| 摘要 |
<p>In-situ, nondestructive examination to test the efficacy of a rivetted, bolted or bonded join of metal sheets (10, 11), or to test whether a composite material is faulted, or to test whether a multi-layer patch is bonded properly, involves the creation of an interferometric hologram. A photographic film or plate (20) is mounted closely above, and isolated vibrationally from a region (21) of a structure containing the join, composite material or patch. The film or plate is illuminated by the expanded beam (26) from a laser (24) firstly when the structure is in a first state of stress, and secondly when the structure is in a second state of stress (the change in stress being sufficient to deform the region over which the film or plate is mounted). At each illumination, a diffraction pattern is produced in the emulsion of the plate or film by interference between the illuminating beam and that part of it which is reflected from the region. The two diffraction patterns combine to produce a fringe pattern in the emulsion. That fringe pattern, when compared with the fringe pattern produced by a good join or a non-faulted composite body or patch, provides information about the region under investigation. The technique is especially useful for testing the joins in the skin of an aircraft fuselage, or potentially damaged composite structures.</p> |
