Basic Concepts of FMEA and FMECA
Failure Mode 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, to assess the risk associated with those failure modes, to rank the issues in terms of importance and to identify and carry out corrective actions to address the most serious concerns.
Although the purpose, terminology and other details can vary according to type (e.g. Process FMEA, Design FMEA, etc.), the basic methodology is similar for all. This article presents a brief general overview of FMEA / FMECA analysis techniques and requirements.
FMEA / FMECA Overview
In general, FMEA / FMECA requires the identification of the following basic information:
- Effect(s) of Failure
- Cause(s) of Failure
- Current Control(s)
- Recommended Action(s)
- Plus other relevant details
Most analyses of this type also include some method to assess the risk associated with the issues identified during the analysis and to prioritize corrective actions. Two common methods include:
- Risk Priority Numbers (RPNs)
- Criticality Analysis (FMEA with Criticality Analysis = FMECA)
Published Standards and Guidelines
There are a number of published guidelines and standards for the requirements and recommended reporting format of FMEAs and FMECAs. Some of the main published standards for this type of analysis include SAE J1739, AIAG FMEA-4 and MIL-STD-1629A. In addition, many industries and companies have developed their own procedures to meet the specific requirements of their products/processes. Figure 1 shows a sample Process FMEA in the Automotive Industry Action Group (AIAG) FMEA-4 format.
Basic Analysis Procedure for FMEA or FMECA
The basic steps for performing an FMEA/FMECA analysis include:
- Assemble the team.
- Establish the ground rules.
- Gather and review relevant information.
- Identify the item(s) or process(es) to be analyzed.
- Identify the function(s), failure(s), effect(s), cause(s) and control(s) for each item or process to be analyzed.
- Evaluate the risk associated with the issues identified by the analysis.
- Prioritize and assign corrective actions.
- Perform corrective actions and re-evaluate risk.
- Distribute, review and update the analysis, as appropriate.
Risk Evaluation Methods
A typical FMEA incorporates some method to evaluate the risk associated with the potential problems identified through the analysis. The two most common methods, Risk Priority Numbers and Criticality Analysis, are described next.
Risk Priority Numbers
To use the Risk Priority Number (RPN) method to assess risk, the analysis team must:
- Rate the severity of each effect of failure.
- Rate the likelihood of occurrence for each cause of failure.
- Rate the likelihood of prior detection for each cause of failure (i.e. the likelihood of detecting the problem before it reaches the end user or customer).
- Calculate the RPN by obtaining the product of the three ratings:
RPN = Severity x Occurrence x Detection
The RPN can then be used to compare issues within the analysis and to prioritize problems for corrective action.
The MIL-STD-1629A document describes two types of criticality analysis: quantitative and qualitative. To use the quantitative criticality analysis method, the analysis team must:
- Define the reliability/unreliability for each item, at a given operating time.
- Identify the portion of the items unreliability that can be attributed to each potential failure mode.
- Rate the probability of loss (or severity) that will result from each failure mode that may occur.
- Calculate the criticality for each potential failure mode by obtaining the product of the three factors:
Mode Criticality = Item Unreliability x Mode Ratio of Unreliability x Probability of Loss
- Calculate the criticality for each item by obtaining the sum of the criticalities for each failure mode that has been identified for the item.
Item Criticality = SUM of Mode Criticalities
To use the qualitative criticality analysis method to evaluate risk and prioritize corrective actions, the analysis team must:
- Rate the severity of the potential effects of failure.
- Rate the likelihood of occurrence for each potential failure mode.
- Compare failure modes via a Criticality Matrix, which identifies severity on the horizontal axis and occurrence on the vertical axis.
Applications and Benefits
The FMEA / FMECA analysis procedure is a tool that has been adapted in many different ways for many different purposes. It can contribute to improved designs for products and processes, resulting in higher reliability, better quality, increased safety, enhanced customer satisfaction and reduced costs. The tool can also be used to establish and optimize maintenance plans for repairable systems and/or contribute to control plans and other quality assurance procedures. It provides a knowledge base of failure mode and corrective action information that can be used as a resource in future troubleshooting efforts and as a training tool for new engineers. In addition, an FMEA or FMECA is often required to comply with safety and quality requirements, such as ISO 9001, QS 9000, ISO/TS 16949, Six Sigma, FDA Good Manufacturing Practices (GMPs), Process Safety Management Act (PSM), etc.
ReliaSoft's Xfmea software facilitates analysis, data management and reporting for failure mode and effects analysis (FMEA) and failure modes, effects and criticality analysis (FMECA). The software supports all major standards (AIAG FMEA-3, J1739, ARP5580, MIL-STD-1629A, etc.) and provides extensive customization capabilities for analysis and reporting, allowing you to configure the software to meet your organization's specific analysis and reporting procedures for all types of FMEA / FMECA.