Root Cause Failure Analysis...

Forensic and Metallurgical Analysis
Failure Mechanisms
Root Cause Determination
Mitigation Guidelines

Forensic and Metallurgical Analysis

The first step in a root cause failure analysis is forensic investigation, data and material gathering. It is critical that an independent metallurgical expert be included in the initial field investigation team to document the failed parts by detailed photography, gather critical parts, other material and data.

These steps need to be completed prior to any clean-up or alterations are conducted to the equipment and the scene. Such information is not only critical for the engineering investigation, but also will help with potential insurance claims.

Discussions with operators present at the time of failure and maintenance staff about the circumstances of failure need will be necessary. Recorded data from the control and monitoring systems on the various operational variables leading up to the failure incident need to be gathered. The critical questions to be answered are: 1) how did this failure happen?, 2) why did this failure occur?, and 3) what can we do to prevent this failure occurring again?

The main objectives of the metallurgical analysis are to determine the following:

  1. Location(s) of failure origin, fracture initiation & propagation mechanism(s),
  2. If the material(s) meet the respective material specification requirements (such as chemical composition, microstructure, hardness, tensile properties, creep properties, toughness, etc.), and
  3. The presence of environmental contaminants and their nature (size distribution, metallic or nonmetallic, chemical composition, type of compounds, etc.)

After the failed parts are brought to the metallurgical laboratory, further analysis may include detailed close-up photo documentation, collection of any contaminants, nondestructive examinations, study of the fracture surface by optical and scanning electron microscopes and documentation under high magnifications. Hardness tests and metallurgical sectioning at selected locations and planes will reveal the condition of the material microstructure. Chemical analysis of the base alloy, coatings and contaminants may be conducted by selected methods.

Failure Mechanisms

Components can fail by several mechanisms depending on the operating conditions and the environment in which they operate. Some of the common failure mechanisms encountered in turbines are:

  1. Foreign and/or domestic object impact damage
  2. High-cycle fatigue (HCF)
  3. Low-cycle fatigue (LCF) / Thermal mechanical fatigue (TMF)
  4. Creep
  5. Environmental attack (oxidation, corrosion, pitting, erosion, etc.)

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Results from such metallurgical analysis are critical and will help define the next investigation steps. An experienced metallurgist with in-depth experience will be required to conduct such an analysis and determine the most probable failure mechanism.

Root Cause Determination

A simplified RCA information flow diagram is shown here.

The root cause of failure may become apparent at the conclusion of the metallurgical analysis if there is deficiency discovered in the material or aggressive environment is found. More often the failure 'mechanism' is misinterpreted as the 'cause' of failure. In the case of HCF, LCF, TMF and creep failures, the external circumstances (to the component) which drive the failure mechanism in question need to be understood and verified. Further testing and analysis to determine the root cause of failures may include one or more of the following:

  1. Aero-thermal analysis
  2. Stress analysis
  3. Temperature distribution
  4. Vibration and dynamic analysis
  5. Fracture mechanics analysis

Crack initiation may indicate failure in some cases, whereas, crack propagation life may be significant in other cases. Only after the results of such analyses are available, one can determine the most probable 'cause' of the failure.

Mitigation Guidelines

Any RCA concludes with specific recommendations for corrective action to be implemented to prevent future such failures. By definition, if the identified cause is corrected to prevent the reoccurrence of such failure, it becomes the 'root cause' of the failure. In many cases, corrective actions are introduced without a good understanding of the 'root cause.' The failures may reoccur, which may lead to economic loss with potential danger to life and property. Corrective action may include one or more areas as illustrated in the above chart. Corrective actions should address all of the critical issues identified during the RCA process.

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