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FMEA Corner 
This Month's Theme is Interface Matrix
Next month's theme will be "can you anticipate everything?"

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.



in·ter·face ['in-tər-,fās, noun]
The Merriam-Webster Dictionary defines "interface" as "the place or area at which different things meet and communicate with or affect each other."

ma·trix ['mā-triks, noun]
The Merriam-Webster Dictionary defines "matrix" as "something shaped like a pattern of lines and spaces."


What is an FMEA Interface Matrix?

According to the book Effective FMEAs, "FMEA Interface Matrix is a chart with the subsystems and/or components (depending on the scope of the FMEA) on both the vertical and horizontal axes. The chart shows which interfaces must be considered in the analysis and the type of interface".

It is an optional diagram, one of four diagrams that can potentially be used as part of preparation for a System or Design FMEA. The other three diagrams are FMEA Block Diagram (usually required), Parameter Diagram (optional), and Functional Block Diagram (optional). Each has its own unique contribution to the FMEA application. Not all of these diagrams are required, so it is important to know when to use the various diagram tools. (See the "When is an FMEA Interface Matrix used?" section below.)

What exactly is an interface?

An interface is the point or surface where two parts or subsystems meet. There are four primary types of interfaces: physical connection, material exchange, energy transfer and data exchange. Since interfaces can contain up to fifty percent or more of the total failure modes, it is essential that any FMEA carefully considers the interfaces between subsystems and components, in addition to the content of the subsystems and components themselves.

What are examples of the four interface types?

Examples of physical connections include brackets, bolts, clamps and various types of connectors. Examples of material exchange include pneumatic fluids, hydraulic fluids, or any other fluid or material exchange. Examples of energy transfer include heat transfer, friction, or motion transfer such as chain links or gears. Examples of data exchange include computer inputs or outputs, wiring harnesses, electrical signals or any other type of information exchange.

When is an FMEA Interface Matrix used?

In most cases, an FMEA Block Diagram is an essential preparation step to visually show the scope of the FMEA. In some cases, where the FMEA team wants to ensure that all of the various types of interfaces are included in the analysis, an Interface Matrix can also be used. It is most appropriate for systems or subsystems, and is supplemental to the FMEA Block Diagram.

What does an FMEA Interface Matrix look like?

The following is an example of an FMEA Interface Matrix for a fictitious Design FMEA on an all-terrain bicycle. For illustration purposes, the matrix includes both the subsystems of the entire bicycle system, as well as the components of the Hand Brake Subsystem.

FMEA Interface Matrix Example

How is an FMEA Interface Matrix used in an FMEA?

There are a number of ways that an Interface Matrix can be used in an FMEA. For example:

  1. The Interface Matrix may identify interfaces that were missed in the FMEA Block Diagram, in which case the block diagram can be updated.
  2. Interfaces from the Interface Matrix can be input to function descriptions in the FMEA. In this example, there is a physical interface between the brake pad (1.9.3) and the brake caliper (1.9.4). Specifically, the brake pad is attached to the brake caliper with a bolt and nut. The function description in the FMEA worksheet might be "brake pad to brake caliper interface provides fixed and adjustable connection so that brake pad is in correct orientation with wheel rim."

Does Xfmea support FMEA Interface Matrix?

Xfmea easily attaches any document or diagram to the item being analyzed. The FMEA Interface Matrix can be generated in Excel or other worksheet software, and attached to the item being analyzed in Xfmea using the same procedure outlined in last month’s issue of HotWire.

!FMEA Tip of the Month

Use the visual display of the Interface Matrix to brainstorm with the FMEA team any missed interfaces. It is the author’s experience that interfaces are sometimes missed, and the knowledge of a team of subject-matter experts can surface important interfaces.

?Something 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. Your contact information will be kept anonymous.

It has been great to read some of your articles on the internet on the subject of FMEA. I would appreciate an answer to this question.

What is difference between actions taken to reduce severity and actions taken to reduce occurrence? They pretty much look the same.

Carl: I’ll answer your question from the viewpoint of Design FMEAs, but the same techniques can be used for Process FMEAs.

The primary difference between actions taken to reduce severity and actions taken to reduce occurrence is the type of strategies employed to reduce either severity or occurrence risk. Understanding the difference in the strategies should answer your question.

Action strategies to reduce severity risk include design for fail-safe, design for fault tolerance, design for redundancy, and provide early warning. Here is the definition of these action strategies.

Design for fail-safe

A fail-safe design is one that, in the event of failure, responds in a way that will cause minimal harm to other devices or danger to personnel. Fail-safe does not mean that failure is improbable; rather that a system’s design mitigates any unsafe consequences of failure. In FMEA language, fail-safe reduces the severity of the effect to a level that is safe.

Design for fault-tolerance

A fault-tolerant design is a design that enables a system to continue operation, possibly at a reduced level (also known as graceful degradation), rather than failing completely, when some part of the system fails. In FMEA language, fault-tolerance reduces the severity of the effect to a level that is consistent with performance degradation.

Design for redundancy

A redundant design provides for the duplication of critical components of a system with the intention of increasing the reliability of the system, usually in the case of a backup or fail-safe. This means having backup components that automatically "kick in" should one component fail. In FMEA language, redundant design can reduce the occurrence of system failure and reduce system severity to a safe level. This strategy can be employed to address single-point failures.

Provide early warning

Failures that occur without warning are more dangerous than failures with warning. Catastrophic effects can be avoided by adding a warning device to the system design. In FMEA language, adding early warning reduces the severity of the effect, potentially reduces the occurrence of system failure, and increases likelihood of detection of failure mode/cause during in-service usage.

The objective for the above strategies is to lower the severity rating by reducing the risk associated with the severity of the effect of a given failure mode.

Action strategies to reduce occurrence risk include changing the design or manufacturing process to eliminate the failure mode or cause, reducing stress-strength interference, and other design or process improvement actions. They are different from action strategies to reduce severity risk, in that they lower the likelihood of occurrence of the failure mode/cause and the associated occurrence rating.

About the Author

Carl S. CarlsonCarl 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.

Effective FMEAsMaterial 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.