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Track Down The Facts To Analyze Systematic Failures

Looking At The Clues Of Any Mechanical Breakdown Can Lead To Improvement, Not Just Repair.

Ronald L. Hughes

02/14/2013

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As T.S. Eliot so correctly observed, Failure is relativeit is wh at we can make of the

mess we have made of things. It is easy to see that the key to success when

analyzing failure is not to react to a problem, but to be proactive by treating the

failure as an opportunity to learn.

Understanding the causes of failure can sometimes se em to be a daunting task for a

failure scene investigator called on to look at the causes of system or equipment

failure in a manufacturing plant. This is completely understandable given the apparent

chaotic circumstances that usually surround an incident under investigation.

Too often failure is reacted to in a manner that will put everything back in a known

"acceptable condition" as fast as possible without any real consideration given to

actually solving the incident through a well-thought-out investigative process.

Symptoms are not no ted, or ignored, and the evidence is either cleaned up or

destroyed.

When this occurs the failure will again manifest itself and typically reoccur at an

unexpected time. The good news is that when this happens often enough we become

more efficient at reacting to the problem and the refore seemingly better at correcting

the situation more rapidly, thus decreasing dow ntime or mitigating the consequences

of the incident being investigated. This mind-set, or restraining paradigm, is a failure

unto itself.

Analysts must realize that the life of any component is not infinite but predete rmined

based on the stresses being subjected to the component. Therefore, engineering designs not only transform a need into a des cription of

a product, but also take into account the des igns compatibility with the related expected physical stresses induced into the component

based upon its functional requirements. This includes the life of the product (as measured by its performance over time), reliability, andmaintainability.

Equally important to the ana lyst is the realization that failure seldom occurs for a single reason or comes from a single force or input. This

quickly becomes evident when chasing a ll of the poss ibilities that the evidence leads the investigator to explore. Therefore, most failures

are typically a result of a multiplicity of inputs and errors and are depicted by the logic tree in a multiplicity of legs illustrating all the

cause-and-effect relationships o f the failure.

Reading the clues

Every incident analyzed will occur within a specific timeline representing the time between when the anomalous conditions of the failure

first manifested themselves to when the failure was safely isolated. The failure data that is found within this timeline provides the clues

or evidence that is needed to uncover the cause o f any incident or failurebe it sporadic or chronic.

Every piece of data w ill beg a question as to how can this data be in the condition found or the position found. When the investigator

can answer the how can questions, and tie the anomalies to a specific point within the timeline, then he or she has s uccessfully

followed the pa th to failure for the incident under investigation. In short, the investigator has found the root cause (s) of the failure.

Equally as important as understanding that the clues exist within a specific timeline is the fact that any failure can be analyzed by also

understanding the principles of how failure occurs within that timeline.

The following three principles of failure analysis can be use d during an investigation to follow the pa th that led to an incident:

Order and pattern

Determinism

Discoverability

Each can be used during the investigation to follow the path tha t led to the incident.

Order and pattern

There is order and pattern to everything in the universe; the sun

comes up and the sun goes down, the tides go in and the tides go

out, there are four seasons in a year, and so on. There is also order

and pa ttern to failure, and by unde rstanding this simple principle it is

only logical to conclude that an order and patte rn of failure exist w ithin

the timeline of the failure under investigation. The key is to read the

clues to uncover the order and pattern that led to failure.

Determinism

Just as the re is an order and pattern that e xist within the timeline of any failure, there are also determinable effects that exist w ithin the

order and patte rn. To sta te it in simple terms, every input will produce a set of known outputs, and every produced output came from a

known input, that is, the de terminable effects. The key to determinism is to make sure that the inputs and outputs are in the correct

order and patte rn. (The cause is below the effect in the logic tree.)

For example, consider the following:

Does misalignment cause high vibration or does high vibration cause misalignment?

Both are possible but which one occurred? This means that the analyst must determine which cause-and-effect relationship is correct. In

this scenario e ither the equipment w as initially misaligned or it was aligned correctly and became misaligned.

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Once this is de termined then the cause-and-effect relationship is

knownthat is, if misaligned, initially misalignment caused high

vibration, or if aligned correctly and became misaligned, high vibration

caused misalignment.

Discoverability

Seldom is there a single cause or a single path to failure.

Discoverability, when applied in the investigative process , helps the

analyst to ensure that all the possible causes have been explored and

accounted for by the ana lysis. The key is to s tart as broad and all-

inclusive as possible while working through the specifics.

By asking the question how can over and over again, and

systematically working through the cause-and-effect relationship o f

the failure, all the poss ible root cause scenarios are explored and

accounted for in the analysis.

Following the concepts o f order and pa ttern, determinism, and

discoverability makes it easy for the analyst to graphically illustrate

the investigation on the logic tree and document the analysis as

shown in the example below.

The science of the clues

The clues uncovered during the investigation can always be a ccounted

for by a scientific explanation of the anomaly. For example, you can't

have electricity outside the realm of Ohm's law, and you can't have a

fire without a heat or ignition source, a fuel source, and an oxygen

source.

Even something as s imple as color provides scientific evidence for the

analyst. Color changes in materials indicate different exposures to

temperature or corrosive products.

The color of smoke changes with different fuel sources. The color of

lubrication products changes with the loss of a dditives, contamination,

temperature, or pressures that overcome the film barrier.

Another key principle for the analyst is the mechanics o f fractures.

Fracture mechanics, or "fractology," is the study of the propagation of

cracks in materials. It is based on the use o f analytical solid mechanics

to calculate the driving force on a crack and experimental so lid

mechanics to characterize the material's resistance to fracture.

Fracture mechanics is therefore an important tool for determining the

expected mechanical performance o f materials and components.

By applying the physics of stress and s train (in particular the theories of e lasticity and plasticity) to the microscopic crystallographic

defects (morphology of the fractured surface) found in materials, it is easy to predict and understand the macroscopic signatures of

mechanical failure seen on the components fractured surface face.

This technique is used to understand the the oretical causes o f failures and also to verify the forces that must be present based on the

pattern of the fractured surface. In essence, the fractured face te lls what kind of force caused the failure. So if the type of force

necessary to cause the failure can be found, it can be e liminated or mitigated, thus reducing the likelihood o f recurrence at some future

date.

Often when an analyst fails to achieve success, the tendency is to s imply change the definition of success to a level that can be more

easily obtained. Although this will allow the investigator to quickly move on, it obviously limits the payback from the root cause a nalysis

effort. By changing this restraining paradigm to one that seeks the maximum return by only accepting true success and proactively

conducting a fact-based ana lysis driven by the evidence, the paybacks then become expediential instead of incremental.

The quality of the clues of the failure, and the correct interpretation of what the clues tell the analyst, are wha t determines the degree of

success for the incident unde r investigation.

Ronald L. Hughes, ME, is a member of theAmerican Society of Mechanical Engineers and theAmerican Society of Training and Development. He

is currently a senior consultant for the Reliability Center Inc., an engineering and consulting firm.

Logic tree example.

Color tells the story.

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