|
Thermography can be applied in any situation where a
problem or condition can reveal itself by means of a
thermal difference. For many situations, this is
quite easy to apply; a thermal condition can be seen
because the process involves release of thermal
energy. An example of this is inspecting the
condition of electrical distribution equipment. When
electrical current passes through a resistive
element, heat is generated. If the target emissivity
is high enough, we can see that heat with an
infrared camera. Sliding and bolted connections can
become resistive through loosening, corrosion, etc.
This increase in electrical resistance usually
results in increase in heat generation and the
camera can quickly pick it up. Sounds simple, and
often it is. Frequently, it is not simple due to the
nature of heat transfer. Good training is the key to
successful application of infrared technology.
Another example is the inspection of concrete
bridges. As many of us know, concrete can develop
delaminations, which can lead to potholes. When a
pothole develops, it is quite easy to detect;
usually your tire and wheel “find” the hole and you
end up with an unpleasant repair bill. Wouldn’t it
be great if we could find these before they cause
problems? By cleverly using the sun’s energy as a
heating medium, and viewing with an infrared camera;
we find that subsurface delaminations have a
different heating effect than the sound parts of the
deck structure, so the camera can see it. This
example shows that even though the bridge deck
doesn’t generate heat it can still be analyzed with
thermography given the proper conditions.
Here
is another example of an application where we can
use passive heating or cooling. Recently developed
composite aircraft materials are extremely sturdy
and lightweight. These materials are vital to
aircraft performance and airworthiness. However, the
honeycomb structure of this material presents a
potentially dangerous problem: water ingress.
It
has been discovered that certain control surfaces
tend to absorb water in the honeycomb structure, for
reasons that are not fully understood. The problem
is aggravated by the effects of lightning and hail,
which cause barely visible impact damage. The water
enters the honeycomb and freezes when the aircraft
is at high altitude. As the ice expands it breaks
down the cells in the structure. This condition
grows like a cancer and eventually jeopardizes the
entire structural integrity of the component.
Until
recently, the only effective method of diagnosing
the problem was through radiography. While this is
still the most accurate way, it has several
disadvantages: it is expensive in time, equipment,
and manpower, and can expose maintenance personnel
to hazardous ionizing radiation.
Thermography can be an indispensable tool for
inspecting planes for this problem. After the plane
has landed, the ice remains at 0 C while it is
melting. The rest of the plane has warmed to ambient
temperatures on the approach. This provides an ideal
opportunity to search for the ice pockets with a
thermal imaging system while the plane is being
serviced. |
