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



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



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



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



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. •

design - bolt design and avoiding failure

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