FASTENER FACTS

Where does fatigue failure occur?

The most common locations for fatigue failure include

the joint interface (i.e., first "loaded" thread), the fIllet,

the threads, and the thread expiration or "run-out." As

the industry has developed better materials and production

methods to improve fatigue strength, the threads

have become the weakest point of the fastener and currently

account for the highest number of fatigue failures.

How is fatigue strength measured?

The number of variables involved and their interdependence

in fastener performance have made setting

standards for fatigue strength a difficult task. Currently,

the number of "cycles to failure" is used to determine a

relative strength for a series of fasteners. This complex

measurement offers a standard of performance that

encompassesa ll the variables of the fastener,w hich will

fmally fail at its weakest link.

Modified Goodman Diagram

Socket Head Cap Screws, Rolled Threads.

Stress based on the area at the basic thread minor diametel:

The failure of fasteners in industrial and aerospace applications

costs u.s. industry-and the public-billions

of dollars every year in downtime and lost production.

Injuries and even deaths have resulted. More than 60

aircraft accidents have been attributed to fastener failure.

While recent legislation has mandated that suppliers

meet industry standards for these critical components,

no existing regulation for industrial fasteners specifically

addressesth e causeo f an estimated8 5% of all fastener

failures- fatigue.

What is fastener fatigue?

A typical industrial fastener, say, a socket head cap screw,

looks absolutely rigid, but in fact it is-it must bequite

flexible. Due to such factors as design, material,

method of manufacture and heat treatment, a cap screw

will "stretch" when subjected to mechanical and/or

thermal pressure. Such cap screws constantly stretch

and return to their original shape. (If they are subject to

excessive stress, of course, they permanently deform

and eventually destruct.) These stretch-and-returna ctions

are called cycles. A socket head cap screw can be subject

to perhaps 240 cycles a day (e.g. , in an 800-ton

press) all the way up to 1 million cycles a day (e.g., in

an ultrasonic horn).

As this peak-to-peak cycling occurs, the fastener is subject

to stress. Eventually a crack will occur, just as it does

when you rapidly flex a paper clip back and forth. The

crack occurs at the fastener's most vulnerable point, referred

to by engineers as the "maximum stress concentration

area." The crack spreads and fastener fatigue

failure has occurred.

The art of manufacturing industrial fasteners is a constant

search for these various Achilles' heels and an ongoing

development of design and manufacturing methods for

overcoming them. The paradox of that quest is that, once

you've "cured" one area of vulnerability, you have, in

truth, created another. If not replaced, most dynamically

loaded fasteners will suffer fatigue failure eventually;

the only question is when they will fail. The fastener

designer's objective becomes one of extending the

number of cycles to failure at a given dynamic load.

MEAN STRESS, KSI (APPLIED PRELOAD)

Modified Goodman diagrams help designers predict

fastener perfonrzance. The broken diagonal line depicts

the mean of the alternating load for a screw with a

00% probability of enduring 10 million cycles. The

diagonal solid lines show that the maximum deviation

of dynamic load from the mean stress is :t12 ksi when

the screw is preloaded to 100 ksi.

Threads: A generous radius in the root of the thread

reduces the concentration of stress caused by a "flat

root" profIle. Equally important is the proper radius in

the thread run-out. Again, this lessens stress by reducing

sharp comers and improves fatigue strength. Note

that this radiused run-out is not mandated by common

socket screw specifications.

ASTM standards require that threads be formed by rolling

rather than cutting or grinding. Threads formed by

rolling will ensure that the grain flow follows the thread

contour. If the rolling is done after heat treatment the

fatigue life can be increased by several hundred percent,

due to the residual compressives tressesin duced by the

process. Rolled threads provide a smooth finish, reducing

the susceptibility to a fatigue failure that could propagate

from a surface imperfection. ASTM standards

define acceptance criteria for thread laps that can initiate

a fatigue crack. These standards appear to be the

most commonly violated. Although often overlooked,

they are critical to the fatigue life of the fastener.

Heat Treating: While heat treatment is used to produce

stronger parts, improper treatment can result in conditions

that will greatly reduce the fatigue strength of the

fastener. Carburization (increase in surface carbon making

the surface harder than the core) and decarburization

(surface softer than the core) will reduce fatigue performance.

Microstructural changes and cracks can be

caused by insufficient temperature control. The wrong

quenching media or procedure may not produce parts

hardened throughout and can also cause cracking.

Surface Finish: ASTM standardss pecify surfacef inishes

for different parts of a fastener. A rough surface frnish

on the screw threads or body or even a slight deforrnation

in the fillet area represent potential initiation sites

for a fatigue failure.

Guarding against failure

Clearly, design, purchasing and other industrial specifiers

concerned with fasteners must take their own precautions

to guard against fatigue failure. In this endeavor,

responsible manufacturers of fasteners are constantly

seeking out and shoring up the points of vulnerability

referred to earlier.

The ultimate goal is to increase the number of "cycles to

failure." Here are some stages along the trail to that goal.

Head Construction: ASTM standards require heads to

be forged rather than machined. This precludes the planes

of weakness caused by machining and increases the head

fatigue strength. In addition, head height, socket depth

and width, and wall thickness must all fall within strict

tolerances in order to ensure proper key engagement. This

allows the socket head cap screw to be tightened to a

high preload, thereby minimizing the cyclic loads felt by

the fastener. It should be noted that limited hex key engagement

and/or oversize sockets can lead to screw

failure at low wrenching torques.

Fillet Design: A smooth fIllet with the correct radius

for the application will help to reduce fatigue failure by

blending the sharp profIle where the head meets the

shank. An elliptical radius will provide better distribution

of stress and decrease the possibility of fatigue

crack initiation.

By Paul R. Franco

Manager. Product Engineering

SPS Technologies, Unbrako N.A.

Some conclusions

The Fastener Quality Act, as well as a full range of

ASTM, ANSI, and military specifications, offers limited

protection from fatigue failure by providing guidelines

for individual fastener parameters. In order to protect

the end user and the public, designers and specifiers

must go beyond these regulations to ensure the synergistic

match of all facets of fastener production and supply.

Preventing fatigue failure includes starting with proper

design, working with a qualified supplier, taking advantage

of the most modem materials and production capabilities,

and drawing on the manufacturer's extensive

application and engineering experience.

The manufacture of fasteners must be implemented in a

carefully controlled process, taking into account all the

physical, mechanical, and chemical issues raised above.

Perhaps most important is a thorough process control and

assurance program, designed with the end user's application

and tolerances in mind, to guarantee adequate

testing, both in-process and after completion.

Considering the high incidence of fatigue failure and the

possible associated catastrophic costs, end users should

consider using fatigue requirements in their specifications

for critical fasteners. A qualified manufacturer can

build these criteria into the production process and conduct

tests for process verification.

As the future brings even lighter parts, even fewer fasteners

in critical applications, and even more exacting part

specifications, the designers and specifiers of these parts

must be even more aware of the role of fatigue in fastener

failure. By working closely with an experienced, qualified

supplier, designers and specifiers can help to

reduce the incidence-and the cost-of fastener fatigue

failure.