RELIABILITY TESTING

PRODUCT TESTINGRELIABILITY LIFE TESTINGTEST TIME CALCULATIONSBURN IN TESTINGACCEPTANCE TESTINGACCELERATED LIFE TESTINGEXPERIMENTAL DESIGN

PRODUCT TESTING

• It consists of several types of tests.Functional Or Operational Tests

To test the performance requirements and to evaluate design adequacy

Environmental Stress TestFor testing the capability of product to survive at various operating conditions.

Reliability Qualification TestTo measure product reliability

Safety Test To find hazards ,catastrophic occurrences ,loss of life or economic loss by generating faulty actions.

Reliability Growth Testing Consists of reliability testing of prototypes thro’ which the causes for failure is found and those failures are eliminated by design modification

Some other test types are• Maintainability demonstration.• System integration testing.• Operation test and evaluation.

Note:Reliability testing and safety testing use to

generate failures to find failure modes and then eliminate them by other design.

Test parameters should be defined for product testing:

• Condition of the test• Resources used• Apparatus or equipment• Test equipment, tooling, and supplies• Time and duration• Personnel selection• Test conditions• Test materials• Analysis of test data• Reporting of test data

RELIABILITY LIFE TESTING

Life testing is the most important activity in reliability program which is in need of largest fund and manpower.

It is used to design a product and subsequent product.

It also provide information for entire life about maintenance plan corrective actions on design and process for a tested product.

FACTORS TO BE CONSIDERED

• Objective of the test• Type of test to be performed• Operating and environmental conditions.• Number units to be tested(sample size)• Duration of test• Unequivocal definition of failure.

TEST STAGES

Infant stage• ability to fulfill the needDevelopment stage• performance, reliability, cost and modes of behaviorEvolution stage• compare to improve economy, maintenance and use of

std.partsPerformance stage • requirements of performance and reliabilityUser stage• withstand handling, transportation etc..

TEST METHODS

• Marginal testing• Non destructive testing.• Destructive testing.• Accelerated testing.• Burn-in and screen testing.• Acceptance and qualification testing.• Sequential testing.• Experimental design

MARGINAL TESTING

• It is prescribed by designer.• Probability of failure is a imminent one due to

degradation.Example: Electrical equipments are tested.

Isolating faulty parts.

Non-destructive testing

NDT involves comparing a known input to a measured output and comparing to a known model - does not require the sacrifice of the physical system, as would be the case with disassembly, dissection, or failure testing. Such methods are known as Non-Destructive Testing techniques. such techniques are valued for saving time and money. On the other hand, NDT methods do not always reveal hidden defects, and skill is usually needed in interpreting the results.

• Liquid Penentrant • Magnetic Particle • Eddy Current • Ultrasonic• Radiography Neutron • Radiography X-ray

Destructive testing

In destructive testing, tests are carried out to the specimen's failure, in order to understand a specimen's structural performance or material behavior under different loads. These tests are generally much easier to carry out, yield more information, and are easier to interpret than nondestructive testing.

Destructive testing is most suitable, and economic, for objects which will be mass produced, as the cost of destroying a small number of specimens is negligible. It is usually not economic to do destructive testing where only one or very few items are to be produced (for example, in the case of a building).

Types of destructive testing

• Stress tests• Crash tests• Hardness tests• Metallographic tests

The testing component is subjected to abnormal stresses.• If the component survives it is safe to use at

normal stresses.• This is for static situations(structures and

buildings).Example: beam or a truss can withstand lighter

load than test load.

Testing is done in time ordered manner(time ordered observations).

Example: life of electric bulb ,life of ball bearings(weakest will fail simultaneously)

Progressively censored samples

The items that can withstand the given tests are censored throughout the life to find the life of the item.

Two types• Censoring time is predetermined(type I).• Censoring starts after survival drop to

predetermined levels(type II).

Accelerated life testingAccelerated tests are becoming increasingly popular in today’s industry due to

the need for obtaining life data quickly. Life testing of products under higher stress levels without introducing

additional failure modes can provide significant savings of both time and money.

Correct analysis of data gathered via such accelerated life testing will yield parameters and other information for the product’s life under use stress conditions(reference conditions).

It is the test of an item goes for production stage when it is in the research and development stage for adequate life and no gross weakness existence.

Accelerated test conditions are typically produced by testing items at higher levels than normal

Example: temperature, voltage , pressure , vibration etc…

PRACTICAL CONSIDERATIONS

• Engineer should know how to design.• Variables must be varied by which we can

redesign and test and the factors not to changed.

• Materials used for the tests must be kept minimum.

• Model selected should describe stress dependence and life distributions.

• Failure must be clearly defined.

ACCELERATED TESTING MODELS

• Arrhenius and Eyring model (life on temperature)• Larsen-miller relationship (rupture test of metals using

temperature and stress as variables)• Prot model (endurance limit of metals)• Power law (capacitors and electronic components based on

voltage as variable)• Palmgren equation (life of bearings)• Lieblein and zelen models( fiiting palmgren equation based

on assumption)• Kaufman and meador model (dielectric breakdown of

insulating fluids based on inverse power law)

Test time calculation

• For constant failure rate(CFR) modelmean time to failure,

MTTF =T/rwhere,

T-cumulative test time.r-total no of failures.

Length of testFor type ll testing, length of the test depends on• no. of units being tested• no. of failures to be observed• time to failure distribution.Expected test time to generate r failures is

r x MTTFUnder CFR model if n units are placed on test until r failures are observed the

expected test time is given byE(test time)=MTTF x TTRr,n

where , TTRr,n is the test time factor for r failures with n units at risk.

If failure units are replaced and r units are there on test then expected test time to observe r failure is

E(test time)=MTTF (r/n)

• For type II testing the length of test E(r)=n( 1-e-t*/MTTF)t*-length of timep= 1-e-t*/MTTF

E(r)=np

BURN IN TEST

To eliminate or reduce “infant morality” failures by accumulating initial operating hours and resulting failures prior to user acceptance.

The mean residual lifetime can be found.

It requires test of all items so it increases production lead time and costs.

ACCEPTANCE TESTING

The objective of the test is to demonstrate that the system design meets performance and reliability requirements under specified operating and environmental conditions.

Sample size may be predetermined or unspecified.Units from the production line should be randomly

selected for testing.BINOMIAL ACCEPTANCE TESTINGSEQUENTIAL TESTS

EXPERIMENTAL DESIGN

• Efficient collection and analysis of data.• A design is selected which will iedntify the

factors, their levels, the no. of replications at specified levels, randomization of experimental units and use of blocking

FACTORIAL DESIGNRBDCRBD

APPLICATIONS OF TESTING

• Comparative evaluation of new design and new processing techniques.

• Deviation of screening techniques to prohibit failures in future field use.

• Estimation of system failure behavior under different test conditions.

referenceswebsites• www.ndt.net/ndtaz/ndtaz.php - 1k(ndt)• en.wikipedia.org/wiki/Nondestructive_testing - 54k• www.arsymposium.org/southamerica/Simposio2004/Paper_Modelo.doc• www.centerforquality.org/courses/onsite/ara.htm - 13k• www.Pdf-search-engine.com• www.google.com• www.enwikipedia.com• www.encyclopedia.com

Books• An introduction to reliability and maintainability engineering- by charles

E.ebeling• Text book given as Photocopy