Accident-investigation

for finding the causes

Accident investigation Primer

Accident-investigation helps finding failures origins and root cause.

The information is essential for correction of dangerous situations and for prevention of future disasters.

It is amazing to find, in a respected engineering Forum, a question like this:
"We recently found a failure in our equipment
[a short description follows]. What could be the reason?"

With all due respect, the question is not the right one.
One should not attempt to guess the reasons of failures.

One should investigate them.
Only by looking at all evidence and clues, by tying all the facts together, one arrives at the most probable cause of the accident.

Accident-investigations have a most important, if costly, function.

That is to teach all involved what is at fault and what needs remedy and improvement.

By investing in depth, looking at the reality without being clouded by any preconceived theory, one can learn from experience.

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That is why Accident-investigations must always be performed when failures occur in service.

Breakdown Accidents happen when service conditions exceed the circumstances assumed in the design phase of the project or when some hidden weak points grow in time and develop to a full blown failure.

Attentive learning of good Accident-investigation reports can teach a lot about how to conduct such a research.

Anyone might profit from looking somewhat deeper into the Accident-investigation subject.

To anybody interested in the subject of this
page the following Book is recommended:
Understanding How Components Fail ,
3rd Edition.
Click on "ASM store" in the following page:
http://www.asminternational.org/

In the Search Window Type the title above.
Then click on the book title from the list.

For a deeper approach look at the following
Treatise from the same source as above:

ASM Handbook Volume 11:
Failure Analysis & Prevention
R.J. Shipley and W.T. Becker
ASM International, 01-Dec-2002
1164 pages

Failures occurring in service can be due to one or more of the following origins: weld defects or inadequate welding procedures, corrosion attack, wear damage, fatigue cracks, overload burst, overheat or run out service conditions.

In other cases, also to faulty design, wrong material selection, metallurgical breakdown.

The good news is that not all service failures can be imputed to the welder, or to the shop, or to the procedure.

The bad news is that those who manufactured the failed item may have to prove it.

That is why it makes sense to understand what an investigation is and how it is done.

Tip!: When an accident arises it is imperative and also good practice to leave everything as it was at the time of occurrence, until the expert arrives.

Any needless manipulation of the fractured surface and surroundings could erase important clues that the investigator would have used to reach his/her conclusions.

Responsibility and damage assessment

Why an expert is needed to perform an Accident-investigation? Sometimes because of legal requirements, especially if life or property were endangered. Other times because of insurance claims.

An Accident-investigation is needed also in every other case whenever the origins of the failures are not obvious, to find how to prevent new cases.

A well known example is that of Accident-investigation in flight disasters, that often cause temporary grounding of all aircraft of the same type of that involved in the crash.

At least until specific controls are established, and it is ascertained that no common damage is likely to affect the whole group.

There is a need for a wealth of knowledge and experience that form the tools of the expert's trade.

Would you like to know how an Accident-investigation is conducted?
Find hereafter a few outlines for orientation.

As for any metallurgical Accident-investigation one should first collect all the available evidence including witnesses reports, details on time and circumstances of failure occurrence, on operating conditions before failure, was there fire, explosion or any other event connected.

Were there victims or collateral damage?

What were the operating fluids and their temperature and pressure, and other auxiliary materials, if any. Notes on shut down procedure for the facility and removal of heat or load or both should be included.

Description of the appearance and first assessment of failure importance,  photographic documentation of the cracked or fractured surfaces is part of the expert's work and should be left under his/her responsibility.

Then one should collect all documentation available including drawings, on the manufacture of the items involved, on base material and filler material specification and certificates, on the welding process used, on the welding procedure, on the welder involved (welders identification code marked near the welds is required by many specifications).

One should look for documentation on all the tests and inspections records that were performed, including radiographs of the failed area, if any exist: these records must be preserved for a specified number of years, according to binding specifications.

Final load test report at the time of manufacture, if used, should also be made available.

If documentation is available, one should confirm by analysis and tests that the chemical composition of the material and its heat treatment condition were in effect as prescribed and requested by design.

If such documentation is not available, the expert will use all of his/her knowledge and experience to obtain examination of materials, condition, hardness, mechanical properties, and especially of the appearance of the fractured surfaces.

Importance of unbiased observation

Failures at weld locations will be the object of special attention.

One should assess if the welded assembly was realized as prescribed by drawing and relevant specifications.

Then one should review if design took into account the working stresses as applicable for the service involved.

One should also investigate if working conditions were significantly different from those presumed in design.

After having collected all available information the investigator is now ready to inquire into the Accident.

First a thorough visual inspection is needed, by the naked eye but also with some low power enlarging stereo-microscope.

The nature of many failures resulting in fractures can be determined by examining the fracture surface by the techniques of fractographic analysis.

The investigator shall observe and document if fractures appear ductile or brittle, if signs of fatigue propagation or of corrosion products are present, if there are wear marks, gross deformation, burst or other remarkable aspects.

The important features must be documented by photographic imaging. The appearance of fractured surfaces is studied by a discipline called Fractography.
See our page on Fractographic Examination.

For a deeper approach look at the following Treatise:

ASM Handbook Volume 12:
Fractography
Publisher: ASM International
Published: 1987
Pages: 517

One should investigate if the physical condition of the surrounding area presents evidence of local damage by mishandling or by some other accident.

Subtle clues show major culprits

Only when all studies of visual characteristics have been completed, time is ripe for metallographic sections.

These sections through fractures, duly prepared with grinding and polishing, are examined under the optical microscope after etching, to reveal the form and consistency of metallic grains near the fracture.

The actual position of the fracture in relation to weld location can hint at possible metallurgical problems to be addressed by metallographic examination.

If welds are involved, shape and dimensions of beads and appearance of the heat affected zone are examined, for presence of possible defects or unwanted metallurgical phases.

In certain cases where clear cut results cannot be obtained, the expert may have recourse to more sophisticated means like scanning electron microscope (SEM) and other specialized techniques.

In many cases any expert will profit from unbiased observations proposed by a curious and thorough observer, no matter what his/her official rank or preparation, familiar with the facility and its operation.

Especially when they appear as striking deviations from usual instructions or behavior.

Common service failures likely to surface in any Accident-investigation, are burst fractures, overload failures, deformation, corrosion generated, wear or heat dependent failures, metallurgical defects, faulty design failures, wrong material selection and inadequate welding procedures.

Only expert metallurgists, familiar with this type of inquiries and knowledgeable with the disciplines involved can conduct a thorough Accident-investigation.

Burst service failures, usually of ductile character, can derive from overpressure, or overload, if the original thickness and soundness of the material at the failure place is still in good condition, not undermined by wear or corrosion.

Deformation caused by faulty assembly could easily build up excessive stresses capable of inducing fatigue cracks and fractures, especially if combined with superficial stress raisers, in presence of vibrations or shocks.

Corrosion diffused on the surface or concentrated in fissures should be evaluated, if connected to the failure location.

Stress corrosion is a dangerous combined condition that could have been easily avoided if anticipated.

An Article on Thermal Fatigue was published (2)
in Issue 16 of Practical Welding Letter for December 2004.
To read it click on PWL#016.

An Article on Welding Failure Investigation was published (2)
in Issue 66 of Practical Welding Letter for February 2009.
Click on PWL#066 to see it.

The Mid February 2012 Bulletin 70 -
Resources on Weld Failure Prevention
was published as PWL#102B. Click on the link to see it.

An Article on Asset Integrity Management was published (11) in Issue 140 of Practical Welding Letter for April 2015.
Click on PWL#140 to see it.

An Article on Advice to the New Welding Engineer was published (2) in Issue 143 of Practical Welding Letter for July 2015.
Click on PWL#143 to see it.

An Article on Example of a Failure Analysis was published (7) in ISSUE 147 of Practical Welding Letter for November 2015.
Click on PWL#147.

Watch this Weld Failure Investigation Video!

Thermo Compressor Failures: RCA Case History

http://www.youtube.com/watch?v=6B6Jx3o_vas

NEW RESOURCES

Find some interesting links in a special Mid Month Bulletin Page of our PRACTICAL WELDING LETTER, designed to offer you, our interested readers, the opportunity to search the web quickly and effectively on the subject of Corrosion and Corrosion Prevention.

We urge you to explore this rich source of essential knowledge.

Online Resources on Corrosion and Corrosion Prevention , presenting Articles, Downloads, Links, Previews and Information is now available by clicking on PWL#039B.
For Online Resources on Failure Analysis click on PWL#053B.

Looking for more Online Reference Links? Click on Welding Resources

* * *

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To reach a Guide to the collection of the most important Articles from Past Issues of Practical Welding Letter, click on Welding Topics.

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