John G. Merkle

The heavy-section steel technology pressurized-thermal-shock experiment, PTSE-1☆

Abstract A pressurized-thermal-shock (PTS) facility was developed in the Heavy-Section Steel Technology Program at Oak Ridge National Laboratory for performing experiments that challenge predictions of analytical methods applicable to full-scale reactor pressure vessels under combined loading. The first experiment (PTSE-1) was designed to address three principal issues: (1) warm-prestressing phenomena; (2) crack propagation from brittle to ductile regions; and (3) transient crack stabilization in ductile regions. The paper presents a description of the PTS facility at ORNL and a review of the objectives and results of the first test. Also included are elastodynamic finite-element analyses of the two crack run-arrest events that occurred in the second and third phases of the test. Finally, some conclusions and recommendations are presented based on the outcome of the first experiment.

Technical Basis for the Master Curve Concept of Fracture Toughness Evaluations in the Transition Range

An American Society for Testing and Materials (ASTM) standard method (E 1921-97) has been developed that exclusively uses fracture mechanics test practices and advanced statistical methods to establish the ductile-to-brittle transition range of fracture toughness for structural steels. The development of suitably accurate analyses had been slowed in the past due to an incomplete understanding of the operational mechanisms that control the fracture toughness behavior of structural steels. New perspectives taken are (1) that dominant linear-elastic conditions need not be rigidly enforced in specimens and (2) that the effect of specimen size on fracture toughness performance is mostly controlled by a weakest-link mechanism instead of being completely controlled by crack tip constraint conditions. The weakest-link behavior is defined from local cleavage crack initiators such as precipitates, inclusions, and grain boundary embrittlement, namely, all microstructural features in steel. Statistical models can be built upon such mechanisms that result in defined fracture probability levels and, when coupled to a master curve concept, can more accurately define the true location of the ductile-to-brittle transition temperature. An integral part of the ASTM test standard development work has been the production of a supporting technical basis document. This document presents substantial background data and supporting theoretical aspects that have been used to justify the method development. The paper will include some of the salient features presented.

Test of thick vessel with a flaw in residual stress field

Intermediate test vessel V-8, a 152-mm-thick vessel fabricated of SA533, grade B, class 1 steel, was pressurized to failure at -23/sup 0/C. The vessel contained a fatigue-sharpened notch adjacent to a half-bead weld repair that had not been stress relieved. Residual stresses and fracture toughnesses were determined before the pressure test by measurements on a prototypical weld, and fracture predictions were made by linear elastic fracture analysis. Predictions agreed well with test results, demonstrating the important influence of high residual stresses on fracture behavior.

Pressurized Thermal Shock Experiments with Thick Vessels

Information is provided on the series of pressurized-thermal-shock experiments at the Oak Ridge National Laboratory, motivated by a concern for the behavior of flaws in reactor pressure vessels having welds or shells exhibiting low upper-shelf Charpy impact energies, approx. 68J or less. (JDB)

Fracture analyses of heavy-section steel technology wide-plate crack-arrest experiments

A series of six wide-plate crack-arrest tests was recently completed by the Heavy-Section Steel Technology program at the National Bureau of Standards, Gaithersburg, MD, using tensile-loaded specimens of A533 Grade B Class 1 steel. Crack-arrest data were obtained at temperatures in the transition range and above the onset of the Charpy upper shelf, thereby providing a basis for the development and evaluation of improved fracture-analysis methods. The 1 by 1 by 0.102-m single-edge-notched (SEN) specimens were welded to long straight pull tabs and subjected to a transverse linear temperature gradient before loading. The crack tips were sharpened by hydrogen-charging an electron-beam weld. The tests were designed to obtain crack arrest near the middle of the specimen where the temperature would produce a high-toughness level in the upper transition region of the material. The specimens were instrumented with strain gages and thermocouples. Initial static design calculations were made using textbook formulas. Additional calculations, using an assumed set of K I D versus a and T relations and an effective stress wave concept, confirmed the reasonableness of tentative design parameters. Pretest and posttest dynamic finite-element calculations were performed for each test. Computed results are compared with transient data for crack-line strains, crack speed, crack-opening displacement, arrest location, and postarrest tearing. Results from both application-mode and generation-mode dynamic analyses are presented. The arrest toughness values calculated from the test data are summarized for temperatures ranging from the transition into the Charpy upper-shelf range.

Applications of Energy Release Rate Techniques to Part-Through Cracks in Experimental Pressure Vessels

In nonlinear applications of computational fracture mechanics, energy release rate techniques are used increasingly for computing stress intensity parameters of crack configurations. Recently, deLorenzi used the virtual-crack-extension method to derive an analytical expression for the energy release rate that is better suited for three-dimensional calculations than the well-known J-integral. Certain studies of fracture phenomena, such as pressurized-thermal-shock of cracked structures, require that crack tip parameters be determined for combined thermal and mechanical loads. A method is proposed here that modifies the isothermal formulation of deLorenzi to account for thermal strains in cracked bodies. This combined thermo-mechanical formulation of the energy release rate is valid for general fracture, including nonplanar fracture, and applies to thermo-elastic as well as deformation plasticity material models. Two applications of the technique are described here. In the first, semi-elliptical surface cracks in an experimental test vessel are analyzed under elastic-plastic conditions using the finite element method. The second application is a thick-walled test vessel subjected to combined pressure and thermal shock loadings.

Analytical applications of the J-integral

It has recently been shown by experiment that a quantity known as the J-Integral may be a useful fracture criterion in the inelastic range. The mathematical definition of the J-Integral is first used here, in combination with some simplifying assumptions, to derive the known relationship between the conventional elastic stress concentration factor of a sharp notch and the elastic stress intensity factor for a crack of the same shape. The same assumptions, together with a generalization of Neubers equation for the inelastic stress and strain concentration factors for a sharp notch, are then used to derive a relationship between the J-Integral and the parameter K I c d of the Equivalent Energy Method. The purpose of this derivation is to identify a set of assumptions which, in combination, lead to an explicit relationship between the two parameters. A graphical procedure for estimating upper and lower bounds on the full restraint value of the fracture toughness is also discussed.

Performance of low-upper-shelf material under pressurized-thermal-shock loading

Abstract The second pressurized-thermal-shock experiment (PTSE-2) of the Heavy-Section Steel Technology Program was conceived to investigate fracture behavior of steel with low ductile-tearing resistance. The experiment was performed in the pressurized-thermal-shock test facility at the Oak Ridge National Laboratory. PTSE-2 was designed primarily to reveal the interaction of ductile and brittle modes of fracture and secondarily to investigates the effects of warm prestressing, A test vessel was prepared by inserting a cracklike flaw of well-defined geometry on the outside surface of the vessel. The flaw was 1 m long by ≈ 15 mm deep. The instrumented vessel was placed in the test facility in which it was initially heated to a uniform temperature and was then concurrently cooled on the outside and pressurized on the inside. These actions produced an evolution of temperature, toughness, and stress gradients relative to the prepared flaw that was appropriate to the planned objectives. The experiment was conducted in twoseparate transients, each one starting with the vessel nearly isothermal. The first transient induced a warm-prestressed state, during which K I , first exceeded K Ic . This was followed by repressurization until a cleavage fracture propagated and arrested. The final transient was designed to produce and investigate a cleavage crack propagation followed by unstable tearing. During this transient, the fracture events occurred as had been planned.

An Approximate Method of Elastic-Plastic Fracture Analysis for Nozzle Corner Cracks

Two intermediate test vessels with inside nozzle corner cracks have been pressurized to failure at Oak Ridge National Laboratory (ORNL) by the Heavy Section Steel Technology (HSST) Program. Vessel V-5 leaked without fracturing at 88°C (190°F), and Vessel V-9 failed by fast fracture at 24°C (75°F) as expected. The nozzle corner failure strains were 6.5 and 8.4 percent, both considerably greater than pretest plane-strain estimates. The inside nozzle corner tangential strains were negative, implying transverse contraction along the crack front. Therefore, both vessels were reanalyzed, considering the effects of partial transverse restraint by means of the Irwin β I c formula. In addition, it was found possible to accurately estimate the nozzle corner pressure-strain curve by either of two semi-empirical equations, both of which agree with the elastic and fully plastic behavior of the vessels. Calculations of failure strain and fracture toughness corresponding to the measured final strain and flaw size are made for both vessels, and the results agree well with the measured values.

Heavy-Section Steel Technology and Irradiation Programs—Retrospective and Prospective Views

In 1965, the Atomic Energy Commission (AEC), at the advice of the Advisory Committee on Reactor Safeguards (ACRS), initiated the process that resulted in the establishment of the Heavy Section Steel Technology (HSST) Program at Oak Ridge National Laboratory (ORNL). In 1989, the Heavy-Section Steel Irradiation (HSSI) Program, formerly the HSST task on irradiation effects, was formed as a separate program, and, in 2007, the HSST/HSSI Programs, sponsored by the U.S. Nuclear Regulatory Commission (USNRC), celebrated 40 years of continuous research oriented toward the safety of light-water nuclear reactor pressure vessels (RPV). This paper presents a summary of results from those programs with a view to future activities.