Southwest Research Institute 1997 Annual Report

The Institute is an international leader in developing and applying nondestructive evaluation technology for clients in industry and government. SwRI engineers employ various inspection techniques using methods such as cylindrically guided waves, magnetostrictive sensors, and eddy current technology. Over the past year, projects have focused on applications in government and utility infrastructure, pipelines, nuclear power plants, and military aircraft components.

Inspection of reactor vessels is important for nuclear power safety and is required by federal regulation. For some boiling water reactors (BWRs), welds are not easily accessible, and inspections must be conducted from within the reactor vessel in close, restricted areas using sophisticated robotic equipment. The SwRI-developed Enhanced Data Acquisition System (EDAS^TM) is used for enhanced test reliability and in probing for possible flaws in underclad regions of structures in aging nuclear reactors. Ishikawajima-Harima Heavy Industries (IHI) Inc., of Japan, recently developed a small, simple underwater inspection robot named the AIRIS 21 to replace older, bulkier equipment. This robotic scanner weighs less than 30 pounds and is less than 30 inches in diameter. Institute engineers, working in conjunction with IHI, have integrated EDAS^TM and qualified ultrasonic techniques into the AIRIS 21 to provide a complete and fully qualified inspection capability. Held to the vessel wall by water suction, the device can maneuver in any direction, while a unique tracking system provides location information. Together, AIRIS 21 and EDAS^TM reduce the difficulty of performing BWR weld inspections inside the reactor. The technology also is being investigated for inspecting other BWR vessel internal components, such as core shroud assembly welds.

SwRI engineers, in collaboration with EDM, Inc., of Fort Collins, Colorado, have developed a cylindrically guided wave technique (CGWT) to inspect anchor rods for high-voltage transmission towers. These rods, ranging from 10 to 20 feet in length and 0.75 to 2 inches in diameter, are buried in the ground or in concrete and are susceptible to environmentally induced corrosion. When these rods fail, a transmission tower may fall, resulting in a power outage and expensive repairs. The CGWT can detect corrosion damage at levels as small as 20 percent.

SwRI, working with the U.S. Federal Highway Administration, has addressed the problem of aging reinforced concrete bridges. The Institute has applied magnetostrictive sensor (MsS) technology for nondestructive evaluation of the reinforcing steel members within these bridges. Magnetostrictive sensors, embedded in the concrete or placed on a bridge surface, can transmit and detect elastic waves in the reinforcing steel. Corrosion or debonding of the steel from the concrete can be identified and quantified by analyzing the detected elastic waves.

The Institute recently developed an improved eddy current method for mapping corrosion damage in cast iron pipe or other conductive surfaces. Damage in cast iron is characterized by a corrosion product that remains on the outside surface of the pipe wall, and has the same appearance as sound metal, making visual determination of the depth or extent of the damage difficult. SwRI developed an eddy current inspection method that uses custom coil design and advanced signal processing to improve the resolution that can be obtained with currently available technology. A finite-element method-based model was used to determine coil design factors that affect spatial resolution for probes applied at long distances from the pipe surface. The resulting design allows inspection at a greater distance from the pipe surface and provides improved spatial resolution.

Reliable nondestructive inspection of aircraft skins for cracks around fasteners is critical for ensuring the structural integrity of the aircraft. In a U.S. Air Force-funded program, SwRI evaluated the effectiveness of a new inspection device based on magneto-optical imaging of eddy currents. This program involved developing test panels representative of an actual aircraft structure that contained fatigue cracks. Test results from these panels were used to determine the probability of detection of cracks so that the Air Force could assess the capability of this instrumentation for routine inspections.

The U.S. Air Force retirement-for-cause inspection system for F100, F101, and F110 jet engines uses surface eddy current probes on a number of engine disks with rough surfaces. These surfaces cause probes to wear and often produce an excessive probe failure rate. SwRI is developing new materials, including ion beam-assisted diamond-like coatings, to increase the life of these probes and improve their wearability.

SwRI was funded to develop eddy current probes for improved inspection of the blade attachment slots in commuter jet engines. The SwRI-designed 16-coil array provides complete inspection of highly stressed areas susceptible to fatigue cracking, while requiring minimal reinsertions of a probe. Since the coils are individually suspended, each maintains contact with the inspection surface, and allows for slot geometry and probe manufacturing variations. SwRI engineers designed and configured the coils to minimize response to the slot edge, allowing closer inspection at the edge, where fatigue cracks tend to form. In addition, an Institute-designed multiplexer allows the 16-channel probe to be linked with existing four-channel eddy current instrumentation for data transmission.

Although ultrasonic testing is widely used in nondestructive evaluation, it requires use of a liquid couplant and cannot be applied easily in extreme temperatures. During the past few years, several university studies have shown that high-power laser pulses can generate ultrasonic waves in materials while laser interferometry can detect the ultrasonic waves without liquid couplants. SwRI currently is developing this laser-generated ultrasonic testing technology for industrial applications such as the inspection of hot steel ingots.

An SwRI-developed magnetic sensor for accurate measurements of cladding thickness has been used successfully in nuclear and petrochemical pressure vessels. Nondestructive evaluation of ferritic nuclear reactor pressure vessels clad with austenitic stainless steel traditionally is performed using ultrasonic testing from the inside surface of the vessel. To evaluate the hazards of detected flaws, the depth of the flaw into the base metal must be determined. Ultrasonic testing can determine total flaw depth, but not its depth into the base metal unless the cladding thickness is known.

The U.S. Air Force has determined that more than 50 percent of accidents involving aircraft damage result from high-cycle fatigue (HCF). HCF is the high-frequency, low-intensity stress that increases the potential for low-cycle fatigue and foreign object damage, perhaps resulting in catastrophic failure. The Air Force needs to be able to examine nondestructively various engine materials to detect the presence of HCF, which is assumed to produce very small defects. To address this issue, SwRI identified and evaluated those nondestructive evaluation methods that could be used to detect the smallest HCF-type defects. Technologies evaluated included surface acoustic waves, surface skimming ultrasonic waves, angle-beam ultrasonics, eddy current, and the Krypton Emission Technique^TM. Results indicated that most of the techniques could only detect defects greater than 50 micrometers long. Eddy current techniques detected defects as small as 20 micrometers.

Copyright© 1997 by Southwest Research Institute. All rights reserved under U.S. Copyright Law and International Conventions. No part of this publication may be reproduced in any form or by any means, electronic or mechanical, including photocopying, without permission in writing from the publisher. All inquiries should be addressed to Communications Department, Southwest Research Institute, P.O. Drawer 28510, San Antonio, Texas 78228-0510.

Nondestructive Evaluation of Materials and Structures