Every month in FMEA Corner, join Carl Carlson, a noted expert in the field of FMEAs and facilitation, as he addresses a different FMEA theme (based on his book Effective FMEAs) and also answers your questions.
Questions and answers are a great way to learn about FMEAs, for both experienced and less experienced FMEA practitioners. Please feel free to ask any question about any aspect of FMEAs. Send your questions to Carl.Carlson@EffectiveFMEAs.com, and your contact information will be kept anonymous. All questions will be answered, even if they are not featured in the FMEA Corner.
fail·ure [fāl-yuhr, noun]
The Merriam-Webster Dictionary defines "failure" as "a state of inability to perform a normal function."
mech·a·nism [me-kə-ni-zəm, noun]
The Merriam-Webster Dictionary defines "mechanism" as "a process or system that is used to produce a particular result."
What are failure mechanisms?
"Failure mechanisms are the physical, chemical, thermodynamic or other processes that result in failure. Failure mechanisms are categorized as either overstress or wear-out mechanisms. Overstress failure arises because of a single load (stress) condition, which exceeds a fundamental strength property. Wear-out failure arises as a result of cumulative damage related to loads (stresses) applied over an extended time." (From the Physics-of-failure-based prognostics for electronic products paper by Michael Pecht and Jie Gu. Published in the Transactions of the Institute of Measurement and Control. 2009, Sage Publications, Ltd., pages 309-322.)
Application of Failure Mechanisms in FMEAs
"Nature never breaks her own laws." - Leonardo da Vinci
For Design FMEAs at the component level, causes can be further defined and developed by understanding the underlying failure mechanisms. Causes are the circumstances that induce or activate a failure mechanism.
Wherever possible, for high-risk issues the FMEA team should define the cause at the failure mechanism level. This means that for System FMEAs or Subsystem FMEAs, the FMEA team should either proceed with a Component FMEA that can drill down to the precise failure mechanism that explains the failure mode and include it in the description in the cause column, or continue with the “Five Whys” until isolating the cause at the mechanism level. No matter how the FMEA team chooses to proceed, wherever possible the FMEA team should properly define the cause at the failure mechanism level for high-risk issues.
The FMEA team should include representatives that understand the specific underlying failure mechanisms for the type of items being analyzed.
FMEA Tip of the Month
One of the common mistakes is to describe the failure mechanism without also describing the cause of failure. When we say, "the FMEA team should properly define the cause at the failure mechanism level for high-risk issues," we mean that both the failure mechanism and the cause are described and included in the FMEA.
Application of Failure Mechanisms in Xfmea
In Xfmea, you can include the cause/failure mechanism description either in the FMEA "Cause" column or you can add a separate "Failure Mechanism" column.
If you want to include a separate column in Xfmea for "Failure Mechanism" associated with the cause:
- Open the FMEA Properties window by choosing FMEA > Tools > Properties Customization.
- In the window, click the Causes page, then change the Display Name for one of the user defined text fields to "Failure Mechanism," and click the Enabled cell to change it to "Yes." Then close the window.
In the FMEA worksheet view, the "Failure Mechanism" column displays at the far right by default. As shown below, you can move it to the desired location, such as next to the "Potential Cause(s) of Failure" column.
Scenario: You are part of an FMEA team that is working on a Design FMEA for the "Surface burner coil" item, which heats the burner plate on an electric range.
Problem: A portion of the Design FMEA for the item is shown below:
For the "Burner surface stays cold" failure mode, the team has identified two cause descriptions, including associated failure mechanisms.
Cause 1: Burner coil connector corrodes due to moisture intrusion.
Cause 2: Fatigue cracks in burner coil create open circuit due to wrong coil material.
Identify which portion of the cause description is the failure mechanism in both of these cause descriptions. [Show/Hide Answer]
Solution: Cause 1: Burner coil connector corrodes (failure mechanism) due to moisture intrusion (cause).
Cause 2: Fatigue cracks (failure mechanism) in burner coil create open circuit due to wrong coil material (cause).
I’ve always wanted to know about FMEAs
The important thing is not to stop questioning. - Albert Einstein
A HotWire reader submitted the following question to Carl Carlson. To submit your own question about any aspect of FMEA theory or application, e-mail Carl at Carl.Carlson@EffectiveFMEAs.com.
Our company is in the phase to work with an updated FMEA procedure based on your course that we went to last year.
I would like to have your thoughts regarding risk matrix and risk matrix ranking.
Carl: Glad to hear your company is working on an updated FMEA procedure.
The following is information from our technical support team:
Xfmea allows for extensive customization of the Risk Ranking process by providing the Risk Ranking Logic option for assessing risk based on Severity, Occurrence and Detection. Xfmea provides the framework for custom ranking based on simple or complex rankings or on combinations of provided values. Additionally the Xfmea software provides for additional ranking results if more than three levels are required. For example, you could have a Negligible, Marginal, Significant, Critical and Catastrophic scale for varying combinations instead of a Low, Medium, High scale.
Utilizing custom risk ranking logic avoids the pitfalls of RPN thresholds in general terms by allowing for the possibility of establishing a rating for every combination of values allowable.
Additional information regarding risk ranking logic is available from the Xfmea help file at: http://help.SynthesisPlatform.net/xfmea9/define_priority_levels_window.htm.
As can be seen, risk ranking logic has the ability to highlight with colors and additionally has the ability to color code beyond the typical three rating scale (Low, Medium, High). For example, "Negligible" could be green, "Marginal" could be light green, "Significant" could be yellow, "Critical" could be orange and "Catastrophic" could be red. Any of those color selections could be modified to show different gradients of the same color or any other color scheme. In addition, risk ranking logic can highlight either the RPN or a Risk Ranking Logic field, which returns the label for each category (Negligible, Marginal, Significant, etc.).
Once risk ranking logic has been set up, the results are query-able throughout the database, which gives practitioners and management an excellent tool to manage risk.
About the Author
Carl S. Carlson is a consultant and instructor in the areas of FMEA, reliability program planning and other reliability engineering disciplines. He has 30 years of experience in reliability testing, engineering and management positions, and is currently supporting clients of ReliaSoft Corporation with reliability and FMEA training and consulting. Previous to ReliaSoft, he worked at General Motors, most recently senior manager for the Advanced Reliability Group. His responsibilities included FMEAs for North American operations, developing and implementing advanced reliability methods and managing teams of reliability engineers. Previous to General Motors, he worked as a Research and Development Engineer for Litton Systems, Inertial Navigation Division. Mr. Carlson co-chaired the cross-industry team that developed the commercial FMEA standard (SAE J1739, 2002 version), participated in the development of SAE JA 1000/1 Reliability Program Standard Implementation Guide, served for five years as Vice Chair for the SAE's G-11 Reliability Division and was a four-year member of the Reliability and Maintainability Symposium (RAMS) Advisory Board. He holds a B.S. in Mechanical Engineering from the University of Michigan and completed the 2-course Reliability Engineering sequence from the University of Maryland's Masters in Reliability Engineering program. He is a Senior Member of ASQ and a Certified Reliability Engineer.
Material for the FMEA tips, problems and solutions is excerpted from the book Effective FMEAs, published by John Wiley & Sons, ©2012. Information about the book Effective FMEAs, along with useful FMEA aids, links and checklists can be found on www.effectivefmeas.com.