Reliability

It is defined as the probability that a given system will perform it’s function adequately for it’s specified period of lifetime under specified operating conditions.

Common measures are :

1. Failure rate.

2. Mean time between failures(MTBF)

3. Survival percentage.

Failure Rate:

Rate which components of population fail.

R(t)=N_s(t)/N_f(t)

Where,

N_s(t)- No. of components that survived during time ’t’

N_f(t) – No. of failures that occurred during the same time.

Mean time between Failures(MTBF):

The reciprocal of the failure rate(1/λ).

Where

λ —-Failure rate.

Failure rate = (No.of failure )/(Time period during all components were exposed to failure)

 

Reliability Analysis:

Failure Mode and Effect Analysis(FMEA)

Purpose :

1. To recommend design changes.

2. To identify design weakness.

3. To help in choosing alternatives.

Four Stages:

I^st Stage – System boundaries and the scope of the analysis is decided.

II^nd Stage – Data Collection

Ex: Specification,Operating Procedure,Working Conditions.

III^rd Stage– Preparing the component or parts list.

IV^th Stage – Failure frequency and the functions of the part identified,causes of       failures,Failure detection.

 

 

 

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