D. S. Kupperman

Thermal residual strains and stresses in silicon carbide-fiber-reinforced silicon nitride composites

Abstract Neutron diffraction was used to measure thermal residual strains in SiC-fiberreinforced reaction-bonded silicon nitride composites as a function of volume fraction of fibers. The measured values are in agreement with those estimated by finite-element modeling. Due to the anisotropy of the thermal expansion coefficient of SiC, residual axial strain (stress) is tensile and residual transverse strain (stress) is compressive in the fibers. The variation in the measured residual axial strain in the matrix can be related to the density of the composite. In addition, neutron diffraction was used to study the effects of fiber coatings and processing on the thermal residual strains.

Application of Neutron Diffraction Time-of-Flight Measurements to the Study of Strain in Composites

Neutron diffraction time-of flight measurements in the Intense Pulsed Neutron Source at Argonne National Laboratory have been used to study strain (from which stress is calculated) in various metal-and ceramic-matrix composite structures. For example, the effect of fabrication procedures (e.g., thermal cycling) on strain in the constituents of a metal-matrix composite have been studied. In this example, neutron diffraction experiments were used to quantify the process-induced reduction of strain in composite fibers. Measurements carried out to 900°C on this material have been used to validate theoretical assumptions in the prediction of fabrication-induced residual stress. Neutron diffraction studies of a high-temperature ceramic superconducting composite consisting of yttrium-barium-copper-oxide (YBCO) with various volume fractions of silver have also been carried out. The results of these studies have provided information on the effect of Ag content on stoichiometry, strength of interface bonding between Ag and YBCO (indicated by the tensile strains in the Ag), and creep mechanisms during fabrication. Ceramic-matrix composites with randomly dispersed ceramic whiskers have also been studied. Comparison of measured data with analytical-model predictions gave important clues to the micromechanisms involved in the creation of residual stresses. For example, the measured residual strain, as a function of angle, in hexagonal silicon nitride whiskers embedded in an alumina matrix is in excellent agreement with the predictions of a simple linear elastic model. For samples of silicon carbide whiskers embedded in an alumina matrix, temperature-dependent experiments allow determination of the stress-free temperature used in elastic analyses. Problems with determining strain-free lattice parameters and uncertainties in absolute measurements will be discussed.

Residual strain in advanced composites

Abstract The mechanical properties, such as strength and toughness, of advanced ceramic and metal-matrix composites can be greatly influenced by the presence of fabrication-induced residual stresses. Stresses are created when two different materials used to form the composite are fired and cooled. Differential contraction can lead to one constituent (e.g., the matrix) compressing the other (e.g. fiber). Very large residual stresses (up to 2 MPa) can result. It is important in the optimization of mechanical properties to know the stress levels and how to modify them. Central to the problem is the ability to measure changes in the lattice spacing when the material is elastically strained. If the strain is in the elastic region and we know the elastic moduli of the material, we can determine the stress inside the material, or the average stress of a constituent of a composite. Neutron diffraction can provide a bulk measurement of the lattice spacing of the constituents of a composite. If both macrostresses (...

Steam generator mock-up for assessment of inservice inspection technology

A steam generator mock-up has been assembled for round-robin (RR) studies of the effectiveness of currently practiced inservice inspection technology for detection of current-day flaws. The mock-up will also be used to evaluate emerging inspection technologies. The 3.66 m (12 ft)-tall mock-up contains 400 tube openings, each consisting of nine test sections that can be used to simulate current-day field-induced flaws and artifacts. Included in the mock-up are simulations of tube support plate (TSP) intersections and the tube sheet (TS). Cracks are present at the TSP, TS, and in the free span sections of the mock-up. For initial evaluation of the RR results, various eddy current methods, as well as multivariate models for data analysis techniques are being used to estimate the depth and length of defects in the mock-up. To ensure that the RR is carried out with procedures as close as possible to those implemented in the field, input was obtained from industry experts on the protocol and procedures to be used for the exercise. One initial assembly of the mock-up with a limited number of flaws and artifact has been completed and tested. A second completed configuration with additional flaw and artifacts simulations will be used for the round robin.

Evaluation of NDE Round-Robin Exercises Using the NRC Steam Generator Mockup at Argonne National Laboratory

This paper discusses round-robin exercises using the NRC steam generator (SG) mock-up at Argonne National Laboratory to assess inspection reliability. The purpose of the round robins was to assess the current reliability of SG tubing inservice inspection, determine the probability of detection (POD) as function of flaw size or severity, and assess the capability for sizing of flaws. For the round robin and subsequent evaluation completed in 2001, eleven teams participated. Bobbin and rotating coil mock-up data collected by qualified industry personnel were evaluated. The mock-up contains hundreds of cracks and simulations of artifacts such as corrosion deposits and tube support plates that make detection and characterization of cracks more difficult in operating steam generators than in most laboratory situations. An expert Task Group from industry, Argonne National Laboratory, and the NRC have reviewed the signals from the laboratory-grown cracks used in the mock-up to ensure that they provide reasonable simulations of those obtained in the field. The mock-up contains 400 tube openings. Each tube contains nine 22.2-mm (7/8-in.) diameter, 30.5-cm (1-ft) long, Alloy 600 test sections. The flaws are located in the tube sheet near the roll transition zone (RTZ), in the tube support plate (TSP), and in the freespan. The flaws are primarily intergranular stress corrosion cracks (axial and circumferential, ID and OD) though intergranular attack (IGA) wear and fatigue cracks are also present, as well as cracks in dents. In addition to the simulated tube sheet and TSP the mock-up has simulated sludge and magnetite deposits. A multiparameter eddy current algorithm, validated for mock-up flaws, provided a detailed isometric plot for every flaw and was used to establish the reference state of defects in the mock-up. The detection results for the 11 teams were used to develop POD curves as a function of maximum depth, voltage and the parameter mp , for the various types of flaws. The POD curves were represented as linear logistic curves, and the curve parameters were determined by the method of Maximum Likelihood. The effect of both statistical uncertainties inherent in sampling from distributions and the uncertainties due to errors in the estimates of maximum depth and mp was investigated. The 95% one-sided confidence limits (OSL), which include errors in maximum depth estimates, are presented along with the POD curves. For the second round robin a reconfigured mock-up is being used to evaluate the effectiveness of eddy current array probes. The primary emphasis is on the X-Probe. Progress with the X-Probe round robin is discussed in this paper.Copyright

Application of Neutron Diffraction NDE to High-Temperature Superconducting Composites

Since the discovery of superconducting materials with relatively high transition temperatures (Tc), there has been a considerable effort both to understand the reason for the high Tc and to improve the mechanical properties, (the latter has been a limiting factor for practical applications). The YBa2Cu3O7-δ (YBCO) compounds have received considerable attention because of their high Tc and high upper critical magnetic field [l–3]. Additions of silver have recently been shown to improve the mechanical properties (toughness and strength) of these compounds [4]. Furthermore, the addition of the Ag can improve the conductive path between grains of superconducting YBCO and possibly help reduce the “weak-link” effect [5]. Note that whereas the addition of a low-volume fraction of silver does not adversely affect the superconductivity the introduction of transition metals to YBCO can have a detrimental effect on the superconducting properties. Also, the addition of silver has a minimal affect on the stress free lattice spacing. During fabrication of YBCO/Ag composites, differential thermal expansion upon cooling can lead to potentially troublesome residual stresses. Since the Ag contracts more than the YBCO, good bonding between the ceramic and silver could lead [6] to tensile stresses in the silver and compressive stresses in the YBCO for relatively small percentages of Ag. These residual stresses could lead to premature failure of the composite, debonding of the YBCO and Ag, and/or microcracking, which will affect the flow of superconducting current and the life expectancy of components made from this material. An understanding of the nature and magnitude of these stresses will help improve the design of these composites.

Workshop on residual stress at Argonne

Abstract On May 22-23, 1990, a Gorkshop on the application of neutron diffraction to the determination of residual stress in engineering materials was held at Argonne National Laboratory, Argonne. Illinois. The workshop was sponsored by the Materials and Components Technology and Intense Pulsed Neutron Source Divisions with partial support from the Division of Educational Programs. Talks were given by recognized experts in the field with ample time provided for attendees and speakers to address issues and concerns regarding the use of neutron diffraction. Examples discussed covered a wide range of materials and geometries from pipe welds to advanced composites, with applications in a wide variety of industries.

Origin of Spurious Ultrasonic Echoes in Stainless Steel Piping with Weld Overlay

The initiation and growth of intergranular stress-corrosion cracking (IGSCC) in the heat-affected zone of welds in stainless steel reactor piping has been a subject of concern to electric utilities for over ten years. This type of crack can be detected with ultrasonic shear waves during normal maintenance periods with a reliability of up to 80% [1]. Often, when crack indications have been found, the utility has been allowed to apply a weld metal overlay as a temporary repair measure. However, the complex, elastically anisotropic microstructure of the overlay considerably reduces the reliability of subsequent ultrasonic inspections. This paper addresses the problems arising because of the overlay.