CRITICALITY

CRITICALITY is a measure of the frequency of occurrence of an effect.

                       – May be based on qualitative judgement or

– May be based on failure rate data (most common)

Qualitative analysis:

–Used when specific part or item failure rates are not available.

Quantitative analysis:

–Used when sufficient failure rate data is available to calculate criticality numbers.

Qualitative Approach:

• Because failure rate data is not available, failure mode ratios and failure mode probability are not used.
• The probability of occurrence of each failure is grouped into discrete levels that establish the qualitative failure probability level for each entry based on the judgment of the analyst.
• The failure mode probability levels of occurrence are:

–Level A – Frequent

–Level B – Reasonably Probable

–Level C – Occasional

–Level D – Remote

–Level E – Extremely Unlikely

Quantitative Approach

Failure Mode Criticality (CM) is the portion of the criticality number for an item, due to one of its failure modes, which results in a particular severity classification (e.g. results in an end effect with severity I, II, etc…).

• Category I – Catastrophic: A failure which may cause death or weapon system loss (i.e., aircraft, tank, missile, ship, etc…)
• Category II – Critical: A failure which may cause severe injury, major property damage, or major system damage which will result in mission loss.
• Category III – Marginal: A failure which may cause minor injury, minor property damage, or minor system damage which will result in delay or loss of availability or mission degradation.
• Category IV – Minor: A failure not serious enough to cause injury, property damage or system damage, but which will result in unscheduled maintenance or repair.

The quantitative approach uses the following formula for Failure Mode Criticality:

Cm = βαλpt

Where

Cm = Failure Mode Criticality

β = Conditional probability of occurrence of next higher failure effect

α = Failure mode ratio

λp = Part failure rate

T = Duration of applicable mission phase

FMEA

Failure Mode – A particular way in which an item fails, independent of the reason for failure.

 Failure Mode and Effects Analysis (FMEA) – A procedure by which each credible failure mode of each item from a low indenture level to the highest is analyzed to determine the effects on the system and to classify each potential failure mode in accordance with the severity of its effect.

Indenture Levels – The hierarchy of hardware levels from the part to the component to the subsystem to the system, etc.

Redundancy – More than one independent means of performing a function.  There are different kinds of redundancy, including:
(1) Operational – Redundant items, all of which are energized during the operating cycle; includes load-sharing, wherein redundant items are connected in a manner such that upon failure of one item, the other will continue to perform the function.  It is not necessary to switch out the failed item or switch in the redundant one.

            (2) Standby – Items that are inoperative (have no power applied) until they are switched in upon failure of the primary item.

            (3) Like Redundancy – Identical items performing the same function.

            (4) Unlike Redundancy – Non identical items performing the same function

THE FMEA PROCESS

• Define the system to be analyzed.  A complete system definition includes identification of internal and interface functions, expected performance at all indenture levels, system restraints, and failure definitions.  Also state systems and mission phases not analyzed giving rationale for the omissions.

• Indicate the depth of the analysis by identifying the indenture level at which the analysis is begun.

• Identify specific design requirements that are to be verified by the FMEA.

• Define ground rules and assumptions on which the analysis is based.  Identify mission phases to be analyzed and the status of equipment during each mission phase.

• Obtain or construct functional and reliability block diagrams indicating interrelationships of functional groups, system operation, independent data channels, and backup or workaround features of the system.

• Identify failure modes, effects, failure detection and workaround features and other pertinent information on the worksheet.

• Evaluate the severity of each failure effect in accordance with the prescribed severity categories.

FMEA Flow Diagram:

History:

The FMECA was originally developed by the National Aeronautics and Space Administration (NASA) to improve and verify the reliability of space program hardware.

FMECA Flow Diagram: ( Failure Mode, Effects and Criticality Analysis )

Criticality Analysis Flow:

Who is the Team ?

 

Areas to be represented are:

• Quality

• Logistics

• Engineering

• Purchasing

• Manufacturing

• Sales

• Tooling

• Marketing

• Customer

• Supplier

FAILURE ANALYSIS

• Why ?

As the standards of our industry rise due to increasing globalization and competition, there is an ever growing need for consistency and reliability. Breakdown of any unit, system or equipment is an avoidable and costly occurrence and must be prevented or minimized. Analysis of such failures becomes a resourceful and affordable tool in addressing such unwanted occurrences.

To establish whether the cause of component failure lay on:

a) Service conditions
b) Design considerations
c) Material and its specification
d) Improper processing and assembly procedures or
e)  Combinations of these.

Only the real “Root cause” can ensure the effectiveness of corrective and preventive actions and avoid recurrence of failure.

• Stages Of Failure Analysis

1. Understanding and assimilation of background data and selection of samples.
2. Examination and documentation of the failed part by the following

1. Visual examination of parts, location (if necessary) and relevant photographs as well.

2.  Non destructive testing by means of Radiography, Dye      penetrant, Magnetic particle testing etc.

3. Mechanical Testing for various physical properties.

3. Vital specimens are selected, classified, and subjected to:

1. Macroscopic examination and analysis. This involves examining the fracture surfaces, secondary cracks, deposits and other such elements
2. Microscopic examination and analysis of fracture surface (by Scanning Electron Microscopy, if required).

4. Chemical analysis of material for conformation to specifications.

5. Chemical analysis of corrosion products, deposits, contaminants etc.

6. The actual state of the failed part and the failure mode are established.

7.  Fracture mechanics study if found necessary.

8. A simulation of the identical working environment to determine if any external      factors have contributed to the failure

9. Conclusions are determined after compiling all evidences and analysis and       then the report is generated.
10. Follow-up recommendations are also provided.

3D SIMULATION

Design Validation:

• Accelerate new product development
• Switch to alternate or cheaper material
• Reduce Prototyping costs
• Improve product quality and performance
• Enhance reliability

Responding To Design Challenges:

• Improve complex product designs
  • Enhanced function and performance
  • Meet product specification and / or regulation

• Reduce Re-Design
  • Design right first time
  • Weight and shape optimization
• Early problem detection and correction
  • Avoid field failures
  • Study alternative designs
• Greater product quality
  • Efficient and reliable
  • Reduced liability

CAE Solutions:

• Development of indigenous technologies and products
• Enumeration of methods for Analysis to test correlations
• Procedure for Failure mode and Prediction and Life Calculations
• Value Addition and Value Engineering (VAVE)
• Reduction in Cost and Product development time
• Elimination of Performance problems
• Improvement in performance efficiency

COSMOS Salient Features:

• Theory in Finite Element Analysis including procedure for performing FEA
• Practical solution to complex problems involving multi-domain interaction
• Correlation to real world problems and phenomena
• Advanced training on Fatigue, Non-Linear FEA and Vibrations

Why COSMOS for Design Validation:

• CAD Integrated Design validation
• Easy to use and Shorter Learning Curve
• Evaluate multiple Design scenarios in one stroke
• Integrated Kinematic Analysis using Cosmos Motion
• Seamless transfer of loading from COSMOS Motion to COSMOS  Works for FEA
• Multi Domain Analysis in Integrated CAD Environment

SolidWorks / COSMOS Simulation benefits:

• Easy-to-Use Simulation toolset – Enables designers to concentrate on designs not tools
• Automatic Report Generation
• Multiple configurations of designs can be studied automatically – enables Design of Experiments
• Unlimited Model size – limited only by Computational resources
• True Contact simulation for accurate load transfer
• Sensors and Probes to compare results with Real-World Test Data
• Fast, Accurate and Reliable – Backed by almost 3 Decades of experience