Diagnosing Engineering Problems with Neutrons
- PDF / 1,746,384 Bytes
- 7 Pages / 604.8 x 806.4 pts Page_size
- 95 Downloads / 208 Views
IN/DECEMBER 1999
perature, environment, and the type and magnitude of any applied loads are determined by the engineering problem. In many cases, it is not possible to scale down the structure or alter the experi mental conditions because the engineer ing relevance is then lost. In this aspect, neutron diffraction has shown itself to be an adaptable servant. New dedicated instruments are providing faster rates of data acquisition, higher spatial and strain resolution, and increasing experimental flexibility. At the same time, engineering confidence in the technique is growing with the definition of industrial Stan dards for delivering the quantifiable levels of accuracy and repeatability essential for engineering application. The field of stress measurement by neutron diffraction is now about 15 years old. Early reviews were provided by Allen et al. 1 and in MRS Bulletin by Krawitz and Holden. 2 The subject was the topic of a North Atlantic Treaty Organization (NATO) meeting in 1992. 3 Currently, there are two main thrusts in the field. One is the recognition in engineering that finite element modeling represents a cost-effective way to simulate the stress State of components and estimare their lifetime in service. There is a clear need to benchmark these calculations. The second is the need to standardize meth ods of measurement at different laboratories throughout the world. In this article, we will introduce the basis of the tech nique, illustrate the capabilities and limitations of it through practical applications, and point out future directions.
Basis of the Technique Practical measurements of stress by diffraction have been carried out for
more than 70 years.4 The basic idea is that the lattice spacing provides an intrinsic strain gauge for testing the State of stress in a sample. The relationship between the lattice spacing d\M of the atomic planes described by Miller indices (hkl), the angle of the corresponding diffraction peaks (26/,w), and the wavelength of the radiation A is given by Bragg's law: A = 2dhk,sinQ,lki.
(1)
The particular advantage of neutron dif fraction is that the absorption of neutrons by the nuclei is, for the most part, fairly low. T h i s m e a n s that n e u t r o n beams penetrate centimeters into most components, permitting nondestructive measurements of lattice spacing at depth. Of course, it is not possible to evaluate the strain at a point; like all other meth ods, neutron diffraction can only provide a measure of the strain over a region. This region, the gauge volume, is delineated by t h e i n t e r s e c t i o n of n a r r o w beams of incident and diffracted neutrons, defined by slits or radial collimators, and is fixed in space. The apertures e n s u r e that only n e u t r o n s diffracted from locations within the gauge volume Vo can be detected. By placing the com ponent under study on a computercontrolled positioning table, any point in the sample may be moved into the gauge volume. In this way, a map of lattice spacing with position can be built. If a ref
Data Loading...
