Benchmarking PRAISE-CANDU 1.0 With xLPR 1.0

A probabilistic fracture mechanics code, PRAISE-CANDU 1.0, has been developed under a software quality assurance program in full compliance with CSA N286.7-99, and was initially released in 2012 June. Extensive verification and validation has been performed on PRAISE-CANDU 1.0 for the purpose of software quality assurance. This paper presents the benchmarking performed between PRAISE-CANDU 1.0 and xLPR (eXtremely Low Probability of Rupture) version 1.0 using the cases from the xLPR pilot study. The xLPR code was developed in a configuration management and quality assured manner. Both codes adopted a state-of-art code architecture for the treatment of the uncertainties. Inputs to the PRAISE-CANDU were established as close as possible to those used in corresponding xLPR cases. Excellent agreement has been observed among the results obtained from the two PFM codes in spite of some differences between the codes. This benchmarking is considered to be an important element of the validation of PRAISE-CANDU.

Three-Dimensional Finite Element Analysis of Rigid Flanged Shaft Coupling Subjected to Twisting Moment

A reamer bolt is commonly used when clamping a rigid shaft coupling subjected to large shear force. Although some joint design procedures assume that the applied shear force transmits only through the reamer surface, it is also supported by the friction force on the contact surfaces. Accordingly, to design the coupling clamped by reamer bolts, it is important to evaluate the ratio of the shear forces supported by the reamer surface and the friction force, which is defined as shear force transfer ratio (SFTR) here. In this study, distributions of SFTR and the bending stresses along the reamer surface are analyzed by three-dimensional FEM, focusing on the effects of the fit between the reamer bolt and bolt hole, the scatter of initial bolt stress and the misalignment of the connecting shafts. Numerical results quantitatively clarify how the amounts of the SFTR and the bending stresses as the friction coefficients, the fit and the magnitude of misalignment are changed. As for an offset misalignment, it is found that its effect on the bending moment generated in the shaft body is negligibly small, if the offset between two shafts in radial direction is less than 10mm which is 1% of the total shaft length.

Spiral Wound Gasket Compressibility and Pressure Ratings

The industry practice of using pressure rating nomenclature to describe spiral wound gasket compressibility can contribute to some level of confusion regarding the proper design and selection of these gaskets. This situation can result in a misconception that a more easily compressed “soft” gasket (for example, “0–999 psi” rating) cannot be used in a higher pressure application. This is not necessarily true, and in many cases a softer (less dense) gasket construction can actually be beneficial in both high and low pressure applications. This article addresses both the terminology used to describe spiral wound gasket compressibility and the design characteristics of these gaskets in an effort to improve the understanding of this subject.

Improved Gasket Testing

At present, the European Standards EN 1591-1 which is used for the calculation of bolted flanged joints and EN 13555 in which the determination of the required gasket characteristics are defined are under revision. Several studies on gasket testing and on flange calculation have been performed in order to verify the suggested modifications of the standardization working groups. As a result of these studies several optimizations in respect of the testing procedures, the definition of the gasket characteristics and the calculation procedure can be derived. In this paper, additional or modified gasket characteristics such as the stiffness-independent gasket deflection, caused by creep of the gasket material or formulas for the description of the gasket compression curves, are presented. Several tests with diverse gasket materials were performed in order to generate generally admitted procedures to determine these required characteristics which are the basis for the advanced calculation procedure which will be presented in a second paper.

Gasket Selection and Assembly Criteria for Internal Sealing Manways and Handholes

Internal sealing manways, hand-holes, drum doors, etc. present very unique sealing and assembly challenges. Unlike other conventional gasketed connections, the majority of the gasket compression forces are developed during operation, and not during assembly. This creates several critical issues that must be understood and overcome in the original gasket selection process and the assembly itself. Even if considered and addressed in the original gasket selection and assembly process, these internal sealing manways will likely still require a post start-up retorque. The timing of the re-torque is of critical importance in ensuring worker safety and continued equipment reliability and uptime.

A New Experimental Approach for Measuring Friction Coefficients of Threaded Fasteners Focusing on the Repetition of Tightening Operation and Surface Roughness

Torque method is widely used when tightening threaded fasteners. Although it has a great advantage of easy operation, the scatter of bolt preloads inevitably occurs due to the scatter of friction coefficients. Friction coefficients on the contact surfaces are affected by various factors such as joint materials, surface roughness, tightening speed, etc. To evaluate the effects of those factors with high accuracy, experimental errors must be suppressed as low as possible. In this study, a new simple test equipment for measuring the friction coefficients is designed, in which the strain gages are attached to the equipment to eliminate the experimental errors caused by the gages being attached to each test specimen. Using the equipment, friction coefficients on the thread surface and nut loaded surface are measured separately. Experimental results show that the surface roughness has a smaller effect when using threaded fasteners made of stainless steel than the case of carbon steel fasteners. As for the repetition of tightening operations, it is found that the removal of metal power, which is generated by the galling between the mating surfaces, is effective for reducing the scatter of friction coefficients.

An Innovative Precise Integration Method in Solving Structural Dynamic Problems

An innovative time integration method that incorporates spurious high-frequency dissipation capability into the so called “high precision direct integration algorithm” is presented, and its numerical stability and accuracy is discussed. The integration algorithm is named “high precision” to emphasize its numerical capability in reaching computer hardware precision. The proposed procedure employs the well-known state space approach to solve the simultaneous ordinary differential equations, the exact solution of which contains an exponential matrix to be efficiently computed using the truncated Taylor series expansion together with the power-of-two algorithm. The proposed method, belonging to the category of explicit methods, is found to provide better accuracy than many other existing time integration methods, and the integration scheme remains numerically stable over a wide range of frequencies of engineering interest. This paper is also devoted to study numerical accuracy of the Precise Integration Method in solving forced vibration problems, particularly near resonance conditions. The numerically obtained transfer functions are then compared with the analytical exact solution to detect spurious resonance. Finally, numerical examples are used to illustrate its high performance in numerical stability and accuracy. The proposed method carries the merit that can be directly applied to solve momentum equations of motion with exactly the same procedure.

Behavior of Flange Joints Under Combined Internal Pressure and Thermal Loading: The Case of Using Metal-to-Metal Contact Type Gaskets

In a bolted flange joint, metal-to-metal contact type gasket takes over only part of the bolt load to achieve seating stress, and the additional bolt load is transmitted to the metal-to-metal contact to compensate for the unloading effects due to internal and external loadings. Due to this advantage, flange joints with metal-to-metal contact type gaskets are gradually used in chemical industry, nuclear power industry, etc. A three-dimensional nonlinear finite element model is developed to highlight the complex behavior of the flange joint with metal-to-metal contact type gasket under combined internal pressure and thermal loading. Despite of the common perception that the gasket stress in the metal-to-metal contact type gasket stays constant, reduced gasket stress is concluded due to flange rotation and joint thermal expansion.

Mesh Convergence Studies for Thick Shell Elements Developed by the ASME Special Working Group on Computational Modeling

The ASME Special Working Group on Computational Modeling for Explicit Dynamics was founded in August 2008 for the purpose of creating a quantitative guidance document for the development of finite element models used to analyze energy-limited events using explicit dynamics software. This document will be referenced in the ASME Code Section III, Division 3 and the next revision of NRC Regulatory Guide 7.6 as a means by which the quality of a finite element model may be judged. One portion of the document will be devoted to a series of element convergence studies that can aid designers in establishing the mesh refinement requirements necessary to achieve accurate results for a variety of different element types in regions of high plastic strain. These convergence studies will also aid reviewers in evaluating the quality of a finite element model and the apparent accuracy of its results. In this paper, the authors present the results of a convergence study for an impulsively loaded propped cantilever beam constructed of LS-DYNA thick shell elements using both reduced and selectively reduced integration. A large load is applied to produce large deformations and large plastic strains in the beam. The deformation and plastic strain results are then compared to similar results obtained using thin shell elements and hexahedral elements for the beam mesh.

Determination of Transitional Size of Metal Clusters

We investigated the transitional size of metal clusters where the electronic effect and the size effect on the ground state structure become weaker. Identification of a transitional size cluster provides the means to efficiently determine the ground state structure of large clusters using density functional theory. Beyond the critical size of clusters, geometrical effects become important and the putative global minimum obtained from an empirical method can be used to determine the true ground state structure where the size effect on structures is less significant. We identified the lowest-energy structure using a first principles method in combination with the global search algorithm. We then used the similarity function to quantify structural difference and similarity between the global minimum obtained from an empirical method and the true ground state structure. Two structures become similar beyond a certain critical size. To investigate low-lying structures of metal clusters, we used a Monte Carlo simulated annealing method which employs the Aggregate-Volume-Bias Monte Carlo (AVBMC) algorithm. Incorporated in the Monte Carlo method is an Embedded Atom Method (EAM) potential developed by the authors.