design - bolt design and avoiding failure
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JO INT DES IGN ED ITED BY STEPHAN IEM URASK I JOHNSON
To ensure threaded fasteners w ill bear their load, designers mustspecify m ore than the quantity and size of bolts.
Why bolts fail
JOHN BUDA
Product Line Manager
Unbrako Div.
SPS Technologies
Cleveland, Ohio
The fact that threaded fasteners can
be removed from an assembly, per-
mitting joints to be disassembled,
makes them more convenient than perma-nent methods of fastening, such as welding
or riveting. But this convenience also makes
them more complex, increasing the poten-
tial for failure either of the fastener or of the
joint. Bolt failure is usually a visible, atten-
tion-getting event, but the ways in which a
joint can fail can be more subtle.
S t r e s s e d o u t
Bolts generally fail due to
one of four causes: overstress,
fatigue, corrosion, and em-brittlement. Overstress is per-
haps the simplest cause to un-
derstand - the loads on the
bolt, whether in tension,
shear, or bending, are simply
too high. Most designers' pri-
mary consideration is tensile
load, a combination of
preload, or the tension in-
duced during installation, and
some additional in-service
load. Preload is essentially in-
ternal and static, compressingthe joint components. Service
loads are the external, often
cyclic, forces experienced by the fastener.
Tensile loads attempt to pull the joint as-
sembly apart. If these loads exceed the
bolt's yield limit, they will stretch the bolt
beyond its elastic range into the plastic re-
gion. This, of course, causes the bolt to de-
form permanently, so the original preload
cannot be regained when the external load is
removed. Similarly, external loads on the
bolt that exceed its ultimate tensile strengthwill fracture the bolt.
To obtain the desired preload, most bolts
are torqued into place. Overtorque, the re-
sult of inducing an excessive torsional com-
ponent during installation, can also over-
stress a fastener. This reduces the fastener's
axial tensile strength. That is, a continu-
ously torqued bolt yields at a lower value
than the same bolt pulled in straight tension.
Thus, a bolt may not be able to be torqued to
a preload corresponding to its publishedminimum tensile strength. Torquing to
NOVEMBER 21, 1994 M AC HIN E D ES IG N 85
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I
J O IN T D E S IG N
\Torque - i nduced
tens ion
S tr aig h t t en s io n
a ne r to rq uin g
fopre load
T or qu in g a fa ste n er to i nduce
te ns io n c re ate s d iffe re nt y ie ld
c h ar ac te ris tic s t ha n p ro d uc in g
te ns io n b y p ullin g it d ir ec tly .
yield can maximize bolt preload, minimizejoint relaxation, and improve preload con-sistency. However, bolts should not betorqued to yield unless there is sufficientdifference between yield and ultimate ten-
86 MACHIIIEDESISIINOVEMBER 21,1994
sile strength.Shear loading exerts a force perpendicu-
lar to the bolt's longitudinal axis. Manyaerospace designers prefer shear loadingbecause it takes advantage of the bolt's ten-sile and shear strengths. Because it actsmainly as a pin, a fastener in shear results in
a relatively simple joint. However, draw-backs prevent shear joints from being usedmore often. They require more material andspace, and accurate material data frequentlyare not available to convert tensile stressesinto design shear loads.Fastener preload affects the integrity of
shear joints. The lower the preload, the eas-ier it is for joint plies to slip, placing them incontact with the bolt. Shear load capacity iscomputed by multiplying the number ofbolts in the joint by their shear strength, andthen multiplying by the number of trans-
verse planes (one shear plane is known assingle-shear and two shear planes as dou-ble-shear). These planes should traverse abolt's unthreaded shank. Designing a shearplane through the thread is not recom-mended because the fastener's shearstrength can be overcome by stress concen-trations as the cross section changes.Whenrating afastener's shear strength, some de-signers use tensile stress area, while othersprefer minor diameter area.
Ifthe bolts in shear joints are torqued to
spec, mating surfaces or plies cannot beginto slide until the external forces exceed thefrictional resistance (preload times contactsurfaces' friction coefficient). Increasingfriction between mating surfaces can en-hance joint integrity, sometimes limitingthe number of bolts that must be used.In addition to tensile and shear loads,
bolts are subjected to bending stresses.Bending stresses arise from bearing andmating surfaces that are not perpendicularto the bolt's longitudinal axis, and the loca-
tion and direction of external
forces. Here, especially, thesimpler the joint, the greaterits integrity and reliability.
T h e c o m m o n c a u s e
Fatigue is less straightfor-ward, but it is the major causeof failure, estimated to ac-count for approximately 85%of bolt failures. Most of thesefailures occur in "tension-ten-sion" applications, where the
bolts are subjected to a smallpreload and an alternating
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J O IN T D E S IG N
service load. These cyclic stresses can cause
bolts to fail at loads less than their rated ten-
sile strength under near-static conditions.
Fatigue life depends on the number and
magnitude of loading cycles. Although not
reported as frequently, joints subject to
compression, such as those in presses,
stamping equipment, and molding machin-
ery, can also exhibit fatigue failures. Here,
the operating dynamic is preload stress mi-
nus cyclic compression stresses. As in ten-
sion-tension applications, the number and
magnitude of stress changes induce the fa-
tigue.
Another cause of bolt failure is corrosion,
which can take a variety of forms, including
chemical decomposition, galvanic corro-
sion from dissimilar metal contact, and
stress corrosion cracking. Chemical decom-
position results from exposure to agents
ranging from rainwater to acid. Designersshould review metals compatibility tables
or eliminate electrolytes to prevent galvanic
corrosion. Stress corrosion cracking is rela-
tively limited. It affects primarily high-
strength alloy steel fasteners under high ten-
sile loads in the presence of corrosive
agents. In these failures, fractures typically
start as cracks at surface corrosion pits. Cor-
rosion assists crack propagation at a rate de-
termined by the stress on the bolt and the
fracture toughness of the material. Com-
plete failure occurs when cracks have made
the remaining functional area too small to
bear the applied stress.
Higher strength steel fasteners (generally
Rockwell C36 hardness and above) are
more prone to a condition known as hydro-
gen embrittlement. Simply stated, atomic
hydrogen is introduced into and diffused
throughout the material. Shortly after a load
is applied, the hydrogen migrates to the
highest stress locations, settles between
grain boundaries, and fractures the fastener.
Hydrogen can be introduced during acid
cleaning, pickling, electroplating, and expo-sure to hydrogen-rich environments such as
those found in chemical plants and laborato-
ries. Steel corrosion also produces hydro-
gen as a by-product.
When a fastener contains a critical mass
of hydrogen before installation, it usually
fails in less than 24 hours. Unfortunately,
time to failure is virtually impossible to pre-
dict if hydrogen is introduced after a fas-
tener is installed. Designers specifying fas-
teners prone to embrittlement should select
a supplier with the expertise, resources, and
procedures to minimize the potential for
embrittlement.
Joint failure is not always directly related
to catastrophic fastener failure. A number of
fastener-related factors, such as loss of
preload or fastener yield, can cause a joint
NOVEMBER 21, 1994 M AC HIN E D ES IG N 87
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J O IN T D E S IG N
to wear, shift
alignment, create
noise, leak, re-
quire unplanned
maintenance, or
otherwise fail.
Vibration, for ex-
ample, can re-duce thread fric-
tional resistance.
Also,joints canre-
lax after installa-
tion due to appli-
cation of service
loads. These and
bolt creep (es-
pecially at ele-
vated tempera-
tures) can cause
preload loss.
Sometimes jointfailure can be at-
tributed to through holes that are
too large, bearing areas that are too
small, materials that are too soft, and
loads that are too high. None of these
conditions are the result of direct failure of
the bolt, but they can cause loss of joint in-
tegrityor eventual bolt failure.
D ete ctive w ork
Proper diagnosis is as im-
portant as understanding thecauses of bolt failure. The first
step in analyzing the failure is
to collect all broken parts.
Some may indicate the pri-
mary cause of failure, while
others may have broken as a
consequence of the initial
fracture. Thus, it is necessary
to examine all the remaining
pieces to reduce the risk of
drawing the wrong conclu-
sion about the type of failure.
And don't reassemble theparts before the examination; this
could damage the evidence. Finally,
find out where the parts were at the time
of failure, and identify the portion of the
bolt that initially failed.
Some features to look for during an anal-
ysis include:
• Failure in the head area, which may
indicate the presence of bending
stresses or fatigue.
• Reduced area (necking) between the
underhead and first engaged thread(the grip length), which suggests over-
stress. This grip length can also be the
site of shear failures, which do not
exhibit the same necking or stretching.
• Cratering, pitting, and visible by-
products such as rust indicate
corrosion.
• Failures at the first engaged thread,
which has the highest concentration ofstress, are virtually always the result
of fatigue.
• Hydrogen embrittlement can beindicated when bolts are electroplated.
Other clues include a clean, flat
fracture occuring shortly after installa-
tion, and an absence of necking or
bending.
Armed with this information, the joint
designer can prevent the same type of fail-
ures from recurring. Tensile failure, for ex-
ample, can be overcome by designingagainst overstress. Most basically, select
fasteners of the right size and strength level
to accommodate anticipated loads. Also
consider installation variables such as lubri-
cants, plating, and adhesives, as well as
bearing surface and internal thread materi-
als. These factors can be used to determine
the torque coefficient needed to calculate
the correct installation torque.
Proper torque is critical to preventing
failures. Overtorquing can lead to tensile
failure. Undertorquing can result in a loosejoint subject to slippage, leaks, and ulti-
mately fatigue failure. Because torque is a
means of controlling bolt preload, the de-
signer should begin with the specified
preload and then derive installation torque.
Some industries use the following equation
to calculate installation torque: T =KDP,where T = tightening torque (lb-in.); K =
torque coefficient; D = nominal fastener di-ameter (in.); and P = bolt preload (lb). The
empirically derived torque coefficient, K,
attempts to account for every variable in-
cluding friction, bearing surfaces, thread se-ries, material hardness, and surface texture.
Shear failures are straightforward. They
can be prevented by using more, larger, or
stronger fasteners. Moreover, higher
preload increases frictional resistance be-
tween mating surfaces, and helps hold the
joint together.
Fatigue is the most frequently reported
cause of bolt failure, probably because it
is the most difficult to prevent. Fastener
features that can reduce stress concentra-
tions, and thus lessen the chance of fatigue,include forged heads, rolled threads with
B B MACH INEOES I 6NNOVEMBER 21, 1994
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J OIN T D E SIG N
radiused roots and
runouts, elliptical fillets,
controlled head-to-shank
perpendicularity, good surface
finish, and properly proportioned
bearing area. Special additional process-
ing, such as rolling threads and fillets after
heat treating, provides even greater fatigue
resistance.Installation preload is also critical to pre-
venting failures. Generally, in tension-ten-
sion joints where external forces increase
bolt load, the higher the preload the better.
High preloads in stiff joints transfer less of
externally applied loads to the bolt, reduc-
ing cyclic loading. In contrast, tension-com-
pression applications involve cyclic loads
that relieve bolt load. In these instances, low
preload can reduce the amplitude of cyclic
loading on the bolt. However, a locking ele-
ment may be necessary to keep the bolt
from loosening, falling out of the assembly,or becoming fatigued. A crucial factor in
designing against fatigue, whether in ten-
sion-tension or tension-compression appli-
cations, is to minimize the amplitude of al-
ternating stresses, keeping them well below
the limit the bolt can bear.
M a te ria l c ho ic es
Protective coatings or corrosion-resistant
fastener materials can prevent corrosion-re-
lated fastener failures. Protective barriers
include phosphate treatments for carbon
and alloy-steel fasteners, rust-inhibiting
90 MACH INE D E S I G N NOVEMBER 21, 1994
oils, and coatings applied either to the fas-
tener before installation or to the entire joint
afterwards. They also include electroplating
the fastener with a sacrificial coating such
as cadmium, zinc, or tin. Silver plating is of-
ten used for high-temperature applications
and, because of its lubricity, prevents
galling of stainless-steel fasteners.
Corrosion-resistant fastener materials in-
clude various grades of stainless steel; su-
peralloys such as Aerex 350 alloy, from
SPS Technologies; Hastelloy, from Haynes
International; and 20Cb-3 from Carpenter
Technology; plus nonferrous metals such as
brass, titanium, and aluminum. For applica-
tions requiring both high strength and re-
sistance to stress corrosion cracking,
materials such as A-286 (UNS-S-
66286), Inconel 718, and SPS's Multiphase
alloys MP35N and MP159 are possibilities.
Not all these materials resist corrosion inevery environment, and the most corrosion-
resistant material for a particular environ-
ment may not be suitable for a fastener.
However, identifying materials already per-
forming well in identical corrosive environ-
ments can help in selecting a fastener mate-
rial. Also, the decision whether to use plated
standard alloy materials or custom-manu-
factured corrosion-resistant materials is pri-
marily economic. When selecting materials,
a designer should keep in mind that thread
stripping can have the same catastrophic re-
sults as bolt fracture. Thread stripping is a
function of the relative strengths of internal
and external thread materials and length of
engagement.
All the above precautions may not pre-
vent failures, however, if the joint fails not
because of defective bolts, but because the
right type of fasteners were not specified
initially. A joined assembly should be
viewed concurrently as a system, not as iso-
lated elements such as bolt and nut or screw
and tapped hole. As a result, the joint de-
signer must consider:• what fasteners are available
• what standards the engineer must use
• how to keep fasteners easy to reach
• weight of the completed assembly
• how the product is packaged, and
• the cost of installing and maintaining
the joint.
Joint performance also is affected by fas-
tener properties, installation technique and
accuracy, lubrication, adhesives, locking el-
ements, service environment, and field
maintenance. •