American Rifleman - March 2015 - 65

Are Your Guns

FATIGUE FAILURES

d and
Tire
ssed?
Stre
Sometimes, through no fault of their owners' use, gun parts simply fail. In many cases, "fatigue
failures" are to blame. So, can they be predicted? A veteran firearm engineer answers that
question-and explains how modern gun companies strive to avoid such failures altogether.

BY GEORGE E. KONTIS, PE

AMERICANRIFLEMAN.ORG

stress-analysis techniques, attention to component details
and extensive endurance testing, they can be effectively
prevented or delayed.

The Stress Riser

Just a few months after my visit to the Burge, I was
invited to go through the firearms collection at the
Milwaukee Public Museum. I came across an old top-break
revolver with the identical failure experienced by the
shotgun at the Burge. In both guns, the crack started
at the surface and then found its way to a hole before
continuing to grow. Why did the cracks pass through a
hole? Because holes are regions of high stress-called stress
risers. Failures always start at points of highest stress, and
as cracks grow they follow these weak areas-as though
they were "connecting dots." Besides holes, there are many
other types of stress risers, including: sharp internal and
external corners, sharp edges, tool marks, grooves and even
rough surfaces. Such areas can lower resistance to fatigue
by as much as 20 percent. Anywhere there is an abrupt
discontinuity, there is a stress riser, and where they occur
the stresses are higher than the surrounding area. As a
case in point, grooves in threads are high-stress regions. A
fatigue crack that starts in the groove of one thread often
propagates through adjacent grooves as it enlarges. Cracked
parts lead to broken parts after the crack grows so large
that the remaining material can no longer support the
load. When parts do break, it's usually without warning.
Since it's not possible to design gun parts without holes,
edges and corners, how does the gun designer determine
whether or not the stress riser is going to cause an early
failure? It's not difficult if the proper engineering software
is used. At Knight's Armament Co., for example, gun parts
and their associated mechanical loads are analyzed by a
MARCH 2015

65

Photos courtesy of author

arly last year I was invited to a hunt at the Burge
Club, one of middle Georgia's finest shooting
preserves for quail, pheasant and partridge. The
moment I entered the clubhouse, I noticed something new
had been added since my last visit, and it was attracting a
lot of attention. A group of shooters crowded around an old
double-barreled shotgun that had been unceremoniously
wired to the wall. It lacked engraving and was not a particularly fine make. Given that many of these hunters own some
of the finest shotguns produced, I became even more curious
as to what had piqued their interest. A closer look revealed
the answer. A large, ugly crack had propagated through most
of the receiver. "How do you suppose that happened?" asked
one hunter. The barrel was not bulged, there was no sign of
high pressure, nor was there evidence the shotgun had been
in an accident or a victim of abuse.
I went over for a closer look and immediately knew the
cause. The metal in the receiver of that old shotgun had
just gotten tired and stressed. Even the proper term for
what had befallen that gun helps explain its plight: fatigue
failure. Metallurgists have determined that 50 to 90 percent
of all mechanical failures are fatigue failures. The number
for firearms is around 80 percent. Fatigue failures occur after
parts are subjected to repeated loads that, individually, are
not large enough to cause a failure. After many repetitions,
however, a failure can occur.
Fatigue failures were first identified in the 1840s when
railway axles all across Europe began failing in epidemic
proportions. Engineers were sure they had designed the axles
to be strong enough to carry the intended load, but the
huge number of broken axles was undeniable and cause for
further study. Fatigue happens in all materials, but is best
understood in metals. Sadly there is not a simple analytical
method to prevent fatigue failures, but with modern


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American Rifleman - March 2015

Table of Contents for the Digital Edition of American Rifleman - March 2015

Contents
American Rifleman - March 2015 - CW1
American Rifleman - March 2015 - CW2
American Rifleman - March 2015 - Cover1
American Rifleman - March 2015 - Cover2
American Rifleman - March 2015 - 1
American Rifleman - March 2015 - Contents
American Rifleman - March 2015 - 3
American Rifleman - March 2015 - 4
American Rifleman - March 2015 - 5
American Rifleman - March 2015 - 6
American Rifleman - March 2015 - 7
American Rifleman - March 2015 - 8
American Rifleman - March 2015 - 9
American Rifleman - March 2015 - 10
American Rifleman - March 2015 - 11
American Rifleman - March 2015 - 12
American Rifleman - March 2015 - 13
American Rifleman - March 2015 - 14
American Rifleman - March 2015 - 15
American Rifleman - March 2015 - 16
American Rifleman - March 2015 - 17
American Rifleman - March 2015 - 18
American Rifleman - March 2015 - 19
American Rifleman - March 2015 - 20
American Rifleman - March 2015 - 21
American Rifleman - March 2015 - 22
American Rifleman - March 2015 - 23
American Rifleman - March 2015 - 24
American Rifleman - March 2015 - 25
American Rifleman - March 2015 - 26
American Rifleman - March 2015 - 27
American Rifleman - March 2015 - 28
American Rifleman - March 2015 - 29
American Rifleman - March 2015 - 30
American Rifleman - March 2015 - 31
American Rifleman - March 2015 - 32
American Rifleman - March 2015 - 33
American Rifleman - March 2015 - 34
American Rifleman - March 2015 - 35
American Rifleman - March 2015 - 36
American Rifleman - March 2015 - 37
American Rifleman - March 2015 - 38
American Rifleman - March 2015 - 39
American Rifleman - March 2015 - 40
American Rifleman - March 2015 - 41
American Rifleman - March 2015 - 42
American Rifleman - March 2015 - 43
American Rifleman - March 2015 - 44
American Rifleman - March 2015 - 45
American Rifleman - March 2015 - 46
American Rifleman - March 2015 - 47
American Rifleman - March 2015 - 48
American Rifleman - March 2015 - 49
American Rifleman - March 2015 - 50
American Rifleman - March 2015 - 51
American Rifleman - March 2015 - 52
American Rifleman - March 2015 - 53
American Rifleman - March 2015 - 54
American Rifleman - March 2015 - 55
American Rifleman - March 2015 - 56
American Rifleman - March 2015 - 57
American Rifleman - March 2015 - 58
American Rifleman - March 2015 - 59
American Rifleman - March 2015 - 60
American Rifleman - March 2015 - 61
American Rifleman - March 2015 - 62
American Rifleman - March 2015 - 63
American Rifleman - March 2015 - 64
American Rifleman - March 2015 - 65
American Rifleman - March 2015 - 66
American Rifleman - March 2015 - 67
American Rifleman - March 2015 - 68
American Rifleman - March 2015 - 69
American Rifleman - March 2015 - 70
American Rifleman - March 2015 - 71
American Rifleman - March 2015 - 72
American Rifleman - March 2015 - 73
American Rifleman - March 2015 - 74
American Rifleman - March 2015 - 75
American Rifleman - March 2015 - 76
American Rifleman - March 2015 - 77
American Rifleman - March 2015 - 78
American Rifleman - March 2015 - 79
American Rifleman - March 2015 - 80
American Rifleman - March 2015 - 81
American Rifleman - March 2015 - 82
American Rifleman - March 2015 - 83
American Rifleman - March 2015 - 82b
American Rifleman - March 2015 - 83b
American Rifleman - March 2015 - 82c
American Rifleman - March 2015 - 83c
American Rifleman - March 2015 - 82d
American Rifleman - March 2015 - 83d
American Rifleman - March 2015 - 82e
American Rifleman - March 2015 - 83e
American Rifleman - March 2015 - 82f
American Rifleman - March 2015 - 83f
American Rifleman - March 2015 - 84
American Rifleman - March 2015 - 85
American Rifleman - March 2015 - 86
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American Rifleman - March 2015 - 88
American Rifleman - March 2015 - 89
American Rifleman - March 2015 - 90
American Rifleman - March 2015 - 91
American Rifleman - March 2015 - 92
American Rifleman - March 2015 - Cover3
American Rifleman - March 2015 - Cover4
American Rifleman - March 2015 - CW3
American Rifleman - March 2015 - CW4
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