reliability & availability calculation methods
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Reliability & Availability Calculation Methods
Sharif University Of TechnologyAerospace Engineering Department
Aircraft performance courseby Dr. M.B Malaek
Presented by Javad Ghasempoor SISStd. No: 89105241
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MTBF Introduction
MTBF as a reliability term based on :
a) Assumptionsb) definition of failurec) attention to these details
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What is a Failure?
1) The termination of the ability of the product as a whole to perform its required function
2) The termination of the ability of any individual component to perform its required function but not the termination of the ability of the product as a whole to perform.
Infinite definitions of failure depending on the type of product andmanufacturers
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Engineers are never wrong; they just make bad assumptions !
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MTBF definition
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MTTR definition Mean Time to Repair (or Recover) :
expected time to recover a system from a failure
include the time it takes to : diagnose the problem get a repair technician onsite physically repair the system
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Reliability - Availability DefinitionReliability is the ability of a system or component
to perform its required functions under stated conditions for a specified period of time
Availability, on the other hand, is the degree to which a system or component is operational and accessible when required for use
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An example
- 500,000 25-year-old humans- Throughout the year, 625 people failed (died)- operational life of the population is 500,000 x 1 year =
500,000 people years- The failure rate is 625 failures / 500,000 people years =
0.125% / year- The MTBF is the inverse of failure rate or 1 / 0.00125 =
800 years
extremely high reliability(MTBF) but a low expected service life
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Rate of failure Vs Time
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- failures at ‘useful life period’ could include undetectable defects low design safety factors higher random stress than expected human factors natural failures
prevent the decay curve shown in the "wear outperiod”
Ample burn-in periods for components by the manufacturers
proper maintenance proactive replacement of worn parts
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Methods of Predicting and Estimating MTBF
Predict : calculate a value based only on a system design
Estimate : calculate a value based on an observed sample
of similar systems
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Reliability Prediction Methods
1. MIL-HDBK 217 handbookcontains failure rate models for the various part types used in electronic systems
Parts Count Prediction Parts Stress Analysis Prediction
2. Telcordia
3. HRD54. RBD (Reliability Block Diagram)5. Markov Model6. FMEA / FMECA7. Fault Tree8. HALT
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Reliability Estimation Methods
Similar Item Prediction MethodField Data Measurement Method
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RelCalc for Windows software
automate the MTBF reliability prediction calculation procedure of Telcordia SR-332, or MIL-HDBK-217
Benefits • Save time: reduce prediction times by 60-95% over manual
methods• Save money • Integrate reliability analysis for better quality products
an alternative to tedious, time consuming, and error prone manual methods
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RelCalc graphs
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Sample report
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Applications
Military and commercial product applicationsProposal, feasibility studiesDesign phase trade-offsFind likely problem spotsFinal design predictionsCost reduction analysisWarranty analysis Compare Design Alternatives Track Reliability Improvement
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version updates include
Latest Telcordia and 217 handbook revision including many revised device failure rates some new device types new environments (GL) and revised
environment descriptions
- Windows 7 compatible
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Library Over 44,000 IC, Semiconductor, and Other
Parts Hundreds of Thousands of
Resistors/Capacitors Various Package Types, Including SMT Various Device Manufacturers
- MTBF Consulting Services
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Thanks for your attentation