirally finned cylinders are investigated, which have pitch-to-root diameter ratio (p/Dr) ranging between 0.384 ≤ p/Dr ≤ 1. A new equivalent diameter equation (Dc) has been developed to better capture the vortex shedding frequency emanating from the crimped spirally finned cylinders. The addition of crimped spiral fins reduces the coherence of the vortex shedding process as compared to that of a bare cylinder. Moreover, the addition of crimped spiral fins causes an elongation in the vortex formation region, as well as induces a larger velocity deficit in the near-wake. Reduction in the pitch-to-diameter ratio (p/Dr) leads to a progressive increase in the strength and coherence of the vortex shedding process. It also results in a gradual reduction in the vortex formation length and velocity deficit. The near-wake flow characteristics of the crimped spirally finned cylinders inherently affect the sound pressure during flow-excited acoustic resonance. Furthermore, the helical fins impose an asymmetrical inclination of the acoustic particle velocity. This hinders the flow-acoustic coupling, leading to a weakened energy transfer between the flow and sound fields. The findings of this investigation provide better understanding of the complex flow-sound interaction mechanism from crimped spirally finned cylinders in heat exchanger tube bundle.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(5):051302-051302-10. doi:10.1115/1.4040698.

The structural integrity of a reactor pressure vessel (RPV) is important for the safety of a nuclear power plant. When the emergency core cooling system (ECCS) is operated and the coolant water is injected into the RPV due to a loss-of-coolant accident (LOCA), the pressurized thermal shock (PTS) loading takes place. With the neutron irradiation, PTS loading may lead an RPV to fracture. Therefore, it is necessary to evaluate the performance of RPV during PTS loading to keep the reactor safety. In the present study, optimization of RPV maintenance is considered, where two different attempts are made to investigate the RPV integrity during PTS loading by employing the deterministic and probabilistic methodologies. For the deterministic integrity evaluation, three-dimensional computational fluid dynamics (3D-CFD) and finite element method (FEM) simulations are performed, and stress intensity factors (SIFs) are obtained as a function of crack position inside the RPV. As to the probabilistic integrity evaluation, on the other hand, a practically more useful spatial distribution of SIF on the RPV is calculated. By comparing the distribution thus obtained with the fracture toughness included as a part of the master curve, the dependence of conditional failure probabilities on the position inside the RPV is obtained. Using the spatial distribution of conditional failure probabilities in RPV, the priority of the inspection and maintenance is finally discussed.

Commentary by Dr. Valentin Fuster

Research Papers: Materials and Fabrication

J. Pressure Vessel Technol. 2018;140(5):051401-051401-8. doi:10.1115/1.4040789.

Additive creep rate model has been developed to predict creep strain-time behavior of materials important to engineering creep design of components for high temperature applications. The model has two additive formulations: the first one is related to sine hyperbolic rate equation describing primary and secondary creep deformation based on the evolution of internal stress with strain/time, and the second defines the tertiary creep rate as a function of tertiary creep strain. In order to describe creep data accurately, tertiary creep rate relation based on MPC-Omega methodology has been appropriately modified. The applicability of the model has been demonstrated for tempered martensitic plain 9Cr-1Mo steel for different applied stresses at 873 K. Based on the observations, a power law relationship between internal stress and applied stress has been established for the steel. Further, a higher creep damage accumulation with increasing life fraction has been observed at low stresses than those obtained at high stresses.

Topics: Creep , Steel , Stress , Damage , Modeling
Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(5):051402-051402-6. doi:10.1115/1.4040642.

The applicability of miniature compact tension (Mini-C(T)) specimens to fracture toughness evaluation of neutron-irradiated reactor pressure vessel (RPV) steels was investigated. Three types of RPV steels neutron-irradiated to a high-fluence region were prepared and manufactured as Mini-C(T) specimens according to Japan Electric Association Code (JEAC) 4216-2015. Through careful selection of the test temperature by considering previously obtained mechanical properties data, valid fracture toughness, and reference temperature (To) was obtained with a relatively small number of specimens. Comparing the fracture toughness and To values determined using other larger specimens with those determined using the Mini-C(T) specimens, To values of both unirradiated and irradiated Mini-C(T) specimens were found to be the acceptable margin of error. The scatter of 1T-equivalent fracture toughness values of both unirradiated and irradiated materials obtained using Mini-C(T) specimens did not differ significantly from the values obtained using larger specimens. The correlation between the Charpy 41 J transition temperature (T41J) and the To values agreed very well with that of the data in the literature, regardless of specimen size and fracture toughness of the materials before irradiation. Based on these findings, it was concluded that Mini-C(T) specimens can be applied to fracture toughness evaluation of neutron-irradiated materials without significant specimen size dependence.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(5):051403-051403-8. doi:10.1115/1.4040790.

Although S∼N curve-based approaches are widely followed for fatigue evaluation of nuclear reactor components and other safety critical structural systems, there is a chance of large uncertainty in estimated fatigue lives. This uncertainty may be reduced by using a more mechanistic approach such as physics based three-dimensional (3D) finite element (FE) methods. In a recent paper (Barua et al., 2018, ASME J. Pressure Vessel Technol., 140(1), p. 011403), a fully mechanistic fatigue modeling approach which is based on time-dependent stress–strain evolution of material over the entire fatigue life was presented. Based on this approach, in this work, FE-based cyclic stress analysis was performed on 316 nuclear grade reactor stainless steel (SS) fatigue specimens, subjected to constant, variable, and random amplitude loading, for their entire fatigue lives. The simulated results are found to be in good agreement with experimental observation. An elastic-plastic analysis of a pressurized water reactor (PWR) surge line (SL) pipe under idealistic fatigue loading condition was performed and compared with experimental results.

Commentary by Dr. Valentin Fuster

Research Papers: Operations, Applications and Components

J. Pressure Vessel Technol. 2018;140(5):051601-051601-8. doi:10.1115/1.4040893.

Water hydraulics relief valves are essential components of hydraulic systems. These valves maintain the desired pressure and thereby prevent other components from being damaged. During operation of the relief valve, the water flow often cavitates in the valve port owing to the rapid decline in pressure, affecting the stability and safety of the hydraulic system. To improve relief valve performance, an optimal design of the valve was determined. Using a computational fluid dynamics approach, the effects of the valve core design and the nonsmooth groove structure of the valve seat on the jet flow structure were modeled and tested. The anti-cavitation structure was optimized parametrically, and the ideal valve port structure was determined. Tests were conducted to compare cavitation in the water hydraulics relief valve with and without the anti-cavitation structures. Results of these tests showed evident enhancement of cavitation performance.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(5):051602-051602-8. doi:10.1115/1.4041060.

A new integrity pressure relief device in a nonrefillable steel gas cylinder is proposed and tested. Instead of a rupture disk welded on the opening of the head, the new integrity pressure relief device is machined by stamping a circular groove on the vessel head, which not only avoids an additional penetration on the head but also reduces the manufacture cost. To ensure the safety and reliability of the device, its performance is evaluated using a reliability method based on material properties and burst pressure. The effect of stamping pressure on the groove depth is investigated, and then, the material properties taken from different locations are tested. Tensile properties taken along the circumferential direction of the cylinder are suggested to be used to predict burst pressure of the new integrity pressure relief device. The tolerance of the burst pressure in a percentage is analyzed, and a probabilistic model is built. The reliability analysis shows that the batch of cylinders with the integrity pressure relief device has a very high qualified probability.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(5):051603-051603-8. doi:10.1115/1.4040361.

A direct spring loaded pressure relief valve (DSLPRV) is an efficient hydraulic structure used to control a potential water hammer in pipeline systems. The optimization of a DSLPRV was explored to consider the instability issue of a valve disk and the surge control for a pipeline system. A surge analysis scheme, named the method of characteristics, was implemented into a multiple-objective genetic algorithm to determine the adjustable factors in the operation of the DSLPRV. The forward transient analysis and multi-objective optimization of adjustable factors, such as the spring constant, degree of precompression, and disk mass, showed substantial relaxation in the surge pressure and oscillation of valve disk in a hypothetical pipeline system. The results of the regression analysis of surge were compared with the optimization results to demonstrate the potential of the developed method to substantially reduce computational costs.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(5):051604-051604-9. doi:10.1115/1.4041056.

The United States Department of Energy's Savannah River Site (SRS) in Aiken, South Carolina, is dedicated to promoting site-level, risk-based inspection practices to maintain a safe and productive work environment. Protective suits are worn by personnel working in contaminated environments. These suits require that cooling be applied to keep the interior temperature within safe and comfortable limits. A vortex tube, also known as the Ranque-Hilsch vortex tube (RHVT), can provide the necessary cooling. As mechanical devices void of moving components, vortex tubes separate a compressed gas into hot and cold streams—the air emerging from the “hot” end reaching a temperature of 433.2 K and the air emerging from the “cold” end reaching a temperature of 241.5 K (Hilsch, 1946, “Die Expansion Von Gasen Im Zentrifugalfeld Als Kälteprozeß,” Z. Für Naturforsch., 1, pp. 208–214). Routing the cold stream of the vortex tube to the user's protective suit facilitates the required cooling. Vortex tubes currently in use at SRS are preset, through modification solely by and within the SRS respiratory equipment facility (REF), to provide a temperature reduction between 22.2 and 25.0 K. When a new model of vortex tube capable of user adjustment during operation recently became available, prototype testing was conducted for product comparison. Ultimately, it was identified that similar cooling performance between the old and new models is achievable. Production units were acquired to be subjected to complete product analysis at SRS utilizing a statistical test plan. The statistical test plan, data, thermodynamic calculations, and conclusions were reviewed to determine acceptability for site use.

Commentary by Dr. Valentin Fuster

Research Papers: Pipeline Systems

J. Pressure Vessel Technol. 2018;140(5):051701-051701-12. doi:10.1115/1.4040994.

Composite pipes are currently being used in a multitude of applications varying from civil to oil and gas applications. Pipes are generally connected together by means of pipe elbows that in turn are subjected to bending moment and pressure loading. This study looks into the effect of combined loading on the first ply and ultimate failure load of pipe elbows. The influence of pressure loading followed by a bending moment versus first applying bending moment followed by subsequent pressure loading, on the ultimate catastrophic failure load, is investigated through numerical models. The combined bending moment and pressure load ramping is also studied. Design by analysis finite element damage mechanics numerical methods are applied to investigate post first ply failure (FPF) and stress redistribution. The study shows that different loading combinations can give rise to different damage mechanisms and ultimately failure loads. A safe design load envelope for different fiber-reinforced pipe elbows based on FPF and ultimate catastrophic load is identified and discussed.

Topics: Pressure , Stress , Pipes , Failure
Commentary by Dr. Valentin Fuster

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