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Failures of Metals


Crankshaft that failed due to fatigue. Photo Credit: Peter Lewis: Fatigue of Material. English Wiki.

The potential consequences of failed metal components can hardly be overstated. Economic damage, potential injury, and
loss of production capability are all likely outcomes of a metal component failure. For this reason, metal failures are often
investigated in-depth with the intent of avoiding a repeated failure in the future. Determining the cause of failure requires a
systematic, step-by-step investigation. Fortunately the science of forensic engineering is well established and allows the
determination of the root cause of failures with a high degree of accuracy.

The primary types of metal failures are:
• Overload
• Fatigue
• Stress-corrosion cracking
• Hydrogen embrittlement

OVERLOAD
Overload failures are caused by loads that stress the metal beyond its ultimate strength. These types of failures are generally
found in components that were simply loaded well beyond their design specifications, whether purposefully, or accidentally.

There are two categories of overload failures. Ductile failures occur when the component stretches or bends to some degree
before failure. Brittle failure occurs when the component breaks with little or no distortion. The manner of failure is determined
by the type of material, its heat treatment, etc. Overload failures occur very rapidly.

While dangerous and inconvenient, overload failures are not the most common type of metal failure, and are normally avoided
by simply keeping the component loading levels well below design specifications.

FATIGUE
Fatigue failures are caused by repeated or fluctuating loads and may occur even when the part is stressed below its yield
strength. Fatigue failures are the most common types of metal failures. Fatigue failures occur very slowly and yield
recognizable markings on the fracture surface. Repeated loading cycles allow a small material failure to grow unchecked into
a larger one, potentially leading to total component failure. The fatigue crack begins at the surface of the material, usually at a
point such as a machine mark, corrosion pit, surface scratch, or other imperfection that allows the crack to gain a foothold and
propagate. The crack then progresses slowly across the fracture face leaving a distinctive pattern called “beach marks” which
indicates the direction of progression. Final failure occurs when the remaining unaffected area is insufficient to carry the
applied load.


Beach marks showing the progression and direction of a fracture in a failed railroad rail. Source: Transportation
Safety Board of Canada. www.tsb.gc.ca

STRESS-CORROSION CRACKING
Stress-corrosion cracking can occur where stresses acting on a part, in combination with a chemical environment,
can lead to failure. This type of failure normally occurs under circumstances where loading alone would not cause
failure without the addition of the chemical influence. The failure can be sudden and unexpected in normally ductile
materials. This can be especially pronounced in metals experiencing exposure to chemical agents at high
temperatures or pressures.


HYDROGEN EMBRITTLEMENT
Hydrogen embrittlement can lead to brittle fracture by the unintentional absorption of hydrogen during forming and
finishing processes such as chromium plating. It occurs most frequently in high strength steel.

Each of the above failure types leaves unique identifiers, which can help identify the exact nature of the metal
failure. Each case has its unique properties, but in common, they share the characteristic that they can be difficult
to anticipate, and account for during the design process.

Because each of the above failure types can occur unexpectedly, and sometimes occur while the material is
loaded below design specifications, thorough testing under conditions that simulate the anticipated environment is
often the best way to identify where potential failures may occur.

 

 



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