FMECA

Failure modes, effects, and criticality analysis (FMECA) is a methodology to identify and analyze:

• All potential failure modes of the various parts of a system
• The effects these failures may have on the system
• How to avoid the failures, and/or mitigate the effects of the failures on the system

FMECA is a technique used to identify, prioritize, and eliminate potential failures from the system, design or process before they reach the customer

Initially, the FMECA was called FMEA (Failure modes and effects analysis). The C in FMECA indicates that the criticality (or severity) of the various failure effects are considered and ranked. Today, FMEA is often used as a synonym for FMECA. The distinction between the two terms has become blurred.

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.

Types of FMECA

• Design FMECAis carried out to eliminate failures during equipment design, taking into account all types of failures during the whole life-span of the equipment
• Process FMECA is focused on problems stemming from how the equipment is manufactured, maintained or operated
• System FMECA looks for potential problems and bottlenecks in larger processes, such as entire production lines

Two approaches to FMECA

• Bottom-up approach – The bottom-up approach is used when a system concept has been decided. Each component on the lowest level of indenture is studied one-by-one. The bottom-up approach is also called hardware approach. The analysis is complete since all components are considered.
• Top-down approach – The top-down approach is mainly used in an early design phase before the whole system structure is decided. The analysis is usually function oriented. The analysis starts with the main system functions – and how these may fail. Functional failures with significant effects are usually prioritized in the analysis. The analysis will not necessarily be complete. The top-down approach may also be used on an existing system to focus on problem areas.

FMECA main steps

• FMECA prerequisites
• System structure analysis
• Failure analysis and preparation of FMECA worksheets
• Team review
• Corrective actions