Ü
(
A moulded chair was subjected to various loadings in
order to examine the ways in which it deformed under
load, to identify any possible discontinuities in
elastic behaviour due to material properties. A
typical pattern of deflection, produced by a 100 g
weight is shown in Figure 5.
Figure 5 Fringe pattern on under-side of plastics
chair, due to loading with 100 g weight,
obtained by interference holography.
References
3 Archbold E and Ennos A E., "Applications of
holography and speckle photography to the
measurement of displacement and strain"
J Strain Analysis, 1974, 9, No 1, pp 10-16.
4 Ennos A E and Archbold E., "Coherent optical
technique for studying polymers" Plastics and
Rubber: Materials and Applications 1976,
September, pp 116-120.
3 LASER SPECKLE PHOTOGRAPHY FOR MEASURING TANGENTIAL
DISPLACEMENTS OF SURFACE ELEMENTS
E Archbold, A E Ennos (3,5,6)
When a diffusely reflecting surface is illuminated with
coherent light from a laser, and the surface is
photographed with a conventional camera but
preferably using ultra-fine grain film) the record is
overlaid with a speckle pattern that is derived from
the many randomly-distributed surface elements.
Although the pattern shows no regularity and appears
to carry little information in itself, it is quite
uniquely determined by the microstructure of the
surface and the geometrical characteristios of the
recording system, and if the surface moves or is
distorted, the pattern changes. Two successive
exposures, made in rather the same way as the double
exposure in "frozen fringe" holography, will give a
doubled pattern in regions where small displacements
in the plane of the surface occur.
A structural component of a measuring machine was
found to have developed a crack in a welded joint,
and a 35 mm camera loaded with Agfa-Gevaert 10E75
holographic film was used to make a double exposure,
before and after loading the structure, as shown in
Figure 6.
90
35mm Camera
‚Laser
Illumination
Figure 6 Laser photography in use io study movements
near a crack in a load-bearing box section
structure.
After processing the film, a narrow beam of
monochromatic light is directed through a point on the
film corresponding to each point on the structure
to be examined (Figure 23.
Figure 7 The displacement is measured from a speckle
pattern derived from the double exposure film.
The speckle pattern scatters the beam into & cone and
if there is doubling of the speckles, the brightness
of the patch of scattered light is modulated into a
series of bright and dark bands (Figure 8) whose
angular relation g to the direction of the
beam is given by
sin g = nA/d,
where n is an integer, À is the wavelength of the
light, and d is the "doubling" separation on the film.