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All failures are caused by human errors, of which there are
three general types:
Errors of knowledge
Errors of performance (which might be caused
by negligence), or
Errors of intent (which may come down to acts
of greed)
What are often called “acts
of God” are more or less widely spaced natural events, such
as the flooding associated with unusually large storms, earthquakes, and so
forth. In terms of geologic time rather than the very short human experience,
these events are certainties, not exceptions. They will happen, given enough
time. Failures associated with “acts of God” are, again, the results of
under-design for the actual conditions the component or system faces in service.
Errors of knowledge usually involve insufficient knowledge,
education, training, and/or experience. Here are a few examples of such errors
of knowledge
Ancient Romans used lead in their wine
goblets. Using them over long periods produced lead poisoning and ultimately
insanity. 19th century Arctic explorers repeated this failure in their food
containers.
Dendrite growth on metals in conductive ionic
environments produces short circuits in electronic components for computers.
Hydrogen Embrittlement (HE) causes otherwise
stable high strength steel components to fail.
Degassing produces bubbles and ultimately
corrosion in coated cast iron pipes.
Internal and external corrosion of gas lines
in the early 20th century caused frequent urban explosions.
NASA Shuttle disasters involved both O-ring
and ceramic insulation failures.
Errors of performance result from lack of sufficient care
or from negligence. Negligence involves such things as misreading of drawings,
inadequate specifications, and defective manufacturing and workmanship. Some
examples are:
Recent NASA failures in a Mars mission
involved the incorrect conversion from the English to the Metric System of
measurement in a computer program.
The Chernobyl Nuclear Power Plant accident
involved a major failure in design of the safety system.
Failures in Human Breast Implants involve an
insufficiently durable packaging for the silicone materials of which the
implants were made. Explosions of natural gas have been caused by the spark
from a car’s ignition being started next to a leaking pipeline.
Errors of intent very commonly involve greed. Greed leads
to actions usually carried out with a conscious or unconscious denial of full
knowledge of the potential consequences. In other words, the perpetrators
convince themselves that their actions will not have serious impacts. For
example:
Cost reduction driving design of military
vehicles causing premature failures.
Exxon Valdez and many other oil tanker spills
were caused by using single hulls in super tankers.
Aloha stadium superstructure corrosion
failures were caused by lack of surface preparation and poor materials and
coating selection.
Failure of bonding of steel belts in Firestone
radial tires on Ford SUV’s caused many roll-over accidents.
An interesting example of combining various types of errors
is found in the production of
galvanized steel. Since the
1930's it has been
known that introduction of approximately 0.15% of aluminum into a hot-dip
galvanizing bath will cause the formation of a thin aluminum-iron-zinc
intermetallic layer at the steel surface. This intermetallic layer acts as a
barrier to iron migration into the zinc, preventing the formation of brittle
iron-zinc intermetallics. With this Al-Fe-Zn layer in place, when the galvanized
component is bent during service, the zinc layer deforms plastically, rather
than fracturing. When manufacturers experience high rates of cracking in their
galvanizing layers, they have often let the aluminum concentration in their
galvanizing bath slip out of control.
This failure may be a combination of all three kinds of
errors. The manufacturer may simply not know that the aluminum concentration is
so vital to the success of his product, he may just be letting his quality
control function slip out of negligence, or he may be unwilling to spend the
money necessary to mount an effective quality control program in his plant.
These ideas provide the philosophical underpinnings for a study of failures. It is important to recognize that many failures are preventable if we understand the materials and their intended applications well enough and are willing to pay the required costs for safety and durability.
