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Metal Fatigue Part
II:
What Does Fatigue on
a Fracture Surface Look Like?
Metal fatigue
is one fracture mechanism that can
easily be identified, even by “amateurs”. The
second part of this article will give the reader
some of the visual clues that can be used to
identify a fracture created by metal fatigue.
As indicated previously, a fatigue
crack will grow as the component is used. Conversely, without
cyclic/alternating stress (or a sufficient level of
cyclic stress) a fatigue crack in a component or
structure will not grow. Thus, if a record is
available and/or can be predicted, i.e., number of
flights, hard landings, severe winds, etc. and when
and where severe stress occurred in the structure
and then periodic inspections can occur and those
inspections will be timed to catch fatigue cracks.
That is to say, things, components, structures
(aircraft) can and will have cracks in them. Isn’t
it comforting to know the airplanes we fly have
fatigue cracks in them, yet they continue to
function perfectly? The secret to safe use and
a most
important concept in fatigue analysis is critical
crack size. That is, a fracture (complete
separation) will not occur until the crack grows to
a size that is critical. Another way of saying this
is, a crack can exist, and even grow, but the metal
will not separate or fracture just because a crack
is present. The metal will separate when the crack
reaches critical size. This is the “whole” concept
behind the inspection of aircraft. You find/catch
the cracks, remove/repair/blunt the cracks before,
in fact, long, long before, they reach critical
size. That is, the (NDI) inspector might miss the
crack during the first inspection (say February),
but the next inspector will find it on the second
inspection (say June). Even if the second inspector
also misses the crack, a number of additional
inspections will occur before the crack reaches
criticality.
Let’s try to visualize the concepts behind metal
fatigue. Photograph A is a pin, used to swing or
pivot a large electronic gate at a very large, high
security facility in the DFW area.
Photograph A:
Fracture surface from a large
electronic gate pivot pin. The white arrows indicate
the fatigue crack origin.
As the gate swings open and closed, cyclic stresses
are applied to the gate hinge pins. Because of a
machining/design/manufacturing error, a fatigue
crack starts in one of the hinge pins at the 10:30
o’clock position. In operation, this pin is in a two-way bending.
To help visualize two-way bending, take the ends of
a pencil, one end in each hand, and bend/bow it down
(without breaking) and then up. As the pencil is
bowed up, the top of the pencil will be arched.
Think of fibers oriented longitudinally along the
pencil. These fibers are stretched and thus they are
under a tension/pulling stress.
Actually, another fatigue crack did start at the
4:30 o’clock position (see arrow at 4:30 o’clock
position in Photograph A). Every time the gate
cycled between open and closed the subject gate pin,
experienced tension first, (lets say, as the gate is
opening) on one side of the pin and then as the gate
swings closed, tension is experienced on the other
side of the pin. The fatigue crack grew first
because of the cyclic tension stress, from the 10:30
o’clock position and then from the 4:30 o’clock
position. The fatigue crack grows at each position
only when a tensile stress is present, i.e., the top
or bottom fibers in the bent pencil are stretched.
As the gate is used, the cracking grows into the
pin, from the surface toward the center. As the
crack gets larger, there is less metal to take up
the applied stress each time the gate cycles and the
crack grows faster and faster. Finally the crack,
or in this case the two opposing cracks, reach
critical size and there is insufficient metal left
in the pin to take up or transfer the stress from
opening and closing the gate and fast fracture
occurs (the remaining metal rapidly fails in an
instant). The pin separates unexpectedly and
possibly catastrophically. The final fast fracture
is the darker band in the center of the pin
fracture, which is oriented from about 7:30 o’clock
up to 1:30 o’clock.
--top--
Part I -
Part II - Part III
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