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Research Papers: Design and Analysis

J. Pressure Vessel Technol. 2018;140(3):031201-031201-11. doi:10.1115/1.4039125.

The stress intensity factor (SIF) solutions for subsurface flaws near the free surfaces of components, which are known to be important in engineering applications, have not been provided yet. Thus, in this paper, SIF solutions for subsurface flaws near the free surfaces in flat plates were numerically investigated based on the finite element analyses. The flaws with aspect ratios a/ℓ = 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5, the normalized ratios a/d = 0.0, 0.1, 0.2, 0.4, 0.6, and 0.8, and d/t = 0.01 and 0.10 were taken into account, where a is the half flaw depth, ℓ is the flaw length, d is the distance from the center of the subsurface flaw to the nearest free surface, and t is the wall thickness. Fourth-order polynomial stress distribution in the thickness direction was considered. In addition, the developed SIF solutions were incorporated into a Japanese probabilistic fracture mechanics (PFM) code, and PFM analyses were performed for a Japanese reactor pressure vessel (RPV) containing a subsurface flaw near the inner surface. The PFM analysis results indicate that the obtained SIF solutions are effective in engineering applications.

Commentary by Dr. Valentin Fuster

Research Papers: Fluid-Structure Interaction

J. Pressure Vessel Technol. 2018;140(3):031301-031301-8. doi:10.1115/1.4038725.

Flow-induced vibrations of tubes in two-phase heat exchangers are a concern for the nuclear industry. Electricité de France (EDF) has developed a numerical tool, which allows one to evaluate safety margins and thereafter to optimize the exchanger maintenance policy. The software is based on a semi-analytical model of fluid-dynamic forces and dimensionless fluid force coefficients which need to be evaluated by experiment. A test rig was operated with the aim of assessing parallel triangular tube arrangement submitted to a two-phase vertical cross-flow: a kernel of nine flexible tubes is set in the middle of a rigid bundle. These tubes vibrate as solid bodies (in translation) both in the lift and drag directions in order to represent the so-called in-plane and out-of-plane vibrations. This paper outlines the experimental results and some detailed physical analysis of some selected points of the experiment series: the response modes are identified by means of operational modal analysis (OMA) (i.e., under unmeasured flow excitation) and presented in terms of frequency, damping, and mode shapes. Among all the modes theoretically possible in the bundle, it was found that some of them have a higher response depending on the flow velocity and the void fraction. Mode shapes allow to argue if lock-in is present and to clarify the role of lift and drag forces close to the fluid-elastic instability (FEI).

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(3):031303-031303-7. doi:10.1115/1.4039696.

Liquid slug flow driven by pressurized air in an inclined pipe with a downstream elbow is investigated numerically. As the liquid slug hits the elbow, the impact pressure and the associated force generated at the elbow may damage pipe supports as well as the pipe itself. It is essential for the design engineers of pipeline systems to accurately predict the pressure trace during the impact for safe operation. The slug arrival velocity and slug length (i.e., mass) at the elbow directly affect that pressure. In order to calculate these slug parameters just before the impact, an improved one-dimensional (1D) model proposed in the literature is used. At the elbow, pressure variation with respect to time is calculated by a recently developed computer code which uses a two-dimensional (2D) smoothed particle hydrodynamics (SPH) method. In the numerical setup, two representative initial slug lengths, one for short slugs and one for long slugs, and three different initial air tank pressures are used. The obtained numerical data are validated with available experimental results. For both short and long slugs, calculated peak pressures show great agreement with measured peak pressures.

Commentary by Dr. Valentin Fuster

Research Papers: Materials and Fabrication

J. Pressure Vessel Technol. 2018;140(3):031401-031401-7. doi:10.1115/1.4039346.

In structural integrity analysis of reactor pressure vessels (RPVs), a postulated shallow crack is subjected to biaxial far-field stresses. However, the fracture toughness Kc or Jc, which is an important material property for the structural integrity assessment of RPVs, is usually obtained from testing deeply cracked compact tension (C(T)) or single-edged bending (SE(B)) specimens under uniaxial loading. Thus, the fracture toughness data do not reflect the biaxial loading state that cracks in a RPV are subjected to. Cruciform bending specimen was therefore developed to simulate the biaxial stress state. In this paper, a series of finite element (FE) simulations of the cruciform specimens containing different crack geometries and of different material properties are conducted. The crack tip stress fields are analyzed, and the constraint is investigated using the J–A2 theory. The results show that the biaxial effect is material property dependent which could be useful for the optimization of the test method and the better design of cruciform specimens. The trends about the biaxial loading effect revealed in this study would also be helpful in estimating the safe operating life of RPVs.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(3):031402-031402-9. doi:10.1115/1.4039392.
FREE TO VIEW

Ductile failure in steels is highly controlled by the stress state, characterized by the stress triaxiality (T) and the Lode parameter (L). The ASME Boiler and Pressure Vessel Code requires pressure vessels to be designed to resist local ductile failure. However, the standard does not account for the Lode parameter dependence in its failure locus. In this study, the influence of the stress state, characterized T and L, on the ductility of ASME tubular product steel grades is investigated. Two seamless pipes of midstrength carbon steel SA-106 Gr. B and high-strength superduplex steel SA-790 were considered. Ring specimen geometries for plane strain (PS) stress state (L = 0) and tensile stress (TS) state (L = −1) are utilized to establish the ductile failure locus in terms of T and L for the two steels. The experimental results (EXP) show that the effect of the Lode parameter on the failure locus for the SA-106 Gr. B steel is insignificant, whereas for the SA-790 steel, the effect is rather significant. A parameter SL is introduced in order to quantify the sensitivity of the failure locus to the Lode parameter. It is found that for materials with ultimate strength lower than about 550 MPa, the sensitivity to L is insignificant (SL ≈ 1), whereas for materials with ultimate strength higher than 550 MPa, the sensitivity to L could be significant (SL > 1). Scanning electron microscopic (SEM) analysis of the fracture surfaces revealed that the sensitivity to L is closely associated with the rupture micromechanisms involved.

Topics: Steel , Stress , Failure
Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(3):031403-031403-7. doi:10.1115/1.4039345.

This work investigates the behavior of 316 stainless steel (SS) under stress-controlled low cycle fatigue loading. Several fatigue experiments are conducted under different environment such as in air at 300 °C and primary loop water conditions for a pressurized water reactor (PWR). Two different loading conditions are also employed to examine the effect of stress rate on material hardening and ratcheting. During PWR water test, actuator position measurements are used to determine the strain of the specimen. Under PWR environment, 316 SS is found to ratchet to a significantly greater degree compared with in air. At slow stress rate, higher amount of cyclic hardening is observed in 316 SS, and slow stress rate increases the rate of ratcheting. Results also indicate that 316 SS exhibits asymptotic strain response at higher stress loading which can cause material to behave very differently under same stress cyclic loading.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(3):031405-031405-13. doi:10.1115/1.4039779.

The aim of this paper is to investigate different factors, including dwell time, strain range, and strain ratio on creep-fatigue endurances in nickel-based Inconel 718 and GH4169 superalloys. We also summarize classic approaches for life assessments based on the generalizations of Coffin–Manson equation, linear damage summation (LDS), and strain-range partitioning (SRP) method. Each approach does have some degree of success in dealing with a specific set of creep–fatigue data. In order to evaluate the prediction capabilities of the validated approaches, a Bayesian information criterion (BIC) allowing for maximum likelihood and principle of parsimony is used to select the best performing model.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(3):031406-031406-12. doi:10.1115/1.4039699.

Polyethylene (PE) pipe is widely used for oil and gas transportation. Slow crack growth (SCG) is the main failure mechanism of PE pipes. Current SCG resistance testing methods for PE pipes have significant drawbacks, including high cost, time-consuming, and uncertain reliability. Alternative method is in need to reduce the testing time and cost. In this paper, a numerical model is proposed by taking the viscoelastic and damage effect of PE material into account. The material behavior is described on the basis of linear viscoelastic integral constitutive model, along with the damage effect in effective configuration concept. A three-dimensional (3D) incremental form of a viscoelastic and damage model is derived and implemented by abaqus UMAT. It is found that the curve of tensile displacement versus time, as well as the curve of crack opening displacement (COD) versus time from numerical results fit well with those from the standard Pennsylvania Notch Test (PENT; ASTM 1473). Based on the proposed model, SCG failure process is analyzed, and the effects of damage parameters on SCG process are furtherly studied and discussed.

Commentary by Dr. Valentin Fuster

Research Papers: Pipeline Systems

J. Pressure Vessel Technol. 2018;140(3):031701-031701-8. doi:10.1115/1.4039070.

Underground gas storage (UGS), a key component of a natural gas pipeline network, can not only be used as an emergency gas source under a pipeline system failure situation but it is also available for seasonal peak shaving under pipeline system normal operation. Therefore, in order to meet the natural gas needs, it is of vital importance to safeguard the security of UGS operation and assess the reliability of UGS. The aim of the overall study is to develop an integration method for assessing operational reliability of UGS in a depleted reservoir under different injection-production scenarios, whereas existing studies only assess a single component or subsystem reliability. According to function zoning, UGS is separated into reservoir, well system, and surface system, and reservoir and surface system are connected through well system. The well system contains multiple injection/production wells. For the first step of the reliability assessment, the hydraulic calculation, including the gas injection process calculation and the gas production process calculation, is adopted to obtain the operational parameters of each component in UGS. Next, the reliability of the reservoir, injection/production well, and equipment in surface system is evaluated using operational parameters and a Monte Carlo approach. The reliability of the subsystem, such as the well system and surface system, is then calculated according to system reliability theory. Finally, operational reliability of UGS is obtained, which reflects the capacity of performing gas injection-production function. Two test cases are given to illustrate the integration method.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(3):031702-031702-10. doi:10.1115/1.4039780.

Fatigue cracking is a key type of defect for liquid pipelines, and managing fatigue cracks has been a top priority and a big challenge for liquid pipeline operators. The existing inline inspection (ILI) tools for pipeline defect evaluation have large fatigue crack measurement uncertainties. Furthermore, the current physics-based methods are mainly used for fatigue crack growth prediction, where the same or a small range of fixed model parameters is used for all pipes. They result in uncertainty that is managed through the use of conservative safety factors such as adding depth uncertainty to the measured depth in deciding integrity management and risk mitigation strategies. In this study, an integrated approach is proposed for pipeline fatigue crack growth prediction utilizing ILI data including consideration of crack depth measurement uncertainty. This approach is done by integrating the physical models, including the stress analysis models, the crack growth model governed by the Paris’ law, and the ILI data. With the proposed integrated approach, the finite element (FE) model of a cracked pipe is built and the stress analysis is performed. ILI data are utilized to update the uncertain physical parameters for the individual pipe being considered so that a more accurate fatigue crack growth prediction can be achieved. Time-varying loading conditions are considered in the proposed integrated method by using rainflow counting method. The proposed integrated prognostics approach is compared with the existing physics-based method using examples based on simulated data. Field data provided by a Canadian pipeline operator are also employed for the validation of the proposed method. The examples and case studies in this paper demonstrate the limitations of the existing physics-based method, and the promise of the proposed method for achieving accurate fatigue crack growth prediction as continuous improvement of ILI technologies further reduces ILI measurement uncertainty.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Pressure Vessel Technol. 2018;140(3):034501-034501-5. doi:10.1115/1.4039207.

Fatigue crack growth thresholds ΔKth define stress intensity factor range below which cracks will not grow. The thresholds ΔKth are useful in industries to determine durability lifetime. Although massive fatigue crack growth experiments for stainless steels in air environment had been reported, the thresholds ΔKth are not codified at the American Society of Mechanical Engineers (ASME) Code Section XI, as well as other fitness-for-service (FFS) codes and standards. Based on the investigation of a few FFS codes and review of literature survey of experimental data, the thresholds ΔKth exposed to air environment have been developed for the ASME Code Section XI. A guidance of the thresholds ΔKth for austenitic stainless steels in air at room and high temperatures can be developed as a function of stress ratio R.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2018;140(3):034502-034502-5. doi:10.1115/1.4039391.

Flow-induced vibration analysis of the San Onofre Nuclear Generating Station (SONGS) replacement steam generators (RSG) is made using nonproprietary public data for these steam generators on the Nuclear Regulatory Commission public web site (www.NRC.gov). The analysis uses the methodology of Appendix N Section III of the ASME Boiler and Pressure Vessel Code, Subarticle N-1300 Flow-Induced Vibration of Tubes and Tube Banks. First, the tube geometry is assembled, and overall flow and performance parameters are developed at 100% design flow; then, the analysis is made to determine the flow velocity in the gap between tubes and tube natural frequencies and mode shapes. Finally, the mass damping and reduced velocity for tubes on the U bend are assembled and plotted on the ASME code Figure N-11331-4 fluid elastic stability diagram.

Commentary by Dr. Valentin Fuster

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