Failure Modes, Effects and Criticality Analysis
[Editor's Note: This article has been updated since its original publication to reflect a more recent version of the software interface, and updates to the quantitative criticality analysis discussion.]
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-4. 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:
Figure 1: Sample FMEA report from the Xfmea software
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.
Figure 2: Xfmea interface with item and FMEA hierarchies
Figure 3: Xfmea Cause properties window
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)
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:
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
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 = 22,291.83) and therefore the expected failures at the operating time of interest (t = 2,000) 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.
Figure 4: Xfmea Criticality Analysis utility
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, 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, RCM++ and RBI software support these basic procedures, the major published industry standards (e.g., J1739, MIL-STD-1629A, etc.) and also provide the flexibility to customize the analysis and reports to meet the user’s needs for a particular application.
Many references for FMEA/FMECA analysis are available in print and on the web. Some useful references include: