DEFORMATION AND WARPING IN MAGNETIC TAPES BY LASER SPECKLE TECHNIQUE

Laser speckles appear as a pattern of fine randomly distributed dots r esulting from multiple interference when an optically ''rough'' surfac e is illuminated by a coherent laser beam. By photographing a speckle pattern, the positions of all points on a surface are ''fingerprinted' ' and any changes in position can be detected with high sensitivity. S uch speckle patterns can be used as a non-contact, nondestructive, qua ntitative tool to measure in-plane deformations in magnetic tapes subj ected to tension. Because such tapes not only stretch but also warp, t he obtained data reflect the projection of the deformation vector in t he longitudinal plane of the tape. By geometrical relations, all three components of the deformation vector at any point of interest can be determined. This article presents data that were obtained for a commer cial computer data cartridge tape (40 Mb Format). It is shown that wit h such data, stress-strain relations can be established and the overal l deformation behavior of the material can be understood.