In our system, the high sensitivity is obtained by using the in-plane holographic moire' technique as the operating principle. The instrument is composed of two optical heads. A writing head, fed by an He-Cd laser through an optical fiber, generates on the surface of the component a reference grating in photoresist. This is used as a mask for the electroforming of a nickel permanent grating intended to endure temperature and corrosion during the life of the component.
A reading optical head is used to perform the periodical comparison between the same reference grating, provided by the reading head itself, and the deformed grating electroformed on the component. Both heads have an identical isostatic base for precise repositioning by means of three reference spheres soldered to the component, so that no optical aligment is required by the operator.Any difference between the pitch of the grating on the component and the reference grating from the reading head will generate moire fringes with a spacing 
= p/
, where p is the spacing of the reference grating. The sensitivity of the system is in the range of 60 - 5000 µstrain when using a grating spacingof 2.5 µm.Moire fringes are imaged by a CCD camera and analyzed by a dedicated software that gives directly the value of , while the sign ambiguity is removed by a calibrated offset of the moire fringe pattern in the reading head opticalsystem.
A complete description of thesystem and the results obtained duringin-field tests on power plant componentswill be given in the paper.
 Fig.: The writing head on a steam pipe during in-field testing |
Abstract Source:
Book of Abstracts, 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN: 87-986898-0-00
Full-Text Source:
Proceedings of the 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN:
Publication Contact:
7th ECNDT, Park Alle 345, DK-2605 Broendby, Denmark, Fax: +45 46 26 70 11, Email: 101373.3414@compuserve.com
| NDTnet |