Posted tagged ‘occurrence’

CRITICALITY

September 16, 2011
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)

01-static criticality experiment facility-uranium enriched solution

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.

01-qualitative vs quantitative analysis-part-item failure rates-calculate criticality number

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

01-criticality analysis-example-Plutonium criticality cell pit removal

FAILURE ANALYSIS

August 23, 2011

01-tank-failure-failure analysis-visual examination-scanning electron microscopy-metallography-materials technology


• 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.

01-RootCause-root cause analysis cycle-problem solving steps-avoidance of recurring problems

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

01-CauseEffect-analysis-bottom up predictive-ishikawa - fishbone diagram-prediction analysis

• 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.

01-visual examination-metallographic examination-appearance of the parts-calibrated metallurgical microscope equipment-image analysis-microstructure

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

01-NDT-non destructive testing-cold process-radiography-die penetrant techniques-magnetic particle testing
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).

01-scanning-electron-microscopy-vital specimens-fracture surfaces-secondary cracks-microscopic examination

4. Chemical analysis of material for conformation to specifications.

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

01-corrodedmetal-corrosion in metals-material technology-material science and metallurgy-iron oxidization-low affinity with oxigen-electrochemical corrosion-oxidation

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

7.  Fracture mechanics study if found necessary.

01-connection_failure_analysis-comprehensive failure analysis-analysis and testing-investigation of failure-design life check-failure mechanisms-identification of causes of failure
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.