TECHNICAL REPORT ISO/TR 61 94 First edition 2017-04 Pneumatic fluid power — Assessment o f component reliability by accelerated life testing — General guidelines and procedures Transmissions pneumatiques — Évaluation de la fiabilité du composant par essai de durée de vie accélérée — Lignes directrices générales et modes opératoires Reference number ISO/TR 16194:2017(E) © ISO 2017 ISO/TR 61 94: 01 7(E) COPYRIGHT PROTECTED DOCUMENT © ISO 2017, Published in Switzerland All rights reserved Unless otherwise specified, no part o f this publication may be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on the internet or an intranet, without prior written permission Permission can be requested from either ISO at the address below or ISO’s member body in the country o f the requester ISO copyright o ffice Ch de Blandonnet • CP 401 CH-1214 Vernier, Geneva, Switzerland Tel +41 22 749 01 11 Fax +41 22 749 09 47 copyright@iso.org www.iso.org ii © ISO 2017 – All rights reserved ISO/TR 61 94: 01 7(E) Contents Page Foreword iv Introduction v Scope Normative references Terms and definitions Symbols and units Concepts o f reliability and accelerated life testing Failure mechanism and mode Strategy o f conducting accelerated li fe testing Design o f accelerated life testing 8.1 8.2 8.3 8.4 8.5 8.6 Normal use conditions Preliminary tests Levels of accelerated stress Sample size Data observation and measurement Types o f stress loading 9.1 9.2 9.3 Minimum number of failures required Termination cycle count Suspended or censored test units End o f test 10 Statistical analysis Analysis o f failure data 10.2 Life distribution 10.3 Accelerated life testing model 10 10.4 Data analysis and parameter estimation 10 10.1 11 Reliability characteristics from the test data 1 12 Test report Annex A (informative) Determining stress levels when stress is time-dependent Annex B (informative) Life-stress relationship models Annex C (informative) Verification o f compromise Weibull slopes Annex D (informative) Calculation procedures for censored data Annex E (informative) Examples o f using accelerated life testing in industrial applications Annex F (informative) Palmgren-Miner’s rule Annex G (informative) ALT experimental results for pneumatic cylinder Bibliography © ISO 2017 – All rights reserved iii ISO/TR 61 94: 01 7(E) Foreword ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies) The work o f preparing International Standards is normally carried out through ISO technical committees Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters o f electrotechnical standardization The procedures used to develop this document and those intended for its further maintenance are described in the ISO/IEC Directives, Part In particular the different approval criteria needed for the di fferent types o f ISO documents should be noted This document was dra fted in accordance with the editorial rules of the ISO/IEC Directives, Part (see www.iso org/directives) Attention is drawn to the possibility that some o f the elements o f this document may be the subject o f patent rights ISO shall not be held responsible for identi fying any or all such patent rights Details o f any patent rights identified during the development o f the document will be in the Introduction and/or on the ISO list of patent declarations received (see www.iso org/patents) Any trade name used in this document is in formation given for the convenience o f users and does not constitute an endorsement For an explanation on the voluntary nature o f standards, the meaning o f ISO specific terms and expressions related to formity assessment, as well as in formation about ISO’s adherence to the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following URL: www.iso org/iso/foreword html ISO/TR 16194 was prepared by Technical Committee ISO/TC 131, Fluid power systems iv © ISO 2017 – All rights reserved ISO/TR 61 94: 01 7(E) Introduction This document is being released to document progress that the working group has developed for accelerate d te s ti ng It is a new me tho d with wh ich the worki ng group memb ers have ver y l ittle exp erience, but s b e en u s e d b y i n s titutiona l l ab oratorie s and taught at academ ic level s S ome e xp eri mentation on r c yl i nders h as b e en done at the Kore an I n s titute o f M ach i ner y a nd M ateri a l s (KI M M ) , but the appl ic ation to pneumatic comp onents i n genera l h as no t b e en eva luate d T h i s c u ment i s o ffere d to memb ers a s a re ference a nd mo del pro ce dure, s o that they c a n develop experience with its use in their own laboratories © ISO 2017 – All rights reserved v TECHNICAL REPORT ISO/TR 61 94: 01 7(E) Pneumatic fluid power — Assessment o f component reliability by accelerated life testing — General guidelines and procedures Scope This c u ment provide s for genera l pro ce du re s a s s e s s i ng the rel iabi l ity o f pneu matic flu id p ower components using accelerated life testing and the method for reporting the results These procedures app ly to d i re c tiona l control va lve s , c yl i nders with pi s ton ro d s , pre s s u re re gu l ators , a nd acce s s or y device s – the s a me comp onents covere d by the I S O 19 s erie s o f s tandard s This c u ment I n s te ad , it e s explains no t the accelerated test method p ro vide s p e c i fic va r i ab i l i t y fo r p ro ce du re s a mo ng me tho d s a nd T he me tho d s s p e ci fie d i n th i s c u ment apply to the fi rs t accelerate d p ro vide s fai lu re, l i fe te s ti n g g u idel i ne s fo r o f co mp o nents de ve lo p i n g an without rep a i rs Normative re ferences There are no normative references in this document Terms and definitions For the pur p o s e s o f th i s c u ment, the term s a nd defi n ition s given i n I S O the fol lowi ng apply I S O a nd I E C mai ntai n term i nolo gic a l datab a s e s for following addresses: — IEC Electropedia: available at http://www.electropedia org/ — ISO Online browsing platform: available at http://www.iso org/obp 5598 , I SO 19 -1 a nd u s e i n s ta nda rd i z ation at the B x li fe l i fe o f a comp onent or a s s embly that s no t b e en a ltere d s i nce its pro duc tion, where its rel i abi l ity i s ( 100 − x ) %; or the time at which ( 100 − x ) % of the population has survived N o te to entr y: T he c u mu l ative fa i lu re frac tion is pro b abi l ity o f 10 % x % For example, if x = 10, the B 10 life has a cumulative failure 3.2 acceleration factor AF ratio between the life at the normal use stress level and the life at the accelerated stress level 3.3 accelerated li fe test ALT pro ce s s i n wh ich a comp onent i s force d to fa i l more quickly that it wou ld have u nder norma l u s e conditions and which provides information about the component’s life characteristics © ISO 2017 – All rights reserved ISO/TR 61 94: 01 7(E) destruct limit stress level at which one or more of the component’s operating characteristics is no longer within specification or the component is damaged and cannot recover when the stress is reduced Note to entry: Destruct limits are classified as a lower destruct limit and upper destruct limit 3.5 failure mechanism physical or chemical process that produces instantaneous or cumulative damage to the materials from which the component is made failure mode manifestation of the failure mechanism resulting from component failure or degradation Note to entry: The failure mode is the symptom o f the aggressive activity o f the failure mechanism in the component’s areas of weakness, where stress exceeds strength failure λ rate requency at which a failure occurs instantaneously at time t, given that no failure has occurred before t f 3.8 highly accelerated li fe test HALT process in which components are subjected to accelerated environments to find weaknesses in the design and/or manufacturing process Note to entry: The primary accelerated environments include pressure and heat model for accelerated li fe testing model that consists of a life distribution that represents the scatter in component life and a relationship between life and stress Note to entry: Li fe distribution examples: Weibull, Lognormal, Exponential, etc Note to entry: Li fe and stress examples: Arrhenius, Eyring, Inverse Power Law, etc 10 normal use conditions test conditions at which a component is commonly used in the field, which can be less strenuous than rated conditions 11 termination cycle count number o f cycles on a test item when it reaches a threshold level for the first time © ISO 2017 – All rights reserved ISO/TR 61 94: 01 7(E) Symbols and units Symbol a B10 η F(t) β R(t) a λ(t) Definition Time at which 10 % of the population is estimated to fail Scale parameter (characteristic life) of the Weibull distribution Probability o f failure o f a component up to time t Shape parameter (slope) of the Weibull distribution Reliability o f a component at time t; R(t) = – F(t) Failures per unit time Other symbols could be used in other documents and so ftware Units of measurements are in accordance with ISO 80000-1 Concepts o f reliability and accelerated li fe testing Reliability is the probability (a percentage) that a component does not fail (for example, exceed the threshold level or experience catastrophic failure) for a specified interval o f time or number o f cycles when it operates under stated conditions This reliability can be assessed by test methods described in the ISO 19973 series Generally, reliability analysis involves analysing time to failure o f a component, obtained under normal use conditions in order to quanti fy its li fe characteristics Obtaining such li fe data is o ften di fficult The reasons for this di fficulty can include the typically long li fe times o f components, the small time period between design and product release, and the necessity for testing components under normal use conditions Given this di fficulty and the need to observe failures o f components to better understand their li fe characteristics, procedures have been devised to accelerate their failures by overstress, thus forcing components to fail more quickly than they would under normal use conditions The term accelerated life testing (ALT) is used to describe such procedures However, a relationship between the reliability o f a component determined by ALT, and its reliability at normal use conditions, is necessary This can be assessed by extrapolating the test results obtained from an accelerated life test and comparing it to that obtained from testing at normal use conditions Figure shows the graphical concept for this relationship © ISO 2017 – All rights reserved ISO/TR 61 94: 01 7(E) Figure — Graphical explanation o f relationship between S-N curve and accelerated li fe testing NOTE Distributions in this concept Figure are not defined In Figure , failures under normal use conditions are represented by the distribution S , and the accelerated conditions are distributions S1 and S2 Their relationship is shown by the connecting line(s) Failure mechanism and mode The failure mechanism is the physical or chemical process that produces instantaneous or cumulative damage to the materials from which the component is made The failure mode is the manifestation of the failure mechanism resulting from component failure or degradation The failure mode is the symptom o f the aggressive activity o f the failure mechanism in areas o f component weakness where the stress exceeds the strength It is necessary that the failure modes observed in accelerated li fe test conditions are identical to those defined for normal use conditions Strategy o f conducting accelerated li fe testing Be fore starting an accelerated li fe test, it is important to identi fy the types o f failures that might occur in service; especially any feedback from the field Several methods are available to assist in this e ffort: design analysis and review using the quality function deployment (QFD), fault tree analysis (FTA), and failure modes and e ffect analysis (FMEA) Another method is a qualitative test like highly accelerated li fe testing (HALT) Qualitative tests are used primarily to reveal probable failure modes, but they not quanti fy the li fe (or reliability) o f the component under normal use conditions © ISO 2017 – All rights reserved ISO/TR 61 94: 01 7(E) Table G.1 — 000 cycle pressure step stress data for company A T he s ame te s t was rep e ate d with a no ther two c yl i nders , but u s i ng s tep s o f 10 0 c ycle s ( Table G ) This demonstrates that failure is sensitive to the length of time at a stress level – the longer the interval, less than the 1,2 bar threshold level, at 25 bar operating pressure Thus, it was determined that the failure occurred at 25 bar the lower i s the de s truc tive s tre s s For the 10 0 c ycle s tep te s t, the m i n i mu m op erati ng pre s s u re wa s Table G.2 — 10 000 cycle pressure step stress data for company A 46 © ISO 2017 – All rights reserved ISO/TR 61 94: 01 7(E) In both of these tests, the mode of failure was extrusion of the cushion seal – not wear as had been observed in the tests at normal conditions (see Figure G.7) It can be concluded that this is a different mode of failure, even though the performance characteristics measured for threshold comparison were the basis for determining the destructive stress level Because of this difference the maximum stress levels for accelerated testing need to be lowered until failure modes are the same as in the normal condition tests Thus, the pressure operating limit was chosen below 20, at 16 bar The chosen pressure operating limit o f 16 bar is 133% o f the specification limit Figure G.7 — Cushion seal failure mode Another two specimens from company A were then used to conduct step stress tests for temperature Pressure was held constant at 6,3 bar for this series of tests, and temperature increased as shown in the table This series o f tests were conducted in 000 cycle steps until failure occurred at 140 °C The failure mode in this case was piston seal wear – same as observed at normal conditions The failure occurred at 140 °C where the minimum operating pressure and the total leakage fell below the threshold level © ISO 2017 – All rights reserved 47 ISO/TR 61 94: 01 7(E) Table G.3 — 000 cycle temperature step stress data for company A T h i s te s t wa s then rep e ate d with 10 0 c ycle s tep s a nd fai lu re o cc u rre d at ° C; where the m i ni mu m operating pressure and the total leakage fell below the threshold level See Table G.4 T hu s , the temp eratu re op erati ng l i m it wa s cho s en as ° C where the s ame typ e o f fai lure mo de o cc u rs T he va lue o f temp eratu re op erati ng l i m it, ° C , i s % o f the s p e c i fic ation l i m it 48 © ISO 2017 – All rights reserved ISO/TR 61 94: 01 7(E) Table G.4 — 10 000 cycle temperature step stress data for company A The same series o f tests were conducted on cylinders from company B, and an abbreviated set o f results for pressure testing is shown in Table G.5 Conclusions were similar to that obtained for company A, except that the mode of failure was piston seal wear – same as at normal conditions © ISO 2017 – All rights reserved 49 ISO/TR 61 94: 01 7(E) Table G.5 — Pressure step stress data for company B Table G.6 f f f total leakage and the stroke time fell below the threshold level i s an abbrevi ate d s e t o re s u lts the 0 a nd 10 0 c ycle s tep te s ts , the 50 or temp eratu re s tep s tre s s te s ti ng for comp any B For e ach o f a i lu re o cc u rre d at ° C a nd ° C , re s p e c tively; where the © ISO 2017 – All rights reserved ISO/TR 61 94: 01 7(E) Table G.6 — Temperature step stress data for company B G.4 Accelerated testing With data from the destructive step stress tests, a test plan for accelerated testing for company A can be developed as shown in Table G.7 Pressure and temperature stress levels are selected to be at levels below the destructive levels discovered from the step stress tests, and limited to conditions at which the failure modes are the same as found at normal conditions At this time, tests at the stress levels shown in black number of units have been completed, and tests in red are in progress Currently, KIMM is putting together test plans for the stress levels that are marked in blue as TBT All of these are at single variable stress conditions – holding pressure or temperature at one level while testing variations at the other levels Dual stress conditions are also planned as shown elevated above the normal conditions - and will also exceed the catalogue ratings However, they are © ISO 2017 – All rights reserved 51 ISO/TR 61 94: 01 7(E) Table G.7 — Overall test plan for company A Li kewi s e, a te s t pla n for comp any B i s develop e d as s hown i n shown in black number of units have been completed Table G.8 Again, tests at the stress levels Table G.8 — Overall test plan for company B 52 © ISO 2017 – All rights reserved ISO/TR 61 94: 01 7(E) The principles of accelerated testing are shown in the two graphs of Figure G.8 – one for pressure stress and one for temperature The blue distribution curves describe a theoretical failure distribution at normal conditions of pressure and temperature Testing would be conducted at stress levels higher than normal conditions, and their li fe distributions would be as shown in the three yellow curves labelled first, second and third stress levels A characteristic o f each distribution (mean, B 10 , or characteristic li fe), would be projected up to the normal stress level by extrapolation using the equation shown below each graph This extrapolation provides the equivalent life at normal conditions Accuracy o f the process is obtained from comparison to real experimental results conducted at normal conditions With experience, and knowledge of the components, testing at normal conditions could eventually be eliminated Then, the benefits o f reduced test time for reliability by accelerated testing are realized Figure G.8 — Accelerated li fe test concept Some of the test results at higher stress levels are shown to demonstrate the process of accelerated testing The bar graph in Figure G.9 shows results for 12 bar pressure at 23 °C temperature for company A pneumatic cylinders In this test, some o f the specimens were continued on test a fter observing their first failures as shown on the black bars However, at this time, only the first failure results are used in the analysis Similar data was obtained from testing at bar pressure – with longer lives, as expected © ISO 2017 – All rights reserved 53 ISO/TR 61 94: 01 7(E) Figure G.9 — Accelerated pressure test data for company A Partial results are now shown in a composite Weibull graph of Figure G.10 Distributions from the and 12 bar tests are shown plotted, and their characteristic life points, B 10 life points, and B10 life at the lower % one s ide d fidence i nter va l are j oi ne d b y c u r ve s T he s e c u r ve s are de s crib e d b y the i nvers e p ower law a nd thei r proj e c tion provide s the ex trap olation T he proj e c te d B 10 for l i fe at the % lower fidence level i s 4, 70 x 10 a fter the fi na l d i s tribution from va lue s at the norma l cond ition c ycle s T h i s figure m ight change accelerate d te s ti ng at 14 b ar i s i ncluded For comparison, data from testing at normal conditions provides the distribution labelled “real experimental result.” Its B10 f l i e at the % lower fidence level i s ,9 162 x 10 c ycle s – i nd ic ati ng that with on ly p a r tia l te s t re s u lts avai lable at th i s ti me, the acc u rac y o f the proj e c tion i s 54 85 7% © ISO 2017 – All rights reserved ISO/TR 61 94: 01 7(E) Figure G.10 — Comparison o f results for company A using pressure stress and inverse power law model The bar graph in Figure G.11 describes results from one of the accelerated temperature tests Note that the test time was quite short © ISO 2017 – All rights reserved 55 ISO/TR 61 94: 01 7(E) Figure G.11 — Accelerated temperature test data for company A The Weibull graph in Figure G.12 describes the same type of results as the previous one, except that it uses temperature stress – which is governed by the Arrhenius equation for extrapolation This also has only two sets o f tests completed at this time, and its projection to the normal conditions is shown in the green distribution However, these results not compare favourably to the results from direct testing at normal conditions, as shown in the solid purple distribution It is observed that the 95% lower confidence curve is quite bent and this requires further examination This is an example of the need for experience in selecting the stress levels and understanding the conduct of testing 56 © ISO 2017 – All rights reserved ISO/TR 61 94: 01 7(E) Figure G.12 — Comparison of results for company A using temperature stress and Arhenius model G.5 Conclusions It was pointed out that accelerated testing reduces the test time, so the amount of time required from the several tests conducted this far is shown in Table G.9 Testing at normal conditions required about one ye a r, and te s ti ng at i ncre as e d pre s s ure level s re duce d th i s b y ab out 10 days , or more Te s ti ng at elevate d temp eratu re s re s u lte d i n the mo s t s ign i fic ant re duc tion i n te s t ti me, but (at th i s ti me) has an i s s ue with acc u rac y that ne e d s more development © ISO 2017 – All rights reserved 57 ISO/TR 61 94: 01 7(E) Table G.9 — Comparison o f test time reduction T he te s t pro gram at KI M M h as i n itiate d re s e a rch te s ti ng T he pro gram u s e s comp onents s tage o f e xploration B a s el i ne te s ti ng from for for the flu id p ower i ndu s tr y i n accelerate d rel iabi l ity two manu fac tu rers to e xp a nd the variabi l ity i n th i s e a rly de term i ni ng acc u rac y o f the i s ne ce s s ar y i n th i s e a rly s tage, and s b e en u s e d for accelerate d te s t proj e c tion s the i n iti a l comp ari s on s – the pre s s u re typ e i s encouraging; the temperature one is not Destructive testing has been educational, but does not have f f be a good guideline It is imperative that failures at the high stress levels be examined to determine if f f be used for information The temperature stress method appears to have the most advantage for time for a better evaluation to b e conti nue d I t i s l i kely that te s ti ng to 3 % o c ata lo gue rati ngs they are the s ame mo de as at norma l cond ition s I or the h ighe s t s tre s s level s wou ld no t, the data i s no t qua l i fie d or a na lys i s but c an s avi ngs , but a l s o app e ars to b e the le a s t acc u rate at th i s e a rly s tage H owever, there i s much ye t to tr y I t i s i mp or tant that acc u rac y b e e s tab l i s he d i n the accelerate d te s t me tho d T h i s re qu i re s s ome norma l cond ition te s ti ng for comp ari s on s b e fore fidence i s develop e d and e xp erience gai ne d E ventua l ly, the baseline normal condition testing can be phased out and the advantages of reduced test time from accelerated testing can be realized A new area for exploration is combining pressure and temperature stress to see what advantages and complexities occur The temperature acceleration also needs to be evaluated to determine what prac tice s i mprove acc u rac y O ther lab oratorie s ne e d to b e gi n te s t pro gra m s o f thei r own s o that they c a n b egi n to acqu i re exp erience 58 © ISO 2017 – All rights reserved ISO/TR 61 94: 01 7(E) Bibliography [1] ISO 19973-1:2015, Pneumatic fluid power — Assessment of component reliability by testing — Part : Gen eral procedures [2] ISO 13849-1, Safety of machinery — Safety-related parts of control systems — Part 1: General principles for design [3] [4] [5] E lsayed E.A Reliability Engineering Addison Wesley, 1996 M c Linn J.A Practical Accelerated Li fe Testing ASQ, 2000 ReliaSoft, Accelerated Life Testing Reference, ReliaSoft Publishing, 2007 [6] Wayne Nelson, Accelerated Testing Wiley, 2004 [7] [8] [9] M eeker W.Q., & E scobar L.A Statistical Methods for Reliability Data Wiley, 1998 ISO 5598, Fluid power systems and components — Vocabulary ISO 19973-2, Pneumatic fluid power — Assessment of component reliability by testing — Part 2: Direction al control valves [10] ISO 19973-3, Pneumatic fluid power — Assessment of component reliability by testing — Part 3: Cylinders with piston rod [11] ISO 19973-4, Pneumatic fluid power — Assessment of component reliability by testing — Part 4: Pressure regulators [12] ISO 19973-5, Pneumatic fluid power — Assessment of component reliability by testing — Part 5: Non-return valves, shuttle valves, dual pressure valves (AND function), one-way adjustable flow control valves, quick-exhaust valves [13] ISO 80000-1, Quantities an d units — Part : Gen eral © ISO 2017 – All rights reserved 59 ISO/TR 61 94: 01 7(E) ICS 23.100.01 Price based on pages © ISO 2017 – All rights reserved