Often overlooked
in any listing of NDT methods, visual inspection is one of the most common and
most powerful means of non-destructive testing. Visual testing requires
adequate illumination of the test surface and proper eye-sight of the tester.
To be most effective visual inspection does however, merit special attention
because it requires training (knowledge of product and process, anticipated
service conditions, acceptance criteria, record keeping, for example) and it
has its own range of equipment and instrumentation. It is also a fact that all defects
found by other NDT methods ultimately must be substantiated by visual
inspection.
Visual testing
can be classified as direct visual testing, remote visual testing and
translucent visual testing. The most common NDT methods MT and PT are indeed
simply scientific ways of enhancing the indication to make it more visible.
Often the equipment needed is simple Figure 1.1 a portable light, a mirror on
stem, a 2´ or
4´ hand
lens, one illuminated magnifier with magnification 5x or 10´. For internal
inspection, light lens systems such as borescopes allow remote surfaces to be
examined . More sophisticated devices of this nature using fibre optics permit
the introduction of the device into very small access holes and channels. Most
of these systems provide for the attachment of a camera to permit permanent
recording.
(a) Mirror on stem: may be flat
for normal view or concave for limited magnification.
(b) Hand magnifying glass
(magnification usually 2–3´).
(c) Illuminated magnifier; field
of view more restricted than D (magnification 5–10´).
(d) Inspection glass, usually
fitted with a scale for measurement; the front surface is placed in contact
with the work (magnification 5–10´).
(e) Borescope or intrascope with
built-in illumination (magnification 2–3´).
The applications of visual
testing include:
(1) Checking of the surface
condition of the test specimen.
(2) Checking of alignment of
mating surfaces.
(3) Checking of shape of the
component.
(4) Checking for evidence of
leaking.
(5) Checking for internal side
defects.
source : International Atomic Energy Agency; TRAINING COURSE SERIES No. 11
Visual Inspection is one of the most widely used Non-Destructive Testing methods for the detection of discontinuities before they cause major problems, e.g. poor welding, surface defects, corrosion pits, general condition, degradation, blockages and foreign materials.
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