Modes, Effects and Criticality Analysis
[Editorial Note: In
the online version of this article, we have updated the discussion of the
Quantitative Criticality Analysis calculation method to be consistent with
the latest research.]
Failure Modes and Effects Analysis (FMEA) and Failure Modes, Effects and Criticality Analysis (FMECA) are methodologies designed to identify potential failure modes for a product or process
before the problems occur, to assess the risk associated with those failure modes and to identify and carry out measures to address the most serious concerns.
This article presents a brief general overview of FMEA and FMECA analysis techniques and applications. ReliaSoft’s Xfmea software has been designed to automate and facilitate the FMEA/FMECA
process and provide flexible data management and reporting capabilities.
FMEA/FMECA Analysis Overview
There is a great variety within industry as to the specific implementation details for individual FMEA/FMECA analyses. A number of standards and guidelines have been developed to set the
requirements for the analysis and each organization may have a unique approach to the analysis. Some common FMEA/FMECA guidelines/standards include the U.S. Department of Defense’s
MIL-STD-1629A, SAE International’s J1739 and ARP5580 documents (for automotive and non-automotive applications, respectively) and the Automotive Industry Action Group’s (AIAG) FMEA-3. In
addition, some practitioners distinguish various types of FMEA/FMECA analysis based on the item or process that is analyzed, the stage in the manufacturing/development process when the analysis is
performed and/or whether the analysis is performed on the hardware or the functions that the item is expected to perform. Some commonly acknowledged FMEA types include, but are not limited to,
Design FMEA (DFMEA), Process FMEA (PFMEA), Functional FMEA and System FMEA.
Even though there are many different types and standards, most FMEAs/FMECAs consist of a common set of procedures. In general, FMEA analysis is conducted by a cross-functional team at various
stages of the design, development and manufacturing process and typically consists of the following:
- Item/Process: Identify the item or process that will be the subject of the analysis, including some investigation into the design and reliability characteristics. For FMEA analysis of a product
or system, the analysis could be performed at the system, subsystem, component or other level of the system configuration.
- Functions: Identify the functions that the item or process is expected to perform.
- Failures: Identify the known and potential failures that could prevent or degrade the ability of the item/process to perform its designated functions.
- Failure Effects: Identify the known and potential effects that would result from the occurrence of each failure. It may be desirable to consider the effects at the item level (Local Effects),
at the next higher level assembly (Next Higher Level Effects) and/or at the system level (End Effects).
- Failure Causes: Identify the known and potential causes for each failure.
- Current Controls: Examine the control mechanisms that will be in place to eliminate or mitigate the likelihood that the potential failures will occur (e.g. end of line inspections, design
- Recommended Actions: Identify the corrective actions that need to be taken in order to eliminate or mitigate the risk and then follow up on the completion of those recommended actions.
- Prioritize Issues: Prioritize issues for corrective action according to a consistent standard that has been established by the organization. Risk Priority Number (RPN) ratings and Criticality
Analysis are common methods of prioritization and they are described in more detail later in this article.
- Other Details: Depending on the particular situation and on the analysis guidelines adopted by the organization, other details may be considered during the analysis, such as the operational
mode when the failure occurs or the system’s intended mission.
- Report: Generate a report of the analysis in the standard format that has been established by the organization. This is generally a tabular format similar to the one shown in Figure 1. In
addition, the report may include block diagrams and/or process flow diagrams to illustrate the item or process that is the subject of the analysis. If applicable, the criticality analysis may be
included in a separate table and various plots/graphs can be included to display statistics on the modes and rankings.
Sample FMEA report from the Xfmea software [Click to Enlarge]
Figure 2 shows ReliaSoft’s Xfmea interface with the functions, failures, effects and causes displayed in a hierarchical fashion. The software also provides a “Worksheet View” of the
analysis, which is similar to the tabular report output. Figure 3 shows the properties window for the Failure Cause, which can be used for data entry and display.
2: Xfmea interface with item and FMEA hierarchies [Click to Enlarge]
3: Xfmea Failure Cause Properties window [Click to Enlarge]
Prioritize Issues Based on RPN and/or Criticality
As mentioned previously, most FMEA/FMECA analyses include some effort to prioritize issues in order to determine the sequence and time-frame for the corrective actions that will be performed.
Although the methods used to set this priority may vary by organization, two commonly used methods are described next: Risk Priority Numbers and Criticality Analysis.
Risk Priority Numbers: The risk priority number (RPN) system is a relative rating system that assigns a numerical value to the issue in each of three different categories: Severity (S), Occurrence
(O) and Detection (D). The three ratings are multiplied together to determine the overall RPN for the issue. The rating scales typically range from 1 to 5 or from 1 to 10 and the criteria used in
each rating scale will be determined based on the particular circumstances for the product/process that is being analyzed. Because all issues are rated according to the same set of rating scales,
this number can be used to compare and rank issues within the analysis. However, because the ratings are assigned relative to a particular analysis, it is generally not appropriate to compare RPN
numbers among different analyses. The RPN is calculated as follows:
RPN = (S)(O)(D)
- Severity (S): A rating of the severity or seriousness of each potential failure effect.
- Occurrence (O): A rating of the likelihood of occurrence for each potential failure cause.
- Detection (D): A rating of the likelihood of detecting the failure cause.
For example, consider the following partial FMEA for a battery, which uses ten point rating scales to rank the severity, occurrence and detection:
The following rating criteria are applicable to the battery failure mode:
- Severity: 8 - Extreme Effect. Product inoperable but safe. Customer very dissatisfied.
- Occurrence: 5 - Low. Occasional number of failures likely; expect about 2.7 failures per 1000 due to this cause.
- Detection: 1 - Almost Certain. The operator will almost certainly be able to detect the failure.
The RPN for the issue is (8)(5)(1) = 40. This risk priority number is then compared with the ratings for other issues to help determine which areas to focus on for improvement.
Criticality Analysis: The Criticality Analysis method is similar to the RPN rating system except that it calculates the rankings in a different way. Criticality Analysis takes into account the
probability of failure for the item and the portion of the failure likelihood that can be attributed to a particular failure mode. The Criticality is calculated for each failure mode as follows:
Mode Criticality = Expected Failures x Mode Ratio of
Unreliability x Probability of Loss
- Expected Failures: The expected number of failures at a given
operating time, calculated based on the reliability characteristics that
have been defined for the item (i.e. statistical lifetime
distribution and parameters or fixed probability that does not vary with
- Failure Mode Ratio of Unreliability (FMFR): The ratio of the item unreliability that can be attributed to the particular failure mode. For example, if an item has four failure modes, then one
mode may account for 40% of the failures, a second mode may account for 30% and the two remaining modes may account for 15% each.
- Probability of Loss (PL): The probability that the failure mode will cause a system failure (or will cause a significant loss). This is an indication of the severity of the failure effect and
may be set according to the following scale:
- Actual Loss = 100%
- Probable Loss = 50%
- Possible Loss = 10%
- No Loss = 10%
For example, consider a criticality analysis for the partial FMEA on the battery. The reliability of the battery
can be described with a 2 parameter Weibull distribution (beta = 1.3 and eta
= 22291.83) and therefore the expected failures at the operating time of
interest (t = 2000) can be estimated as .0435. The
portion of the item unreliability that can be attributed to the given failure mode is 25% (or 25% of the item failures are likely to be due to this particular failure mode). The probability of
loss is 100% because the occurrence of the failure mode will cause a system failure. The Criticality for the failure mode is
then calculated as (.0435)(.25)(1.00) = .010875. As with the RPN method, this Criticality
value can be compared with the Criticalities for other failure modes to help rank the issues that must be addressed.
Figure 4 displays the Xfmea Criticality Analysis utility, which can also be used to generate FMECA charts and reports.
4: Xfmea Criticality Analysis utility [Click to Enlarge]
Applications and Related Analyses
FMEA/FMECA techniques are used throughout industry for a variety of applications and the flexible analysis method can be performed at various stages in the product life cycle. FMEA/FMECA analysis
can be employed to support design, development, manufacturing, service and other activities to improve reliability and increase efficiency. For example, there is widespread use of both design and
process FMEAs within the automotive industry and documentation of this analysis is a common requirement for automotive suppliers. This methodology is also widely used in the aerospace, medical and
other manufacturing industries.
The MSG-3 procedures used by the airline industry incorporate FMEA techniques into the analysis procedure. (Reliability Edge Volume 3, Issue 1 contains an article on MSG-3 and ReliaSoft’s MPC 3,
software designed to automate the process.) Likewise, Reliability Centered Maintenance (RCM) procedures incorporate FMEA as a primary component of the analysis.
In addition, the FMEA reporting structure can be used to provide a centralized location for reliability-related information for the system/process. For example, the FMEA can be incorporated into
an effective Reliability Growth management policy by providing a structure to organize information about product failures and assisting with efforts to identify the failure modes that have been
observed during reliability growth testing and the failure modes that may still yet be observed.
FMEA/FMECA analysis is a flexible process that can be adapted to meet the particular needs of the industry and/or the organization. However, most analyses include the basic procedures and data
requirements described in this article. ReliaSoft’s Xfmea software supports these basic procedures, the major published industry standards (e.g., J1739, MIL-STD-1629A, etc.) and also provides the
flexibility to customize the analysis and reports to meet the user’s needs for a particular application. On the Web at http://www.ReliaSoft.com/xfmea.
Many references for FMEA/FMECA analysis are available in print and on the Web. Some useful references include:
- Kececioglu, Dimitri, Reliability Engineering Handbook Volume 2. Prentice-Hall Inc., Englewood Cliffs, New Jersey, 1991. Pages 473-506.
- MIL-STD-1629A: Procedures for Performing a Failure Mode Effects and Criticality Analysis. U.S. Department of Defense, Washington, D.C., November 28, 1984. Note: This standard was cancelled by
the DoD in August 1998.
- SAE Aerospace Recommended Practice ARP5580: Recommended Failure Modes and Effects Analysis (FMEA) Practices for Non-Automobile Applications. SAE International, Warrendale, PA, 2001.
- SAE Surface Vehicle Recommended Practice J1739: Potential Failure Mode and Effects Analysis in Design (Design FMEA), Potential Failure Modes and Effects Analysis in Manufacturing and Assembly
Processes (Process FMEA), and Potential Failure Mode and Effects Analysis for Machinery (Machinery FMEA). SAE International, Warrendale, PA, June, 2000.
- Stamatis, D.H., Failure Mode and Effect Analysis: FMEA from Theory to Execution. American Society for Quality (ASQ), Milwaukee, Wisconsin, 1995.