FAILURE MODES, EFFECTS & CRITICALITY ANALYSIS … ·  · 2016-07-07Failure Mode any design flaw, out-of-spec condition, or change in a product / process which prevents it from functioning

FAILURE MODES, EFFECTS & ANALYSIS - FMEA

A product should have…

Long service life

During this life, it should have few failures

A product should be Reliable

To achieve a required level of reliability, certain techniques are

necessary, one of which is Failure Modes and Effects Analysis (FMEA).

Failure modes and effects analysis 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.

- Omdahl (1988)

Failure Mode

any design flaw, out-of-spec

condition, or change in a product /

process which prevents it from

functioning properly

Example:

Process: ATM Pin Authentication

Failure mode: Unauthorized access; Authentication

failure

Key Elements

Timeliness

- “Before the event action” and not an “After

the fact” exercise

Teamwork

- Reviewed by a team of individuals with

diverse experience & expertise

Documentation

- “Living Document”

Benefits

Reduces the number of product failures

experienced by the customer

Product/ Process Robustness

Reduced costs (materials, re-work, late change,

recalls etc.)

Standardized approach to risk assessment and

reduction

Enhances teamwork, understanding and cross-

functional working relationships

Identification of critical areas of the system (Root

Cause Analysis)

FMEA answers the following Qs:

What function(s) is the product suppose to perform

How could the product fail to perform that function

What effect would the failure have on the end product and the end user

How severe is the effect

What could cause the failure

How likely is the cause to actually result in the failure mode

What is being done to prevent or detect the cause

How effective is this prevention or detection method

What is the overall risk associated with the failure mode

What corrective actions can be taken to eliminate the cause of the failure or

to improve the prevention or detection measure, and thus reduce the risk

Types

Design FMEA Process FMEA System FMEA

Carried out to

eliminate failures

during product

design, taking into

account all types of

failures during the

whole life-span of

the equipment

Focused on

problems stemming

from how the

product is

manufactured,

maintained or

operated

Looks for

potential

problems and

bottlenecks in

larger processes,

such as entire

production lines

Bottom-up approach

• The bottom-up approach is used when a system concept

has been decided. Each component on the lowest level of

components are studied one-by-one.

• Also called the Hardware approach

Approaches to FMEA

Top-down approach

• Mainly used in an early design phase before the whole

system structure is decided

• Analysis start with the main system function- and how these

may fail.

• Functional failures with significant effects are usually

prioritized in the analysis

Functional Block Diagram

• A functional block diagram is used to show how the different parts of the system

interact with one another to verify the critical path.

• It is recommended to break the system down to different levels.

• Review schematics and/or other engineering drawings of the system to show

how different parts interface with one another by their critical support systems to

understand the normal functional flow requirements.

• A list of all functions of the equipment is prepared before examining the potential

failure modes of each of those functions.

• Operating conditions (such as; temperature, loads, and pressure), and

environmental conditions may be included in the components list.

• A bottom-up approach is used where by analysis begins at the component

level, followed by analysis of subsequent or higher system levels.

Functional Block Diagram

Elements of the FMEA Form

1. In the first column, a unique reference to an element (subsystem or component) is given. It

may be a reference to an id. In a specific drawing, a so-called tag number, or the name of the

element.

2. The functions of the element are listed. It is important to list all functions. A checklist maybe

useful to secure that all functions are covered.

3. The various operational modes for the element are listed. Example of operational modes are:

idle, standby, and running. In applications, where it is not relevant to distinguish between

operational modes, this column maybe omitted.

4. For each function and operational mode of an element, the potential failure modes have to

be identified and listed. Note that a failure mode should be defined as a non-fulfillment of the

functional requirements of the functions specified in column 2.

5. The failure modes identified in column 4 are studied one-by-one. The failure mechanisms (e.g.,corrosion, erosion, fatigue) that may produce or contribute to a failure mode are identified

and listed. Other possible causes of the failure mode should also be listed.

Elements of the FMEA Form

6. The various possibilities for detection of the identified failure modes are listed. These may

involve diagnostic testing, different alarms, proof testing, human perception, and the like.

Detection - The ability of the current control scheme to detect or prevent a given cause

Detection is sometimes termed EFFECTIVENESS. It is a numerical subjective estimate of the

effectiveness of the controls to prevent or detect the cause or failure mode before the failure

reaches the customer. The assumption is that the cause has occurred.

Elements of the FMEA Form

7. The effects each failure mode may have on other components in the same subsystem and on the subsystem as such (local effects) are listed.

8. The effects each failure mode may have on the system (global effects) are listed. The resulting

operational status of the system after the failure may also be recorded, that is, whether the

system is functioning or not, or is switched over to another operational mode. In some

applications, it maybe beneficial to consider each category of effects separately, like: safety

effects, environmental effects, production availability effects, economic effects, and soon.

9. Failure rates for each failure mode are listed (Occurrence).

Elements of the FMEA Form

10. The severity of a failure mode is the worst potential (but realistic) effect of the failure considered on the system level (the global effects)

11. Possible actions to correct the failure and restore the function or prevent serious consequences are

listed. Actions that are likely to reduce the frequency of the failure modes should also be recorded.

12. The last column may be used to record pertinent information not included in the other columns.

Risk Ranking

The risk related to the various failure modes is often presented either by a :

Risk matrix, or a

Risk priority number (RPN)

RISK MATRIX

Risk Priority Number

O = the rank of the occurrence of the failure mode

S = the rank of the severity of the failure mode

D = the rank of the likelihood that the failure will be detected before the

system reaches the end-user/customer

RPN = O x S x D

The smaller the RPN the better – and – the larger the worse.

Selection of Actions

The risk may be reduced by introducing

Design changes

Engineered safety features

Safety devices

Warning devices

Procedures/training

RPN Reduction

Step 1: For Each Input, Determine the Potential Failure Modes

Step 2: For Each Failure Mode, Identify Effects and Assign Severity

Step 3: Identify Potential Causes of Each Failure Mode and Assign Score

Step 4: List Current Controls for Each Cause and Assign Score

Step 5: Calculate RPNs

Step 6: Develop Recommended Actions, Assign Responsible Persons, and Take Actions

Step 7: Assign the Predicted Severity, Occurrence, and Detection Levels and Compare RPNs

- END

Additional Readings on FMEA http://www2.warwick.ac.uk/fac/sci/wmg/ftmsc/modules/modulelist/peuss/slides/section_10b_fmea_lecture_slides_compatibility_mode.pdf