CONVERSION FACTORS OF THE TEMPERATURE EFFECT ON THE SHEAR STRENGTH OF ADHESIVELY-BONDED STEEL JOINTS

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1 JOUNAL OF CIVIL ENGINEEING AND MANAGEMENT ISSN / eissn Volume 22(5): do: / CONVESION FACTOS OF THE TEMPEATUE EFFECT ON THE SHEA STENGTH OF ADHESIVELYBONDED STEEL JOINTS Hartmut PASTENAK, Samer SAHELLIE Department of steel and tmber constructons, Faculty of Archtecture, Cvl Engneerng and Urban Plannng, Brandenburg Unversty of Technology, K.WachsmannAllee 2, Cottbus, Germany eceved 23 Feb 2015; accepted 07 May 2015 Abstract. Ths paper presents the nvestgatons on lap shear galvanzed steel jonts bonded by two structural adhesves (acrylc and epoxy) wth two thcknesses of the bondlne. The effect of the temperature on the shear strength of the adhesves when the jonts are shorttermloaded s the man objectve of the nvestgatons. The partal factors of the lmt states as well as the converson factors that cover the use condtons and crcumstances and partcularly the temperature commonly used n the respectve standards; the drect evaluaton method and predcton modelbased method. A comparson between the results obtaned by these methods s presented. Keywords: structural adhesves, steel jonts, partal factors, converson factors, temperature effects, Bayesan method. Introducton Despte the advantages of the adhesve bondng technque, are stll not able to use t n ther practcal applcatons besteel jonts. These doubts are manly because of the lack of standards for verfyng such jonts n steel constructons and also the lack of descrbng the loss of strength and durablty of adhesve materals due to the envronmental effects, manly temperature and humdty, and the longterm loadng. ecently, efforts have been made to n some applcatons n steel and alumnum structures (Pasternak, Menz 2007, 2008; Pekarczyk, Grec 2012). These researches have shown that ths technque s promsng to be used n steel structures. However, for wder use, valdaton of the desgn rules for structural adhesve bonded jonts accordng to the respectve standards has to be done. Usng allowable stress method n the desgn of adhesve jonts has the dsadvantage of not takng the statstcal nature of both load actons and the resstance nto account (Van Straalen et al. 1998). Consequently, concept, nstead, can be used for consderng the statstcal nature wth consstent safety margns. The ncrease of temperature, accordng to some repropertes such as ultmate strength (Osana, es 2012; Pasternak et al. 2012), elastcty modulus and stffness (Cease et al. 2006). It results not only n change of the adhesve behavor from brttle nto ductle, but also n change of the falure mode of bonded connectons from cohesve to adhesve (Sahelle, Pasternak 2013). Moreover, adhesve materals can be consdered as senstve to the socalled rheologcal phenomena such as creep and shrnkage whch may result n shortenng of ther durablty (Majda, Skrodzewcz 2009; Boyes 1998). These effects have to be taken nto consderaton when desgnng the bonded jonts by determnng approprate quanttatve parameters. Ths can be done by the use of the structural relablty method (Van Straalen 2001). Ths method guarantees the relablty level of the structure (the bonded jont) by takng nto account the stochastc nature of the strengths of the jont components. It can also be used to valdate a predcton model (analytcal model) that descrbes the structural behavor of the bonded jonts under prescrbed condtons (Van Straalen, Van Tooren 2002; Pasternak, Cupack In ths paper, the partal factors as well as the converson factor of the temperature effect on the shear strength of lap shear steel jonts bonded by acrylc and epoxy adhesves are determned for two thcknesses of the bondlne 0.35 mm and 0.65 mm and wthn a tem tve values are determned by evaluatng the tests results Correspondng author: Hartmut Pasternak Emal: hartmut.pasternak@tucottbus.de 666 Copyrght 2016 Vlnus Gedmnas Techncal Unversty (VGTU) Press

2 Journal of Cvl Engneerng and Management, 2016, 22(5): data at dfferent temperatures usng the drect evaluaton predcton models that descrbe the change of the shear strength of the studed adhesves due to the temperature change followng to the standard procedure recommended by EN 1990:2002 (2002) together wth the systematc approach developed by Van Straalen (2001). 1. elabltybased evaluaton for structures One of the evaluaton methods, that s based on the structural relablty method, s the partal factor format whch s commonly utlzed to valdate the use of structural materals and the desgn rules for structural applcatons. The strength of a structural materal can be evaluated ether drectly from the test results (drect evaluaton method), or by evaluatng predcton models descrbng the change modelsbased evaluaton). One of the methods that may be appled n the partal factor format s the Bayesan method whch determnes the representatve values (characterstc and desgn values) of the strength values (ISO and the converson factors of the lmt states can be estmated for an ntended structural relablty level (EN 1990: ). the structure s no longer safe. It can be mathematcally expressed by: Z E, (1) n whch, s the resstance and E s the acton effect. If Z 0 (or E ), then the structure s safe and no falure occurs. The relablty of the structure s valdated when the hghest predcted acton effect s stll less than or equal to the smallest predcted resstance over the ntended lfetme of the structure. Ths can be expressed by the desgn and characterstc values of the acton effect E d, E k and of the resstance, as shown n Eqn (2):. k E or. E, (2) d d E k where E and are the partal factors of the acton effect and the resstance respectvely whch take the stochastc nature of the acton effect and the resstance nto account, whle s the converson factor that takes nto account the change of the resstance over the lfetme ntended. The desgn and characterstc values of the acton effect and the resstance have to be probablstcally and These values of the resstance, for example, could be taken at target probabltes of 5% and 0.1% respectvely, further detals are found n the mentoned standards. 2. Shear by tenson tests of adhesvelybonded steel jonts 2.1. Studed jont Double lap shear jonts, whose geometry shown n Fgure 1, were selected. The common hotdp galvanzed be the external and nternal adherends, respectvely. The nvestgated adhesves are the twocomponent coldcure area for each sde s mm. Two thcknesses of the adhesve layer (0.35 and 0.65 mm) were studed. These thcknesses were acheved by the use of onesded adhesve strps, as shown n Fgure 1. The adhesves were appled accordng to the recommendatons of the manufacturer 3M ScotchWeld TM. After bondng, the specmens were left at room temperature 7 days for epoxy and 5 days for acrylc adhesve to be cured Test procedure bonded specmens were put n a clmate chamber at the tonng, specmens were tested by means of tensle testng machne near to the clmate chamber. The nstallaton of a specmen up to the end of the test was done wthn 2 mnutes. Temperature was measured usng a laser thermometer at the surface of the specmen and only changed ng machne out of the clmate chamber. Seven specmens Fg. 1. Double lap shear jonts black areas represent bonded areas

3 668 H. Pasternak, S. Sahelle. Converson factors of the temperature effect on the shear strength of adhesvelybonded steel jonts Fg. 2. Test equpment: left: the clmate chamber close to the testng machne; rght: the laser thermometer used for measurng the temperature at the surface of the specmen temperature range studed Fg. 3. Falure modes of AC (left) and EP (rght) for each temperature were tested; Fgure 2 shows the clmate chamber besde the testng machne and the laser thermometer used. The speed rate of the crosshead was set to 1.27 mm/ mn. Accordng to We et al. of the jonts has a close relatonshp wth the average stress level of the adhesve layers. Hence, the shear stress was consdered regularly dstrbuted over the bondlne and calculated by dvdng the recorded appled force by the twosded bonded areas,.e = 512 mm 2. It s worthwhle to menton that all specmens faled ether cohesvely (CF) or specal cohesvely (SCF) (EN ISO 10365: ), see Fgure Test results The mean and standard devaton values of the maxmum These values were calculated after checkng that the test results are normally dstrbuted and excludng the outlers of each group at the studed temperatures. AndersonDarlng test (ADtest) was used for normalty (omeu 2005) and outlers (the extreme values do not appear to repre ance test (EPA QA 1998) as well as by Dxon extreme value test (Dxon 1950). It has to be mentoned that AC/ EP ndcate the acrylc/epoxy adhesves and 0.35/0.65 ndcate the bondlne thckness nvestgated. 3. Drect evaluaton of the maxmum shear strength values The resstance of the adhesvelybonded jonts or the shear strength of the adhesves can drectly be evaluated To determne the characterstc and desgn values of the maxmum shear strength results, the Bayesan method was appled. Once these values have been determned, the correspondng partal factors can be estmated. Ths procedure was also followed by Van Straalen (2001). The characterstc and desgn values k, can be determned by Bayesan method usng the followng equatons: k, k t d, d t 1 n ; (3) n whch: and are the mean value and the standard devaton of the maxmum shear strength of the adhesve. t, k and t, d dstrbuton to be used for estmatng the characterstc In the absence of other nformaton, the characterstc value s assumed to be at the target probablty of 0.05 the target probablty of 0.1% (Van Straalen 2001). The degree of freedom ( ) equals to n n s the number of tests or the sample sze. desred degree of freedom ( ) and the probablty target can be determned by the use of Excel software or can be found n the lteratures. The characterstc and desgn values k, at each temperature for AC0.35, AC0.65, EP0.35, and EP n 3.1. Determnaton of the partal factor The partal factor ( ) of the shear strength of the ad mated usng Eqn (5) and ts values are 1.08, 1.29, 1.08, and 1.13 for AC0.35, AC0.65, EP0.35, and EP0.65, respectvely. k. (5) d

4 Journal of Cvl Engneerng and Management, 2016, 22(5): Table 1. Characterstc and desgn values k, at each temperature resulted from drect evaluaton method Temperature [ o C] AC0.35 AC0.65 EP0.35 EP Determnaton of the converson factor In EN 1990:2002 (2002), the converson factor that takes the addtonal effects such as the mosture, the tempera k. (6) d In our case, the desgn value at any temperature can be related to the desgn value at room temperature by the converson factor ; therefore, can be estmated by dvdng the desgn value at each temperature by ts value at the room temperature. To cover all temperature range studed, the mnmum value has to be consdered. The values of AC0.65, EP0.35, and EP0.65 respectvely. 4. Evaluaton of the maxmum shear strength values on the bass of an analyss model The above procedures were appled on small populatons. Each one was consdered at dfferent temperature. The sample sze, therefore, was of seven values as maxmum (when no outlers detected). The rsk of usng such small sample sze s when there s an extreme value relatve to the rest values of the data set whch s not detected by the checkng of outlers tests. Hence, the parameters of the normal dstrbuton used wll be affected by these extreme values. Ths wll normally result n gettng a hgh scatterband that wll surely affect the characterstc and desgn values as well as the partal factor accordngly calculated. To avod havng such problem, usng larger populatons that contan same features of the samples s recommended. In our case, four larger populatons were created by mergng the data obtaned for all temperatures. Each of them has the results of the specmens of the same adhesve materal and the same thckness of the adhesve layer. These populatons are: AC0.35(M), AC0.65(M), EP0.35(M), and EP0.65(M). The letter (M) ndcates the dependng on the number of the outlers excluded. One of the methods to evaluate the results of the tests s the analyss modelbased method that can be mplemented by the procedures explaned n EN 1990:2002 PhDstudy has also developed a systematc approach for relable desgn rules for bonded jonts (Van Straalen 2001). Hs approach s formulated to be used for all knds of adhesve bonded jonts under varous actons and based on current knowledge about the behavour of these jonts and on structural relablty method. The analyss models to descrbe these populatons are formulated by consderng that the maxmum shear strengths are functons of the temperature,.e. max f T ; therefore, the mean values of ( max ) at each temperature versus the correspondng temperature (T) were graphcally plotted. The predcton models were then found by a regresson analyss usng Excel software as shown n Fgure 5 (for AC0.35(M) and AC0.65(M)) and n Fgure 6 (for EP0.35(M) and EP0.65(M)). In ths work, to evaluate the test results by the proposed models, the methodology developed by Van Straalen (2001) was used. tfy the dfferences between the test values test, and the correspondng values calculated by the model model, for each data set by determnng the multplcaton factors K for all data ponts usng Eqn (7): K test, model,. (7) New data sets of the calculated multplcaton factors K wll have been obtaned and can be statstcally treated. By applyng the Bayesan method, the characterstc and desgn values of K can be determned. Table 2 lsts the characterstc and desgn values of K for each populaton. The characterstc and desgn values of the shear strength at each pont can be calculated by Eqns (8) and (9). The results are summarzed n Table 3.

5 670 H. Pasternak, S. Sahelle. Converson factors of the temperature effect on the shear strength of adhesvelybonded steel jonts Fg. 5. Shear strength vs. temperature for AC0.35(M) and AC0.65(M) Fg. 6. Shear strength vs. temperature for EP0.35(M) and EP0.65(M) Table 2. Characterstc and desgn values of Characterstc value Desgn value K AC0.35(M) AC0.65(M) EP0.35(M) EP0.65(M) K k K d Table 3. Characterstc and desgn values k, at each temperature resulted from analyss modelbased evaluaton method Temperature [ o C] AC0.35 AC0.65 EP0.35 EP K. ; (8) k, k model, K.. (9) d, d model, Determnaton of the partal and converson factors: The partal factors at room temperature that cover all temperature range studed were estmated by Eqns (5) and (6). The values of partal factors obtaned are 1.10, 1.17, 1.18, and 1.32 whle the mnmum converson factors are 0.69, 0.72, 0.76, and 0.82 for AC0.35, AC0.65, EP0.35, and EP0.65 respectvely.

6 Journal of Cvl Engneerng and Management, 2016, 22(5): Conclusons In ths paper, the nvestgatons on the temperature effect on the shear strength of two structural adhesves, used for bondng lap shear galvanzed steel jonts, were performed. Two thcknesses of the adhesve layer were used 0.35 mm and 0.65 mm. Two relabltybased methods were appled to evaluate the test results and to determne the partal factors of the lmt states as well as the converson factors of the shear strength of the adhesves. Throughout the work, t was found that the partal factors, for AC0.35, EP0.35 and EP0.65, obtaned by the predcton modelbased method are generally hgher than those gven by the drect evaluaton method, however, for AC0.65, a 10% hgher value was recorded. Both for the studed adhesves. The dfference between the converson factor values of both methods was notced to be relatvely hgh. However, the values of the predcton modelbased method seem to be more convenent and realstc than those obtaned by the drect evaluaton methmethod (drect evaluaton method) to the extreme values whch could not be detected as outlers to be excluded by the respectve outler tests; consequently, the determnaton of the desgn values was affected by the relatvely large standard devaton of the scattered small samples at some temperatures. It s also clear that the conver ly corresponded to the hghest temperature as shown for EP0.65 (Table 1) whch can also be explaned by the Among the avalable lterature, only one case (converson factor values of EP0.35) can be used for the sake of comparng the results of ths work. The factors of be n a good agreement when compared wth the conver As a concluson for all cases studed n ths work, the drect evaluaton method proposes a converson factor of the temperature effect of 0.29 whle the predcton modelbased method proposes a more convenent value whch s Acknowledgements provded by the German Academc Exchange Servces (DAAD). eferences Boyes, Adhesve bondng of stanless steel: strength and durablty UK. Cease, H.; Derwent, P.; Dehl, H.; Fast, J.; Fnley, D Measurement of mechancal propertes of three epoxy adhesves at cryogenc temperatures for CCD constructon. Ferm Natonal Accelerator Laboratory, FermlabTM 2366A, Batava. 19 p. Cupack, Y.; Pasternak, H Calbraton of desgn rules n accordance wth Eurocode usng the example of adhesve jonts, Baungeneur DIN EN 10327:2004. Contnuously hotdp coated strp and sheet of low carbon steels for cold formng. DIN Deutsches Dxon, W. J atos nvolvng extreme values. Contract versty of Oregon. EN 1990:2002. Eurocode Bass of structural desgn. European Commttee for Standardzaton. Brussels, p. EN ISO 10365:1995. Adhesves Desgnaton of man falure patterns. European Commttee for Standardzaton. Brussels, p. ISO 2394:1998. General prncples on relablty for structures. Internatonal Organzaton for Standardzaton. Geneva, epoxy adhesve at ambent temperature, Internatonal Journal of Adheson and Adhesves Osana, K..; es, M. L Temperature effect on the strength of adhesvely bonded sngle lap jonts, Engenhara Térmca based desgn rules for adhesve bonded jonts, Internatonal Journal of Adheson and Adhesves advantages and possbltes, n Proc. of the 9 th Internatonal Conference Modern Buldng Materals, Structures and Technques 10 p. jonng technque also n steel constructon, n The 5 th Internatonal Conference on ThnWalled Structures Pasternak, H.; Cupack, Y.; Dlger, K.; Hanssen, E Entwcklung enes Eurocodebaserten Bemessungskonzepts für Klebverbndungen m Stahlbau (n Anlehnung an DIN EN 1990) Fnal report, Germany. Pekarczyk, M.; Grec, Applcaton of adhesve bondng n steel and alumnum structures, Archves of Cvl Engneerng for small samples assumptons, STAT (Selected topcs n Assurance elated Technologes) Sahelle, S.; Pasternak, H Temperature effect on mechancal behavour of adhesve bonded steel jonts under short and longterm loadng, n the 13 th Conference (VSU 2013) Unted States Envronmental Protecton Agency (EPA QA) Gudance for data qualty assessment practcal methods for data analyss verson).

7 672 H. Pasternak, S. Sahelle. Converson factors of the temperature effect on the shear strength of adhesvelybonded steel jonts Van Straalen, I. J Development of desgn rules for structural adhesve bonded jonts a systematc approach: PhD thess. Netherlands Delft Unversty of Technology. Van Straalen, I. J.; Van Tooren, M. L Development of desgn rules for adhesve bonded jonts, Heron Van Straalen, I. J.; Wardener, J.; Vogelesang, L. B.; Soetens, F. draftng desgn rules, Internatonal Journal of Adheson and Adhesves dstrbuted adhesve propertes on the overall mechancal response of metallc adhesvely bonded jonts, Internatonal Journal of Adheson and Adhesves Hartmut PASTENAK. Prof. DrIng. habl., Brandenburg Unversty of Technology). He s nvolved n teachng, research and desgn of steel structures for more than 25 years. He s member n several Natonal and Internatonal Commttees (e.g. full member of the ECCS Techncal Commttee 8 Structural Stablty, member of German subcommttee DASt015 Grders wth thn webs, member of COST C25 acton Sustanablty of Constructons, member of the workng group Eurocode 3 Crane supportng structures ). He has partcpated n numerous research projects (e.g. on thnwalled members, weldng stresses and adhesve bonded jonts). Under hs supervson 10 PhD s were completed. He has more than 130 publcatons n Journals and at Conferences and s steel structures. Samer SAHELLIE. Structural Engneerng, Tshreen Unversty, Syra. Hs research nterests nclude adhesvebonded jonts n lghtweght steel constructons, strength and behavour of materals, structural modelng and analyss, buldng desgn and constructon, rehabltaton of steel and concrete structures.