Original Date: 01/27/1997
Revision Date: 01/18/2007
Best Practice : Nondestructive and Mechanical Evaluation
Lawrence Livermore National Laboratory (LLNL) maintains a technology base related to the nondestructive inspection and characterization of materials in applications areas of interest to its laboratory and its customers. Nondestructive evaluation (NDE) is becoming an increasingly more important element of the design and manufacturing processes. Due to the ever rising cost of material and labor, emphasis is being placed on the use of NDE early in the design and fabrication processes. Often parts and components are too costly to permit the luxury of destructively testing a number of them to demonstrate design goals.
Evaluation methods may be based on acoustics (sound), penetrating radiation (x-ray, gamma rays, beta particles, protons, neutrons), light (ultraviolet, infrared, visible), and electric and magnetic fields. NDE encompasses material characterization, real-time monitoring during manufacturing, flaw and damage detection in components, inspection of assemblies for tolerances, alignment, and in-service monitoring of flaw and damage growth in order to determine the maintenance requirements and continued safe operation of the part. LLNL has applied these techniques to unique inspection problems that require the applications of innovative NDE.
Computer axial tomography (CAT) measures the volume densities of materials and provides pictorial views of an object’s internal structure of materials and fabricated parts. In computed tomography, the object being investigated is translated and rotated in the path of the radiation, with transmission measurements made at each position. The data is then reconstructed into images using computer algorithms. LLNL uses the signal and image processing program, VIEW, and other programs to compute the tomograms and display the images. Several slices through the object can be restructured to provide a 3-D view of the internal structure. Operational tomography systems, developed by LLNL, range from the KCAT, with an energy of 70 to 125 keV and a resolution of 20 micrometers, to the HECAT, with an energy of 9 MeV and a resolution of 0.5 millimeter.
Imaging technology and image analysis are an ever increasing part of nondestructive testing. LLNL has assembled and developed tools which couple image processing with computational NDE algorithms. Focus areas include edge detection and image enhancement for digitized radiographs; focused wave mode calculations for ultrasonic inspections; in-depth examinations of image reconstruction; and statistical studies of different NDE algorithms.
Radiation gauging is used to measure areal densities of materials, where areal densities are defined as thickness times density. The information provided is similar to that obtained from film radiography. Radiation gauging can be highly accurate and quantitative because the information is obtained point-to-point, allowing for small variations in material density or thickness to be measured because of the high degree of accuracy.
Radiography covers a broad range of materials, components, and assemblies. Typical inspections include x-ray and gamma-ray energies from 3 keV to 9 MeV, using both film and electronic imaging. Current and future emphasis is on quantitative and micro-evaluations based on a broad range of photon energies and beam geometries coupled with sophisticated digital signal and image processing. Radiography evaluates the internal characteristics of a specimen. The data is presented as a 2-D image in analog form. Radiation is routinely used to inspect machined hemispheres for voids, cracks and inclusions. Low density materials, such as foam and composites, are inspected for composition, density, and uniformity. Assembly inspection radiograph is used to inspect assemblies ranging in size from large rocket motors to electronic circuit boards and chips.
Ultrasonic NDE evaluates material properties and conditions by probing the material with high-frequency sound waves. Pulses of ultrasonic waves are radiated into the material and subsequently detected using specially-designed transducers. The sound waves are altered as they travel into and through material, and therefore provide a change to the detected pulse which is then displayed, processed, and interpreted. Ultrasonics are usually applied to detect thickness and search for flaws in metals, such as cracks and voids; however, ultrasonics can also be applied to ascertain grain size, measure residual stress, and determine bond quality.
LLNL has an established and diverse customer base throughout the U.S. for applying its NDE technologies. Approximately 50% of its applications serve LLNL programs and the remaining 50% is distributed over industry clients, other Department of Energy (DOE) laboratories, federal agencies, and California agencies. LLNL works routinely with production plants in a flexible manufacturing environment, transfers developed technology from the laboratory to industry, and actively promotes the role of NDE in concurrent engineering. Successes include the evaluation of heart valves; automobile brakes and gears; aircraft structures; aquifers in six meters of sand; mines buried in 12 centimeters of sand; weapon valves; tool bits; aircraft corrosion; mammography; turbine blades; engine components; munitions; pistons; bridge cables; plastic explosives; waste drums; laser welding; and laser cutting.
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