DELAMINATION IN BLAST RESISTANT LAMINATED GLASS

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1 21 st Interntionl Conference on Composite Mterils Xi n, th August 2017 DELAMINATION IN BLAST RESISTANT LAMINATED GLASS Mohmmd Amin Smiein 1, Dvid Cormie 2, Dvid Smith 2, Will Wholey 2, Bmber R.K. Blckmn 1, Pul A. Hooper 1, John P. Der 1 1 Deprtment of Mechnicl Engineering, Imperil College London, SW7 2AZ, UK 2 Arup Resilience Security & Risk, 13 Fitzroy Street, W1T 4BQ, UK Keywords: Blst, Delmintion, Lminted Glss, Sndwich composite ABSTRACT Lminted glss is used in structures for protection ginst blst lods. Lminted glss fcilittes its sfety mechnism through delmintion of the glss from the interlyer. However, the mount of delmintion is importnt. An experimentl study hs tken plce to quntify the delmintion energy in lminted glss composites. Through-crcked tensile tests on lminted glss nd lso tensile tests on the polymer interlyer lone hve been crried out. The results were used to clculte n dhesive frcture energy. The tests were crried out t temperture rnge of C for three different interlyer thicknesses. The dhesive frcture energy ws found to be independent of temperture nd interlyer thickness for the rnge of tempertures tested. 1 INTRODUCTION Lminted glss composite properties my chnge once environmentl conditions diverge from the design point. In such lmintes, polyvinyl butyrl (PVB) interlyer is sndwiched between lyers of nneled glss. In some climtes, ir tempertures cn rech up to 50 C nd surfce tempertures cn exceed this vlue. As with most polymers, PVB s mechnicl response is temperture dependnt [1, 2]. This cn led to significnt divergence in performnce from the initil design nd predictions. Lminted glss is n importnt component in blst protective design. When n explosion occurs, the glss frctures lmost instntly fter blst wves interct with the window. Then the interlyer crries the lods through its deformtion. Protection is provided when glss frgments sty dhered to the interlyer nd thereby preventing the blst pressure from entering the building. In blst scenrio, dhesion between the interlyer nd the glss is importnt. Low dhesion will men tht shrp glss frgments detch from the interlyer nd trvel t high velocities, cusing dditionl threts to the people in the vicinity. With high dhesion, displcements will be low, thus, leding to premture tering of the interlyer. This is becuse for greter displcement, greter length of delminted PVB is required. Therefore blnce between the two extremes is essentil. Common experiments usully ssocited with ssessing dhesion re the peel nd pummel test. The pummel test does not led to n ccurte result such s n dhesive strength for comprison; it is subjective test nd requires humn judgement [3]. The peel test is better pproch thn the pummel test, s it provides n dhesive frcture energy. Kinloch et.l. hve provided test method nd nlysis techniques for peeling of flexible lmintes [4]. However, this method requires quntifiction of the energy lost vi other routes, such s bending of the peel rm. An lterntive method tht hs recently been used to ssess dhesion is the through-crcked tensile test. The glss is pre-crcked perpendiculr to the loding direction on both sides of the lminte nd tested in tension (Fig. 1). This removes the complictions ssocited with bending of the peel rm. Severl uthors hve previously worked on this [5-10]; some of which hve not provided n ccurte nlysis of the frcture energy. Furthermore, issues such s strin rte nd temperture dependency hve not been ddressed. The through-crcked tensile test ws originlly used by Sh et l. [6] to chrcterise the dhesion between lminted glss nd the PVB interlyer. They termed this test s the tension dhesion test. Similrly, through crcked tensile tests were lter used by Seshdri et l. [7] to ssess the mechnicl behviour of crcked lminted glss. They stted tht if the interlyer constitutive model is known, the interfcil frcture toughness nd energy relese rte cn be clculted from the tensile tests on crcked lminted glss. Seshdri et l. [11] lter extended their work by ssuming hyperelstic

2 Mohmmd Amin Smiein, Dvid Cormie, Dvid Smith, Will Wholey, Bmber R.K. Blckmn, Pul A. Hooper, John P. Der mteril model to develop finite element model which would llow nlysis of more complex geometries other thn the tensile specimens they tested. Delince et l. [12], however, could not reproduce the experimentl results of Seshdri. They proposed this ws becuse the specimens spent very short time in stedy stte delmintion. They lso did not successfully extend the method to other interlyers such s SentryGls Plus (SGP). The focus of the current study is to clculte n dhesive frcture energy for PVB t different tempertures. The glss thickness either side of the lminte ws kept constnt t 3 mm. Three different PVB interlyer thicknesses were considered, nmely 0.76 mm, 1.52 mm nd 2.28 mm. The experiments were crried out over temperture rnge of C. The outline of this pper is s follows. The theory behind how the dhesive frcture energy is clculted is explined. This is followed by description of the experimentl setup for the throughcrcked tests nd the tensile tests on the interlyer. Finlly the dhesive frcture energy is clculted from the experiments crried out nd discussion on how it vries with interlyer thickness nd temperture is presented. 2 THEORY Figure 1: Schemtic sketch of the through-crcked tensile test specimen. Kinloch et l. hve followed n energy bsed nlysis for clculting the dhesive frcture energy in the peel test [4]. Their nlysis cn be modified to llow the dhesive frcture energy to be clculted from the through-crcked tensile tests. In the through-crcked tensile test, s the lod is pplied in tension, the interlyer begins to delminte from the glss. The delminted interlyer ligment crries the lod undergoing tensile deformtion (Fig. 2).

3 21 st Interntionl Conference on Composite Mterils Xi n, th August 2017 Figure 2: Specimen with 1.52 mm interlyer tested t 20 C t test rte of 1 m/s, viewed through polrizing filters. The dhesive frcture energy, G, in the delmintion of the interlyer cn be clculted through the following energy blnce pproch. G 1 du b d ext du d s du d Where du ext is the externl work, du s is the stored energy in the delminted interlyer, du dt is the energy dissipted during the tensile deformtion of the delminted interlyer ligment, b is the smple width nd is the delmintion length. The externl work is clculted from the throughcrcked tensile test. The stored energy nd tht of tensile deformtion re clculted from tensile tests performed on unlminted PVB. The through-crcked tensile test is similr to peel test conducted t 0 loding ngle. For n dhesive filure lod, P, interlyer thickness, h, nd smple width, b, undergoing displcement dδ, the energy terms cn be clculted from Eqn. 2 nd Eqn. 3 respectively. du du s ext du dt (1) Pd Pd (2) dt bhd 0 d Where σ is the stress nd ε is the tensile strin in the delminted interlyer ligment. The lod, P, cn be clculted s the lod plteu from the through-crcked tensile test (shown s the dshed line in Fig. 3). The tensile strin in the PVB ligment, ε, cn be clculted from the tensile test on the PVB s the strin corresponding to the stress cused by the lod P. (3)

4 Mohmmd Amin Smiein, Dvid Cormie, Dvid Smith, Will Wholey, Bmber R.K. Blckmn, Pul A. Hooper, John P. Der Figure 3: Through-crcked tensile test t 20 C for the 0.76 mm interlyer (dshed line represents the dhesive filure lod, P) The dhesive frcture energy cn be determined by substituting the energy terms from Eqn. 2 nd Eqn. 3 into Eqn. 1. The finl G vlue for the through-crcked tensile tests is obtined s qurter of the G vlue in Eqn. 4. This is becuse in the through-crcked test, s the lod is pplied, four new crck surfces (delmintion surfces) re formed. G P b h d 3 EXPERIMENTAL METHODS AND DATA ANALYSIS 3.1 Through-crcked tensile test The test specimens were prepred from two plies of 3 mm nneled glss lminted together with PVB. Three different PVB interlyer thicknesses were used, nmely 0.76 mm, 1.52 mm nd 2.28 mm. A totl of 19 specimens were tested (Tble 1). The dimensions of the specimen were 150 mm long by 60 mm wide. A single coincident crck ws creted perpendiculr to the loding direction on both sides of the specimen. For tests where repet ws performed, n verge ws tken for the dhesive filure lod. Tests were conducted in the temperture rnge of C. Tble 1: Number of through-crcked tensile specimens tested t ech condition. Thickness of PVB Temperture ( C) (mm) The experiments were crried out t test speed of 1 m/s. At room temperture for the 0.76 mm interlyer this corresponds to strin rte of 21 s -1 in the delminted ligment. This is representtive vlue compred to the mximum strin rte the lminted pne experiences under blst conditions [13]. The experiments were crried out using n Instron high rte servo-hydrulic testing mchine (Fig. 4). The lod ws pplied through lost-motion device; this llowed for the loding trin to ccelerte to the desired velocity prior to pulling the smple. The lod ws mesured using piezoelectric lod cell (PCB model 222B). To ensure uniform temperture cross the smple, it ws plced in n Instron 0 (4)

5 21 st Interntionl Conference on Composite Mterils Xi n, th August 2017 environmentl chmber nd ws soked for 24 hours prior to testing. The ir temperture ws mesured using sensors built into the environmentl chmber to ± PVB Tensile test Figure 4: Through crcked tensile test setup. Tests on the PVB mteril (unlminted) were conducted t speed producing similr strin rte to the through crcked tensile tests (Fig. 5). The tests were conducted t n pproximte velocity of 1 m/s; this corresponds well with the strin rte in the through crcked tensile tests. The tests were conducted t temperture rnge of C. Three repets were conducted t every temperture. The lod ws mesured using piezoelectric lod cell (PCB model 208A03). The displcement ws mesured using opticl techniques. High speed video recording nd imge processing on MATLAB ws used to trck the mrked lines on the smple representing the guge length.

6 Mohmmd Amin Smiein, Dvid Cormie, Dvid Smith, Will Wholey, Bmber R.K. Blckmn, Pul A. Hooper, John P. Der Figure 5: PVB Tensile test setup. The specimen ws punched out with die (Fig. 6), ccording to dimensions given in the ASTM stndrd D [14]. The thickness of the specimen ws 0.76 mm. 4 RESULTS AND DISCUSSION 4.1 Through-crcked nd tensile tests Figure 6: PVB tensile test smple dimensions. The dhesive filure lod ws found to drop with temperture increse (Fig. 7). At higher tempertures, the polymer is more complint (less stiff). This results in lower forces for given displcement. The thicker interlyers generlly showed greter dhesive lod, n ccount of their greter effective stiffness. As the temperture increses, the thickness of the interlyer becomes less importnt. At room temperture, the difference in dhesive lods between the three interlyers re much greter thn t 50 C or 60 C.

7 21 st Interntionl Conference on Composite Mterils Xi n, th August 2017 Figure 7: Adhesive filure lod ginst temperture. A typicl result from the tensile tests is shown in Fig. 8. There is steep rise in stress up to certin vlue (pproximtely 4 MP in this cse). After this vlue, the rte of increse of stress is reduced by bout n order of mgnitude for given strin. This behviour is s expected for the viscoelstic mterils such s PVB [2]. Figure 8: Engineering stress ginst engineering strin with three repets (1 m/s t 30 C). 4.2 Adhesive frcture energy Through the use of the lod vlue from the through-crcked tensile tests, n dhesive frcture energy vlue ws clculted for ech repetition of the tensile tests t ll conditions. An verge ws tken nd the dt were plotted ginst temperture for every interlyer thickness with error brs representing one stndrd devition (Fig. 9).

8 Mohmmd Amin Smiein, Dvid Cormie, Dvid Smith, Will Wholey, Bmber R.K. Blckmn, Pul A. Hooper, John P. Der Figure 9: Adhesive energy ginst temperture for different interlyers. Del Linz et l. developed model bsed on experimentl dt to clculte the dhesion energy t room temperture [5]. Bsed on this model, t rte of 20 s -1 it is estimted tht the dhesive frcture energy is 2800 J/m 2. This is in good greement with the results from the experiments crried out. Despite the fct tht the 1.52 mm interlyer is consistently bove tht of the 0.76 mm interlyer, from the results obtined in this experiment, the dhesive frcture energy is shown to be thickness independent. This is becuse s the thickness ws incresed to 2.28 mm, similr trends could not be observed. Similrly in the temperture rnge tested, the dhesion energy lso demonstrted to be temperture independent. The dhesive energy becomes more dispersed between the different interlyers s the temperture increses. This could be becuse the smple spent much lower time in the stedy stte. At greter tempertures, the interlyer is more complint. This resulted in much greter deformtion of the interlyer compred to the stedy stte propgtion of the delmintion front. 5 CONCLUSION In this study the influences of temperture nd interlyer thickness on the dhesive energy of PVB were studied. The effect of incresing the temperture from 20 C to 60 C on the dhesive energy did not show cler trend. Moreover, the chnge of interlyer thickness did not show cler corroltion with dhesive energy. However s the temperture ws incresed, the spred in dt between the different interlyer thicknesses showed n increse. Further repetitions of the through-crcked tensile tests will ssist to vlidte the results t higher temperture, where lower frction of the test ws spent in the stedy stte delmintion region. Future work will include quntifiction of dhesion for greter temperture rnge nd lso different strin rtes. For purposes of prcticl pplictions, it cn be deduced tht the dhesion between the glss nd PVB will not vry significntly in the tempertures tested. Therefore it is resonble to ssume tht prt from ny influence of stiffness, the mount of shrp glss frgments tht will sty dhered t higher tempertures will be similr to tht of the design conditions. However, the more importnt fctor is how much energy the crcked lminte cn bsorb from the blst lod without tering. Quntifiction of the lod nd displcement cpcities of the interlyer t higher tempertures in future studies will help in the understnding of the post-crck response of the lminted glss t higher tempertures. ACKNOWLEDGEMENTS The uthors would like to thnk the EPSRC (the Engineering nd Physicl Sciences Reserch

9 21 st Interntionl Conference on Composite Mterils Xi n, th August 2017 Council) nd Arup Resilience, Security nd Risk for finncilly supporting Mohmmd Amin Smiein since the beginning of his PhD. The Authors re lso grteful to Leroy Reed t Kite Glss (UK) for providing the lminted glss nd PVB mteril. REFERENCES [1] W. Cllister, D. Rethwisch, Mterils science nd engineering, 8th Edition, John Wiley & Sons, New York, [2] P. Hooper, B. Blckmn, J. Der, The mechnicl behvior of poly (vinyl butyrl) t different strin mgnitudes nd strin rtes, J Mter Sci, 47, 2012, pp (doi: /s ). [3] S. Huo, H. Reis, Estimtion of dhesive bond strength in lminted sfety glss using guided mechnicl wves: Prt I. An energy velocity pproch, Insight: Non-Destructive Testing nd Condition Monitoring, 50(3), 2008, pp (doi: /insi ). [4] A.J. Kinloch, C.C. Lu, J.G. Willims, The peeling of flexible lmintes, Interntionl Journl of Frcture, 66, 1994, pp (doi: /BF ). [5] P. Del Linz, P.A. Hooper, H. Aror, Y. Wng, D. Smith, B.R.K. Blckmn, J.P. Der, Delmintion properties of lminted glss windows subject to blst loding, Interntionl Journl of Impct Engineering, 105, 2017, pp (doi: /j.ijimpeng ). [6] Y. Sh, C. Y. Hui, E. J. Krmer, P. D. Grrett, J. W. Knpczyk, Anlysis of dhesion nd interfce debonding in lminted sfety glss, Journl of Adhesion Science nd Technology, 11, 1997, pp (doi: / X01010). [7] S. Murlidhr, A. Jgot, S. J. Bennison, S. Sigl, Mechnicl behviour in tension of crcked glss bridged by n elstomeric ligment, Act Mterili, 48, 2000, pp (doi: /S (00) ). [8] R. Iwski, C. Sto, J. Ltilldend, P. Viot, Experimentl study on the interfce frcture toughness of PVB (polyvinyl butyrl)/glss t high strin rtes, Interntionl Journl of Crshworthiness, 12 (3), 2007, pp (doi: / ). [9] J. Frnz, J. Schneider, Through crcked tensile tests with polyvinylbutyrl (PVB) nd different dhesion grdes, in: Engineering trnsprency, Interntionl conference t glsstec, 2014, pp [10] C. Butchrt, M. Overend, Delmintion in frctured lminted glss, Engineered trnsprency, Interntionl Conference t Glsstec, [11] M. Seshdri, S.J. Bennison, A. Jgot nd S. Sigl, Mechnicl response of crcked lminted pltes, Act Mterili, 50, 2002, pp (doi: /S (02) ). [12] D. Delince, D. Sonck, J. Belis, D. Cllewert, R. Vn Impe, Experimentl investigtion of the locl bridging behviour of the interlyer in broken lminted glss, Interntionl symposium on the ppliction of rchitecturl glss, Munich, 2008, pp [13] P. A. Hooper, Blst performnce of silicone bonded lminted glss, PhD Thesis, Deprtment of Mechnicl Engineering, Imperil College London, [14] ASTM stndrd D638-02A, Stndrd test method for tensile properties of plstics, ASTM Interntionl, West Conshohocken, PA, 2003 (doi: /D A).