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Location/Shipping Address:
925 Industrial Drive South
Sauk Rapids, MN 56379
(View Map)
Mailing Address:
Engel Metallurgical Ltd.
PO Box 6100
St. Cloud, MN 56302
E-mail Address:
engelmet@engelmet.com
Phone Numbers:
320-253-7968
1-800-258-7968
Fax: 320-253-7917 |
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Preserving Failed Components
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| When
a part fails, what should you do? |
It might be more appropriate to ask, "what should you not do?" When in doubt, do nothing. Put another way: look but don't touch, because what may appear as "junk" to the untrained eye could well be valuable evidence to those skilled in failure analysis.
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Getting
Started
Failure analysis is an extremely complex subject involving areas of mechanics,
physics, metallurgy, chemistry, electrochemistry, manufacturing processes,
stress analysis, design analysis, and fracture mechanics, to name a few.
Since it is nearly impossible for any single person to be an expert in
all these fields, it is extremely important to know when and where to
seek help. A competent failure analyst can assess the situation and bring
into play the appropriate specialists to help reach an accurate conclusion.
Utmost care must be taken with the failed components as the retrieval
and analysis investigative processes begin. Step number one is observation.
Simply study the affected area visually. The second step should be to
contact an expert in the field of failure analysis. Very often that expert
can advise you over the phone as to what you should do next. In most cases,
photographic documentation will be necessary. In some cases, an on-site
inspection by the analyst would also be prudent. The third and most important
step is to preserve all the evidence.
Preservation and Shipping
Keeping in mind the old axiom, "a little knowledge can be a dangerous
thing, " we'd like to outline in general terms some ordinary, common sense
things that you can do to prevent any inadvertent destruction or distortion
of clues that could be vital to the final determination as to the cause
of failure.
The greatest
of care must be taken in the proper selection, preservation, and cleaning
of fracture surfaces. Equal care must be given in the transportation of
a fractured part for analysis. And make sure you send everything. If a
bolt breaks, send both halves, the nut, and preferably the complete assembly
to which the bolt was fastened. It's necessary that the entire item be
examined whenever possible. When a fracture occurs, it seems natural to
put the mating pieces back together. Whatever else you do, you must restrain
that very natural tendency. Even in the best of circumstances, fracture
surfaces are extremely delicate and can be easily damaged. It may appear
to the naked eye that no damage would take place, but that's not the way
a microscope would see it. Many microscopic examinations have been frustrated
by careless repositioning of the parts, by careless packaging and shipping,
and by inadequate protection from corrosion, including contact with fingers.
The surface of the fracture must remain as uncontaminated as possible
and protected from further damage.
Let's
assume that an on-site inspection by the failure analyst was determined
to be unnecessary or impractical. Instead, you have been asked to ship
the critical parts for inspection. At that point, you will undoubtedly
be instructed on the specifics of protecting the components during removal
and transit. You will probably be asked to wrap each component separately
to assure that the damaged areas of each piece are thoroughly protected.
You can imagine the damage that would occur if two metal components were
simply tossed into a carton with no protective padding and allowed to
bang against each other while being shipped. Corrugated cardboard, newspaper
or bubble pack securely bound around larger components might be recommended
- plastic wrap or a zip-lock bag for small parts. Each situation is different,
however. It's far better to ask and be certain than to assume and make
an error.
To help
avoid damage from corrosion, handle the specimens as little as possible,
Try to keep all vital components out of the weather. A common recommendation
is to dry specimens with a jet of warm air and then pack them with a suitable
desiccant to absorb moisture. And if you suspect that the problem itself
was one of corrosion, never smear the part with grease in an attempt to
preserve it. Check with the analyst first, because appropriate, timely
action is necessary for the most accurate result in corrosion analysis.
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| Major
Losses |
SEM Photo of Aluminum Fracture
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Note loss
of detail after four light brushings
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Don't be in a hurry to clean up the mess. In the case of a major loss you can expect competing interests to be vying to affix blame. The wise route to follow would be to contact an unbiased failure analysis expert and bring her or him into the picture at the start, because the obvious answer may not be the correct answer. Was it a faulty beam that caused the collapse of that structure or could it have been a bad batch of concrete in the pier? A few snapshots aren't always going to give you the answer after the bulldozers have been through. It takes careful and thorough examination of all possibilities while the elements are still in place. Even in major cases, the principal rules still apply: look but don't touch, call an expert, and preserve the evidence.
Just the Facts Please
Discovering the cause of component failure is like unraveling a complex
mystery. A large number of factors, often interrelated, must be understood
to determine the cause of the primary failure. But as in finding the solution
to any mystery, it is first necessary to get all of the facts out on the
table in their original state of failure whenever possible. Knowing what
not to do is just as important as knowing what to do.
Failure Analysis and the SEM |
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