This thesis presents a combination of numerical and experimental studies performed to assess the influence of the warm prestress effect on the cleavage fracture toughness of two ferritic pressure vessel steels. The aims of the research are to gain a detailed knowledge of the materials low temperature response under uniaxial and fracture conditions; to examine, using the finite element method, crack tip stress fields during warm prestress LUCF load cycles; and provide a clear and consistent method of classifying the warm prestress effect. An experimental programme investigated the room temperature and low temperature response of two candidate steels, A533B and BS1501. These steels were tested uniaxially under monotonic and cyclic conditions, and in the cracked condition in the as-received and warm prestressed conditions. Application of a three parameter statistical model to the experimental data showed that the distribution of data in the as received and warm prestressed conditions can be described accurately. The shift in the cleavage toughness distribution following warm prestressing was predicted by combining the statistical model with a validated analytical model of the warm prestress effect. Repeated proof loading was shown to increase cleavage toughness in A533B steel, providing the loading was load controlled. There were negligible effects of repeated proof loading on BS1501 steel. Some further enhancement of cleavage fracture toughness was observed when sub critical crack extension was introduced following warm prestressing, although the results were highly scattered. The finite element method was employed to simulate experimental fracture events. It was found from these simulations that fracture occurs following warm prestressing, when the reloaded crack tip stress distribution matches the as-received fracture crack tip stress distribution. The stress matching was observed to occur well into the elastic stress field ahead of the crack tip. This fracture criterion was employed to provide predictions of cleavage toughness following varying applied preload levels. The results were compared to experimental data sets and various analytical models. The Chell model of the warm prestress effect was observed to provide the best agreement with the finite element predictions. Crack tip blunting during the preload steps was found to have no influence on the predictions of cleavage fracture toughness. Differences in hardening response of the material was also shown to have little influence of the predictions of cleavage toughness. Simulations incorporating sub critical crack extension prior to reloading to fracture demonstrated that cleavage 'toughness can be enhanced further by limited crack extension. Large increments of crack growth were shown to reduce the warm prestress effect. The finite element predictions were validated against the appropriate analytical solution proposed by Chell and experimental results.

This investigation seeks to perform a comparative study between the combined effects of internal pressure and piping loads versus external pressure and piping loads on a pressure vessel. There are currently several well-known and widely-used procedures for predicting the stress situation and the structural stability of pressure vessels under internal pressure when external piping loads (due to thermal expansion, weight, pressure, etc.) are applied at the nozzles. This project familiarises one with several international pressure vessel design Codes and standards, including AS ME (American Society of Mechanical Engineers) pressure vessel code sections and WRC (Welding Research Council) bulletins. It has been found that many vessels are designed to operate under normal or steam-out conditions (in vacuum). The combined effect of the external atmospheric pressure and the piping loads at the nozzle could be catastrophic if not addressed properly - especially when the stability of the structure is a crucial consideration, i.e. when buckling is a concern. The above-mentioned codes and standards do not directly address procedures or provide acceptance criteria for external loads during vacuum conditions. The approach to the study was, firstly, to investigate the effects of internal pressure and piping loads at the shell-to-nozzle junction. Theoretical stresses were compared with Finite Element results generated using the software package MSC PATRAN. Finite Element Methods provide a more realistic approach to the design of pressure vessels as compared to theoretical methods. It was necessary to determine if the theoretical procedures currently used were adequate in predicting the structural situation of a pressure vessel. Secondly, the buckling effects of vessels subjected to external atmospheric pressure and piping loads were also investigated. Buckling of the shell-to-nozzle region was explored with the aid of Finite Element software. The results gained were used to develop appropriate procedures for the design of vessels under external atmospheric pressure and piping loads. The design is such that it indicates if buckling will occur at the shell-to-nozzle junction. These design procedures form the basis for future exploration in this regard. / Thesis (M.Sc.Eng)-University of Natal, Durban, 2003.

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A combined theoretical and experimental study of the criteria governing the failure analysis of pressure vessels with defects has been performed. The fields of fracture mechanics (linear elastic and elastic-plastic behaviour), failure governed by large scale plastic deformation and energy balance methods are critically reviewed. All three approaches are shown to have relevance in the complete failure analysis of a structure with defects. An experimental study of the failure mechanisms of precracked polycarbonate and maraging steel plates and model polycarbonate vessels is presented. Room temperature, static loading tests are performed on 85 mm wide, 5 mm thick compact specimens of polycarbonate (0.013 andle; ^aandfrasl;_W andle; 0.810). For comparison, 51 mm wide maraging steel compact specimens are monotonically loaded at room temperature. The influence of through-thickness constraint on the fracture toughness and slow crack growth characteristics of the steel is investigated using plates of varying thickness (3.1 mm-25.4 mm) and initial crack length (0.386 andle; ^aandfrasl;_W andle; 0.766 for 3.1 mm thick sheet); various ancillary studies (scanning electron microscopy, surface deformation studies) complement the results. The crack growth behaviour of longitudinal through (0.704 ≤ c / andradic;‾DT/2</span> ≤ 1.434) and deep part-through (^dandfrasl;_T = 0.700 and 0.878) cracks in 50.8mm and 102mm diameter (5mm and 6mm wall thickness) polycarbonate cylinders is also studied. Bowling and Townley's two-criteria approach to failure i.e. LEFM on the one hand and limit analysis on the other, is shown to provide a useful method for assessing the relative importance of crack initiation, in the presence of limited crack tip plasticity and general yield as failure criteria for a given sized defect. Thus, for crack tip plasticity fully contained by an outer elastic field i.e. not general yield, the LEFM parameter, K_Ic (with the possibility of a plasticity correction factor for thin sheet) can be used to predict crack initiation. For the low strain-hardening maraging steel, Irwin's plane strain plasticity correction, ¹andfrasl;_6π (K_{Ic/σ_y})² is shown to be applicable to sheet thicknesses comparable to ¹andfrasl;_π (K_{Ic/σ_y})² i.e. twice Irwin's plane stress plastic zone radius.

Improved methods of determining temperature transients in reactor systems are desired because of recent interest in Pressurized Thermal Shock (PTS) issues. The research presented herein was performed in support of the Nuclear Regulatory Commission's effort to re-evaluate its existing PTS rules. These rules are particularly important to the re-licensing of aging nuclear power plants. The much advanced computational power available to industry may offer a tool that allows the accurate calculation of temperatures inside the reactor vessel while not being inaccessibly expensive. It is proposed that an off-the-shelf Computational Fluid Dynamic (CFD) code, STAR-CD, can be a competitive tool in solving the thermal hydraulic domain of a reactor system. A comparison of the methodology and accuracy of the code types that have been previously used in PTS and one that has not been used extensively, CFD, is provided. A review of the literature shows that computer codes have been validated for solving PTS scenarios. The highly specialized program, REMIX, has been utilized extensively from 1986 to 1991 to interpret accident scenarios in reactor systems. Other programs are also available that can calculate downcomer temperatures including system and CFD type codes. Three codes representing the three different types of programs available are described in detail in the literature review section. Data appropriate for assessing a program's ability to calculate the response of a system to a PTS scenario is available from the current matrix of PTS tests being completed at the APEX-CE facility of the Oregon State University Nuclear Engineering department. The facility is a reduced scale integral test facility originally built for modeling the then-proposed AP-600 plant designed by Westinghouse. For the current test series, the facility was modified to model the Palisades nuclear power plant, a Combustion Engineering Pressurized Water Reactor (PWR). Two of the tests were chosen for their PTS typical conditions to compare with calculations of STAR-CD, REMIX, and RELAP. The computer models in each of the programs were either created, modified from a previous version, or the calculations for the comparisons were contributed. The downcomer temperatures at several locations and cold leg temperature gradients, where available, were extracted from the data and calculations and compared. Comparisons are presented in chapter 5 with graphs, along with some interpretation of the comparisons. It was found that STAR-CD agreed best with the data set in the downcomer and is the only program that calculated the temperature gradient in the cold legs. The agreement of STAR-CD with the cold leg data is also very good. REMIX and RELAP calculations agreement with data for downcomer temperatures are found to be good for all comparisons made, qualitatively more than quantitatively when contrasted with the STAR-CD calculations. / Graduation date: 2002

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Thesis (Ph.D.)--University of Wollongong, 2003. / Typescript. Includes appendices. Bibliographical references: leaf 251-266.

Thesis (M.S.)--Ohio University, June, 2004. / Includes bibliographical references (p. 105-107).