IEC 61 747 40 2 Edition 1 0 201 5 01 INTERNATIONAL STANDARD Liquid crystal display devices – Part 40 2 Mechanical testing of display cover glass for mobile devices – Uni axial flexural strength (4 poi[.]
I E C 61 47 -40 -2 ® Edition 201 5-01 I N TE RN ATI ON AL S TAN D ARD Li q u i d crys tal d i s pl a y d e vi ces – P art 40 -2 : M ech an i cal te s ti n g of d i s pl ay co ve r g l as s for m obi l e d e vi ces – U n i - IEC 61 747-40-2:201 5-01 (en) axi al fl e xu ral s tre n g th (4-poi n t be n d ) T H I S P U B L I C AT I O N I S C O P YRI G H T P RO T E C T E D C o p yri g h t © I E C , G e n e v a , S wi tz e rl a n d All rights reserved Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from either IEC or IEC's member National Committee in the country of the requester If you have any questions about I EC copyright or have an enquiry about obtaining additional rights to this publication, please contact the address below or your local I EC member National Committee for further information IEC Central Office 3, rue de Varembé CH-1 21 Geneva 20 Switzerland Tel.: +41 22 91 02 1 Fax: +41 22 91 03 00 info@iec.ch www.iec.ch Ab ou t th e I E C The I nternational Electrotechnical Commission (I EC) is the leading global organization that prepares and publishes I nternational Standards for all electrical, electronic and related technologies Ab o u t I E C p u b l i ca ti o n s The technical content of IEC publications is kept under constant review by the IEC Please make sure that you have the latest edition, a corrigenda or an amendment might have been published I E C Catal og u e - webstore i ec ch /catal og u e The stand-alone application for consulting the entire bibliographical information on IEC International Standards, Technical Specifications, Technical Reports and other documents Available for PC, Mac OS, Android Tablets and iPad I E C pu bl i cati on s s earch - www i ec ch /search pu b The advanced search enables to find IEC publications by a variety of criteria (reference number, text, technical committee,…) It also gives information on projects, replaced and withdrawn publications E l ectroped i a - www el ectroped i a org The world's leading online dictionary of electronic and electrical terms containing more than 30 000 terms and definitions in English and French, with equivalent terms in additional languages Also known as the International Electrotechnical Vocabulary (IEV) online I E C G l os sary - s td i ec ch /g l oss ary More than 60 000 electrotechnical terminology entries in English and French extracted from the Terms and Definitions clause of IEC publications issued since 2002 Some entries have been collected from earlier publications of IEC TC 37, 77, 86 and CISPR I E C J u st Pu bl i s h ed - webstore i ec ch /j u stpu bl i sh ed Stay up to date on all new IEC publications Just Published details all new publications released Available online and also once a month by email I E C C u stom er S ervi ce C en tre - webstore i ec ch /csc If you wish to give us your feedback on this publication or need further assistance, please contact the Customer Service Centre: csc@iec.ch I E C 61 47 -40 -2 ® Edition 201 5-01 I N TE RN ATI ON AL S TAN D ARD Li q u i d crys tal d i s pl a y d evi ce s – P art 40 -2 : M ech an i cal te s ti n g of d i s pl ay co ver g l as s for m obi l e d evi ce s – U n i axi al fl exu ral s tren g th (4-poi n t ben d ) INTERNATIONAL ELECTROTECHNICAL COMMISSION ICS 31 20 ISBN 978-2-8322-221 0-2 Warn i n g ! M ake su re th a t you obtai n ed th i s pu bl i cati on from an au th ori zed d i s tri bu tor ® Registered trademark of the International Electrotechnical Commission –2– I EC 61 747-40-2:201 © I EC 201 CONTENTS FOREWORD I NTRODUCTI ON Scope Norm ative references Terms, definitions and abbreviations Terms and definitions Abbreviations General Apparatus Testing environment and pre-conditioning Testing frame Test fixture and setup Test fixture dim ensions 5 Loading rate Procedure Safety Hazard – Broken glass Crush hazard – Onl y one person may operate the testing frame Crush hazard – Take care when installing or rem oving a specim en Hazard – Press the em ergency stop button when an unsafe condition is observed Fl ying debris hazard – Ensure that specim ens are installed correctly Hazard – Protect control cables from damage or disconnection Sam ple I ndividual specim en Complete the report Calculations 7 Strength calculations 7 Statistical calculations Reporting 8 I nform ation to be reported for each test 8 I nform ation to be made available upon request 9 Specifications Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure – Testing frame – Support assembly (side view) – Support assembly (top view) – Load assem bly (side view) 1 – Load assem bly (bottom view) 1 – Exam ple load traces for appropriate and inappropriate span settings – Edge fracture originating from between the load bars – Edge fracture originating from underneath the load bar – Surface fracture originating from between the load bars – Example Weibull plot I EC 61 747-40-2:201 © I EC 201 –3– INTERNATI ONAL ELECTROTECHNI CAL COMMI SSION L I Q U I D C R YS T AL D I S P L AY D E VI C E S – P a rt -2 : M e c h a n i c a l te s ti n g o f d i s p l a y c o ve r g l a s s fo r m o b i l e d e vi c e s – U n i -a xi a l fl e xu l s t re n g th ( - p o i n t b e n d ) FOREWORD ) The I nternati on al Electrotechni cal Comm ission (I EC) is a worl d wid e organization for stan dardization com prisin g all n ation al el ectrotechnical comm ittees (I EC National Comm ittees) The object of I EC is to prom ote internati onal co-operation on all questions concerni ng stand ardi zati on in the el ectrical an d electronic fields To this end and in additi on to other acti vities, I EC publish es I nternational Stan dards, Techn ical Specifications, Technical Reports, Publicl y Avail abl e Specificati ons (PAS) an d Gu ides (h ereafter referred to as “I EC Publication(s)”) Thei r preparation is entrusted to technical comm ittees; any I EC National Comm ittee interested in the subj ect dealt with m ay partici pate in this preparatory work I nternational, governm ental an d n on governm ental organ izations l iaising with th e I EC also participate i n this preparation I EC collaborates closel y with the I ntern ational Organi zation for Stand ardization (I SO) in accordance with ditions determ ined by agreem ent between th e two organi zati ons 2) The form al decisions or ag reem ents of I EC on tech nical m atters express, as n earl y as possible, an i nternati onal consensus of opi nion on the rel evant subjects since each technical com m ittee has representati on from all interested I EC N ational Com m ittees 3) I EC Publications have the form of recomm endations for intern ational use an d are accepted by I EC National Com m ittees in that sense While all reasonable efforts are m ade to ensure that the tech nical content of I EC Publications is accu rate, I EC cann ot be h eld responsi ble for th e way in which th ey are used or for an y m isinterpretation by an y en d u ser 4) I n order to prom ote intern ational u niform ity, I EC National Com m ittees und ertake to apply I EC Publications transparentl y to the m axim um extent possible i n their national an d regi on al publicati ons Any d ivergence between an y I EC Publication and the correspondi ng national or regi on al publ icati on shall be clearl y in dicated in the latter 5) I EC itself d oes n ot provi de an y attestation of conform ity I n depend ent certificati on bodies provi de conform ity assessm ent services and, in som e areas, access to I EC m arks of conform ity I EC is not responsi bl e for an y services carri ed out by ind ependent certification bodi es 6) All users shou ld ensure that th ey h ave the l atest editi on of thi s publicati on 7) No liability shall attach to I EC or its directors, em ployees, servants or ag ents inclu din g in divi du al experts and m em bers of its tech nical com m ittees and I EC Nati onal Com m ittees for any person al i nju ry, property d am age or other dam age of any n ature whatsoever, whether di rect or indirect, or for costs (includ i ng leg al fees) and expenses arisi ng out of the publ ication, use of, or relian ce upon, this I EC Publicati on or any other I EC Publications 8) Attention is drawn to th e N orm ative references cited in th is publ ication Use of the referenced publ ications is indispensable for the correct applicati on of this publication 9) Attention is drawn to the possibility that som e of the elem ents of this I EC Publication m ay be the su bject of patent rig hts I EC shall not be held responsibl e for identifyi ng any or al l such patent ri ghts I nternational Standard I EC 61 747-40-2 has been prepared by technical committee 1 0: Electronic display devices The text of this standard is based on the following docum ents: CDV Report on votin g 1 0/568/CDV 1 0/608A/RVC Full inform ation on the voting for the approval of this standard can be found in the report on voting indicated in the above table This publication has been drafted in accordance with the I SO/I EC Directives, Part –4– I EC 61 747-40-2:201 © I EC 201 A list of all parts in the I EC 61 747 series, published under the general title Liquid crystal display devices , can be found on the I EC website The comm ittee has decided that the contents of this publication will remain unchanged until the stability date indicated on the I EC web site under "http://webstore iec.ch" in the data related to the specific publication At this date, the publication will be • • • • reconfirm ed, withdrawn, replaced by a revised edition, or amended A bilingual version of this publication m ay be issued at a later date I EC 61 747-40-2:201 © I EC 201 –5– INTRODUCTION Mobile electronic devices have becom e increasingl y sophisticated and often include displays for the purposes of user interface and viewing Such displays commonl y incorporate a transparent cover glass which aids in protecting the display against the introduction of damage through routine device transport and use, as well as occasional or accidental m isuse The purpose of this standard is to provide m echanical testing procedures for cover glasses utilized in such applications Such glasses can be strengthened, for exam ple via an ionexchange process, which acts to increase m echanical strength through the introduction of a surface com pressive layer –6– I EC 61 747-40-2:201 © I EC 201 L I Q U I D C R YS T AL D I S P L AY D E VI C E S – P a rt -2 : M e c h a n i c a l te s ti n g o f d i s p l a y c o ve r g l a s s fo r m o b i l e d e vi c e s – U n i -a x i a l fl e xu l s t re n g th ( -p o i n t b e n d ) S cop e This part of I EC 61 747-40 is a m echanical perform ance testing procedure for cover glass used in electronic flat panel displays in m obile devices This standard is focused on the measurement of as-received edge strength via uni-axial flexure generated by a four-point bend N o rm a t i ve re fe re n c e s The following docum ents, in whole or in part, are normativel y referenced in this docum ent and are indispensable for its application For dated references, onl y the edition cited applies For undated references, the latest edition of the referenced docum ent (including an y amendments) applies I EC 61 747-40-1 , Liquid crystal display devices – Part 40-1 : Mechanical testing of display cover glass for mobile devices – Guidelines I EC 61 649: 2008, Weibull analysis T e rm s , d e fi n i t i o n s a n d a b b re vi a t i o n s T e rm s a n d d e fi n i t i o n s For the purposes of this document, the following term s and definitions apply sp eci m en individual piece of glass to be tested to failure sam pl e group of specim ens sharing a comm on pedigree (such as manufacturing process and period of production), for which failure statistics can be generated and reported sam pl e si ze number of specimens in a sample n om i n al val u e value about which a tolerance range is specified s t re n g t h normalized load at which fracture occurs I EC 61 747-40-2:201 © I EC 201 –7– Note to entry: For gl asses without a com pressive surface layer, strength can be equ ated to the fracture stress I n cases where non -li nearities prevent a suitable conversi on from load to stren gth (or stress), load at fracture should i nstead be reported Ab b revi ati on s PTFE pol ytetrafluoroeth ylene G en eral This test m easures edge strength by forcing a pair of parallel load bars (anvils) through a specimen that is supported by a pair of parallel support bars The two pairs of bars shall be parallel and the applied force shall be perpendicular to the surface formed by the support bar pair The loads at fracture are measured and converted to strength Strength calculations are found in Clause The test is applied to a sample of several specimens The sample statistics of the strength values are defined in Clause The statistical values to be reported or specified are given in Clauses and The specimens to be tested are typicall y 44 mm to 60 m m in width and 60 mm to 1 mm in length, with thickness ranging from 0, 55 mm to 2, mm When the specimen is placed on the support bars, a pair of opposite edges shall be aligned to be parallel with the support bars Before the specim en is placed on the support bars, the surface that will contact the load bars is covered with a layer of pol ym eric adhesive tape to preserve the fracture surface, reduce the scattering of glass fragm ents upon fracture, and minimize contact dam age The support rods are also covered with PTFE tape and may be rollers, also to m inim ize contact dam age and friction The general requirements of the apparatus are given in Clause Apparatus dim ensions can be affected by specimen dimensions These relationships are outlined in I t should be noted that direct comparisons cannot be m ade between specimens tested under different apparatus/fixture dimensions or at different cross head traverse rates Testing procedures are given in Clause I t is assum ed that all measurem ents are perform ed by personnel skilled in the general art of mechanical property m easurements Furthermore, it should be assured that all equipm ent is suitabl y calibrated as is known to skilled personnel and that records of the calibration data and traceability are kept 5 App aratu s Tes ti n g en vi ron m en t an d p re -co n d i ti o n i n g The standard testing environment is specified in I EC 61 747-40-1 Specim ens shall be stored in such an environment for at least four hours before testing Tes ti n g fram e The testing frame provides the aspects needed for the controlled vertical movem ent of som e mechanical elements relative to a test fixture surface I t also includes a load cell to indicate the applied force of these m echanical elements against other mechanical elements that are attached to the test surface and detectors to indicate displacem ent from the start of m otion A controller is also required to coordinate the necessary m otions These may be driven by external comm ands or by load cell responses Examples of motion directives include: • • Jog up or down Slow m anual up or down until the stop button is pushed 8 ã ã ã I EC 61 747-40-2:201 â I EC 201 Return to a preset start of test Traverse downward at a fixed rate until fracture is detected, then stop Emergency stop In addition to providing motion control and measurem ent of load, the controller shall, at minimum, report the fracture load, allow the setting of the load rate and the display load as a function of time and/or deflection from the start of the test Other features can include: • • The collection, organization, storage, and reporting inform ation entered for the sam ple, its specim ens, fracture load data, and statistical analysis results Calculating individual specimen fracture strength using data input for the specimen and the fracture load The main structural elem ents of the testing fram e shall be m ade of steel with dim ensions large enough so com pliance is essentiall y zero wi th respect to the m axim um force that will be applied This maximum force depends on the specim ens being tested, but should generall y be less than kN These elem ents include: • • • • • the testing surface, vertical support colum ns, a cross head, motion tractors within the support columns, load cell assembl y and attachment mechanisms The m inim um and m aximum rates of m otion shall encom pass the range of 0, 001 m m/m in and 000 mm/m in The controller shall sample the load cell and displacement detectors at a minimum rate of kH z The load cell shall be calibrated against a known weight, force gauge or load cell and be linear to within % over the m axim um applied force The load cell shall be capable of being reset to zero after the attachm ent of m echanical elem ents to it before the apparatus setup is com plete I t shall also include an attaching mechanism Figure illustrates some of the elements of the testing frame I EC 61 747-40-2:201 © I EC 201 –9– B C D E A IEC Ke y A Test surface B Support colum ns C Cross head (m oves up and down) D Load cell assem bly E Control l er F i g u re – Te s ti n g fra m e Tes t fi xtu re an d s etu p Subclause outlines the generic test fixture elem ents and setup procedure Subclause identifies the requirem ents of the dim ensional characteristics, which can depend on specimen characteristics The prim ary test fixture elements are the support assembl y and the load assembl y These are illustrated from Figure to Figure Other test fixture elements include: • • • • Clamps used to fasten the support assem bl y to the test frame testing surface PTFE tape used to cover the support bars Machined installation gage to align the support and load assemblies Microm eter with flat anvil faces and resolution of 0,002 m m or better – 10 – I EC 61 747-40-2:201 © I EC 201 A C D B IEC NOTE The key to the l etters i s provid ed in Fi gu re Fi g u re – Su pport assem bl y (si d e vi ew) A C B IEC NOTE The key to the l etters i s provid ed in Fi gu re Fi g u re – Su pport assem bl y (top vi ew) I EC 61 747-40-2:201 © I EC 201 – 11 – G E F IEC NOTE The key to the l etters i s provid ed in Fi gu re Figu re – Load assembl y (si d e vi ew) E F IEC Ke y A Support bars with opti onal rol l ers B Support bar separation, dS C Support assem bl y attachment surface D Support assem bly bod y E Load bars F Load bar separation, dL G Com pression bar Fi gu re – Load assembl y (bottom view) Setup steps include: a) b) c) d) e) f) Attach the load assembl y to the test fram e load cell assembl y at the compression bar Reset to zero on the load cell Cover the support bars with PTFE tape Roughl y center the support assembl y below the load assembly Place the installation gage onto the support bars so the bottom grooves contain the bars Slowl y, lower the cross head and adj ust the support assembly until the load bars are contained in the installation gage top grooves – 12 – I EC 61 747-40-2:201 © I EC 201 g) Clam p the support assem bly onto the test fram e testing surface h) Raise the cross head until the separation of the load and support bars is suitable for loading a specim en and set the start point The installation gage bar is a m achined metal block with top and bottom grooves corresponding to the load bars and support bars respectivel y I t is used to assure parallel alignment of the two assemblies When assembled, the bars shall be parallel to 0, 05 mm per 25 mm in length and concentric to within 0,25 mm The parallelism requirem ent applies to: • • • Support bar to support bar Load bar to load bar Support bars to load bars Test fi xtu re di m en sion s The test fixture dimensions are driven by the specim en dim ensions: length, l, width, w , and thickness, h The radius of the load and support bars shall be greater than h/2 The load and support bar lengths shall be large enough to accomm odate the maximum specimen width The height of the support assem bl y bod y shall be sufficient to accommodate the m axim um specimen deflection This is influenced by several factors such as the following, and best determined experimentally: • • • Young’s modulus of the material under test Fracture strength of the specimens under test Specimen thickness and width The support bar separation, dS , and load bar separation, dL , are set in terms of the nom inal specim en length, lnom , as in equations (1 ) and (2) dS dL ≤ = l nom (1 ) d S (2) The support bar separation is critical to obtaining comparable testing results because it can affect the Weibull scaling param eter (see 7.2) Unless otherwise specified, the support bar separation is 36 mm ± 0, m m These dimensions are partl y intended to prevent non-linearities in loading behavior due to large specim en deflection, slippage and/or run-out failure The latter occurs when the deflection is large enough to draw the specim en ends past the support bars I n this case, the failure load does not correspond to specimen fracture, but rather to the load at which the specim en slipped out of the support bars The associated load values shall be suspended (see 2) This failure mode can often be observed as an unusual discontinuity in the applied load graph Figure shows a normal load versus tim e graph, as well as a case of large deflection with associated slippage due to non-optimized span distances Support bar separation of 36 m m is needed to avoid excessive run-out failure on strengthened glasses I t provides 20 % excess glass on each side of the support bar rather than the base recom mendation of % excess glass I EC 61 747-40-2:201 © I EC 201 – 13 – IEC Appropriate span settin gs are req uired to avoid non-li neariti es due to l arge specim en defl ections, slippage an d/or run -out failu re The fin ely dash ed lin es are sim ply a gu ide to the eye Figure – Example load traces for appropriate and inappropriate span settings 5.5 Loading rate The loading rate during testing shall be constant I t is given in term s of the cross head traverse rate U nless otherwise specified the rate is mm /m in As with fixture dimensions, data generated from testing at different loading rates are not directl y comparable Procedure 6.1 Safety 6.1 Hazard – Broken glass Wear safety glasses with side panels at all times Wear gloves when hand ling broken glass 6.1 Crush hazard – Only one person may operate the testing frame Operator injury m ay occur if more than one person operates the system Before working in the hazard area between the upper and lower fixtures, ensure that other personnel cannot operate the computer system or other controls 6.1 Crush hazard – Take care when installing or removing a specimen Installation or rem oval of a specim en involves working inside the hazard area between the fixtures • • Keep clear of the fixture at all times Never enter the hazard area between the fixtures while the cross head is moving – 14 – H a z a rd I EC 61 747-40-2:201 © I EC 201 – P re s s t h e e m e rg e n c y s t o p b u t t o n w h e n a n u n s a fe c o n d i t i o n is o b s e rv e d If an unsafe condition is observed, press the emergency stop button Correct the unsafe condition before resuming testing F l yi n g d e b ri s h a z a rd – E n s u re t h a t s p e c i m e n s a re i n s t a l l e d c o rre c t l y Always install the specimens in the center of the fixtures in line with the load path I ncorrect specim en installation creates stresses which can break the fixtures The high energies involved can proj ect broken parts som e distance from the test area H a z a rd – P ro t e c t c o n t ro l c a b l e s fro m d am ag e o r d i s c o n n e c ti o n Protect all cables, particularl y transducer cables, from dam age Damage to control and feedback cables can induce an open loop condition which m ay drive the cross head to the extrem es of its m otion S am p l e The sam ple size is 30, excluding an y specim ens rejected for pre-existing dam age Values associated with test run-out failure, surface breaks or edge breaks originating from outside of the load bar are included in the sample, but the values are treated as late suspensions (see 2) I f there are m ore than suspensions, the testing fails and a new sam ple shall be selected Upon receipt of a new sam ple, the following steps shall be taken: a) Determine and record the following inform ation: • sample identification , • sample specim en nom inal dimensions: length, width, thickness, • requesting person b) Determine whether existing test fixtures are com patible I f not, change them (Clause 5) c) Record the fixture dimensions or identification number d) Set and record the loading rate e) Determine and record whether width is to be measured on each specimen I f specimen width varies less than % of average, the average width m ay be used for each specim en f) Set the start-of-test height for the testing frame g) Zero the micrometer This is done by gentl y closing the anvils with no specimen present, and pressing the reset button I n d i vi d u al s peci m en Com plete the following steps on each specimen of the sample The working surfaces should be clean and free of an ything that can induce damage a) b) c) d) Determ ine and record the specimen identification num ber I nspect for an y dam age I f damaged, report this, but not include in the testing I f required (see 2), m easure and record the width Measure and record the thickness to the nearest 0, 001 m m This m easurem ent shall be done at a location on the specimen that is away from an y edge and in a location that will not be stressed (the overhang area) e) Cut a section of pol ymeric adhesive tape that can cover the specimen and place it sticky side up on the working surface f) Gentl y attach one edge of the specimen to one end of the tape I EC 61 747-40-2:201 © I EC 201 – 15 – g) Gentl y lower the rest of the specim en onto the rest of the tape h) Carefull y trim the excess tape away from the specim en i) Return the test fram e to the start-of-test position and place the specimen onto the support bars with the pol ym eric adhesive tape side up NOTE The l oad bars will touch the polym eric adh esive tape side j) Start the test sequence k) Observe the load vs time graph and other signs of testing progress for any abnorm alities • I f abnormalities are observed (such as run-out failure), this m ay be cause to suspend the result l) Record the failure load Following this, calculate and record the failure stress (see 7.1 ) m) I nspect the fracture pattern to determ ine whether the break source originated from the edge between the load bars, the edge underneath the load bars, the edge outside of the load bars, or away from an y edge All but the first and second type listed, in which failure originates from an edge either underneath or between the load bars, shall be suspended Figure illustrates a normal edge fracture pattern illustrating the desired failure mode Figure illustrates an edge failure underneath the load bar, while Figure illustrates a surface fracture n) Rem ove the broken glass and thoroughl y clean the area for the next test IEC NOTE Such a fractu re represents the desi red fail ure m ode, an d thus the resu lting data point is treated as nonsuspend ed The dark vertical bars indicate th e approxim ate load bar positions, whi le the white arrow tip represents the fail ure ori gin l ocation Figu re – Edge fracture originating from between the load bars – 16 – I EC 61 747-40-2:201 © I EC 201 IEC NOTE Such a fracture orig in ates from underneath the load bar, an d thus the resulti ng data poi nt is treated as nonsuspen ded The dark vertical bars in dicate th e approxi m ate load bar positi ons, whil e the white arrow ti p represents the fail ure origi n location Figu re – Edge fracture originating from underneath the load bar IEC NOTE Such a fracture represents an und esired failu re m ode, and thus the resul ti ng data point is suspend ed The dark vertical bars indicate the approxim ate load bar positi ons, wh ile th e white arrow tip represents the fail ure ori gin location Figure – Surface fracture originating from between the load bars 6.4 Complete the report The report shall include, at a minim um, the information found in I ndividual custom ers m ay require additional information I EC 61 747-40-2:201 © I EC 201 – 17 – Depending on the capabilities of the testing fram e controller, preparation of the report may require the statistical anal ysis items in to be calculated, stored, and printed on another computer Calculations 7.1 Strength calculations The following equations have been found applicable for the types of glass for which this standard is intended Other types of glass m ay have non-linearities at elevated deflections Strength, σ , (MPa) is given as a function of fracture load (N) as equation (3) σ = L (d S − dL ) wh (3) where: dS = support bar separation dL = load bar separation w = specim en width (mm ) h = specim en thickness (m m) L = fracture load (N) 7.2 Statistical calculations The Weibull anal ysis standard, I EC 61 649, shall be used to calculate the following param eters: • • • Weibull scale param eter, η (MPa) Weibull shape param eter, β the th percentile failure stress, B (MPa) The m aximum likelihood estimate (MLE) m ethod of calculation shall be used for the Weibull parameters See of I EC 61 649: 2008 The following equations are adapted from eq uations (1 7) and (1 8) of I EC 61 649: 2008 in order to accomm odate suspensions as variable censoring This is j ustified by considering a surface break I f the surface flaw had not been present, the largest edge flaw would have failed at a larger stress than the failure stress that is calculated for the surface flaw I n a sense, the testing of the edge flaw was term inated before failure Following the notation of I EC 61 649, let t represent valid failure strength values, with i = to r, and let T represent the suspended values, with j = to s i j The shape parameter is the value of β that satisfies equation (4) r s i =1 j =1 s β β ∑ ti ln ti + ∑ T j ln T j r β ∑ ti i =1 + − β ∑ Tj β − r r ∑ ln ti = i =1 j =1 Given this value for the shape param eter, the scale parameter is given as equation (5) (4) – 18 – η = r I EC 61 747-40-2:201 © I EC 201 s r β t T jβ + i j =1 i =1 ∑ ∑ 1/β (5) The point estimate of the th percentile, B , is calculated using 9.8 of I EC 61 649: 2008 and equation (20) of I EC 62649: 2008, which is repeated here as equation (6) I n Clause 9, this parameter is recomm ended to be specified for performance standards I t com bines the scale and shape parameters to single, m eaningful, metric B1 1/β = η [− ln (0, )] (6) I n addition to the calculation of statistical param eters, a Weibull plot is required (see 8.2) See of I EC 61 649: 2008 for instructions on how to produce such a plot An exam ple Weibull plot is shown in Figure Probability pl ot of fou r-poi nt bend perform ance Failu re probability (%) 99 90 80 70 60 50 40 30 20 10 Failu re stress (MPa ) F i g u re – E xa m p l e W e i b u l l p l o t 8 Re p o rt i n g I n fo rm a t i o n t o b e re p o rt e d fo r e a c h test Report the following for each test • • • • • Sample identification and nominal dim ensions Support bar separation, dS (mm) Weibull scale parameter, η (MPa) Weibull shape param eter, β Fracture strength th percentile, B (M Pa) IEC