An Introduction to Reliability testing

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PEUSS 2011/2012 Reliability testing Page 1

An Introduction to Reliability

testing

Dr Jane Marshall

Product Excellence using 6 Sigma Module

Objectives of this session

• Reliability testing

• Accelerated life testing • RET testing

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Product life cycle

Development

Design

FMECA, FTA, PoF,RBD FE,accelerated life test

Development Test Manufacture SPC Test ESS, Burn-in Use

Field data analysis FRACAS

What is a test ?

• A test has been defined as

– "a subjection to conditions that show the real character of the thing”

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Purposes of testing

• The purposes of testing are numerous.

– A test may be performed to see whether a certain configuration or item is feasible.

– A test may be used to determine which of several configurations is the optimum with respect to performance, reliability, cost, modes of behaviour under varying conditions, etc.

– A test maybe used to make more sensitive comparisons to further improve economy, maintenance, use of standard parts, and so on. – A test may be used to demonstrate whether the item is adequate to meet

the requirements of performance and reliability.

– A test can be used as thorough investigation of the latent capabilities of the item under severer or more diverse conditions than those immediately anticipated.

 Define exactly what requires testing - bearing in mind that the objective is to minimize the number of tests required for cost reasons

 The test types may be categorized as follows: -Mandatory

Testing based on pedigree or experience

Data driven test requirements.

Goal: To minimize the number of tests required.

What should we be testing ?

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 Mandatory Testing

Tests based on regulatory requirements - e.g. Road worthiness

Tests based on safety requirements

Tests based on customer requirements

Tests based on competitor benchmark

What should we be testing ?

 Testing based on pedigree or experience:

-A historical product with similar physical and performance attributes and similar testing may be referred to as a guide.

Manufacturing and development engineers may have a ‘feel’ for the type of testing required based on previous experience.

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 Need to determine the frequency of testing. Each and every product

Each and every product for a limited period

Periodic testing

Regulatory requirement testing at a specified interval/frequency

Goal: To test units as infrequently as possible.

Frequency of testing

Time based testing

• when an item fails,

• which part in a component causes failure • which component in a system fails.

• the mode or modes of failure as exemplified by performance drift, erratic performance,

catastrophic failure, etc.,

• the mechanism of failure that is, the reasons for failure caused by poor design, part misapplication, etc.

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• In this case samples of the equipment are tested

repeatedly through their cycle of operation until failure. • This testing, which is analogous to time-to-failure

testing, becomes the more meaningful test when

starting and stopping operations are more destructive than the mere accumulation of time.

• The important parameter in this form of testing is mean number of cycles to failure

Event testing

Environmental testing

• As its name implies, this form of testing

represents a survey of the reaction of the item to the various environments.

• It is usually required in qualification tests and is frequently introduced in the development stage, usually at less numerous or less severe

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Tests used during design

and development

• Accelerated life tests • Qualification tests • Software testing

• Development testing – RET, Growth testing, Test, Analyse and fix

– Test to cause failures not to demonstrate successful achievement

Testing for reliability and

durability

• Test to cause failures

• Not test to demonstrate successful achievement • Use concepts of load and strength to determine

test conditions

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Types of Accelerated Test

• High Usage Rate.

• run the product faster than normal, • increase the products duty cycle.

• Care should be taken that the increased usage rate does not increase the other stresses on the product too much, so that the failure mechanism produced are the same as those seen under normal usage rates.

• Over-stress testing

– This consists of running a product at a higher than normal level of some accelerating stress or stresses to shorten product life or to degrade product

performance factor.

– Typical accelerating stresses are temperature, voltage, mechanical load, thermal cycling, humidity and vibration.

– Over-stress testing is the most common form of accelerated testing.

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• Censoring

– This is accelerated testing through a statistical ‘trick’, where the tests are terminated before all the specimens have failed.

– Statistical techniques are then used to predict what the outcome of the test would have been had it run to completion.

– This allows the tests to be shortened considerably and has become common practise.

• Degradation

– Accelerated degradation testing involves over-stress testing but instead of life, product performance is observed as it degrades over time.

– Similar statistical methods to those used for the censoring tests are used to calculate the reliability.

• Specimen Design

– The life of some products can be accelerated through the size, geometry and finish of specimens.

– For instance large specimens fail sooner than small ones, e.g. high capacitance capacitors fail sooner than lower capacitance ones of the same design, this is because the large capacitors have more dielectric area.

Types of Accelerated Test

Constant stress

• The most common form

• Each specimen is run at a constant stress level. • Mimics the stresses seen in actual use in many

cases and can be simple. • The advantages are

– A constant stress level is easy to maintain – Models for acceleration are well developed

and tested.

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Step stress tests

• Progressively increasing stresses are applied to the same sample for constant time intervals while the sample does not fail.

• The fundamental assumption in this kind of test is that the failure probability at each level is independent of previous levels, regardless of previous history.

• Generate data on the product reliability more quickly, but with less precision.

• The step stress test are normally carried out as comparison tests.

Methods of applying stress

• Progressive Stress Tests

– In progressive stress testing, a specimen undergoes a continuously increasing level of stress.

– The method has similar advantages and disadvantages to the step stress test.

• Cyclic Stress

– In use some products repeatedly undergo a cyclic stress loading, for example all AC electrical products see a sinusoidal stress.

– A cyclic stress test for a product repeatedly loads a specimen with the same stress pattern at high stress levels.

– For many products this cycle is sinusoidal, for others the duty cycle repeats but is not sinusoidal.

• Random Stress

– In a random stress test, a random stress loading is applied to the specimen for a random time and then removed another random stress for some other random time and so on.

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‘Elephant’ or proof tests

• Such tests are often called

– torture tests, – shake and bake, – killer tests,

– design margin tests – design limit tests.

• If the product survives the test then it passes and is judged to be a ‘good’ product.

• If the product fails it is usually redesigned to remove the cause of failure.

• Elephant tests are normally performed on a few specimens which are subject to a single severe stress level for a fixed time.

• An elephant test may not produce certain important failures that are seen in practice and so a series of different elephant tests may have to be used.

• Accelerated degradation is concerned with models and data analysis or product performance over time at overstress and design conditions.

• These form of tests have an advantages over accelerated life tests since performance degradation data can be analysed earlier before any specimens fail.

• This is done by extrapolating performance degradation to estimate a time where performance reaches a failure level.

Accelerated Degradation tests

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• Tests that are carried out in the design and development phase of a product to remove defects.

• The standard approach is

– TEST – ANALYSE – FIX

• Various methods exist for analysis of the data

Reliability Enhancement Tests

RET

• Purpose is to improve design and increase durability • Step stress testing – combination of stresses

• Do not simulate environment

• Stresses in excess of those seen in service • HALT – highly accelerated life testing

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What stress limits?

• Increase stress to cause failures but limit by:

– Technological limits – Test capability

• Use unrepresentative stresses because:

– Future causes of failure uncertain – Probabilities to failure uncertain – Time spent on testing expensive

– Finding causes of failure during development less expensive in the long run

Tests used prior to shipment

to the customer

• Environmental Stress Screening – ESS • Demonstration testing

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– This involves accelerated testing of products under a combination of random vibration and thermal cycling and shock - shake and bake.

– It has two major purposes,

• as an elephant test during development

• as an accelerated burn in to improve reliability.

– It is widely used for military, industrial and consumer electronics.

Environmental stress

screening (ESS)

– Burn-in consists of running items under design or accelerated conditions for a suitable length of time. – Burn-in is a manufacturing operation that is intended

to fail short-lived units (defectives, sometimes called freaks).

– If burn-in is successful then the production units will have few early failures.

– Burn-in is primarily used for electronic components

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FRACAS

• Failure reporting and corrective action system • Closed-loop system for identifying, assessing,

and correcting failure-related problems in a timely manner

• Start of project

• Used by all personnel • Review team

Summary

• Introduction to reliability testing

– Design and development • Accelerated life testing • RET • Reliability Growth – Pre-delivery testing • ESS • Demonstration • FRACAS

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Inputs

Outputs

Product

Spec Top Level Process

Airworthi ness requirem ents Tech. Risk analysis Safety & reliability requirem ents Verificatio n reports Declaratio n of design and performan ce Complian ce analysis Define tests to be performed Write test plan Plan resources Complete Testing Analyse Test Results Draw Conclusions external & internal customer requirem ents Additional testing requireme nts

The generic “test” process

Inputs _Outputs

Sub process - Define tests to be performed

List of grouped tests with definition of test method and design Group tests which can be done together List all tests required Define test method e.g. mechanical environmental performance reliability

Define test goal

Identify constraints budget people time Define verification test plan taking into account variation of inputs Identify test inputs Take account of experience data Identify potential relationships between inputs and outputs Identify levels for test inputs

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Inputs Outputs List of grouped tests with definition of test method and procedure

Sub process - Plan resources

Allocate people resource Allocate budget Allocate test equipment Procure test equipment Procure test hardware Define hardware requirements Define test equipment requirements Verify Calibration is up to date for test equipment

Write test plan - include information from test method

test design lessons learned standard formats

Completed test plan with resources including verified test equipment

Sub process - Write test plan

Define tests to be performed Write test plan Plan resources Complete Testing Analyse Test Results Draw Conclusions

Inputs Outputs

Sub process - Complete Testing

Completed test plan with resources including verified test equipment Test Results

Run test as planned

Record Test Results

Record unusual events or observations Define tests to be performed Write test plan Plan resources Complete Testing Analyse Test Results Draw Conclusions

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Inputs _Outputs Analyse Test Results

Test Results Test Report Complianc e Analysis

Use engineering knowledge Use engineering analytical tools

Produce test report

Complete/update compliance analysis Update lessons leaned database Define tests to be performed Write test plan Plan resources Complete Testing Analyse Test Results Draw Conclusions

Inputs Outputs

Sub process - Draw Conclusions

Test Report Complianc e Analysis Verificatio n reports Declaratio n of design and performan ce Complian ce analysis Additional testing requireme nts

Decide if test was successful or not

Decide what additional testing is required