Failure Analysis – the Basics
Failures happen, often despite the best efforts of designers, engineers, and fabricators. The first consideration after a component failure should be the potential for other similar components in service to exhibit the same type of failure.
There are many potential causes of failure and where to begin a related investigation can seem like a daunting task. The investigation of failures is often referred to as a root cause analysis or RCA.
Very often a multidisciplinary approach is needed to perform a root cause analysis. For a structural failure, involvement of an engineer, materials/metallurgy professional and oftentimes a welding specialist is necessary. The engagement of an individual knowledgeable with the process/factors important to the design and the performance of the failed component is also helpful.
An additional consideration is the potential for litigation related to the failure. In such cases, legal counsel should be sought early in the process and failed components must be preserved for examination and involvement by all parties with an interest in the matter in testing and assessment activities. This is particularly important if any destructive testing or inspections are intended.
Tools that may be applied in a structural RCA include: computer simulations based on the Finite Element Analysis (FEA) method or Computational Fluid Dynamics (CFD) algorithms, Scanning Electron Microscopy (SEM) — potentially with Energy Dispersive Spectroscopy (EDS) for metallurgical assessments, fundamental engineering principals and concepts, and critical thinking.
In performing any type of component assessment, some criteria against which to judge the findings is needed. In the case of an RCA, guidance and requirements can be found in Codes and Standards, Customer Specifications and other applicable documents such as Purchase Orders and contracts.
Once a failure analysis has been completed, determination of the actions needed to prevent future occurrences is the next logical step. Such actions may include any or all of the following: design changes, operating parameter modifications, use of alternate materials, changes in fabrication techniques, and more.
The negative aspects of a failure should not overshadow the valuable lessons that can be learned from it. Analyze, understand, and address the cause(s).
Finally, RCA findings and the resolution(s) should be documented for use by peers, co-workers and future generations of engineers and designers who will inevitably ask themselves “I wonder why they did that?”
Interdisciplinary teams are essential for performing most failure analyses, since the root cause is rarely the result of a single variable. Understanding not only the metallurgical properties but also the environmental effects and stresses to a system or component is important in determining the cause of a material failure.
Once we identify component failure modes, we assess component remaining life, recommend inspection methods/ intervals, develop repairs to components and modify equipment designs to reduce the likelihood for future failures and ensuring structural integrity.
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Above – We were asked to investigate the failure of a splined clutch power transmission shaft. This macrophotograph at 7x shows a broken tooth with simultaneous fatigue initiation at the root radius of each side of the tooth. Reverse loading ultimately lead to fatigue fracture.
@ O’Donnell Consulting Engineers