An Introduction to Reliability testing

Transcription

1 An Introduction to Reliability testing Dr Jane Marshall Product Excellence using 6 Sigma Module PEUSS 2011/2012 Reliability testing Page 1 Objectives of this session Reliability testing Accelerated life testing RET testing Reliability growth testing Test, analyse and fix FRACAS PEUSS 2011/2012 Reliability testing Page 2 1

2 Product life cycle Use Field data analysis Design FMECA, FTA, PoF,RBD FE,accelerated life test FRACAS Development Development Test Test ESS, Burn-in Manufacture SPC PEUSS 2011/2012 Reliability testing Page 3 What is a test? A test has been defined as "a subjection to conditions that show the real character of the thing However, testing is a continuing operation to provide information throughout the complete evolution of the system. PEUSS 2011/2012 Reliability testing Page 4 2

3 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. PEUSS 2011/2012 Reliability testing Page 5 What should we be testing? Goal: To minimize the number of tests required. 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. PEUSS 2011/2012 Reliability testing Page 6 3

4 What should we be testing? 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 PEUSS 2011/2012 Reliability testing Page 7 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. PEUSS 2011/2012 Reliability testing Page 8 4

5 Frequency of testing Goal: To test units as infrequently as possible. 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 PEUSS 2011/2012 Reliability testing Page 9 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. PEUSS 2011/2012 Reliability testing Page 10 5

6 Event testing 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 PEUSS 2011/2012 Reliability testing Page 11 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 environmental levels. PEUSS 2011/2012 Reliability testing Page 12 6

7 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 PEUSS 2011/2012 Reliability testing Page 13 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 PEUSS 2011/2012 Reliability testing 14 7

8 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. PEUSS 2011/2012 Reliability testing Page 15 Types of Accelerated Test 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. PEUSS 2011/2012 Reliability testing Page 16 8

9 Types of Accelerated Test 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. PEUSS 2011/2012 Reliability testing Page 17 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. PEUSS 2011/2012 Reliability testing Page 18 9

10 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. PEUSS 2011/2012 Reliability testing Page 19 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. This type of test simulates the real use of a number of products. PEUSS 2011/2012 Reliability testing Page 20 10

11 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. PEUSS 2011/2012 Reliability testing Page 21 Accelerated Degradation tests 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. PEUSS 2011/2012 Reliability testing Page 22 11

12 Reliability Enhancement Tests 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 PEUSS 2011/2012 Reliability testing Page 23 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 Also helps with helps optimise stress screens PEUSS 2011/2012 Reliability testing Page 24 12

13 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 PEUSS 2011/2012 Reliability testing Page 25 Tests used prior to shipment to the customer Environmental Stress Screening ESS Demonstration testing PEUSS 2011/2012 Reliability testing Page 26 13

14 Environmental stress screening (ESS) 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. PEUSS 2011/2012 Reliability testing Page 27 Burn in 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 PEUSS 2011/2012 Reliability testing Page 28 14

15 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 PEUSS 2011/2012 Reliability testing Page 29 Summary Introduction to reliability testing Design and development Accelerated life testing RET Reliability Growth Pre-delivery testing ESS Demonstration FRACAS PEUSS 2011/2012 Reliability testing 30 15

16 The generic test process Inputs Outputs Product Spec external & internal customer requirem ents Airworthi ness requirem ents Tech. Risk analysis Safety & reliability requirem ents Define tests to be performed Plan resources Write test plan Top Level Process Complete Testing Analyse Test Results Draw Conclusions Verificatio n reports Declaratio n of design and performan ce Complian ce analysis Additional testing requireme nts PEUSS 2011/2012 Reliability testing Page 31 Define tests to be performed Plan resources Complete Testing Analyse Test Results Draw Conclusions Inputs Write test plan Sub process - Define tests to be performed Outputs Product Spec external & internal customer requirem ents Airworthi ness requirem ents Tech. Risk analysis Safety & reliability requirem ents List all tests required Identify test inputs Group tests which can be done together Define test method e.g. mechanical environmental performance reliability Identify potential relationships between inputs and outputs Define test goal Identify levels for test inputs Take account of experience data Identify constraints budget people time Define verification test plan taking into account variation of inputs Define Sample size and repetitions required List of grouped tests with definition of test method and design Identify test outputs PEUSS 2011/2012 Reliability testing Page 32 16

17 Plan Define tests to be performed resources Complete Testing Analyse Test Results Draw Conclusions Write test plan Inputs Sub process - Plan resources Outputs List of grouped tests with definition of test method and procedure Define hardware requirements Define test equipment requirements Allocate budget Allocate people resource Allocate test equipment Procure test hardware Procure test equipment Verify Calibration is up to date for test equipment Completed test plan with resources including verified test equipment Write test plan - include information from test method test design lessons learned standard formats Sub process - Write test plan PEUSS 2011/2012 Reliability testing Page 33 Plan Define tests to be performed resources Complete Testing Analyse Test Results Draw Conclusions Write test plan Inputs Sub process - Complete Testing Outputs Completed test plan with resources including verified test equipment Run test as planned Record Test Results Record unusual events or observations Test Results PEUSS 2011/2012 Reliability testing Page 34 17

18 Plan Define tests to be performed resources Complete Testing Analyse Test Results Draw Conclusions Write test plan Inputs Outputs Analyse Test Results Produce test report Test Results Use engineering analytical tools Use engineering knowledge Complete/update compliance analysis Test Report Complianc e Analysis Update lessons leaned database PEUSS 2011/2012 Reliability testing Page 35 Plan Define tests to be performed resources Complete Testing Analyse Test Results Draw Conclusions Write test plan Inputs Sub process - Draw Conclusions Outputs Verificatio n reports Test Report Complianc e Analysis Decide if test was successful or not Decide what additional testing is required Define actions to be taken based on analysis Conclude further reporting Declaratio n of design and performan ce Complian ce analysis Additional testing requireme nts PEUSS 2011/2012 Reliability testing Page 36 18