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Metal Fatigue – One
Way Bending
The swinging gate, of course has two hinges and two
hinge pins. Interestingly, the other hinge was
mounted (welded in place) in such a way that it
experienced mostly one way bending. The fracture
surface for the other pin is shown in Photograph A.
(New article new numbering)
Photograph A
In this case, the fracture initiated at the 1
o’clock position and propagated toward the 7 o’clock
position. A much smaller fatigue crack propagated
(on a lower level) inward on a thread below the main
fracture surface, from the 7 o’clock position.
The final, fast fracture region is the small band,
denoted with small white arrows, which are oriented
from about 9 o’clock to 4:30 o’clock.
Fatigue cannot usually be accurately timed, i.e.,
this crack has been growing for two years, three
months and twenty two days. The inability to age or
time a fatigue crack can be seen by considering a
cross country airplane trip. During takeoff from
NYC, the wing stresses are high (and if small cracks
are present, they may grow microscopically), between
NYC and the Great Lakes, the stresses are much lower
(and there is no microscopic crack growth). Then
passing over the Great Lakes, there are times of
extreme air turbulence (notice we have left out
complicating factors such as jet stream). Then over
the Great Plains, the weather/winds are calm and
wing stress is very low. When crossing the Rocky
mountains, updrafts (and downdrafts) are
experienced. As can be seen, on a six or seven hour
flight, it would be impossible to age any crack
growth to specific stress events in the trip. The
extreme stresses which MIGHT promote crack growth
are not continuous/intermittent and thus to use and
average stress over the time in the air would be
incorrect. However, under certain, very unique
circumstances, “timing” can be accomplished. I have
worked on at least two “wheel off” tractor/trailer
truck cases where aging or timing of the growing
fatigue crack could be accurately predicted. Each
case was unique in that new wheel/lug studs were
replaced when a flat tire was fixed. Thus when the
studs failed on the first trip out, ALL prior
stress history of the truck was irrelevant, i.e.,
the critical component, i.e., the wheel/lug studs
which failed were changed at the last stop.
Sometimes, the fracture surface even at a visual
level, can give us useful information. A close-up
view of a fractured hexagonal shaft is shown in
Photograph B.
Photograph B
In Photograph B, the fatigue fracture surface
exhibits, in classic “beach marks”, “thumbnail
marks”, “stop marks”, and are clearly present in the
darker portion of the fracture surface (see small
white arrows). However, from Photograph C, it can
be seen that newer cyclic fatigue crack evidence (in
the areas denoted “new fracture surface”) is an area
where fatigue crack growth occurred long after the
darker fatigue crack surface shown in Photograph B
was created.
Photograph C
The upper half of the fracture surface is where fast
fracture (final separation) occurred. This final
“fast fracture” region is shown in Photograph D.
Photograph D
Thus, just visually the fracture surface tells us
that an “old fatigue fracture” surface is present,
newer fatigue fracture is present (created at a
distinctly different time and environment) and the
fast fracture region created at the time of final
failure/separation.
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Dr.
R. Craig Jerner, Ph.D., PE specializes in accident
investigation and metallurgical failure analysis,
with over 30 years experience as a metallurgical
consultant and accident investigator. He has
testified as a metallurgical expert in over 250
depositions and more than 70 court appearances. If you or someone you know should need the services of Dr. Jerner and J.E.I. Metallurgical, please visit our web site at the buttons below, or e-mail Dr. Jerner --- 