Stracener_EMIS 7305/5305_Spr08_ System Reliability Analysis - Concepts and Metrics Dr. Jerrell T. Stracener, SAE Fellow Leadership in Engineering EMIS 7305/5305 Systems Reliability, Supportability and Availability Analysis Systems Engineering Program Department of Engineering Management, Information and Systems

Stracener_EMIS 7305/5305_Spr08_ Reliability Definitions and Concepts Figures of merit Failure densities and distributions The reliability function Failure rates The reliability functions in terms of the failure rate Mean time to failure (MTTF) and mean time between failures (MTBF)

Stracener_EMIS 7305/5305_Spr08_ Reliability Concepts, Principles and Methodology Hardware Software Operator Service Product Production/Manufacturing Processes and Equipment Product and Customer Support Systems

Stracener_EMIS 7305/5305_Spr08_ What is Reliability? To the user of a product, reliability is problem free operation Reliability is a function of stress To understand reliability, understand stress on hardware - where its going to be used - how its going to be used - what environment it is going to be used in To efficiently achieve reliability, rely on analytical understanding of reliability and less on understanding reliability through testing Field Problems Stress/Design, Parts and Workmanship

Stracener_EMIS 7305/5305_Spr08_ Definitions of Reliability Reliability is a measure of the capability of a system, equipment or component to operate without failure when in service. Reliability provides a quantitative statement of the chance that an item will operate without failure for a given period of time in the environment for which it was designed. In its simplest and most general form, reliability is the probability of success. To perform reliability calculations, reliability must first be defined explicitly. It is not enough to say that reliability is a probability. A probability of what?

Stracener_EMIS 7305/5305_Spr08_ More Definitions of Reliability Reliability is defined as the probability that an item will perform its intended unction for a specified interval under stated conditions. In the simplest sense, reliability means how long an item (such as a machine) will perform its intended function without a breakdown. Reliability: the capability to operate as intended, whenever used, for as long as needed. Reliability is performance over time, probability that something will work when you want it to.

Stracener_EMIS 7305/5305_Spr08_ Definitions of Reliability Essential elements needed to define reliability are: – What does it do? System, subsystem, equipment or component functions – What is satisfactory performance? Figures of System Allocations &/or subsystem, equipment & component – How long does it need to function? Life: required number of operational units (time, sorties, cycles, etc) – What are conditions under which it operates? Environment Operation Maintenance Support

Stracener_EMIS 7305/5305_Spr08_ Reliability Figures of Merit Basic or Logistic Reliability MTBF - Mean Time Between Failures measure of product support requirements Mission Reliability P s or R(t) - Probability of mission success measure of product effectiveness

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10 Basic Reliability Design and development Basic reliability is a measure of serial reliability or logistics reliability and reflects all elements in a system Measures Air ForceMFHBF - Mean Flight Hours Between Failures MFHBUM - MFHB Unscheduled Maintenance ArmyMFHBE - Mean Flight Hours Between Events NavyMFHBF - Mean Flight Hours Between Failures MFHBMA - MFHB Maintenance Actions Automotive Industry Number of defects per 100 vehicles

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Stracener_EMIS 7305/5305_Spr08_ Mission Reliability Mission Reliability is defined as the probability that a system will perform its mission essential functions during a specified mission, given that all elements of the system are in an operational state at the start of the mission. Measure P s or R(t) - Probability of mission success based on: Mission Essential Functions Mission Essential Equipment Mission Operating Environment Mission Length

Stracener_EMIS 7305/5305_Spr08_ Basic Elements of Reliability Modeling & Analysis Reliability is a probability Therefore a working knowledge of probability, random variables and probability distributions is required for: - Development of reliability models - Performing reliability analyses An understanding of the concepts of probability is required for design and support decisions

Stracener_EMIS 7305/5305_Spr08_ Reliability Humor: Statistics “If I had only one day left to live, I would live it in my statistics class -- it would seem so much longer.” From: Statistics A Fresh Approach Donald H. Sanders McGraw Hill, 4th Edition, 1990

Stracener_EMIS 7305/5305_Spr08_ Failure Density Function associated with a continuous random variable T, the time to failure of an item, is afunction f, called the probability density function, or inreliability, the failure density. The function f has the following properties: for all values of t and

Stracener_EMIS 7305/5305_Spr08_ Failure Distribution Function The failure distribution function or, the probability distribution function is the cumulative proportion of the population failing in time t, i.e.,

Stracener_EMIS 7305/5305_Spr08_ Failure Distribution Function The failure distribution function, F, has the following properties: 1. F is nondecreasing, i.e., if 0  t 1 < t 2 < , then F(t 1 )  F(t 2 ), 2. 0  F(t)  1 for all t 3. in general, but here F(0) = P(a < T  b) = F(b) - F(a)

Stracener_EMIS 7305/5305_Spr08_ Remark The time to failure distribution has a special name and symbol in reliability. It is called the unreliability and is denoted by Q, i.e. Q(t) = F(t) = P(T  t)

Stracener_EMIS 7305/5305_Spr08_ Failure Densities and Distributions Failure Density Failure Distribution f(t) t Area = P(t 1 < T <t 2 ) F(t) t F(t 2 ) F(t 1 ) t2t2 t1t1 P(t 1 < T < t 2 ) = F(t 2 ) - F(t 1 ) 1 0 0

Stracener_EMIS 7305/5305_Spr08_ Percentile The 100pth percentile, 0 < p < 1, of the time to failure probability distribution function, F, is the time, say t p, within which a proportion, p, of the items has failed, i.e., t p is the value of t such that F(t p ) = P(T  t p ) = p ort p = F -1 (p) F(t) p tptp

Stracener_EMIS 7305/5305_Spr08_ Reliability In terms of the failure density, f, of an item, the 100pth percentile, t p, is t p f(t) 0 tptp

Stracener_EMIS 7305/5305_Spr08_ The Reliability Function The Reliability of an item is the probability that the item will survive time t, given that it had not failed at time zero, when used within specified conditions, i.e.,

Stracener_EMIS 7305/5305_Spr08_ Properties of the Reliability Function 1. R is a non-increasing function, i.e., if 0  t 1 < t 2 < , then R(t 1 )  R(t 2 ) 2. 0  R(t)  1 for all t 3. R(t) = 1 at t = 0 4.

Stracener_EMIS 7305/5305_Spr08_ Properties of the Reliability Function The probability of failure in a given time interval, t 1 to t 2, can be expressed in terms of either reliability or unreliability functions, i.e., P(t 1 < T < t 2 ) = R(t 1 ) - R(t 2 ) = Q(t 2 ) - Q(t 1 )

Stracener_EMIS 7305/5305_Spr08_ Reliability Relationship between failure density and reliability

Stracener_EMIS 7305/5305_Spr08_ Relationship Between h(t), f(t), F(t) and R(t) Remark: The failure rate h(t) is a measure of proneness to failure as a function of age, t.

Stracener_EMIS 7305/5305_Spr08_ Properties of the Failure Rate The (instantaneous) failure rate, h, has the following properties: 1. h(t)  0, t  0 and 2.

Stracener_EMIS 7305/5305_Spr08_ The Reliability Function The reliability of an item at time t may be expressed in terms of its failure rate at time t as follows: where h(y) is the failure rate

Stracener_EMIS 7305/5305_Spr08_ Cumulative Failure Rate The cumulative failure rate at time t, H(t), is the cumulative number of failures at time t, divided by the cumulative time, t, i.e., The average failure rate of an item over an interval of time from t 1 to t 2, where t 1 < t 2, is the number of failures occurring in the interval (t 1, t 2 ), divided by the interval length, t 2 - t 1

Stracener_EMIS 7305/5305_Spr08_ Mean Time to Failure and Mean Time Between Failures Mean Time to Failure (or Between Failures) MTTF (or MTBF) is the expected Time to Failure (or Between Failures) Remarks: MTBF provides a reliability figure of merit for expected failure free operation MTBF provides the basis for estimating the number of failures in a given period of time Even though an item may be discarded after failure and its mean life characterized by MTTF, it may be meaningful to characterize the system reliability in terms of MTBF if the system is restored after item failure.

Stracener_EMIS 7305/5305_Spr08_ MTTF MTTF (Mean Time to Failure) or MTBF (Mean Time Between Failures) may be determined from the time to failure probability density function by use of three equivalent methods: 1. definition of MTBF 2. moment generating functions 3. characteristic function

Stracener_EMIS 7305/5305_Spr08_ Relationship Between MTTF and Failure Density If T is the random time to failure of an item, the mean time to failure, MTTF, of the item is where f is the probability density function of time to failure, iff this integral exists (as an improper integral).

Stracener_EMIS 7305/5305_Spr08_ Relationship Between MTTF and Reliability

Stracener_EMIS 7305/5305_Spr08_ Reliability “Bathtub Curve”

Stracener_EMIS 7305/5305_Spr08_ Reliability Humor