© ISO 2016 Glass in building — Determination of the bending strength of glass — Part 1 Fundamentals of testing glass Verre dans la construction — Détermination de la résistance du verre à la flexion —[.]
INTERNATIONAL STANDARD ISO 1288-1 First edition 2016-02-15 Glass in building — Determination of the bending strength of glass — Part 1: Fundamentals of testing glass Verre dans la construction — Détermination de la résistance du verre la flexion — Partie : Principes fondamentaux des essais sur le verre Reference number ISO 1288-1:2016(E) © ISO 2016 ISO 1288-1:2016(E) COPYRIGHT PROTECTED DOCUMENT © ISO 2016, 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 2016 – All rights reserved ISO 88-1 : 01 6(E) Contents Page iv Scope Normative references Terms and definitions Symbols Factors to be taken into account when testing glass 5.1 Glass as a material 5.1.1 General 5.1.2 Effect of surface condition 5.1.3 Effect of rate of loading 5.1.4 Effect of test surface area 5.1.5 Effect of ambient medium 5.1.6 Effect of aging 5.1.7 Effect of temperature 5.2 Bending stress and bending strength 5.2.1 General 5.2.2 Effective stress 5.2.3 Equivalent bending strength 5.2.4 Profile bending strength 5.3 Types o f glass 5.3.1 General 5.3.2 Patterned glass 5.3.3 Laminated glass 5.4 Orientation of the specimens 5.5 Number of specimens in a sample Explanations of the test methods 6.1 Coaxial double ring test for large test surface areas 6.1.1 Elimination of edge effects 6.1.2 Analysis o f the stress development 6.1.3 Testing of patterned glass 11 6.2 Test with specimen supported at two points (four point bending) 11 6.1.4 Limitations 11 6.1.5 Inclusion of edge effects 11 6.1.6 Analysis o f the stress development 11 6.3 Coaxial double ring test for small test surface areas 13 6.1.7 Elimination of edge effects 13 6.1.8 Analysis o f the stress development 13 Range of application of the test methods 7.1 General limitations 14 7.2 Limitations to ISO 1288-2 14 7.3 Limitations to ISO 1288-3 14 7.4 Limitations to ISO 1288-4 15 7.5 Limitations to ISO 1288-5 15 Calibration of the testing machines Recommendations for safe use of test equipment Bibliography Foreword © ISO 2016 – All rights reserved iii ISO 1288-1:2016(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 meaning o f ISO specific terms and expressions related to formity assessment, as well as information about ISO’s adherence to the WTO principles in the Technical Barriers to Trade (TBT) see the following URL: Foreword - Supplementary in formation The committee responsible for this document is ISO/TC 160, Glass in building, Subcommittee SC 2, Use considerations ISO 1288 consists of the following parts, under the general title Glass in building — Determination of the : — Part 1: Fundamentals of testing glass — f — (four point bending) — Part 4: Testing of channel shaped glass — f bending strength of glass Part 2: Coa xial double rin g test on flat specim en s with large test sur ace areas Part 3: Test with specim en supported at two points Part 5: Coa xial double rin g test on flat specim en s with sm all test sur ace areas iv © ISO 2016 – All rights reserved INTERNATIONAL STANDARD ISO 1288-1:2016(E) Glass in building — Determination of the bending strength of glass — Part 1: Fundamentals of testing glass Scope T h i s p ar t o f I S O 8 s p e c i fie s the de term i nation o f the b end i ng s treng th o f monol ith ic gla s s for use in buildings The testing of insulating units or laminated glass is excluded from this part of ISO 1288 This part of ISO 1288 describes — considerations to be taken into account when testing glass, — explanations of the reasons for designing different test methods, — limitations of the test methods, and — give s p oi nters to s a fe ty re qu i rements for the p ers on nel op erati ng the te s t e qu ipment I S O 8 -2 , I S O 8 -3 , I S O 8 - and I S O 8 -5 s p e ci fy te s t me tho d s i n de tai l T he te s t me tho d s s p e ci fie d i n th i s p ar t o f I S O 8 a re i ntende d to provide l arge nu mb ers o f b end i ng strength values that can be used as the basis for statistical evaluation of glass strength Normative references T he fol lowi ng i nd i s p en s able c u ments , i n whole or i n p ar t, are normatively re ference d i n th i s c u ment a nd are for its appl ic ation For date d re ference s , on ly the e d ition cite d appl ie s For u ndate d re ference s , the late s t e d ition o f the re ference d c u ment (i nclud i ng any amend ments) appl ie s ISO 1288-2, Glass in buildin g — Determ in ation o f f th e ben din g stren gth o f glass — Part 2: Coa xial double rin g test on flat specim en s with large test sur ace areas ISO 1288-3, Glass in buildin g — Determ in ation o f f th e ben din g stren gth o f glass — Part 3: Test with specim en supported at two points ( our point ben din g) ISO 1288-4, Glass in building — Determination of the bending strength of glass — Part 4: Testing of channel shaped glass ISO 1288-5, Glass in buildin g — Determin ation o f f th e ben din g stren gth o f glass — Part 5: Coa xial double rin g test on flat specim en s with sm all test sur ace areas ISO 16293-1, Glass in buildin g — Basic soda lim e silicate glass products — Part : Definition s an d gen eral physical and mechanical properties NO TE I S O TC 16 0/S C i s com menc i ng work on s ta nda rd s for “ther m a l l y temp ere d s o da l i me s i l ic ate s a fe ty gl a s s ”, “ he at s treng thene d s o d a l i me s i l ic ate gl a s s ” a nd “chem ic a l l y s treng thene d gl a s s ” Terms and definitions For the pu r p o s e s o f th i s c u ment, the © ISO 2016 – All rights reserved fol lowi ng term s and defi n ition s apply ISO 88-1 : 01 6(E) f l a t g l a s s pro duc t form i ng to any gla s s tran s forme d gla s s made from ISO 162 -2 , ISO 162 -3 , ISO 162 - a nd I S O 162 -5 , or a ny the s e pro duc ts without del ib erately i nduc i ng pro fi le or c u r vatu re 3.2 bending stress tensile bending stress induced in the surface of a specimen N o te to entr y: For te s ti ng p u r p o s e s , the b end i n g s tre s s s hou ld b e u n i for m o ver a s p e c i fie d p a r t o f the s u r face 3.3 effective bending s tress weighte d average o f the ten s i le b end i ng s tre s s e s , c a lc u late d b y applyi ng a fac tor to ta ke i nto accou nt non-u n i form ity o f the s tre s s field bending s trength bending stress (3.2) or effective bending stress (3.3) which leads to breakage of the specimen 3.5 equivalent bending strength apparent bending strength (3.4) of patterned glass, for which the irregularities in the thickness not allow precise calculation of the bending stress (3.2) quotient of the maximum bending moment and the section modulus of a channel shaped glass p r o f i l e b e n d i n g s t r e n g t h stress intensity factor measure of the stress at a crack tip 3.8 pres tressed glass any gla s s pro duc t th at s a s u r face pre s tre s s , i e therma l ly temp ere d s o da l i me s i l ic ate s a fe ty glas s , he at s treng thene d s o da l i me s i l ic ate gla s s and chem ic a l ly s treng thene d glas s Symbols F h L k , K1 K2 M bB p PbB r1 r2 applied load specimen thickness length of side of square test sample constant for calculation of bending stress in ISO 1288-3 constants for calculation of bending stress in ISO 1288-5 maximum bending moment gas pressure applied within loading ring in ISO 1288-2 f MbB/Z radius of loading ring radius of supporting ring pro fi le b end i ng s treng th (o ch an nel sh ap e d gla s s) = N M M M Nm Pa Pa M M © ISO 2016 – All rights reserved ISO 1288-1:2016(E) radius of circular specimen average specimen radius (for evaluation) r3 r3m y0 M M M m3 centra l defle c tion o f s p e ci men section modulus (of channel shaped glass) poisson number of specimen Z μ NOTE For soda lime silicate glass (see ISO 16293-1), a value of 0,23 is used bending stress effective bending stress bending strength equivalent bending strength radial stress tangential stress stress in a direction along the length of the specimen σb σbeff σbB σbeqB σrad σT σL Pa Pa Pa Pa Pa Pa Pa Factors to be taken into account when testing glass 5.1 Glass as a material 5.1.1 General Glass is a homogeneous isotropic material having almost perfect linear-elastic behaviour over its tensile strength range Gla s s s a ver y h igh compre s s ive s treng th a nd the ore tic a l ly, a ver y h igh ten s i le s treng th, but the s ur face o f the gla s s s many i rregu la ritie s wh ich ac t a s we a kne s s e s when gla s s i s s ubj e c te d to ten s i le s tre s s T he s e i rregu laritie s a re c au s e d b y attack from moi s ture and b y contac t with h ard materia l s (e g grit) a nd are conti nua l ly mo d i fie d by moi s tu re wh ich i s a lways pre s ent i n the r Tensile strengths of around 10 000 MPa can be predicted from the molecular structure, but bulk glass norma l ly fa i l s at s tre s s e s s iderably b elow 10 M Pa T he pre s ence o f the i rregu laritie s a nd thei r mo d i fic ation b y moi s tu re contribute s to the prop er tie s o f glass which need consideration when performing tests of strength B e c au s e o f the ver y h igh compre s s ive s treng th, glas s a lways fai l s u nder ten s i le s tre s s Si nce gla s s i n bu i ld i ngs i s ver y rely u s e d i n d i re c t ten s ion, the mo s t i mp or ta nt prop er ty for lo ad re s i s tance i s the tensile bending strength All the tests described in this part of ISO 1288 are intended to evaluate the tensile bending strength of glass T he b end i ng s treng th i s i n fluence d b y the a) b) c) d) fol lowi ng fac tors: surface condition (see 5.1.2); rate and duration of loading (see 5.1.3); area of surface stressed in tension (see 5.1.4); ambient medium, through stress corrosion cracking, as well as healing of surface damage in the glass (see 5.1.5 and Reference [1]); © ISO 2016 – All rights reserved ISO 88-1 : 01 6(E) e) age, i.e time elapsing since the last mechanical sur face treatment or modification to simulate damage (see 5.1.6); f) temperature (see 5.1.7) The influence exerted by factors b) to f) on bending strength has been taken into account in this part of ISO 1288 Effect of surface condition For the purpose of bending strength tests according to this part of ISO 1288, glass behaves as an almost ideally linear-elastic material that fails in a brittle manner This brittleness means that contact with any hard object can lead to sur face damage in the form o f ultra-fine, partly submicroscopic cracks and chips Sur face damage o f this kind, which is practically unavoidable during normal handling o f glass, exerts a notch action which is a major factor in reducing mechanical strength, whereas the chemical composition o f the glass has only a minor, and in some cases, entirely negligible, significance Hence, it follows that the bending strength determined by the methods re ferred to in this part o f ISO 1288 is related largely to the sur face condition o f the specimen to be tested This sur face condition is characterized by the following main features: a) the sur face condition imparted by a particular method o f treatment, which produces a specific damage spectrum and thus, results in a strength which is specific to the finished sur face condition; b) residual stress, e.g in the form o f thermal or chemical prestress intentionally imparted, as well as unintended residual stresses Effect of rate of loading For the interpretation of the bending strength values determined as described in this part of ISO 1288, the rate of loading is of special importance Cracks propagate in glass over a wide range of values of tensile stress (see Reference [2]) There is a lower limit to the stress intensity factor below which cracks not propagate (see Re ference [1]) There is then some subcritical crack propagation at higher levels o f stress intensity factor, which is influenced by humidity, temperature and chemical agents Above a critical stress intensity factor, crack propagation is very rapid and leads to (almost) instantaneous failure The consequence o f the subcritical crack propagation is, for example, that the rate of load increase and/or the duration of static loading influences the bending strength For prestressed glass, this time dependence does not manifest itself until the tensile stress induced in the sur face exceeds the compressive stress permanently present there (see Re ference [3]) Effect of test surface area The decrease in bending strength of glass with increasing size of the test area exposed to high stress is also of importance (see Reference [4]) This area effect is accounted for by the statistical distribution o f sur face de fects varying in e ffectiveness; the larger the test area, the greater is the probability o f its containing a large sur face de fect Consequently, the influence o f the area e ffect increases with decreasing incidence o f de fects in the sur face, so that this influence is more pronounced in the case o f undamaged, e.g fire-finished glass sur faces (see Re ference [5]) Di fferences are likely between the mean values o f the bending strength as measured in accordance with ISO 1288-2 (maximally stressed area: 240 000 mm ), or by using devices R105, R60, R45 and R30 in accordance with ISO 1288-5 (maximally stressed areas: 850 mm , 260 mm , 254 mm and 113 mm2 ), due to the size of the stressed area Depending on surface damage, the results obtained from testing smaller sur face areas may be significantly higher than those obtained from testing larger surface areas, as shown in Table © ISO 2016 – All rights reserved ISO 1288-1:2016(E) Table — Approximate effects of test surface area on the mean measured bending strength Test method ISO 1288-2 ISO 1288-5 ISO 1288-5 ISO 1288-5 ISO 1288-5 Si nce gl as s I S O 8 -3 bu i ld i ngs for Device — R105 R65 R45 R30 Relative bending strength 100 % 120 % to 180 % 125 % to 210 % 140 % to 270 % 145 % to 300 % u s e i n bu i ld i ngs i s o ften i n la rge s i z e s , the te s t me tho d s s p e ci fie d i n I S O 8 -2 and gi ve va lue s wh ich a re more appropriate as the b a s i s T he te s t me tho d s p e ci fie d in ISO 8 -5 can be for u s e fu l de s ign i ng flat gla s s as a me tho d for use in o f eva luati ng the comp arative b end i ng s treng th o f flat gla s s 5.1.5 Effect of ambient medium T he s u rround i ng me d iu m i n wh ich the gl as s i s te s te d has a n i n fluence on the s treng th o f the gla s s , p a r tic u larly i f the moi s tu re level i s ver y low When gla s s i s u s e d i n bu i ld i ngs , the relative hu m id ity typic a l ly range s from % to 10 % With i n th i s range, the e ffe c t on the b end i ng s treng th , a s te s te d according to this part of ISO 1288, is not great However, tests on glass for use in buildings shall be f eliminate this effect when comparing bending strength results u nder ta ken i n te s t cond ition s with relative hu m id ity level s i n the range o 5.1.6 I f the % to 70 % , i n order to Effect of aging gla s s s u r face ne ce s s ar y to is mo d i fie d a l low the fre sh ( b y abras ion, da mage to mo d i fic ation b y moi s ture a ffe c ts e tch i ng , e dge worki ng , he a l b e fore the te s t i s e tc ) b e fore under ta ken the te s ti ng , it i s T he conti nua l s ur face the da mage i n a way that c an re duce a ny we a ken i ng e ffe c t (s e e Reference [1 shall be conditioned for at least 24 h before testing ] ) I n prac tice, gla s s i s h igh ly un l i kely to b e s tre s s e d d i re c tly a fter it s b e en tre ate d, s o it 5.1.7 Effect of temperature T he b end i ng s treng th o f gl as s is a ffe c te d b y cha nge s i n temp eratu re With i n the norma l range o f temp erature s exp erience d b y glas s i n bui ld i ngs , th i s e ffe c t i s no t ver y s ign i fic ant, but to avoid p o s s ible complications in the comparison of bending strength values, testing shall be undertaken in a restricted range of temperatures 5.2 Bending stress and bending strength 5.2.1 General The test methods described in ISO 1288-2, ISO 1288-3, ISO 1288-4 and ISO 1288-5 are designed to induce a uniform bending stress over an area (the test area) of the specimen However, the stresses i nduce d b y the appl ie d lo ad s dep end on the natu re o f the materia l te s te d as wel l a s the lo ad d i s tribution 5.2.2 Effective stress 6.1.6), it can The weighting is beff Where the s tre s s va rie s s ign i fic antly over the te s t are a, as i s the c a s e i n I S O 8 -3 (s e e b e repre s ente d b y a weighte d average s tre s s c a l le d the e ffe c tive b end i ng s tre s s , σ ob ta i ne d b y s tati s tic a l ly eva luati ng the prob abi l ity o f © ISO 2016 – All rights reserved frac ture at a ny p oi nt i n the s tre s s e d a re a ISO 1288-1:2016(E) 5.2.3 Equivalent bending strength Variation s in homo geneity or th ickne s s bending strength, σbB equivalent bending strength, σbeqB o f the s p e c i men a ffe c t the s tre s s d i s tribution H ence, the , i s never enti rely a n acc urate va lue a nd, i n s ome i n s tance s , it i s b e tter terme d the For s ome o f the gla s s typ e s te s te d (for e xample, flo at gla s s) , s uch vari ation s a re ver y s ma l l and the b end i ng s treng th de term i ne d b y the te s ts i s s u fficiently clo s e to the ac tua l b end i ng s treng th difference to be unimportant for the I n the c a s e o f p atterne d gla s s , however, on ly the e qu iva lent b end i ng s treng th c a n b e de term i ne d P r o f i l e b e n d i n g s t r e n g t h When channel shaped glass is tested according to ISO 1288-4, most of the specimens fail from fractures origi nati ng at the corner o f the profi le, where the web and fla nge me e t, and no t at the e xtreme o f the fl ange or s u r face o f the web T h i s i s due to s e condar y s tre s s e s generate d b y the s pre ad i ng o f the fla nge s when the chan nel s e c tion i s b ent I n th i s te s t, the b end i ng s treng th i s b e tter expre s s e d as the pro fi le bending strength, PbB 5.3 Types of glass 5.3.1 General T he te s ts s p e ci fie d i n I S O 8 -2 , I S O 8 -3 and I S O 8 -5 a re for te s ti ng flat gla s s T h i s i nclude s flo at glass, drawn sheet glass, patterned glass, patterned wired glass, polished wired glass and prestressed surface of patterned glass) gl as s , provide d there s b e en no del ib erately i nduce d c u r vature or pro fi le (o ther th an the p atterne d 5.3.2 Patterned glass The coaxial double ring test for large test surface areas (see ISO 1288-2) can be used to determine the equivalent bending strength of patterned glass, provided the maximum and minimum thicknesses no t devi ate b y more th an % or m m, wh ichever i s the lower, from the average th ickne s s T h i s i s b e c au s e o f d i ffic u ltie s i n s e a l i ng the pre s s u re ri ng to a p atterne d s u r face There is no limitation on the depth of pattern if the four point bending test (ISO 1288-3) is used 5.3.3 Laminated glass The testing of the bending strength of laminated glass (see ISO 12543-1) is excluded from this part of ISO 1288 I n a b end i ng te s t, add itiona l s he ar de formation ari s e s i n the ela s tic or pl as tic i nterlayers (sl id i ng o f the hard glas s pl ie s on the i nterl ayer) T h i s e ffe c t me an s that me as u ri ng the b end i ng s treng th o f lam i nate d gl as s i s l i kely to give a s treng th va lue le s s than the ac tua l b end i ng s treng th o f a monol ith ic glas s o f the s ame th ickne s s T h i s s he a r de formation i s p a r tic u l arly s en s itive to the e ffe c ts o f temp erature and loading rate L am i nate d gla s s i s manu fac tu re d from monol ith ic glas s pro duc ts that ca n b e te s te d i nd ividua l ly b y the test methods described in ISO 1288-2, ISO 1288-3 and ISO 1288-5 T he pro ce s s o f ma nu fac tu re i s u n l i kely to c au s e s ign i fica nt ch ange s i n the b end i ng s treng th o f the comp onent gl as s e s , s o it i s un ne ce s s a r y to te s t lam i nate d gla s s wh ich c an b e a s s u me d to h ave b end i ng strengths appropriate to their individual components The load resistance of laminated glass depends on the interactions between the component parts of the comp o s ite s tr uc tu re, wh ich i s b eyond the s cop e o f th i s p a r t o f I S O 8 © ISO 2016 – All rights reserved ISO 88-1 : 01 6(E) Orientation of the specimens Many glass products lack symmetry in their production This may be immediately obvious, such as a patterned glass, which is likely to have one sur face much more deeply patterned than the other and possibly in which the pattern is directional, or it may be less obvious, such as the side on which the wheel cut was made (see ISO 1288-3:—, Figure 1) Where such asymmetry is present, it may be necessary to test the glass in several di fferent orientations in order to determine the bending strength Samples of glass to be tested shall have all the specimens nominally identical 5 Number of specimens in a sample The bending strength o f glass displays a large variation between nominally identical specimens Very little in formation can be obtained by testing only a few specimens, since there is considerable uncertainty about whether the results are representative In statistical terms, this uncertainty can be expressed as confidence limits, values between which there is a given probability that the target being sought will lie Where the target being sought is in the central part of the bending strength distribution (for instance, the mean value), then the confidence limits can be fairly narrow even with just a few specimens An accurate determination o f the tensile stress which leads to a low crack probability can require large numbers of specimens when, for example, a characteristic stress, a permissible stress or a design value of bending strength is to be determined Explanations of the test methods 6.1 NOTE 6.1 Coaxial double ring test for large test surface areas This test is specified in ISO 1288-2 Elimination of edge effects The special feature of the coaxial double ring bending test in accordance with ISO 1288-2 lies in the act that only a circular shaped limited area o f the sur face o f the specimen (not, however, its edges) is subjected to maximum stressing In contrast to other bending tests (for example, see ISO 1288-3), in which the edge o f the specimen is subjected to the maximum stress, the procedure in accordance with ISO 1288-2 is suitable for exclusively subjecting sur faces (or di fferent sur face conditions) to bending f stress The effect of the specimen edge condition is, for the most part, suppressed 6.1 Analysis of the stress development When the deflections are relatively small, the central sur face area is subjected to uni form tensile stressing [see Figure a)], where the radial and tangential stresses are of equal size © ISO 2016 – All rights reserved ISO 88-1 : 01 6(E) a ) b ) I n I n t h t e h e s l m a a r l g l e d d e e f f l l e e c c t t i i o o n n r r a a n n g g e e Figure — Schematic dependence of radial and tangential stresses upon the radius of the specimen, when loaded by a double ring device I f the defle c tion s b e come larger, i e i f they exce e d approxi mately l f the th ickne s s o f the she e t (the precise limit is dependent on the ring ratio, r2/r1), this leads to localized increases in stress below the edge of the loading ring, the extent of which increases as the load rises [see Figure b)] At this stage of the lo ad i ng , the tangenti a l and rad i a l s tre s s e s ch ange d i fferently and a s i mple c a lc u lation o f the s tre s s e s i s no longer p o s s ible T he s tre s s e s c a lc u late d from l i ne ar b end i ng the or y wou ld b e to o h igh f f f 6] to [8]), this increase in stress below the edge of the loading ring can be avoided With a constant piston force, F, the gas pressure, p f Figures and 3) It is, however, not possible to optimize the gas pressure, p, with regard both to the radial and tangential stress development at the same time I t h a s b e en demo n s trate d th at, b y me a n s o a co mb i ne d r i n g a nd s u r ace lo ad (s e e Re erence s [ , c a n b e o p ti m i z e d i n s uch a way th at either the rad i a l o r ta ngenti a l ten s i le s tre s s de velo p s a l mo s t u n i o r m l y with i n the lo ad i ng r i n g (s e e © ISO 2016 – All rights reserved ISO 88-1 : 01 6(E) Key L position of loading ring S position of supporting ring This graph has been measured using the following parameters: Square specimen: 000 mm × 000 mm × mm; r1 r2 F: 22 220 N; Parameter: gas pressure, p This determines that the optimized gas pressure, p, is 33 kPa = 0 m m; = 0 m m; Figure — Radial s tress development along the median of the convexly bent specimen surface If the gas pressure, p, is optimized with regard to the radial stress distribution (as given in Figure 2, curve P f Figure 3, curve P f p , is optimized with regard to the tangential stress distribution (as given in Figure 3, curve P Figure 2, curve P f cause fractures to be triggered-off, the non-homogenous distribution of the radial stress is the decisive factor in the triggering of the fracture For this reason the gas pressure, p regard to the radial stress development for practical test purposes = 3 kPa) , then the tangentia l s tre s s = 3 kPa) I a l l s toward s the lo ad i ng ri ng (a s given i n the ga s pre s s u re, = kPa) , then the rad ia l s tre s s ri s e s towa rd s the lo ad i ng ri ng (a s given i n = kPa) Si nce , i n the c as e o brittle materi a l s , it i s a lways the norma l s tre s s e s wh ich , i s a lways op ti m i z e d with © ISO 2016 – All rights reserved ISO 88-1 : 01 6(E) Key L position of loading ring S position of supporting ring This graph has been measured using the following parameters: Square specimen: 000 mm × 000 mm × mm; r1 r2 F: 22 220 N; Parameter: gas pressure, p This determines that the optimized gas pressure, p, is 15 kPa = 0 m m; = 0 m m; Figure — Tangential stress development along the median of the convexly bent specimen surface However, the advantage of the biaxial stress condition, the two principal stresses of which are of equal 6.3), is ff f f damage has a clear preferential direction (e.g one or more parallel scratches) s i z e, as i s app a rent du ri ng the double ri ng b end i ng pro ce s s i n the c a s e o f s ma l l defle c tion s (s e e lo s t T h i s d i s adva ntage i s , however, more th an o s e t b y the large te s t s ur ace a re a, u n le s s the s u r ace I n the c a s e o f s quare s p e ci men s , the s tre s s d i s tribution i s s l ightly d i re c tiona l; the s tre s s e s a long the med i an s a nd the d i agona l s d i ffer s l ightly, with i n a range o f ab out % The curves in ISO 1288-2:—, Figure and the values in ISO 1288-2:—, Table have been determined from measurements Deviations of individual measured values from the curves or table values are, at most, % 10 © ISO 2016 – All rights reserved ISO 88-1 : 01 6(E) 6.1 Testing of patterned glass Sp e c i men s with one or two p atterne d s ur face s c a n no t b e te s te d by me an s o f a s ma l l te s t are a u s i ng the coaxial double ring bending test (see 6.3 and ISO 1288-5), since the surface spread of the decorations are approxi mately the s ame s i z e as the te s t s u r face are a However, with a double ring bending test with a large test area in accordance with ISO 1288-2, it is possible to test glass with patterned surfaces The permissible structural depth given in 5.3.2 (local deviations from the average thickness a maximum of 30 % or mm, whichever is the lower) is based upon experimental results Where there are one or two p atterne d s ur faces , the cond ition o f the l i ne - shap e d i ntro duc tion o f force by the e dge o f the lo ad i ng ri ng i s violate d by the ne ces s ity to i ntro duce a th icker i nterme d iate layer on the lo ad i ng ri ng s ide T he on ly e ffe c t, however, i s to re duce s omewhat i n s i z e the s u r face are a o f the vi r tua l ly homogeneous radial stress distribution Otherwise, the stress values remain almost unaffected 6.2 NO TE 6.1 Test with specimen supported at two points (four point bending) T h i s te s t i s s p e c i fie d i n I S O 8 -3 Limitations T h i s te s t i s no t appropri ate their thickness 6.1 for pro duc ts wh ich are exp e c te d to have defle c tion s gre ater th an ti me s Inclusion of edge effects This test is performed as a beam bending test using a wide beam The edges of the specimen, over the as the surface If it is required to determine the bending strength of glass where the effects of the edge are important, this test should be used centra l s p an o f nom i na l ly u n i form, u n id i re c tiona l s tre s s , are s ubj e c te d to the ma xi mum s tre s s a s wel l 6.1 Analysis of the stress development Si mple the or y as s u me s that there a re no s tre s s e s develop e d acro s s the width o f a b e a m when it i s s ubj e c te d to b end i ng a long its leng th H owever, wh i le th i s may b e a go o d approxi mation for na rrow b e am s , the Poi s s on e ffe c t generate s s igni fic a nt s tre s s e s ac ro s s the width o f wide b e am s T he s e s tre s s e s i nduce a contra fle xion ac ro s s the width o f the b e am, s o th at the longitud i na l s tre s s c a nno t b e rega rde d as uniform across the width (see Reference [9]) The effect is to increase the tensile bending stress developed at the edges of the beam and decrease the tensile bending stresses at the mid-line of the beam (as shown in Figure 4) © ISO 2016 – All rights reserved 11 ISO 88-1 : 01 6(E) Key + tensile stress - compressive stress X distance from the middle axis of the specimen along the width (mm) f f f f f f f f f 10 f s tres s p redicted F i g ro m s imp le theo ry is , M Pa tens io n o r co mp res s io n s tres s p redicted ro m s imp le theo ry is 43 , M Pa tens io n o r co mp res s io n s tres s p redicted ro m s imp le theo ry is , M Pa tens io n o r co mp res s io n s tres s p redicted ro m s imp le theo ry is 3 , M Pa tens io n o r co mp res s io n s tres s p redicted ro m s imp le theo ry is , M Pa tens io n o r co mp res s io n s tres s p redicted ro m s imp le theo ry is 4, M Pa tens io n o r co mp res s io n s tres s p redicted ro m s imp le theo ry is , M Pa tens io n o r co mp res s io n s tres s p redicted ro m s imp le theo ry is 4, M Pa tens io n o r co mp res s io n s tres s p redicted ro m s imp le theo ry is , M Pa tens io n o r co mp res s io n s tres s p redicted ro m s imp le theo ry is , M Pa tens io n o r co mp res s io n u r e — V a r i a t i o n i n s t r e s s a c r o s s t h e w i d t h o f a n , m m t h i c k s p e c i m e n o f f l o a t g l a s s a t t h e middle of the span I f the e xac t origi n o f frac tu re i s known, it i s p o s s ible to ob tai n, b y complex c a lc u lation, the pre ci s e lo c a l b end i ng s tre s s that s c aus e d frac ture o f the s p e c i men I f, however, the prob abi l ity o f fl aws and the flaw d i s tribution i s s idere d, there i s ano ther appro ach wh ich c an b e ta ken, dep end i ng up on whether the required bending strength is derived from all the bending test results (overall strength), f that the application of a factor, k f bending stress to a “weighted average” of the bending stresses, called the effective bending strength, σbeff The factors are the following: k ks when all the fractures are to be included; or whe ther on ly tho s e re s u lts rom e dge bre a ks a re to b e i nclude d (e dge s treng th) I t c an b e shown , to the c a lc u late d b end i ng s tre s s c an b e u s e d to mo d i y the c a lc u late d = 12 = ,0 © ISO 2016 – All rights reserved ISO 88-1 : 01 6(E) k = ke when only edge fractures are to be included The value of ke depends on the deflection of the specimen at its centre, and values for this parameter are given in ISO 1288-3 6.3 Coaxial double ring test for small test surface areas NOTE 6.1 This test is specified in ISO 1288-5 Elimination of edge effects The special feature of the coaxial double ring bending test in accordance with ISO 1288-5 lies in the fact to maximum stressing In contrast to other bending tests (for example, ISO 1288-3) in which the edge o f the specimen is subjected to the maximum stress, the procedure in accordance with ISO 1288-5 is that only a circular shaped limited area o f the sur face o f the specimen (not, however, its edges) is subjected suitable for exclusively subjecting sur faces (or di fferent sur face conditions) to bending stress The effect o f the specimen edge created by mechanical cold working is for, the most part, suppressed 6.1 Analysis of the stress development The advantages outlined in 6.1.1 and Reference [10] prompted the choice of the coaxial double ring bending test as a test method for determining the bending strength of glass One of these advantages is the uni form, and directionally independent loading o f the specimen with the loading ring, which means that the direction o f possible sur face de fects has no influence on the result However, this only applies up to limited deflections, y0 , at the centre of the specimen Above this limit, excessive local stress may occur under the bearing edges o f the loading ring, the magnitude of which increases as the load becomes greater At the same time, tangential and radial stresses undergo variable modification, which is too complex for simple calculation In this case, the stresses calculated from the linear bending theory are found to be excessively high (see 6.1) For the ring size ratio, r2/r1 = 5, chosen here, the permissible deflection range is given approximately by y0/h < 1,0 The minimum values for the thickness o f specimens specified in ISO 1288-5 have been selected for bending stresses up to 600 MPa in such a way that, for elastic moduli o f not less than 50 GPa, the relative deflections, y0/h , in the centre of the specimen not exceed 0,75 The stress differences in the loading ring region are then less than %, according to Reference [11] and [12] For the purposes of ISO 1288-5, the bending strength can be calculated from the test load using the formulae given in ISO 1288-5, provided that the ring and specimen dimensions and the values of minimum thickness of specimens are adhered to For this quasi-linear range of specimen loading, the following expression applies for the stresses in the sur face o f circular specimens bounded by the loading ring (see Re ference [5]) σ rad = σ T = ( )r + µ ) − µ (1 + µ ) 2π 3( 2 − r12 2 r3 + ln F r1 h r2 (1) Assuming a constant ratio between the values of r1 , r2 and r3 and a Poisson number, μ , for the specimen o f 0,23, the formula used in ISO 1288-5, for a circular specimen, may be derived from the Formula (2): σ rad = σ T = K1 F h (2) Table shows the effect of Poisson number on K1 © ISO 2016 – All rights reserved 13 ISO 1288-1:2016(E) Table — Constant, K1 , as a function of Poisson number, μ Poisson number, μ Constant, K1 , for R30 and R45 rings Error, %, on assuming Constant, K1 , for R60 and R105 rings Error, %, on assuming 1,059 1,065 1,071 1,076 1,082 1,088 1,094 1,100 1,106 2,7 2,1 1,6 1,1 0,5 0,0 0,5 1,1 1,6 0,662 0,667 0,672 0,677 0,681 0,686 0,691 0,695 0,700 3,4 2,7 2,1 1,4 0,7 0,0 0,7 1,4 2,1 0,18 0,19 0,20 0,21 0,22 0,23 0,24 0,25 0,26 μ = 0,23 μ = 0, 23 The Poisson number for soda lime silicate glass (ISO 16293-1) is given as 0,23 For square specimens, Formula (1) applies using the mean specimen radius, as follows: r3 m = (1 + 2) L 2 (3) = 0, L Here, r3m corresponds to the mean of the radii of the circles circumscribing and inscribing the square This leads to constants, in ISO 1288-5, of K2 f K2 f and R105 rings, for square specimens of soda lime silicate glass = ,0 or the R3 and R45 ri ngs a nd = , 674 or the R6 When te s ti ng u s i ng lo ad i ng device s , R45 and R3 , a s s p e c i fie d i n I S O 8 -5 , the tangenti a l ten s i le s tre s s at the e dge o f the glas s i s ab out % o f the ma xi mu m ta ngentia l s tre s s (= rad i a l s tre s s) with i n the loading ring If this results in edge breaks, it is recommended to increase the specimen radius, r3 , or side length, L f r3 /r2 L/2 r2 f f f maximum value (see Reference [10 this case, however, constants K1 and K2 need to be calculated again from Formula (1) , and hence, the proj e c tion o = or the s p e c i men b eyond the s upp or ti ng ri ng With a ratio = , the ta ngentia l s tre s s at the e dge o the s he e t i s defi nitely b elow 10 % o ] ) , s o th at the p o s s ibi l ity o I n i nter-lab orator y te s ts , the c a lc u lation pri nciple gi ven here the e dge bre a ks c an b e vi r tua l ly exclude d I n for s quare a nd c i rc u lar s p e ci men s s le d to s ati s fac tor y agre ement o f the b end i ng s treng th va lue s de term i ne d for b o th form s o f s p e c i men Range of application of the test methods 7.1 General limitations T he te s t me tho d s s p e c i fie d i n th i s p a r t o f I S O 8 a re no t appropri ate glass or insulating units for the te s ti ng o f la m i nate d 7.2 Limitations to ISO 1288-2 T h i s te s t me tho d i s appl ic able on ly to flat gla s s (s e e 3.1) Patterne d gla s s ca n b e te s te d provide d the ma xi mu m and m i n i mu m th ickne s s e s no t deviate b y more than 30 %, but not more than mm, from the mean thickness 7.3 Limitations to ISO 1288-3 T h i s te s t me tho d i s appl ic able on ly to flat gla s s (s e e Patterned glass can be tested without limitations 14 3.1) © ISO 2016 – All rights reserved ISO 88-1 : 01 6(E) This test is not appropriate for products which are expected to have deflections greater than 25 times their thickness 7.4 Limitations to ISO 88-4 This test method is applicable only to channel shaped glass 7.5 Limitations to ISO 88-5 This test method is applicable only to flat glass (see 3.1) Patterned glass shall not be tested by this method Calibration of the testing machines The testing machines used according to ISO 1288-2, ISO 1288-3, ISO 1288-4 and ISO 1288-5 shall have been calibrated by the user within three months be fore a test For the purposes o f calibration o f a testing machine, the glass specimen can be replaced by a metal (e.g steel) plate of appropriate thickness To per form the calibration o f the force measuring instrument, an o fficially calibrated force gauge with independent instrument reading, accuracy ±1 %, shall be placed in series with the force measuring instrument of the testing machine To per form the calibration o f the gas pressure measuring instrument, an o fficially calibrated pressure gauge with independent instrument reading, accuracy ±1 %, shall be placed in parallel to the pressure measuring instrument of the testing machine The force or pressure shall be increased in at least five approximately equidistant steps covering the ranges o f measurement At every step, the instrument readings from the testing machine and that from the o fficially calibrated gauge shall be recorded Both readings shall coincide within ±1 % over the whole measuring range I f the di fferences between the two readings exceed ±1 %, the measuring instrument o f the testing machine shall be adjusted accordingly Only o fficially calibrated gauges shall be used for calibration They shall be o fficially recalibrated every three years Recommendations for safe use of test equipment Where material testing machines are in use, both the user and others may be exposed to hazards arising from the design of the machine and the behaviour of the specimen Material testing machines should be so designed that the user and others are protected, as far as possible, against hazards of all kinds, when the machine is used in the proper manner The tests methods specified in this part o f ISO 1288 are designed to break glass under high stresses, so there is an obvious hazard from the broken glass Appropriate precautions shall be taken during the testing of glass specimens to avoid the particular hazards to both operators and observers from broken glass These could include the following: — providing appropriate protective clothing, e.g sa fety spectacles and gloves, especially for the handling of glass specimens and broken glass fragments; — applying an adhesive film to the sur face o f the glass, which is not being subjected to bending tension, to ensure that all the broken pieces are held together after breakage; — using transparent sa fety screens between observers and/or operators and specimen © ISO 2016 – All rights reserved 15 ISO 88-1 : 01 6(E) If it is not possible, because of — the particular nature of the test process, or — different and unpredictable behaviour of the specimen during the test process, to ach ieve s a fe hand l i ng o f the te s ti ng mach i ne b y de s ign me as u re s or b y add itiona l s a fe ty fe atu re s , the te s ti ng mach i ne shou ld b e re s tric te d to u s e by a l i m ite d group o f p ers on s who are able to a s s e s s the possible risks 16 © ISO 2016 – All rights reserved