TechnologyThe Process for
Investigating
Fastener Failures

When a fastener fails, it is important to gather as muchinformation about the application and details of the installation as
possible as this will not only help determine the root cause of thefailure but how further failures may be prevented.

Photo 1. Ductile Fracture•This is the time we categorize the data we collected. Sometimes wecan tell by the appearance of the samples and fracture surface what may
have happened. This will narrow the questions and failure possibilities.
For instance, a ductile fracture (Photo 1) will exhibit dimples
with an inclusion, or a cups and cones appearance with a dullcolored surface. It will also show some type of distortion, as thematerial will deform plastically before ultimate failure. However,if the load is applied very rapidly, as in a shock or impact load evenwhile tightening, a ductile material may have the appearance of abrittle fracture. This can happen when using unregulated pneumatic
installation tools.
Check the threads of the bolt to determine if there is a change inthread pitch. This will indicate if the bolt was stretched into yield either
by service loads or during installation.
A brittle fracture may be flat, similar to metal fatigue but
is more shiny, exposing grain boundar y cracking, or grainboundary cleaving. Brittle fractures will not show visible signsof distortion. Some fractures may have the characteristic markings ofchevron or waterfall patterns, which point to origin of stress initiation.
Metal fatigue (Photo 2) will display the conchoidal or 'beachmarks' striations across the fracture surface. These will be aseries of bands which may be dark, light or both. The darker bands
signify low frequency impacts or vibration, while the lighter bands areindicative of a higher frequency or more rapid impact loads.
Photo 3 is of a metal fatigue fracture of a wheel stud. There
are several initiation points in the fracture zone which suggests thewheel stud experienced a rotational load and a bending load in a loose
condition.
Typically, fatigue fractures initiate either at the last thread
run-out or at the first unengaged thread protruding from the nut.
Fastener World no.206/2024 惠達雜誌

What type and grade strength of fastener
was used?
••

Did the grade of the nut match thestrength of the fastener?
• Were all of the fasteners used in theconnection the same grade or type?
Was the fastener strength compatible withthe joint service loads?

•

How was the fastener used?

•

Was it a static load?
Was it subject to extreme
environmental conditions?
• Was it in a dynamically loaded joint?
• Were there alternating loads?
• Were the loads axial?
• Was the joint subject to vibration orimpact loads?
• Was the fastener subject to shearloads?
Ancillary components used:•• Flat washer; type, size, coating.
• Nut; standard, locking, grade, coating.
Were there multiple fasteners in the joint?

••

Photo 2. Metal Fatigue

Investigation

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Basic Questions to Discover:•Were the failures adjacent to eachother if multiple failures?

•How were the fasteners installed?

•• By hand wrench• Pneumatic tool
• Torque wrench
Where were the fasteners tightened?

•• Bolt head• Nut• Both
What is the condition of the joint surfaces?

•• Rusty or rough• Painted or smooth
What is the condition of the failed parts?

•Where is the location of the fracture?

•• At the head?
• The thread run-out?
• First thread outside of the nut?
What is the condition of the fracture surface?
•Smooth, dull, bright, rusty?

Technology

Photo 3. Metal fatigue
fracture of a wheel stud

Photo 4. Flat washers

Photo 5-A. Allmetal lock nut

Photo 5-B. Allmetal locknut (inside)

Photo 6. Environment

Ancillary Components Used
This basically means to check the grade strengthor property class of the bolt and nut to be sure the nut
is compatible with the strength of the bolt. If not, there
will be stripped threads and/or fatigue marks on thefracture surface because clamp load was lost.
Clamp load is also lost if a wrought flat washer
was used with a Grade 5 (8.8 metric) bolt or higher.
If the washers have indentations, clamp load was lost.
The washers in Photo 4 have caused a massive lossof clamp load. Metal fatigue from this lost clamp load
will cause a rapid bolt failure.
In addition to being too soft to sustain the service
loads of the joint, the washer on the left of Photo 4 has
an inside diameter that is too large to fully support theloads under the head of the cap screw. This washer istypically called a USS washer. The washer on the rightis an SAE flat washer whose inside diameter will fully
support the loads under the head of the cap screw.
Washers without indentations are made from hardened
steel and will sustain fluctuating loads.
Coatings can make a difference in environmentswith elevated temperatures. Photos 5-A and 5-B areof an all-metal lock nut that came off a turbocharger.
Similar examples have been used in exhaust manifolds
or boiler vessels. The inside will look like Photo B.
The gold colored nuts are cadmium plated and will
fail from Liquid Metal Embrittlement at temperatures
exceeding 400⁰F or 204⁰C. LME is time dependent
so failure will take longer at lower temperatures thanhigher. Zinc plating will also cause LME but at muchhigher temperatures.

Types of External Loads andAmount of Load
Proper bolt selection depends on knowing
what the service loads are. If the bolt is bent, it was
not the correct strength or diameter for the application.
Heavy vibration and impact loads will cause metal
fatigue if the bolts are not properly tightened andmaintained. There should also be enough bolts in theconnection to carry the load and reduce the individual
bolt stress.
Some corrosive environments (Photo 6) canlead to stress-corrosion-cracking, where the grainboundaries become under chemical attack. Standing
water will set up an electrolytic cell and causehydrogen embrittlement.

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Fastener World no.206/2024 惠達雜誌

Photo 7. Failed
stainless steel

Photo 8. Too much
impacting with an air tool
can also crack the adjacentarea around the nut

Photo 9. Hex corners of a nut

Other times, when there is rust in the fracture zone, it can signify that thefailure was not new and had been propagating for some time. This could also cause
other stress initiation sites.
Agricultural machines come under heavy chemical attack, even if properly
washed down every day. Some have replaced their OE bolts for stainless steel
to avoid the frequent replacing of rusty bolts. Unfortunately, in some cases thestainless steel bolts (Photo 7) will fail in a bending mode and metal fatigue
because they weren’t as strong as the OE bolts they replaced.

How was the Fastener Installed?
This makes a huge difference because a fastener can never be evenly tightenedby hand with a hand wrench. Thread frictions vary and so does the installer’s ‘feel’.
Sometimes the handle length of standard wrenches does not provide the properleverage for higher grades of fasteners.
Torque wrenches are fine but not always accurate due to a multitude ofvariables, mostly including friction.
Pneumatic wrenches are mostly unregulated to output torque. They try
to seek a stall point from thread friction. If the threads were lubricated, theywould have either stripped the nut threads or stretched the fastener into yield.
Air wrenches are quick. They can cause galling and thread locking ofstainless steel fasteners, even if they are of different types of stainless:
it is the speed that destroys the fastener. The speed of assembly can also cause anunderloaded joint and subsequent metal fatigue because a high compression rate iscreated as the nut slams into the joint surface. This compressive force has an equaland opposite reaction or recoil which will leave the joint not as tight as expected.
The impact wrench will always leave tell-tale signs of abuse. In Photo
9, the hex corners of the nut clearly display markings from the socket on theinstallation side (right) of the hex corners. There are also markings on the off side(left) which appear not only from removal but from the recoil of the socket as theimpact goes forward and instantly recoils back.
With proper investigative techniques, the cause-effect of failures can be
determined so that preventive measures can be employed.
Copyright owned by Fastener World / Article by Guy Avellon