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Metal
Fatigue Part I:
Taking
the Mystery Out of Metal Fatigue
Cracking
in aircraft structures and/or skins has been a
newsworthy topic recently, so we thought an article
on “Taking the Mystery out of Metal Fatigue” would
be interesting, helpful and timely.
Metal fatigue is
a term that we often hear, but seldom fully
understand. In the past, we heard about the Concorde
losing a portion of its tail rudder on an
around-the-world flight, the United Airlines Flight 811
accident*, an American Airlines 727 that made a
successful wheels-up landing at DFW, and a
Continental Airlines aircraft that was discovered to
have large cracks during a routine inspection at DFW.
Photograph A shows overall view of
United Airlines Flight 811 front
cabin failure.
Photograph A Overall
view of United Airlines Flight 811 with front cabin failure.*
These incidents all made the national and/or local
news and all tend to raise questions and fears about
metal fatigue. What is it, when does it occur, can
it be prevented, can it be found, how is it
identified, etc.?
Metal
fatigue is a failure or fracture mechanism.
Basically, things break. When things break,
certain conditions must exist to make that thing
break by metal fatigue. When things break by metal
fatigue, the fracture surface will have certain
visible characteristics. Fatigue occurs when a metal
or non-metal (plastic or composite) component is
subject to fluctuating or cyclic stress and strain.
The term fatigue has its origin in the early
railroad industry. It was noted in the mid-1800’s
that the axles of railroad cars would separate
without warning (usually causing an accident) after
prolonged use. Evidently, some of the early
investigators postulated that the metal axles “just got tired” and the metal failed because
of fatigue. Although the metal does not “get
tired”, the terminology stuck and failure by cyclic
or alternating stress is still today called metal
fatigue. The misconception that the metal gets
tired and/or crystallizes persists even today.
I had
the privilege of investigating for the United States
Coast Guard during the Ranger 1 accident. The Ranger
1
was an offshore oil rig that collapsed south of
Galveston in 1979. Untold thousands of dollars were
spent during the investigation and many highly
educated, experienced engineers, college professors,
and accident investigators rendered their various
expert opinions to the Coast Guard Accident
Investigation Board. I delivered a multimedia,
three-slide-projector presentation (at the time,
very high tech). The final report consisted of
three volumes, each about an inch thick, containing
literally hundreds of photographs. The Board
was conducting its questioning of me, when the Senior
Coast Guard Officer, Commander Whaley, asked, “Dr. Jerner, is this a case of 'TARD ARN?'” I laughed and
said yes, and the Commander replied “Dr. Jerner, we
have spent thousands of dollars on this
investigation and I commend you for your thoroughness,
but just after the accident we heard some testimony
from the eyewitnesses, one of which was the driller,
and his opinion was that it was 'TARD ARN.'” Now it
seems that when the driller gave his opinion, he was
difficult to understand (probably having a West
Texas, South Louisiana, or just plain oil field
accent) and was asked to repeat "TARD ARN". He was
asked again, this time becoming very frustrated with
the Board, and even more slowly spelled "TARD ARN"
(Tired Iron). To appreciate this story, one has to
imagine a South Texas oilfield driller with probably
40 to 50 years of experience, who has just been
dumped into the Gulf of Mexico by a collapsing oil
rig. He is now sitting in this hearing where these
“dressed up” Coast Guard Officers, in their white
uniforms, cannot understand a simple concept like "TARD ARN!" The findings were indeed “tired iron” or
metal fatigue of the drilling rig leg attachment
joints. The wave action of the water against the legs created an
alternating bending stress which induced the metal
fatigue.
Conditions for metal fatigue are all around us.
A common example/explanation of metal fatigue is bending a
paperclip back and forth (cyclic stress) until it
breaks. Since many of you may try this example- don’t bend it in the
curved section, only in the straight portion of the
paper clip. Failure can also result from
pressurization and depressurization of an aircraft
during takeoff and landing, or high winds causing the
wings to oscillate up and down, winds blowing a bank
of lights at a stadium, large trucks passing by
highway signage, drill pipes being used to drill an
oil well, lug bolts on the wheel of a car or truck,
wire rope passing over a pulley, a surgical nail
installed to assist healing of a broken bone, the
crankshaft of an engine (car, truck, aircraft),
incompletely welded pipeline, and on and on. As can
be seen, metal fatigue is possible in items we use
and rely on daily. You can also see that fatigue or
metal fatigue can be prominent in many failures and
accidents. As previously, indicated metal fatigue
can occur in any place we see cyclic (changing) or
alternating stress.
A
fatigue crack starts at a point, usually on the
surface of the component, and then grows into the
metal as the item is subjected to the cyclic
stress. The time required to start the crack can be very
short, if a surface
defect due to use and/or abuse,
manufacturer’s design, welding, etc. is present.
Sometimes
initiating the crack can take millions of cycles of
stress, even years, in other situations. It is
generally agreed that the crack will grow or extend
if the component is subject to a tensile or pulling
stress. Compressive or pushing stress does not grow
a crack because in a compressive stress situation,
the crack is being closed up on itself.
If metal
fatigue is all around us, can it be prevented?
The answer is yes. Technological advances, resulting
from years of research and development and countless
hours of testing and failure analysis, have led to
numerous ways in which to minimize the probability
of starting fatigue cracks. However, the mainstay
in fighting fatigue is non-destructive inspection.
Non-destructive inspection (NDI) is a technology in
which various inspection techniques are used to
carefully, but non-destructively inspect the
component looking for probable fatigue crack
initiation sites or more likely actual fatigue
cracks in the structure or object. NDI can also be
used during the lifetime of a component (such as an
aircraft) in order to find any fatigue cracks which
might be growing. The idea here is to find and
repair or replace the cracked component before the
crack reaches a critical size. That is why airlines, aircraft owners,
and the government have mandated inspection after a
certain number of flights or flight hours.
*Correction by David Cherbonnier from DLC Engineering, Singapore: The B-747 pictured is United
Airlines Flight 811, not Aloha Airlines
--top--
Part I -
Part II - Part III
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