Evaluation of Sealing Behavior of Gaskets Based on the Test Method HPIS Z104 Proposed in Japan

This paper discusses the gasket testing procedure HPIS Z104 to obtain fundamental sealing behavior of gasket established in Japan. The testing procedure consists of 11 combinations of gasket stresses and a constant internal pressure. It takes about 3 hours to complete one test, which is acceptable for gasket manufacturers. In order to demonstrate the validity of the testing procedure, measurements of leak rates of compressed fiber sheet gaskets were carried out. It has shown that the fundamental sealing behavior can be well characterized using the proposed testing procedure with reasonable time and cost.

Effect of Tightening Speed on the Torque-Tension and Wear Pattern in Bolted Connections

This experimental study investigates the effect of tightening speed and coating on both the torque – tension relationship and wear pattern in threaded fastener applications. The fastener torque – tension relationship is highly sensitive to normal variations in the coefficients of friction between threads and between the turning head and the surface of the joint. Hence, the initial level of the joint clamp load and the overall integrity and reliability of a bolted assembly is significantly influenced by the friction coefficients. The effect of repeated tightening and loosening is also investigated using M12, Class 8.8, fasteners with and without zinc coating. The torque – tension relationship is examined in terms of the non-dimensional nut factor K. The wear pattern is examined by monitoring the changes in surface roughness using a WYKO optical profiler and by using a LECO optical microscope. A Hitachi S-3200N Scanning Electron Microscope (SEM) is used to examine the contact surfaces, under the fastener head, after each tightening/loosening cycle. Experimental data on the effect of variables and the tightening speed, fastener coating and repeated tightening on the nut factor are presented and analyzed for M8 and M12, class 8.8, fasteners.

An Improved Meshfree Method for Fracture Analysis of Cracks

This paper presents an efficient meshless method for analyzing linear-elastic cracked structures subject to single- or mixed-mode loading conditions. The method involves an element-free Galerkin formulation in conjunction with an exact implementation of essential boundary conditions and a new weight function. The proposed method eliminates the shortcomings of Lagrange multipliers typically used in element-free Galerkin formulations. Numerical examples show that the proposed method yields accurate estimates of stress-intensity factors and near-tip stress field in two-dimensional cracked structures. Since the method is meshless and no element connectivity data are needed, the burdensome remeshing required by finite element method (FEM) is avoided. By sidestepping remeshing requirement, crack-propagation analysis can be dramatically simplified. An example problem on mixed-mode condition is presented to simulate crack propagation. The agreement between the predicted crack trajectories by the proposed meshless method and FEM is excellent. In recent years, a class of Galerkin-based meshfree or meshless methods have been developed that do not require a structured mesh to discretize the problem, such as the element-free Galerkin method, and the reproducing kernel particle method. These methods employ a moving least-squares approximation method that allows resultant shape functions to be constructed entirely in terms of arbitrarily placed nodes. Meshless discretization presents significant advantages for modeling fracture propagation. Since no element connectivity data are needed, the burdensome remeshing required by the finite element method (FEM) is avoided. A growing crack can be modeled by simply extending the free surfaces, which correspond to the crack. Although meshless methods are attractive for simulating crack propagation, because of the versatility, the computational cost of a meshless method typically exceeds the cost of a regular FEM. Also in some cases, the MLS which is the bases of the meshless method may form an ill-conditioned system of equations so that the solution cannot be correctly obtained. Hence, in this paper, we propose an improved element-free Galerkin method based on an improved moving least-square approximation (IMLS) method. In the IMLS method, the orthogonal function system with a weight function is used as the basis function. The IMLS has higher computational efficiency and precision than the MLS, and will not lead to an ill-conditioned system of equations. Numerical examples are presented to illustrate the computational efficiency and accuracy of the proposed improved element-free Galerkin method.

Fracture Behavior of Finite Length Part Through Wall Flaws in Zirconium-Niobium Pressure Tubes

Flaws encountered in nuclear pressure tubes must be evaluated to ensure that a delayed hydride cracking (DHC) mechanism is not initiated where the stress concentration at a flaw tip causes diffusion of hydrogen and precipitation of zirconium hydride at the flaw tip. A fracture initiation model for DHC involves a process zone description for the interaction of hydride precipitation with the flaw tip stress distribution. Analytical techniques for this model are practical and accurate for two-dimensional geometry, but cannot be easily applied to the three-dimensional features of finite length surface flaws. Recently, a numerical rendition of the model has been incorporated into a finite element program so that arbitrary geometry and material properties can be managed. The three-dimensional finite length model is applied to specific flaw geometries used in an experimental program. Comparison with corresponding two-dimensional tests demonstrates that the finite length flaw has a significantly higher threshold load than that predicted on the basis of a two-dimensional model.

A Fast Method for Ratchetting Strain Prediction

A time efficient method for predicting ratchetting strain is proposed. By finding the ratchetting rate, at only a few cycles, the ratchetting strain of any cycle can be determined. It is shown that a trajectory of the origin of stress may be defined in the deviatoric stress space as the ratchetting progresses. The method for obtaining this trajectory from a standard uniaxial asymmetric cyclic loading is presented. At the beginning, this trajectory coincides with the initial stress origin and approaches the mean stress, displaying a power law relationship with the number of loading cycles. This path defines a moving frame of reference for stress tensor calculations. Ratchetting rates for different cyclic loading are calculated with the knowledge of this frame of reference and through utilizing a constitutive cyclic plasticity model which incorporates deviatoric stresses and back stresses that are measured with respect to this moving frame. The proposed model is used to predict ratchetting strain of 1070 steel under single step constant amplitude and multi-step loading. The method is also applied to non-proportional loading. Results obtained agree with the available experimental measurements.

Calandria Tube to Tubesheet Roller-Expanded Joint Qualification

The complex calandria tube to calandria tubesheet roller-expanded joint in CANDU nuclear reactors is usually qualified by test. In this paper, a state-of-the-art numerical simulation is undertaken in order to improve the understanding of the behaviour of the joint to support design modifications and provide assurance that the test rig envelopes behaviour of the in-situ reactor assembly. Parameters such as hoop stress, and plastic deformation of the assembly are predicted. The analysis results are also compared with the available test data and in-situ experimental data. The analysis results show that the test performed to qualify the joint using a small plate and single joint is representative of the in-situ reactor configuration.

Large Deformation Analysis by Smoothed Particle Hydrodynamics

Smoothed particle hydrodynamics (SPH)[1] is extended to the elastic-plastic large deformation analysis of metals and the hyper-elastic analysis of rubbers. The elastic-plastic analysis theory and the large deformation theory used in this study are fundamentally similar to those of FEM however the theories are applied at the particle points within a smoothing radius in SPH models. In this study the volume constant condition is imposed on the plastic deformation process using a pressure equation given by the particle density condition in a unit volume. Test problems show that the large deformation analysis by SPH leads to good stability and accuracy comparing with FEM results.

Novel Formulation of the Tightening and Breakaway Torque Components in Threaded Fasteners

New formulas are developed for the torque-tension relationship, various torque components, and for the break-away torque values in threaded fastener applications. The 3-D aspects of the lead helix and thread profile angles, the kinetic and static friction coefficients are all taken into account. Two scenarios of the contact pressure between threads and under the turning fastener head are considered; namely, uniform distributed and linearly distributed contact pressure scenarios. The effect of thread pitch, lead helix and thread profile angles, friction coefficients, and the fastener geometry is discussed. Results from the new formulas are compared with the approximate torque-tension relationship provided in the literature. A percent difference analysis indicates that the new formulas provide significant improvement that would enhance the reliability and safety of bolted connections, especially in critical applications.

Detailed Simulation of an Overlay Repair on a 14” Dissimilar Material Weld

In nuclear reactors, ferritic low alloy steel heavy section components are connected with austenitic stainless steel piping systems. Despite a special manufacturing procedure to ensure a good resistance of the joint, several experiences from the field confirm sensitivity to fatigue and corrosion in this type of junction. Overlay welding is a process widely used to mitigate dissimilar material weld (DMW) stress corrosion cracking by replacing inside tensile stresses by compressive stresses. Taking into account the costs generated by mock-up manufacturing, predictive Finite Element (FE) residual stress calculations are of great interest to prove the effectiveness of the overlay welding and to adjust the parameters of the process and particularly the overlay thickness. This paper presents residual stress computations performed by Framatome-ANP on a 14” pipe geometry, resembling many mid size DMW in the US. Considering 2D axisymmetric hypotheses, the analysis simulates each elementary step of the mock-up manufacturing procedure. In particular, the pass-by-pass welding simulation reproduces the deposit of each bead by thermo-metallurgical and mechanical calculations. Thanks to residual stress measurements carried out on 2 mock-ups (with/without overlay), the numerical approach has been validated and highlights the beneficial overlay effect. However, some discrepancies raise various problems: the backing ring modelling, the machining heating effect, the experimental scatter and the weld material hardening. The simulation being able to analyze the influence of an overlay layer going up to 1 time the original pipe thickness, further work on the stabilization of the residual stress fields obtained here after the deposit of 4 or 5 layers, may lead to a better adjustment of the overlay thickness and to a cut in the operation costs too.

Study of a Major Mechanism for Self-Loosening of Bolted Joints

A recent three-dimensional (3-D) finite element (FE) investigation on self-loosening of bolted joints revealed that two major mechanisms were responsible for the second-stage (nut rotation) self-loosening of bolted joints. One of the mechanisms is the slip-stick contact of the thread surfaces under the combined contact pressure and reversed bending moment exerted from the reversed transverse loading. The current investigation is a detailed study of the slip-stick contact of the thread surfaces with models that mimic the bolt loading condition. The contact pressure and the reversed bending moment are obtained from an earlier simulation for an M12 bolt. The FE simulations indicate that, with the contact pressure on the thread surface of the bolt and nut, the alternating bending moment results in the gradual motion between the contact thread surfaces. A detailed look at the contact surfaces reveals that localized slip along the tangential direction occurs in part of the contact area and the accumulation of this local slip is responsible for the gradual relative motion between bolt and nut. The FE simulations also indicate that the amplitude of the bending moment greatly influences the relative displacement between the bolt and the nut. There exists a threshold below which local slip will not occur. Results from a two-dimensional (2-D) model are discussed and compared with those obtained from a 3-D model.