("Larry Dighera" wrote)
snips
There are a number of points raised in Weldon's comment:

1. The large number of failure modes for the relatively brittle composite
structure used in the 787.

2. The difficulty in testing the composite structure used in the 787.

6. The sensitivity to hot/wet and freeze/thaw conditions an
through-thickness crack growth that represent fatigue-like failure modes
thought to be nonexistent in composites.

7. Visually undetectable impact caused micro-cracking as might occur with
hail damage.


Sounds like just what the Dr. ordered...

From yesterday's e-Hotline
(EAA eHotline Volume 7 Number 49)

NANOTUBES DETECT, REPAIR WING DAMAGE
"Adding even a small amount of carbon "nanotubes" can go a long way toward
enhancing the strength, integrity, and safety of composite structures,
according to a new study at Rensselaer Polytechnic Institute, New York.
Researchers there have developed a simple new technique for identifying and
repairing small, potentially dangerous cracks in high-performance aircraft
wings and many other composite structures. By infusing the polymer with
electrically conductive carbon nanotubes and monitoring the electrical
resistance at different points in the structure, Professor Nikhil Koratkar,
who developed the method, can pinpoint the location and length of even the
tiniest stress-induced crack."

A more detailed story from Science Daily: Oct 4, 2007

http://www.sciencedaily.com/releases/2007/09/070927132550.htm
The majority of failures in any engineered structure are generally due to
fatigue-induced microcracks that spread to dangerous proportions and
eventually jeopardize the structure's integrity, Koratkar said. His research
is looking to solve this problem with an elegant solution that allows for
real-time diagnostics and no additional or expensive equipment.

Koratkar's team made a structure from common epoxy, the kind used to make
everything from the lightweight frames of fighter jet wings to countless
devices and components used in manufacturing and industry, but added enough
multi-walled carbon nanotubes to comprise 1 percent of the structure's total
weight. The team mechanically mixed the liquid epoxy to ensure the carbon
nanotubes were properly dispersed throughout the structure as it dried in a
mold.

The researchers also introduced into the structure a series of wires in the
form of a grid, which can be used to measure electrical resistance and also
apply control voltages to the structure.

By sending a small amount of electricity through the carbon nanotubes, the
research team was able to measure the electrical resistance between any two
points on the wire grid. They then created a tiny crack in the structure,
and measured the electrical resistance between the two nearest grid points.
Because the electrical current now had to travel around the crack to get
from one point to another, the electrical resistance - the difficulty
electricity faces when moving from one place to the next - increased. The
longer the crack grew, the higher the electrical resistance between the two
points increased.

Plus, Koratkar's system features a built-in repair kit.

When a crack is detected, Koratkar can increase the voltage going through
the carbon nanotubes at a particular point in the grid. This extra voltage
creates heat, which in turn melts a commercially available healing agent
that was mixed into the epoxy. The melted healing agent flows into the crack
and cools down, effectively curing the crack. Koratkar shows that these
mended structures are about 70 percent as strong as the original, uncracked
structure - strong enough to prevent a complete, or catastrophic, structural
failure. This method is an effective way to combat both microcracks, as well
as a less-common form of structural damage called delamination.

"What's novel about this application is that we're using carbon nanotubes
not just to detect the crack, but also to heal the crack," he said. "We use
the nanotubes to create localized heat, which melts the healing agent, and
that's what cures the crack."


Montblack
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