A Non-Cyclic Method for Plastic Shakedown Analysis

Shakedown is a cyclic phenomenon, and for its analysis it seems natural to employ a cyclic analysis method. Two problems are associated when this direct approach is used in finite element analysis. Firstly, the analysis typically needs to be stabilized over several cycles, and the analysis of each individual cycle may need a considerable amount of computing time. Secondly, even in cases where a stable cycle is known to exist, the finite element analysis can show a small continuing amount of strain accumulation. For elastic shakedown, non-cyclic analysis methods that use Melan’s theorem have been proposed. The present paper extends non-cyclic lower bound methods to the analysis of plastic shakedown. The proposed method is demonstrated with several example problems.

Methods, Models, and Assumptions Used in Finite Element Simulation of a Pilger Metal Forming Process

The pilger process is a cold-worked mechanical process that combines the elements of extrusion, rolling, and upsetting for the formation of thin-walled tubes. This complex manufacturing process relies on the results of trial and error testing programs, experimental parameter sensitivity studies, and prototypical applications to advance the technology. This finite element modelling effort describes the methods, models, and assumptions used to assess the process parameters used to manufacture thin-walled tubing. The modelling technique breaks down the manufacturing process into smaller computer generated models representing fundamental process functions. Each of these models is linked with the overall process simulation. Simplified assumptions are identified and supporting justifications provided. This work represents proof of principle modelling techniques, using large deformation, large strain, finite element software. These modelling techniques can be extended to more extensive parameter studies evaluating the effects of pilger process parameter changes on final tube stress and strain states and their relationship to defect formation/propagation. Sensitivity studies on input variables and the process parameters associated with one pass of the pilger process are also included. The modelling techniques have been extended to parameter studies evaluating the effects of pilger process parameter changes on tube stress and strain states and their relationship to defect formation. Eventually a complex qualified 3-D model will provide more accurate results for process evaluation purposes. However, the trends and results reported are judged adequate for examining process trends and parameter variability.

Enhancing Probabilistic Risk Assessment Methodology for Solving Reactor Safety Issues

This paper reveals a paradigm of analyzing the consequential effects of severe nuclear reactor accident, radionuclides fraction and source terms release, that will influence the MACCS2 codification [1], by coupling with the results of SAPHIA-PSA Levels l & 2 quantification process [2], MELCORE [3], STCP [4], PST [5], and XSOR [6]. Those codes are mutually exclusive and useful. However, it lacks of the closed interface and linkage for addressing Plant Damage States (PDS), Severe Accident Sequences, and Risk Consequence. Thus, it is imperative to formulate the consistent baseline information for MACCS2, PSA Levels 1, 2 and 3, and then linking to a new algorithm of NCM.

A New Graphical Tool for Building Logic-Gate Trees

Tree structures that use logic gates to model system behavior have proven very useful in safety and reliability studies. In particular process trees are the basic structure used in a decision analysis methodology developed at Los Alamos called Logic Evolved Decision modeling (LED). LED TOOLS is the initial attempt to provide LED-based decision analysis tools in a state of the art software package. The initial release of the software, Version 2.0, addresses the first step in LED — determination of the possibilities. LED TOOLS is an object-oriented application written in Visual Basic for Windows NT based operating systems. It provides an innovative graphical user interface that was designed to emphasize the visual characteristics of logic trees and to make their development efficient and accessible to the subject matter experts who possess the detailed knowledge incorporated in the process trees. This eliminates the need for the current interface between subject matter experts and logic modeling experts. This paper provides an introduction to LED TOOLS. We begin with a description of the programming environment. The construction of a process tree is described and the simplicity and efficiency of the approach incorporated in the software is discussed. We consider the nature of the logical equations that the tree represents and show how solution of the equations yield natural language “paths.” Finally we discuss the planned improvements to the software.

Accelerated Limit Loads Using Repeated Elastic Finite Element Analyses

This paper demonstrates the limitations of repeated elastic finite element analyses (REFEA) based limit load determination that uses the classical lower bound theorem. The r-node method is prescribed as an alternative for obtaining better limit load estimates. Lower bound aspects pertaining to r-nodes are also discussed.

Rigid-Plastic Meshfree Method for Metal Forming Simulation

In this paper, material processing simulation is carried out using a meshfree method. With the use of a meshfree method, the domain of the workpiece is discretized by a set of particles without using a structured mesh to avoid mesh distortion difficulties which occurred during the course of large plastic deformation. The proposed meshfree method is formulated for rigid-plastic material. This approach uses the flow formulation based on the assumption that elastic effects are insignificant in the metal forming operation. In the rigid-plastic analysis, the main variable of the problem becomes flow velocity rather than displacement. A numerical example is solved to validate the proposed method.

Safety Analysis and Risk Assessment for Pressure Systems

There have been many instances where serious injuries and fatalities have resulted from over-pressurization, thermal stress, asphyxiation and other potential hazards associated with testing, handling and storage of compressed gases and pressure facilities at numerous production and research facilities. These hazards are major issues that should be addressed in system design and in materials selection appropriate for high pressure applications. Potential hazards may be mitigated through system analysis and design process which are the major factors in preventing thermal/pressure hazards caused by possible leaks and fragmentation, in the case of rupture. This paper presents a conceptual model and framework for developing a safety analysis which will reduce potential hazards, accidents and legal liabilities. The proposed systematic approach allows to identify hazards provide timely documentation of potential hazards and risks associated with systems, facilities, and equipment. As a result of this hazard analysis process, provisions and actions for hazard prevention and control have been put in place, and all identifiable potential hazards can be reduced to a low risk level.

Risk Assessment for Damaged Pressure Tank Cars

Following an accident or derailment involving a freight train, emergency response, salvage, and repair personnel need to clear the right-of-way, repair the tracks, and remove the damaged rail cars. If the train contains pressure tank cars the potential hazard and safe handling procedures need to be determined prior to salvage and repair activities. The severity of the damage determines the appropriate course of action such as rerailing or unloading the damaged cars. The operations must avoid the risk to response personnel of a delayed rupture. A research program was performed to establish the validity of the existing industry guidelines for assessment of damaged pressure tank cars. The research program first focused on evaluating the technical foundation for the existing guidelines and the degree to which they have been validated. We then designed a program of experiments and analyses to validate the guidelines and estimate their margins of safety. The experimental effort used laboratory specimens to provide material property data as well as validation data for the analyses. The analyses used three different approaches: nonlinear elasto-plastic finite element simulations for modeling denting behavior, elasto-plastic fracture mechanics for analysis of cracks, and nonlinear finite element simulations combined with local fracture theories to quantify the severity of scores, gouges, and rail bums (longitudinal damage features caused by sliding a tank along a rail). This paper emphasizes the latter aspect of the work.

Generalized Application of Periodic Symmetry in Molecular Simulations

Periodic symmetry is widely used in molecular simulations to mimic the presence of an infinite bulk surrounding an N-atom model system. However, the traditional methods of applying periodic symmetry end up enforcing over-restrictive kinematic constraints between the periodic boundaries. After a brief overview of the periodic symmetry, the nature of the constraint is discussed briefly in this paper. Thereafter, the objective is to provide a means to ensure that periodicity is upheld while avoiding unnecessary constraint of the repeating cell boundaries. This paper demonstrates the usual application of periodic symmetry into a molecular simulation algorithm through a typical example. Meanwhile, a novel method is introduced, which uses equivalent external forces applied to physical boundary atoms. Comparisons between the classic treatment and the new method using one-dimensional and two-dimensional models are made. Moreover, the potential application of the new method in regular Finite Element Analysis is discussed.

Failure Mode of Pipes Containing Circumferential Cracks Under Bending and Its Consequence on the Application of NSCM Criterion

The results of some 60 tests performed in Italy on 2”, 4”, 6” and 8” pipes of A 106 B and 304 stainless steel, carrying circumferential through-wall cracks of various size under four point bending conditions (FPB), at room temperature and 300° C, have been analyzed using the Net Section Collapse Moment Criterion (NSCM) and the dimensionless plastic zone parameter (DPZP). Most of the test results have shown that the NSCM applies even though the DPZP is lower than unity. This apparent inconsistency is due to the fact that cracked pipes under bending fail by plastic hinge formation of the type occurring in FPB specimens carrying notches, like the Charpy VN ones, as predicted by the slip line theory. Under these conditions, two half circle plastic zones develop at both sides of the notch while the plastic zone straight ahead the notch tip is almost negligible. FE Calculations have confirmed this behavior: the plastic zone underneath the crack tip has not yet reached the neutral axis when the plastic hinge is formed on the sides of the piping, making the NSCM applicable. This, actually, implies that the DPZP as presently used in the screening criteria is not precisely the proper parameter to adopt in the assessment of the NSCM criterion applicability.