Figure 8 A narrow laser beam passes through a hole
in the mirror and projects a fringe
pattern on to a screen on the far side
of the film.
The displacement of the object point corresponding to
"d" may be calculated from the magnification of the
camera. The orientation of the bands is at right
angles to the plane of the displacements, but the
sign of the displacement cannot be determined from
the film record alone, since the recording process
cannot distinguish which exposure came first.
Figure 9 shows one typical result of loading the
machine with 500 kN.
a
o
Vertical component (opening)
A
eo
o
o
edges [um]
to
o
Horizontal component ( shear)
S
Relative displacement of crack
l | ] 1 1 l 1 1 1 1
0 IO 20 30 40 50 60 70 80 90 100 110
Distance along crack (mm)
Figure 9 The relative vertical and horizontal
movement of the crack edges for a load
of 500 kN.
References
5 Archbold E, Ennos A E, "Laser photography to
measure the deformation of weld cracks under
load" Non Destructive Testing, 1975, August,
pp 181-184.
6 Hughes T, "Deformation measurements by photography"
B Journal Photography 20 June 1975 pp 542-543.
4 MOIRE GRID TECHNIQUES FOR PHOTOGRAPHIC RECORDING
AND MEASUREMENT
J M Burch and C Forno (7,8)
A simple modification to a 35 mm camera produces an
effective resolution up to 600 lines/mm for specially
Structured objects, and leads to an extremely powerful
remote-measuring technique for studying deformations
in large objects up to 10 m or more in size. The
essential principle is that the response of the camera
System is carefully "tuned" by the use of a mask in the
lens of the camera with two parallel slit-apertures, so
designed that periodic structures subtending a
particular angular frequency at the camera are
selectively recorded. Alternatively, two pairs of slots
at right angles may be used to obtain sensitivity in
two orthogonal directions (Figures 10 and 11).
Au,
—9À
—
/ u,
Figure 10 Two-dimensional double-slotted maske
Figure 11 "Pentax" camera with modified lens.
The structure to be studied must have periodic
variation of reflection or transmission in a grid-like
structure (for the two dimensional case). Sometimes
the structure may already have the required form
(for example, the warp and webb of woven textiles) but
more usually it will be necessary to imprint the
surface of the structure with a rectangular pattern,
or stick paper sheets carrying the pattern to the
surface. For example, a brick arch 3.6.m in length
was covered with photocopies of a 65 dots-to-the inch
half tone screen, and the camera distance was chosen
to obtain the correct angular spacinge Two
successive exposures may be made, as in the other
techniques already mentioned, before and after
loading the structure, and the moiré fringe pattern
observed in the first order diffraction patterns of
the reconstructed double images. Figure 12a shows
the brick structure at the start of covering the
surfacee Figures 12b and 12c show the patterns
of in-plane distortion in the two orthogonal
directions, each fringe corresponding to a
displacement movement of approximately 0.4 mm.
For out-of-plane deflections, the same type of camera
can be used in two transversely separated locations,
as in normal photogrammetry, with the additional
advantages and sensitivity that this technique can
bring. The ratio of sensitivity in this mode will be
