© ISO 2016 Glass in building — Determination of the bending strength of glass — Part 2 Coaxial double ring test on flat specimens with large test surface areas Verre dans la construction — Déterminati[.]
INTERNATIONAL STANDARD ISO 1288-2 First edition 2016-02-15 Glass in building — Determination of the bending strength of glass — Part 2: Coaxial double-ring test on flat specimens with large test surface areas Verre dans la construction — Détermination de la résistance du verre la flexion — Partie 2: Essais avec doubles anneaux concentriques sur éprouvettes planes, avec de grandes surfaces de sollicitation Reference number ISO 1288-2:2016(E) © ISO 2016 ISO 1288-2:2016(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 88-2 : 01 6(E) Page Contents iv Scope Normative references Terms and definitions Symbols Principle of test method Apparatus 6.1 Testing machine 6.2 Loading device 6.2.1 Ring load 6.2.2 Surface pressure regulator 6.3 Measuring instruments Sample 7.1 Shape and dimensions of the specimens 7.2 Sampling and preparation of specimens 7.2.1 Cutting and handling 7.2.2 Conditioning 7.2.3 Examination 7.2.4 Adhesive film 7.3 Number of specimens Procedure 8.1 Temperature 8.2 Humidity 8.3 Thickness measurement 8.4 Base plate 8.5 Positioning of specimen and loading ring 8.6 Load application 8.7 Loading rate 8.8 Location of the origin 8.9 Assessment of residual stresses Evaluation 9.1 Limitation of the evaluation 9.2 Calculation of bending strength Test report Foreword 10 Annex A (informative) piston force, Bibliography Example of a device for keeping the gas pressure, p , in line with the F 1 © ISO 2016 – All rights reserved iii ISO 1288-2:2016(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, , Subcommittee SC 2, Use Glass in buildin g considerations ISO 1288 consists of the following parts, under the general title f : — f — — f — f — f Glass in buildin g — Determin ation o th e ben din g stren gth o glass Part : Fun dam entals o testin g glass f 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 ( our point ben din g) Part 4: Testin g o ch an n el sh aped glass f 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 1288-2:2016(E) Glass in building — Determination of the bending strength of glass — Part 2: Coaxial double-ring test on flat specimens with large test surface areas Scope T h i s p ar t o f I S O 8 s p e ci fie s a me tho d for de term i n i ng the b end i ng s treng th o f gla s s buildings, excluding the effects of the edges The limitations of this part of ISO 1288 are described in ISO 1288-1 for use in 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 48, Rubber, vulcan ized or th erm oplastic — Determ in ation o f h ardn ess (h ardn ess between IRHD an d 00 IRHD) ISO 1288-1, Glass in buildin g - Determin ation o f th e ben din g stren gth o f glass - Part : Fun dam entals o f testin g glass Terms and definitions For the pu r p o s e s o f th i s c u ment, the fol lowi ng term s and defi n ition s apply 3.1 bending stress tensile bending stress induced in the surface of a specimen N o te to entr y: For te s ti n g 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 fo rm over a s p e c i fie d p a r t o f the s u r face 3.2 effective bending stress weighte d average o f the ten s i le b end i ng s tre s s e s , ca 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 3.3 bending strength tensile bending stress or effective bending stress which leads to breakage of the specimen 3.4 equivalent bending strength apparent bending strength of patterned glass, for which the irregularities in the thickness not allow precise calculation of the bending stress © ISO 2016 – All rights reserved ISO 88-2 : 01 6(E) Symbols A effective surface area of quasi-uniform stress E mo du lu s o f el as ticity ( Young’s mo du lu s) o f the s p e ci men Fmax NOTE For soda lime silicate glass (see ISO 16293-1), a value of 70 GPa is used piston force piston force upon breakage, “breaking force” Fring force F m2 Pa N N N m m tran s m itte d b y the lo ad i ng ri ng to the s p e c i men, “ri ng lo ad” thickness or average thickness of specimen side length of the square specimens Poisson number of specimen NOTE For soda lime silicate glass (see ISO 16293-1), a value of 0,23 is used h L μ p Pa Pa Pa m m m m Pa Pa Pa s N/s ga s pre s s u re on the s u r face are a defi ne d b y the lo ad i ng ri ng () nominal gas pressure as a function of the piston force nominal gas pressure upon breakage location coordinate radius of loading ring radius of supporting ring average specimen radius (for evaluation) stress bending strength equivalent bending strength time rate at which piston force rises non-dimensional quantities corresponding to F, p p F ( ) p max Fmax r r1 r2 r3m σ σbB σbeqB t F/Δ t Δ *, *, σ* F p and σ [s ee Formulae (1) to (5 ) ] Principle of test method The square specimen, of side length, L f on a supporting ring (a circular ring with a radius r2 means of a loading ring (radius r1 the area, A r r1 is placed under gas pressure, p relationship with the ring load, Fring (see Figure 1) , a nd havi ng vi r tua l ly pla i n p ara l lel s ur ace s , i s place d lo o s ely ) T he s p e ci men i s s ubj e c te d to a lo ad, Fring , by ) , wh ich i s arrange d concentric a l ly to the s upp or ti ng ri ng I n add ition, , defi ne d b y the lo ad i ng ri ng < < , wh ich s a s p e c i fic When the s p e ci men i s s ubj e c te d to the ri ng lo ad a nd the a s s o ci ate d gas pre s s ure, dep end i ng up on the dimensions r1 , r2 , L , and h f f point (r = 0) of the specimen, but decreases as the radius, r, increases , a rad i a l ten s i le s tre s s field , wh ich i s s u ffic iently homo gene ou s for the te s t pu rp o s e , i s develop e d on the conve xly b ent s u r ace over the are a defi ne d b y the lo ad i ng ri ng (s e e Re erence s [ ], [ ], [ ] ) T he tangenti a l ten s i le s tre s s i s e qua l to the rad i a l ten s i le s tre s s at the centra l © ISO 2016 – All rights reserved ISO 1288-2:2016(E) O uts ide the lo ad i ng ri ng , the rad ia l and tangenti a l s tre s s e s fa l l s rply towa rd s the e dge o f the specimen, so that the risk of breakage outside the loading ring is low On the edge of the specimen itself, the radial stress is zero and the tangential stress is a compressive stress, this being the case on both the f f tangential compressive stress (see ISO 1288-1) f F, and the gas pressure, p, the tensile stress in the central part of the specimen 6.1 f conc avely and the convexly b ent s ide s o B y i ncre a s i ng the the s p e c i men T he e dge o the s p e c i men i s thu s a lways u nder orce, i s i ncre as e d at a s tant rate [s e e ( b) ] u nti l bre a kage, s o the origi n o the bre a k c an b e e xp e c te d to o cc u r i n the s u r face are a s ubj e c te d to ma xi mu m ten s i le s tre s s with i n the lo ad i ng ri ng With the test apparatus as shown in Figure 1, a force, pA , acts against the piston force, F, due to the gas pressure, p f f Fring = F - pA Thus a distinction should be made between the piston force and the ring load The bending strength, σbB , or equivalent bending strength, σbeqB , is calculated from the maximum value, Fmax, of the piston force, measured at the time of breakage, and the thickness, h , of the specimen, taking into account the prescribed dimensions of the specimen and various characteristic material values This assumes that the gas pressure, p, follows the piston force, F, according to the nominal function p(F), (see Figure 3) T he orce tra n s erre d by the lo ad i ng ri ng i s Apparatus 6.1 Testing machine The bending test shall be carried out using a suitable bending testing machine, which shall incorporate the following features a) The stressing of the specimen shall be capable of being applied from zero up to a maximum value in a manner which minimizes shock and is stepless b) T he s tre s s i ng device sh a l l b e c ap ab le o f the s p e ci fie d rate o f s tre s s i ng c) The testing machine shall incorporate a load measuring device with a limit of error of ±2,0 % within the measuring range Key loading ring specimen supporting ring Figure — Basic diagram of test apparatus © ISO 2016 – All rights reserved ISO 88-2 : 01 6(E) Key specimen f f r2 ) a ISO 48) a rigid loading ring (radius r1 f f force transmitting component, with a ball mechanism to ensure the force is centred in the loading ring ISO 48) b c f contact circle of the loading ring contact circle of the supporting ring a The radius of curvature of the bearing surface of the ring is mm rigid b as e p late, p re erab ly made o s teel, with s up p o rting ring (radius rub b er p ro file, adap ted to the s up p o rting ring, mm thick, with a hardnes s (40 ± ) I RH D (in acco rdance with ) , p re erab ly made o s teel rub b er p ro file, adap ted to the lo ading ring, mm thick with a hardnes s (40 ± ) I RH D (in acco rdance with adj us tment j aws o r centring the s p ecimen b I n the cas e o f s p ecimens which are p atterned o n the lo ading ring s ide, a s p o nge rub b er p ro file ap p roximately c The j aws are removed b e fo re the b ending tes t is s tarted, in o rder that the edge o f the s p ecimen is no t clamp ed mm thick should also be used to ensure an adequate seal for the gas pressure Figure — Loading device 6.2 6.2 Loading device Ring load The ring load shall be applied using a loading device as shown in Figure The dimensions of the loading device are given in Table Table — D imensions for the loading ring and supporting ring Dimensions in millimetres Lo ading ring r1 300 ± 6.2 Supp orting ring r2 400 ± Effective surface area mm 240 000 Surface pressure regulator The loading device for the surface pressure is shown in Figure © ISO 2016 – All rights reserved ISO 88-2 : 01 6(E) T he regu lator sha l l b e cho s en with regard to acc urac y and flow rate i n s uch a way that the nom i na l function, p(F), as shown in Figure or Table 3, can be met (see Annex A) 6.3 Measuring instruments The following measuring instruments are required: — a measuring instrument enabling the width of the specimen to be measured to the nearest mm; — a measuring instrument allowing the thickness of the specimen to be measured to the nearest 0,01 mm Sample 7.1 Shape and dimensions of the specimens Square specimens of the dimensions shown in Table shall be used T he m i n i mu m th ickne s s given for the s p e c i men s h as b e en c a lc u late d i n s uch a way that the e ffe c t o f the self-weight of the specimen upon the stress distribution can be ignored Table — D imensions of specimens Specimen side length L Dimensions in millimetres Minimum nominal specimen Average specimen radius (for evaluthickness ation) r3m 000 ± The following tolerances for the specimens shall be observed 600 I n the c a s e o f s p e ci men s with flat s u r face s: — the evenness tolerance shall be 0,3 mm; — the parallelism tolerance shall be % of the specimen thickness In the case of specimens with one or two patterned surfaces: f 8.3) shall be not more than % and the local deviations from the average thickness (due to the depth of the pattern) shall be a maximum 30 % or mm, whichever is the lower — the fluc tuation s o 7.2 the plate th ickne s s (s e e Sampling and preparation of specimens 7.2 Cutting and handling T he gre ate s t c a re s l l b e ta ken that the te s t s u r face, wh ich wi l l b e s ub s e quently s ubj e c te d to ten s i le stress, does not come into contact with tools, grinding agents, glass splinters, etc., and also is not damaged during storage NO TE I n order to pre s er ve s p e c i fic s u r face cond ition s , the te s t s u r face c a n b e provide d with a p ro te c tive NO TE T he me tho d o f c utti n g s p e c i men s i s no t s ign i fic a nt a nd no e dge pro ce s s i n g i s ne ce s s a r y coating (glued down) during specimen preparation 7.2 Conditioning Protective coatings shall be removed 24 h before the test (see ISO 1288-1) The specimen shall be stored in the test environment (see 8.1 and 8.2) for at least h before testing © ISO 2016 – All rights reserved ISO 88-2 : 01 6(E) 7.2 Examination B e fore the b end i ng s treng th te s t, a l l s p e c i men s sh a l l b e exa m i ne d over the te s t s u r face are a fau lts 7.2 for a ny wh ich are no t repre s entative o f the qua l ity ch arac teri s tics o f the materi a l te s te d Adhesive film To hold to ge ther the fragments , an ad he s ive fi l m s l l b e fi xe d to the s ide o f the s p e c i men fac i ng the loading ring This facilitates location of the fracture origin and measurement of the specimen thickness 7.3 Number of specimens T he numb er o f s p e ci men s to b e te s te d s l l b e de term i ne d dep end i ng on the fidence l i m its re qui re d , e s p e c ia l ly with regard to e s ti mati ng the e xtreme s o f the s treng th d i s tribution (s e e I S O 8 -1 discussion of numbers of specimens) for a Procedure 8.1 Temperature The coaxial ring bending test shall be carried out at a temperature of (23 ± 5) °C During the test, the temperature of the specimen shall be kept constant to °C, in order to avoid the development of thermal stresses 8.2 Humidity T he co a xia l ri ng b end i ng te s t sh a l l b e c arrie d out at a relative hum id ity b e twe en % and 70 % 8.3 Thickness measurement Since the nominal pressure function, p(F), in accordance with Figure or Table 3, is dependent upon the specimen thickness, h , this shall be determined before starting the test For this purpose, the thickness shall be measured at a minimum of eight points on the edge of the specimen For specimens with one or two ornamental surfaces, both the plate thickness and the core thickness shall be measured The average is taken from all these measured values f h , is used to determine the nominal pressure function, p(F f T he value obtained in this way or the s pecimen thicknes s or the equivalent s pecimen thicknes s , ) B y meas uring the thicknes s on the edge o undes irable damage to the s ur face caused by meas uring to ols es not a ffec t the 8.4 fracture the s pecimen, behaviour Base plate T he b as e plate i s centre d by movi ng down the force tran s m itti ng comp onent (without the lo ad i ng ri ng Figure splinters and other hard and sharp-edged particles shall be cleaned from the supporting ring Damage to the supporting ring shall be eliminated and s p e ci men) i nto the adj u s ti ng cone (s e e 8.5 ) T he b a s e plate sh a l l b e fi xe d i n th i s p o s ition Gla s s Positioning of specimen and loading ring The specimen is positioned with the surface to be tested downwards The loading ring, from which glass splinters and other hard and sharp-edged particles have been removed, is placed on the upper side of the specimen and centred The rubber connection attached to the loading ring shall be checked for its s e a l i ng e ffe c t a nd i f ne ce s s a r y replace d D amage to the lo ad i ng ri ng s l l b e el i m i nate d © ISO 2016 – All rights reserved ISO 88-2 : 01 6(E) 8.6 Load application The piston force, F, and the gas pressure, p, shall be increased continuously until the specimen breaks The inter-relationship, p(F), to be maintained during the loading, shall be determined from the nondimensional representation in Figure (curve p*) or Table The following relationships exists between the non-dimensional parameters, p * and F*, and the values p and F p = p∗ F = F* Eh r 3m ( − µ2 Eh ( ) r32m − µ ) (1) (2) The piston force, F, and the gas pressure, p, shall be monitored up to breakage of the specimen, to check whether the nominal function, p(F), meets that shown in Figure or Table NOTE Only i f the nominal pressure function, p (F), is followed, is a uniform radial tensile stress distribution developed on the convexly bent sur face, in that area o f the specimen defined by the loading ring Keeping the gas pressure, p, in line with the piston force, F, may be carried out manually by means of a control valve, but it is recommended that the gas pressure, p, be controlled automatically as a function o f the piston force, F It is permissible to linearize the function p *(F*) given in Figure for loads close to fracture A suitable arrangement is described in Annex A The maximum force, Fmax, and the associated gas pressure, p max, shall be measured From these two values, the stress at break, σbB or σbeqB , in MPa, shall be determined in accordance with Figure or Table (see 9.2) 8.7 Loading rate The increase with time o f the piston force and the associated gas pressure shall be chosen in such a way that the radial tensile stress in the centre of the specimen increases at a rate of (2 ± 0,4) MPa/s until the specimen breaks Since there is no linear relationship between the stress and the piston force, the permissible loading rate shall be determined in accordance with Figure or Table A preliminary test is recommended in order to determine the rate o f loading NOTE Since the breakage stress is most dependent on the loading rate in the few seconds before fracture, it is permissible to set that loading rate required at loads close to fracture constant over the entire test 8.8 Location of the origin The location o f the origin o f the fracture (see Re ference [5 ]) shall be determined from the fragments The position of the origin of the fracture “inside or outside the contact circle of the loading ring” shall be determined for every specimen NOTE After fracture, further thickness measurements, for control purposes, can be made on fragments rom the centre o f the specimen bounded by the loading ring contact circle, pre ferably as close to the fracture origin as possible f 8.9 Assessment of residual stresses I f the specimens are considered to be free from inherent stresses, (that is, they are o f annealed glass), this condition shall be examined photo-elastically, in the case o f transparent glasses, on specimens or suitable fragments Stress- free specimens placed between cross-polarized polarizing filters shall not show any significant brightness variations when viewed through the cross-section over an optical path length of mm © ISO 2016 – All rights reserved ISO 88-2 : 01 6(E) Evaluation 9.1 Limitation of the evaluation For eva luation pur p o s e s , on ly tho s e s p e c i men s sha l l b e s idere d i n wh ich the origi n o f the frac tu re l ie s with i n the are a defi ne d b y the lo ad i ng ri ng 9.2 Calculation of bending strength The bending strength, σbB , or equivalent bending strength, σbeqB , associated with the fracture force, Fmax, and the associated gas pressure, p max(Fmax), is determined from the non-dimensional Table 3, taking into account the specimen thickness, h For this representation in Figure purpose, the measured variables, Fmax and p max(Fmax), are converted with the aid of the Formulae (3) and (4) into the corresponding non-dimensional factors F* max and p * max (c ur ve σ *) or * F max = F max ( 2 r 3m − µ Eh (3) p max = p max ( Fmax ) ∗ ) ( r34m − µ ) (4) Eh The non-dimensional fracture stress, σ*bB , shall be determined from these values using Figure (curve σ *) or Table and then converted into the bending strength σbB in accordance with Formula (5): * σ bB = σ bB Eh (5) 2 r 3m ( - µ ) Figure — Relationship between the virtually uniform radial tensile stress gas pressure , p*(F*), and the pis ton force r1 : r2 = 1: , F*, , σ* rad, the nominal in a non- dimensional representation (where 1, 3 and r2 : r3 m = 1: 1,5) © ISO 2016 – All rights reserved ISO 88-2 : 01 6(E) Table — Relationship between the virtually uniform radial tensile s tress gas pressure F* 10 15 20 25 30 35 40 45 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 , p*(F*), p and the pis ton force *(F*) 0,5 1,8 3,5 6,0 9,0 12,5 16,5 20,0 24,0 32,5 41,2 51,0 61,5 72,5 83,5 94,7 106,0 117,0 129,0 141,0 153,0 166,0 178,0 191,0 r1 : r2 = 1: σ* rad 1,00 2,05 3,05 3,85 4,60 5,25 5,85 6,40 6,90 7,35 8,20 8,90 9,55 10,15 10,80 11,45 12,05 12,60 13,15 13,70 14,20 14,65 15,15 15,60 16,05 , F*, , σ* rad, the nominal in a non- dimensional representation (where , 3 and r2 : r3 m = 1: 1,5) F* 210 220 230 240 250 260 270 280 290 300 320 340 360 380 400 420 440 460 480 500 p *(F*) 205,0 219,0 231,0 245,0 259,0 272,0 285,0 299,0 312,0 326,0 351,0 377,0 404,0 430,0 456,0 484,0 510,0 536,0 563,0 589,0 σ* rad 16,55 16,95 17,40 17,80 18,20 18,30 19,00 19,35 19,75 20,15 20,85 21,55 22,20 22,85 23,55 24,25 24,95 25,65 26,35 27,00 Test report The test report shall contain the following information: a) a reference to this part of ISO 1288, i.e ISO 1288-2; b) typ e and name o f gl as s; c) pre-treatment and surface condition of the tested specimen surface including the sequence of treatment stages In the case of specimens with one patterned surface, the surface which is placed u nder ten s i le s tre s s (fl at or p atterne d s ide) sh a l l b e i nd ic ate d; d) inherent stress of the specimen, annealed or prestressed glass, including nature and if possible degree of prestressing; e) number of specimens; © ISO 2016 – All rights reserved ISO 88-2 : 01 6(E) f) for each specimen, the following information: 1) thickness, h f f thickness (plate thickness), minimum thickness (core thickness) and average thickness, h , in mm, to the nearest 0,05 mm, in the case of specimens with one or two patterned surfaces; 2) bending strength, σbB or σbeqB , in MPa, rounded off to 0,1 MPa, of each specimen broken in accordance with 9.1; 3) time to breakage in seconds to the nearest s; No average for the measured results shall be given g) number of specimens not broken in accordance with 9.1; , i n m m, to the neares t , mm, i n the cas e o h) 10 a ny deviation from s p eci mens with flat s ur aces; ma xi mum th i s p ar t o f I S O 8 wh ich may h ave a ffe c te d the re s u lts © ISO 2016 – All rights reserved ISO 88-2 : 01 6(E) Annex A (informative) Example of a device for keeping the gas pressure, the piston force, F p, in line with If the gas pressure within the loading ring is applied in accordance with the relevant loading specification (taking the plate and ring geometries into account), the testing procedure corresponds to that o f conventional bending tests, where knowledge o f the force triggering o ff the fracture is su fficient for determining fracture stress When designing the gas pressure control system (the gas pressure depends on the piston force applied), it was the major concern to develop a device which ensured that it was easy to apply in spite o f varying loading specifications I f a typical plot o f the nominal gas pressure against piston force is considered (see Figure A.1), it is apparent that the curve runs almost linearly in the middle and upper regions, and it can be approximated by a straight line without any great error The slope o f a straight line and its zero point displacement can be set with simple electrical devices, while copying a nonlinear function is more complicated and costly As far as ease o f operation is concerned, a linear approximation function brings considerable advantages for control o f the system A linearized loading specification obtained in this way, however, no longer leads to optimized stress distributions in the lower region of the curve, where the deviation from the nominal loading specification becomes greater As long as the stresses generated in this region of the curve not reach values which are critical for the initiation of fracture, this disadvantage is insignificant NOTE The dashed line is the linearized value of p Figure A.1 — Typical plot of force agains t pressure © ISO 2016 – All rights reserved 11 ISO 88-2 : 01 6(E) In cases where the values, p(F), necessary for a series o f measurements, lie in the bent part of the curve (see Figure A.1), linearization on the basis of the least square method, using the values in Table can be carried out This assumes that estimated values for the maximum and minimum fracture stresses, which occur in the series o f measurements, are known Linearization is only permissible in a region in which the deviation between the linearized function and the nominal function remains less than % of the respective nominal value Given these considerations, the electrical part of the control unit (see Figure A.2) was developed and tested An input voltage proportional to the piston force is sent from the load cell to an amplifier The output o f this amplifier is connected to the input o f a function generator which supplies a current proportional to the respective piston force to control a current-pressure transmitter The electrical signal is changed by the transmitter into a pneumatic signal, which is subsequently converted into the pressure range necessary for the test according to the linearized function, p (F) In principle, it is also possible to copy the exact nominal function, p (F), by means o f a microprocessorcontrolled gas pressure regulator By this means a higher degree o f accuracy can be achieved, particularly in the case o f specimens with very low fracture stresses, but at a considerably higher expenditure In theory, any gas available may be chosen as the medium For cost and sa fety reasons, however, the use of compressed air is recommended 12 © ISO 2016 – All rights reserved ISO 88-2 : 01 6(E) Key 10 application of piston force load cell pressure recorder meas uring amp lifier (p res s ure) X-Y reco rder o r memo ry units meas uring amp lifier ( fo rce) force-current converted or microprocessor controlled gas pressure regulator quick- fitting co up ling filter reducing valve reducing valve Figure A — Schematic diagram of the control s ystem © ISO 2016 – All rights reserved 13 ISO 88-2 : 01 6(E) Bibliography [1] I S O 162 -1 , Glass in buildin g — Basic soda lim e silicate glass products — Part : Definition s an d gen eral physical an d m ech anical properties [2 ] R.W S chmi tt Die Doppelrin gm eth ode mit überlagertem Gasdruck als f f f Prü ver ahren zur Bestim mun g der Bruch span nun gen von groß orm atigen eben en Glasplatten klein er Dicke (Th e double rin g m eth od with super-im posed gas pressure as a testin g procedure ur [3 ] for determinin g th e , Diplomarbeit am Institut f f Components and Machine Design), Leiter: Professor Dr H Peeken, RWTH Aachen, 1982 chmi tt R W f f f stress at break o f large-sized M as ch i nenelemente flat sh eets o u nd f glass o f a M as ch i nenge s ta ltu ng sm all thickn ess.) ( T he s i s at the I n s titute or M ach i ner y Entwicklun g ein es Prü ver ahren s zur Erm ittlun g der Biege estigkeit von Glas S f a testin g f th e ben din g stren gth o f glass an d som e aspects o f th e statistical un d A spekte der statistisch en Beh an dlun g der gewonn en en Messwerte (Developm ent o f procedure or th e determin ation o f Diss RWTH Aachen, 1987 R.W Troeder, Chr.: evaluation o test results.), [4] B l ank K , & S chmitt f f Ein m odifiziertes Doppelrin gver ahren zur Bestimmun g f der Biegezug estigkeit gross orm atiger Glasplatten (A m odified coa xial-rin g-ben din g m eth od testin g th e ben din g stren gth o f large glass plates.), for Glas te ch n B er 6K ( 19 ) , volu me , pp 414 - 419 ( Vor trag au f dem I nternationa len Gla s -Kongre s s , H a mbu rg 19 ) [5 ] 14 f F Gesellschaft, Frankfurt/Main 1970, p 209 ff K erkho Bruchvorgan ge in Gläsern (Fracture Processes in Glass.), Deutsche Glastechnische © ISO 2016 – All rights reserved ISO 88-2 : 01 6(E) ICS 81.040.20 Price based on 14 pages © ISO 2016 – All rights reserved