System Suitability

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System Suitability
Lecture Reliability FMECA Lecturer Dr. Dave
Olwell Dr. Cliff Whitcomb, CSEP cawhitco_at_nps.edu
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About the Lecturer

• Dr. David Olwell
• Chair, Department of Systems Engineering, NPS
• Research interests are primarily reliability
  engineering and analysis
• Phone [phone] (W)
• Email dholwell_at_nps.edu

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Objectives

• Be able to construct a FMECA using Risk Priority
  Numbers
• Interpret a Fault Tree
• Be able to perform a simple reliability
  prediction for a series system with independent
  exponential failure times
• Understand the principles of sequential testing

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Reading

• BF (4ed) 12.4-12.6

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FMECA

• Discussion in text (section 12.4.1) is good
• Figure 12.21 can easily be implemented in
  spreadsheet, although for large systems
  commercial software is useful
• Bottom line RPN is used to score failure modes
  on a set of subjective scales, and the score
  orders them. We start by mitigating the ones
  with the highest number, and work down to some
  stopping point.

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FMECA/FMEA

• Failure Modes and Effects Analysis (FMEA) and
  Failure Modes, Effects and Criticality Analysis
  (FMECA) are methodologies designed to identify
  potential failure modes for a product or process
  before the problems occur, to assess the risk
  associated with those failure modes and to
  identify and carry out measures to address the
  most serious concerns.

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Standards

• MIL-STD-1629A
• SAE Internationals J1739 and ARP5580 documents
  (for automotive and non-automotive applications,
  respectively)
• Automotive Industry Action Groups (AIAG) FMEA-3

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Figure
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Common Themes

• Item/Process Identify the item or process that
  will be the subject of the analysis, including
  some investigation into the design and
  reliability characteristics. For FMEA analysis of
  a product or system, the analysis could be
  performed at the system, subsystem, component or
  other level of the system configuration.
• Functions Identify the functions that the item
  or process is expected to perform.
• Failures Identify the known and potential
  failures that could prevent or degrade the
  ability of the item/process to perform its
  designated functions.
• Failure Effects Identify the known and potential
  effects that would result from the occurrence of
  each failure. It may be desirable to consider the
  effects at the item level (Local Effects), at the
  next higher level assembly (Next Higher Level
  Effects) and/or at the system level (End
  Effects).
• Failure Causes Identify the known and potential
  causes for each failure.

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Common Themes

• Current Controls Examine the control mechanisms
  that will be in place to eliminate or mitigate
  the likelihood that the potential failures will
  occur (e.g. end of line inspections, design
  reviews, etc.).
• Recommended Actions Identify the corrective
  actions that need to be taken in order to
  eliminate or mitigate the risk and then follow up
  on the completion of those recommended actions.
• Prioritize Issues Prioritize issues for
  corrective action according to a consistent
  standard that has been established by the
  organization. Risk Priority Number (RPN) ratings
  and Criticality Analysis are common methods of
  prioritization and they are described later.
• Other Details Depending on the particular
  situation and on the analysis guidelines adopted
  by the organization, other details may be
  considered during the analysis, such as the
  operational mode when the failure occurs or the
  systems intended mission.
• Report Generate a report of the analysis in the
  standard format that has been established by the
  organization. This is generally a tabular format
  similar to the one shown in Figure 1. In
  addition, the report may include block diagrams
  and/or process flow diagrams to illustrate the
  item or process that is the subject of the
  analysis. If applicable, the criticality analysis
  may be included in a separate table and various
  plots/graphs can be included to display
  statistics on the modes and rankings.

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Figure
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RPN (Risk Priority Number)

• Severity (S)
• 1-10 scale
• A rating of the severity or seriousness of each
  potential failure effect.
• Occurrence/frequency (O)
• 1-10 scale
• A rating of the likelihood of occurrence for each
  potential failure cause.
• Detection (D)
• A rating of the likelihood of detecting the
  failure cause.
• 1-10 scale, hard to detect being higher
• RPN SOD

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Comment

• Scales are arbitrary
• Implies they can only be used for ranking failure
  modes internal to the system, and may not be not
  useful for comparing across systems
• Must be well defined enough so that different
  people assign the same number

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Example Battery

• Severity 8 - Extreme Effect. Product inoperable
  but safe. Customer very dissatisfied.
• Occurrence 5 - Low. Occasional number of
  failures likely expect about 2.7 failures per
  1000 due to this cause.
• Detection 1 - Almost Certain. The operator will
  almost certainly be able to detect the failure

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Criticality Analysis A different approach

• Criticality (Q)(FMFR)(PL)
• Item Unreliability (Q) The probability of
  failure for the item at the time of interest for
  the analysis.
• Failure Mode Ratio of Unreliability (FMFR) The
  ratio of the item unreliability that can be
  attributed to the particular failure mode. For
  example, if an item has four failure modes, then
  one mode may account for 40 of the failures, a
  second mode may account for 30 and the two
  remaining modes may account for 15 each.
• Probability of Loss (PL) The probability that
  the failure mode will cause a system failure (or
  will cause a significant loss). This is an
  indication of the severity of the failure effect
  and may be set according to the following scale
• Actual Loss 100
• Probable Loss 50
• Possible Loss 10
• No Loss 10

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Battery

• Criticality is (0.08)(0.25)(1.0) (.02)
• Use for relative rankings, not necessarily an
  absolute scale

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Recommendations

• An important use of the FMECA is to document
  mitigation strategies
• The estimated reduction in the RPN is shown if
  the suggested mitigation is adopted
• Useful to compare before and after risk
• In the early FMECA, recommendations suggested
  reduced risk for the first failure mode from 294
  to 28

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Comment

• This FMECA approach is commonly used for
  reliability, but can be applied to other domains
  where risk identification and mitigation are of
  interest

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Fault Trees

• Fault trees emphasize what must occur for system
  failures while reliability block diagrams
  emphasize what must occur for a systems to work
• They are logically equivalent
• Partial FTs can be useful when trying to isolate
  fault structure
• Discussion in book is pretty good, if short

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BF Textbook Example

• Here is an example of the RBDs from Figure 12.10
  reworked as fault trees

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Simple reliability prediction

• Requires either data or assumptions
• If reliability of components is known, use their
  data
• Be careful to verify that the conditions of use
  are the same as those that generated the
  historical data
• If making assumptions, be realistic and
  conservative

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Example

• Here is an example of reliability prediction for
  Figure 12.10(a) using the data from Table 12.2
  and a mission time of 100 hours

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Sequential testing

• Sequential testing allows early termination of a
  reliability test
• If the system exceeds requirements or fails
  requirements by a large margin, the test
  terminates early
• For close calls, it takes more testing to
  accumulate enough data to decide
• Idea covered again in system assessment course

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Example

• The example in the text (Section 12.5.1 and
  Figure 12.26) is good
• Focus on understanding the interpretation of Fig
  12.26
• That example assumes exponential failure times.
• Know the four key elements for constructing the
  figure producer risk, consumer risk, threshold
  MTBF, objective MTBF
• The horizontal axis is total time on test, or
  TTT, and is the sum of all the running time of
  the units that have failed or are still being
  tested

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Operational Testing

• Study section 12.5.4

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Conclusion

• Reliability is a very important element of
  suitability
• Several NPS courses are available for further
  study
• OA4302 Reliability and Weapon System
  Effectiveness Measurement
• SE3321 Reliability Management and Data Systems
• SE3322 Reliability Centered Maintenance
• SE4321 Reliability Growth and Accelerated Testing