RELIABILITYBPT2423 – STATISTICAL PROCESS CONTROL

CHAPTER OUTLINE Fundamental Aspects Product Life Cycle Curve Measures of Reliability

Failure Rate, Mean Life and Availability Calculating System Reliability

Reliability Engineers

LESSON OUTCOMES Known the importance of system reliability Identify on how performance during the life of

a product, process or system is affected by its design and configuration

Able to compute the reliability of systems including systems in series, parallel and hybrid combinations

FUNDAMENTAL ASPECTS Reliability (quality over the long term) is the ability of a

product to perform its intended function over a period of time and under prescribed environmental conditions

Reliability of component dependent on many factors: quality of research performed at its conception the original design and any subsequent design changes the complexity of the design the manufacturing processes the handling received during shipping the environment surrounding its use the end users and etc.

FUNDAMENTAL ASPECTS Reliability testing enables a company to better

comprehend how their products will perform under normal usage as well as extreme or unexpected situations

Reliability programs provide information about product performance by systematically studying the product

Four factors associated with reliability:1. Numerical value2. Intended function3. Life4. Environmental conditions

LIFE-HISTORY CURVE

Life cycle of a product is commonly broken down into 3 phases:

1. Early failure (infant mortality) phase is characterized by failures occurring very quickly

after the product has been produced or put into use by the consumer

the curve during this phase is exponential with the number of failures decreasing the longer the product is in use

some early failures are due to inappropriate or inadequate materials, marginal components, incorrect installation or poor manufacturing techniques

LIFE-HISTORY CURVE

2. Chance failure During the failure, portion of a product’s useful life, failures

occur randomly May be due to inadequate or insufficient design margins Misapplication or misuse of the product by the consumer

can lead to product failure – eg.: overstressing

3. Wear-out Failures increase in number until few, if any of the product

are left Due to a variety of causes such as related to actual product

function or cosmetic (scratched, dented, discolorations, misalignments and interference between components)

LIFE-HISTORY CURVE

MEASURES OF RELIABILITY Overall system reliability depends on the individual

reliabilities associated with the parts, components and subassemblies

Reliability values are sought to determine the performance of a product, reveal any recurring patterns of failure and the underlying causes of those failures

Reliability test determine what failed, how it failed and the number of hours, cycles, actuations or stresses it was able to bear before failure

If result known, decisions can be made concerning product reliability expectations, corrective action steps, maintenance procedures and cost of repair/replacement

MEASURES OF RELIABILITYSeveral different types of test exist to judge the reliability of a product :1. Failure-terminated test

Ended when a predetermined number of failures occur within the sample being tested

Decision concerning whether or not the product is acceptable is based on the number of products that have failed during the test

2. Time-terminated test Concluded when an established number of hours is reached Product is accepted on the basis of how many products

failed before reaching the time limit

3. Sequential Test - relies on the accumulated results of the tests

MEASURES OF RELIABILITYDetermine Failure Rate, λ

Determine Average Life, θ

or

Determine Availability

MEASURES OF RELIABILITYExample :Determine the failure rate for a 90-hour test of 12 items where 2 items fail at 45 and 72 hours, respectively. What is the mean life of the product?

λ = 2

45 + 72 + (10) 90

= 2 / 1017

= 0.00197

θ = 1 / λ = 1 / 0.00197 = 507.6 hours

SYSTEM RELIABILITYReliability is the probability that a product will not fail during a particular time period.

where :s – those units performing satisfactorilyn – total number of units being tested

Reliability in Series

SYSTEM RELIABILITYReliability in Parallel

Reliability in Redundant Systems and Backup Components

r1

rb

SYSTEM RELIABILITYCalculation :Example:

A paranoid citizen has installed the home alert system shown below. What is the overall system reliability?

SYSTEM RELIABILITYExercise :