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Fatigue and Impact: Testing of Composites

The failures of the de Havilland Comet I, demonstrated the importance of materials science and testing.


October 2, 2007
By Frank Lio

Topics

Ultimately, all aircraft will develop cracks; the incredible stresses
produced by takeoffs, landings and other flight-related conditions make
this inevitable. The issue is to identify appropriate materials to use
in manufacturing aircraft and provide corrective maintenance long
before there is any risk of structural failure.

There are, unfortunately, examples of such catastrophic failures due to
material defects. In 1949-1950, the failures of the de Havilland Comet
I, the first commercial jet airliner, demonstrated the importance of
materials science and testing. A unique test at RAe Farnborough on a
grounded Comet showed that, after being subjected to stresses
equivalent to 9,000 hours of actual flying, a split appeared in the
fuselage. It began with a small fracture in the corner of an
escape-hatch window and extended for eight feet. Metal fatigue
initiated the crack at a stress concentration, and low material
fracture toughness caused it to grow rapidly leading to catastrophic
failure.

These failures, along with other notable examples, led
to the science of fracture mechanics — the study of a material’s
ability to sustain cracks under load without sudden fracture – and
could have been avoided through materials testing: the process of
compression, tension, torsion and bending materials to make sure they
stand up to intense stress, strain, fatigue, and impact. Materials
testing takes on even greater importance in today’s aerospace industry,
because one of the primary objectives is to keep the weight of aircraft
structures as low as possible, while maintaining structural integrity
and safety.

Aircraft manufacturers want to use materials such
as carbon-fibre-reinforced polymers (CFRPs) and glass-reinforced
polymers (GFRPs) more extensively; they have a higher stiffness and
strength to weight ratio than most other materials, while providing
better performance and fuel efficiency. It has been estimated that a
mere onepound reduction in weight translates into $1,000 savings in the
cost of fuel during the lifetime of the aircraft. In addition, these
materials can be modeled into new and more radical shapes to take
advantage of aerodynamics and stealth factors.

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But the
inspection of these newer lightweight materials becomes paramount
during the various stages of fabrication. Testing provides the critical
data needed to ensure these materials are not only strong and
lightweight, but also robust and sturdy enough to endure the extreme
conditions associated with aerospace applications.

The most
common tests involve characterizing a material’s fatigue performance
and its reaction to single high-speed events. Fatigue and impact
testing are performed early in the R&D phase to identify
appropriate materials to be used in aerospace applications. Both the
fatigue and impact resistance of a part is, in many applications, a
critical measure of service life.