Failure Mode and Effect Analysis (FMEA)

Customers are placing increasing demands on companies for high quality reliable products. The increasing capabilities and functionality of many products are making it more difficult for manufacturers to maintain quality and reliability. Traditionally, reliability has been achieved through extensive testing and the use of techniques such as probabilistic reliability modeling. These are techniques done in the late stages of development. The challenge is to design in qual­ity and reliability early in the development cycle.

Failure modes and effects analysis (FMEA) is a step-by-step approach for identifying all possible failures in a design, a manufacturing or assembly process or a product or service. Failures are any errors or defects, especially ones that affect the customer and can be potential or actual. “Effects analysis” refers to studying the consequences of those failures. Begun in the 1940s by the US military, FMEA was further developed by the aerospace and automotive industries. Several industries maintain formal FMEA.

FMEA is a methodology for analysing potential reliability problems early in the develop­ment cycle where it is easier to take actions to overcome these issues, thereby enhancing reliability through design. FMEA is used to identify potential failure modes, determine their effects on the operation of the product and identify actions to mitigate the failures. A crucial step is anticipating what might go wrong with a product. While anticipating every failure mode is not possible, the development team should formulate as extensive a list of potential failure modes as possible.

The early and consistent use of FMEAs in the design process allows the engineer to design out failures and produce reliable, safe and customer pleasing products. FMEAs also capture historical information for use in future product improvement.

Failures are prioritized according to how serious their consequences are, how frequently they occur and how easily they can be detected. The purpose of the FMEA is to take action to eliminate or reduce failures, starting with the highest-priority ones.

FMEA also documents current knowledge and actions about the risks of failures for use in continuous improvement. FMEA is used during design to prevent failures. It is also subse­quently used for control, both before and during the ongoing operation of the process. Ide­ally, FMEA begins during the earliest conceptual stages of design and continues throughout the life of the product or service.

1. When to Use FMEA?

FMEA can be used:

• When a process, product or service is being designed or re-designed after quality function deployment.
• When an existing process, product or service is being applied in a new way.
• Before developing control plans for a new or modified process.
• When improvement goals are planned for an existing process, product or service.
• When analysing failures of an existing process, product or service.
• Periodically throughout the life of the process, product or service.

2. Types of FMEAs

There are several types of FMEAs. Some are used more often than others. FMEA should always be done whenever failures mean potential harm or injury to the user of the end item being designed. The different types of FMEA are:

• System—focuses on global system functions
• Design—focuses on components and subsystems
• Process—focuses on manufacturing and assembly processes
• Service—focuses on service functions
• Software—focuses on software functions

3. FMEA-Procedure

The process for conducting an FMEA is straightforward. The basic steps are outlined below:

1. Describe the product/process and its function: An understanding of the product or pro­cess under consideration has to be clearly articulated. This understanding simplifies the pro­cess of analysis by helping the engineer identify those product/process uses that fall within the intended function and the ones that fall outside. It is important to consider both inten­tional and unintentional uses since product failure often ends in litigation, which can be costly and time consuming.
2. Create a block diagram of the product or process: A block diagram of the product/process should be developed. This diagram shows major components or process steps as blocks con­nected by lines that indicate how the components or steps are related.
3. Complete the header on the FMEA form worksheet: FMEA table headers vary since they are supposed to be customized according to the requirements of the companies using them. Generally, the header requires information such as product/process/system name, compo- nent/step name, product designer or process engineer, name of the person who prepared the FMEA form, FMEA date, revision level (letter or number) and revision date. These headings must be modified as needed.
4. Enumerate the items (components,functions, steps, etc.) that make up the product or process: Table 14.1 shows a simplified FMEA sheet. The items that make up the product or process must be listed.

5. Identify all potentialfailure modes associated with the product or process: A failure mode is defined as the manner in which a component, subsystem, system, process, etc. could poten­tially fail to meet the design intent. Examples of potential failure modes include corrosion, hydrogen embitterment, electrical short or open, torque fatigue, deformation and cracking.
6. List down each failure mode using its technical term: A failure mode in one component can serve as the cause of a failure mode in another component. Each failure should be listed in technical terms. Failure modes should be listed for functions of each component or pro­cess step. At this point the failure mode should be identified whether or not the failure is likely to occur.
7. Describe the effects of each of the failure modes listed and assess the severity of each: For each failure mode identified in Column 2, a corresponding effect (or effects) must be identi­fied and listed in Column 3 of the FMEA table. A failure effect is defined as the result of a failure mode in the function of the product/process as perceived by the customer. Examples of failure effects include injury to the user, inoperability of the product or process, improper appearance of the product or process, degraded performance and noise. A numerical rank­ing must be established for the severity of the effect. A common industry standard scale (1 to 10) uses 1 to represent “no effects” and 10 to indicate “very severe” with failures affect­ing system operation and safety without warning. Column 4 of the FMEA table is used for the severity rating (SEV) of the failure mode.

8. Identify the possible cause(s) for each failure mode: A failure cause is defined as a design weakness that may result in a failure. The potential causes for each failure mode should be identified and documented. The causes should be listed in technical terms and not in terms of symptoms. Potential causes are listed in Column 5 and examples include improper torque applied, improper operating conditions, contamination, erroneous algorithms, improper alignment, excessive loading and excessive voltage.

9. Quantify the probability of occurrence (probability factor or PF) of each of the failure mode causes: Every failure cause must be quantified and will be assigned a number (PF), which indicates how likely that cause has the probability of occurring. A common industry standard scale uses 1 to represent “not likely” and 10 to indicate “inevitable.” PF values for each of the failure causes are indicated in Column 6 of the FMEA table.
10. Identify all existing controls (current controls) that contribute to the prevention of the occurrence of each of these failure mode causes: Current controls (design or process) are the mechanisms that prevent the cause of the failure mode from occurring or which detect the failure before it reaches the customer. The engineer should now identify testing, analysis, monitoring and other techniques that can or have been used on the same or similar prod- ucts/processes to detect failures. Each of these controls should be assessed to determine how well it is expected to identify or detect failure modes. Each of the controls must be listed in Column 7 of the FMEA table. After a new product or process has been in use, previously undetected or unidentified failure modes may appear. The FMEA should then be updated and plans must be made to address those failures to eliminate them from the product/process.
11. Determine the ability of each control in preventing or detecting the failure mode or its cause: Detection is an assessment of the likelihood that the current controls (design and process) will detect the cause of the failure mode or the failure mode itself, thus preventing it from reaching the customer. As usual, a number must be assigned to indicate the detection effectiveness (DET) of each control. DET numbers are shown in Column 8 of the FMEA table.
12. Calculate risk priority numbers (RPN): The risk priority number is a mathematical product of the numerical severity, probability and detection ratings:

RPN = (Severity) x (Probability) x (Detection) = SEV x PF x DET

The RPN that is listed in Column 8 of the FMEA table is used to prioritize items required for additional quality planning or action.

13. Determine recommended action(s) to address potential failures that have a high RPN: A high RPN needs immediate attention since it indicates that the failure mode can result in an enormous negative effect, its failure cause has a high likelihood of occurring and there are insufficient controls to catch it. Action items must be defined to address fail­ure modes that have high RPNs. These actions could include specific inspection, testing or quality procedures, selection of different components or materials, de-rating, limiting environmental stresses or operating range, redesign of the item to avoid the failure mode, monitoring mechanisms, performing preventative maintenance and inclusion of back-up systems or redundancy. Column 10 of the FMEA table is used to list applicable action items.

14. Implement the defined actions: Assign responsibility and a target completion date for these actions. This makes responsibility clear-cut and facilitates tracking. The responsible owner and target completion dates must be indicated in column 11 of the FMEA table.
15. Review the results of the actions taken and reassess the RPNs: After the defined actions have been taken, re-assess the severity, probability and detection and review the revised RPNs. The new RPN should help the engineer decide if more actions are needed or if the
    actions are sufficient. Columns 13, 14, 15 and 16 of the FMEA table are used to indicate the new SEV, PF, DET and RPN, respectively.
16. Keep the FMEA table updated: Update the FMEA as the design or process changes, the assessment changes or new information cause the SEV, PF or DET to change.

4. Benefits of FMEA

FMEA is designed to assist the engineer improve the quality and reliability of design. FMEA provides the engineer several benefits when used properly. These benefits include:

• Improve product/process reliability and quality
• Increase customer satisfaction
• Early identification and elimination of potential product/process failure modes
• Prioritize product/process deficiencies
• Capture engineering/organization knowledge
• Emphasizes problem prevention
• Documents risk and actions taken to reduce risk
• Provides focus for improved testing and development
• Minimizes late changes and associated cost
• Catalyst for teamwork and idea exchange between functions

Source: Poornima M. Charantimath (2017), Total Quality Management, Pearson; 3rd edition.