ECO-CORE AND ITS PERFORMANCE IN SANDWICH STRUCTURAL APPLICATION

Transcription

1 ECO-CORE AND ITS ERFORMANCE IN SANDWICH STRUCTURAL ALICATION K. N. Shivakumar and H. Chen Center or Composite Materials Researh Department o Mehanial and Chemial Engineering North Carolina A&T State University Greensboro, NC 7411, USA [email protected] SUMMARY Eo-Core is a ire resistant strutural ore material or omposite sandwih strutures. Strutural perormane o Eo-Core glass/vinyl ester sandwih beams was evaluated or transverse and edgewise loadings. Core shear, ore tension and ae sheet mirobukling were the ailure modes and they an be predited by the existing analytial models. Keywords: Sandwih struture, strutural perormane, Eo-Core, ire resistant, ailure INTRODUCTION Fire has been a major problem or both mobile (mass transit and marine) and immobile (buildings and ivil inrastruture) strutures. With the wide use o polymer omposites in strutural appliations, potential or ire hazards has inreased. Norwegian omposite minesweeper ire [1] demonstrated the vulnerability o omposite ships against ire. September 11, 001 twin tower ire and ollapse have demonstrated the vulnerability o our unproteted steel skysrapers. Although ire annot be ompletely eliminated, it an be mitigated to redue the loss o lie and property. Extensive researh is being onduted to improve ire saety o omposite materials or various appliations. Some o these results are summarized by Sorathia and erez [] or naval appliations. Shivakumar and his o-researhers [3-5] have proposed a method o ire mitigation through bloking/ontaining the ire by using a ire resistant ore material in sandwih strutures. They developed a material alled Eo-Core rom a waste produt rom oal burnt thermal power plants and high har binder. It is well known that sandwih strutures are highly eiient in arrying lexural loads. Sandwih strutures with omposite ae sheet and VC or balsa ore materials are widely used in marine appliations. US and European navies are using or onsidering to use omposite sandwih or building mine sweepers, oastal protetion ships, destroyers, et. In iber reinored plastis (FR) 50% weight is resin and its har yield is less than 15%, resulting in 43% weight o volatiles released in the ase o ire, whih makes it highly suseptible to ire. Like resin, VC ores oer no resistane to ire. On the other hand balsa tolerates or inhibits the growth o ire but it suers rom nonuniorm density (depends on the soure and lie time seasonal variation), large moisture ingression and rotting. Reent studies [6] have shown that about 800% density hange 1

2 and 35% volume hange in balsa are expeted when exposed to sea water, whih makes it suseptible or sel destrution. The novelty o the Eo-Core [3-5] is that it uses little binder (high har yield ontent) and large volume o erami hollow mirobubbles (Cenosphere) together press molded to required size and shape. The Eo-Core has superior mehanial properties, exellent ire resistant properties (passed Mil Spe 031 up to 75 kw/m ) and is non toxi [3-5]. The ous o this researh is a test evaluation o strutural perormane o Eo-Core sandwih panel and its omparison with the preditive models presented in reerene 7. This paper disusses design o test speimens or various types o ailure, namely, shear, lexure and edgewise ompression; veriiation o the design by experiments and identiiation o ailure modes. NOMENCLATURE AND MATERIAL ROERTIES Sandwih beam onsists o a lightweight ore material whih is overed by ae sheets on both sides. Figure 1 is the shemati o the sandwih beam ross setion. The nomenlature used in the design o test speimens is deined: t is the ore thikness, t is the ae sheet thikness and d is the sandwih thikness whih is the distane between two entroids o the ae sheets (d=t +t ). The width o the panel is represented by b. Compression strength, elasti modulus, shear strength and shear modulus o the ore are σ, E τ and G, respetively. Strength and elasti modulus o the ae sheet are σ and E, respetively. The nomenlature used is the same as in the standard text books on sandwih struture [8, 9]. The ae sheet onsidered is FGI 1854 glass/derakane 510A-40 vinyl ester omposite and the ore material is Eo-Core with a nominal density o 0.5 g/. Table 1 lists the mehanial properties o the ae sheet [10] and the ore material [3]. Fae sheet, σ, E d = t + t d Core, σ, E, τ, G t t t D = D + D0 + D 1 3 D = E t 1 E t d D = 0 3 Et D = 1 Figure 1 Shemati diagram o the sandwih ross setion and nomenlature. The test speimens are designed to simulate typial ailures o ore shear, ae yielding, ae wrinkling, shear bukling, ae mirobukling and general bukling that our in sandwih panels as in reerenes 8 and 9. These ailures an be simulated by short beam shear, lexure (4-point bend), and edgewise ompression tests. Test speimens were designed using the material properties listed in Table 1 and equations given in reerene 7.

3 Table 1 Material properties o ae sheet and ore materials. Tensile Compressive Shear Material Modulus Strength Modulus Strength Modulus Strength Ga Ma Ga Ma Ga Ma Fae sheet Eo-Core FAILURE LOAD EXRESSIONS FOR DIFFERENT TEST CONDITIONS Relevant equations to predit ailure loads or the three dierent tests and various ailure onditions are summarized here and are used to ompare with the experiment. All these equations are taken rom reerene 7. Short Beam Shear Test Speimen The short beam shear test was designed to measure shear strength o the ore material. Four-point and 3-point bend loaded testing are ommonly used. Here we hose 4-point bend with quarter point loading. Figure shows the shemati o the short beam shear test. L is the speimen length, S is the span o the support and S/ is the load span. The typial ailure mode in short beam shear test is ore shear, a 45 raking starting rom the mid-thikness as illustrated in Fig.. Critial load at ailure an be related to ore shear strength and test speimen geometry and the equation is shown in Fig.. S/ / / = τ db (1) / S L / Figure Short beam shear test oniguration, ailure mode and ailure load equation. Four-oint Flexure Test Speimen Figure 3(a) shows the shemati o 4-point lexure test with third point loading. Under the lexure loading, sandwih beams are observed to ail by three modes, namely, ae yielding, ore shear and ae wrinkling depending on span to depth ratio and ae sheet- 3

4 ore thikness ratio, and properties o ae sheet and ore material. Figure 3(b) illustrates the potential ailure modes and the ailure loads that an be alulated rom speimen geometry and material properties (Equations -4). S/3 / / / S / L (a) Four-point lexure test oniguration Fae yielding Fae yielding 1D0σ b SdE = () Core shear Fae wrinkling Core shear Fae wrinkling = τ db (3) ( E E G ) 6D0 = b (4) SdE 1 3 Fae wrinkling (b) otential ailure modes and ritial ailure load equations Figure 3 Four-point lexure test oniguration and potential ailure modes. Edgewise Compression Test Speimen The test speimen and the possible ailure modes under edgewise ompression are shown in Fig. 4. The ailure modes are general bukling, shear bukling, ae sheet mirobukling (whih is the same as ae sheet yielding), ae wrinkling and ae dimpling. The assoiated ailure loads or all the ases are summarized in Fig. 4. Based on Equations 1 through 9, ailure loads an be alulated or dierent speimen geometry and material properties. Design plots o ailure load vs. speimen length and the omparison with test results are presented in Testing, Results and Disussion setion. 4

5 L General bukling Shear bukling Fae Fae mirobukling wrinkling Fae dimpling (a) Test oniguration (b) ossible ailure modes General Bukling: Shear bukling: bukling = L π D b π Dt + Gd Gd s = b (6) t (5) Fae sheet mirobukling: r = t σ b (7) Wrinkling: t ( E E G ) 3 b w 1 = (8) Wrinkling [11, 1]: w = t t 3 t E E ( 1 ν ν ) b (9) Figure 4 Edgewise ompression test oniguration and possible ailure modes. FABRICATION OF SANDWICH ANEL AND SECIMEN The sandwih ae sheets were unidiretional 3-ply FGI 1854 glass/derakane 510A-40 vinyl ester omposite abriated by Vauum Assisted Resin Transer Media (VARTM) proess as explained in reerene 13. The thikness o the ae sheet ahieved was 1.4 mm. Eo-Core panel was adhesively bonded to the ae sheets. The adhesive used was Lotite Hysol E-90FL epoxy adhesive. It is a toughened and medium visosity adhesive with tensile strength o 13 Ma, lap shear strength o 5.6 Ma and elongation o 64%. Vauum bag was used or bonding and the pressure applied was 0 inhg (0.068 Ma). The sandwih panel was kept in vauum bag at room temperature or 8 hours or uring o the adhesive. 5

6 TESTING, RESULTS AND DISSCUSION Short beam shear, 4-point lexure and edgewise ompression tests were onduted on sandwih speimens made o FGI 1854 glass/derakane 510A-40 ae sheet and Eo- Core. The thiknesses o the ae sheet and the Eo-Core were 1.4 mm and 5.4 mm, respetively. The tests were onduted in an Instron 40 testing mahine with a 10 kip load ell. During the tests, load and displaement were reorded ontinuously by data aquisition system and the ailures o the speimens were monitored visually, by digital amorder, and a high speed photography or some ases. Short Beam Shear Test The short beam shear test was onduted in aordane with ASTM standard C The speimen length was 15 mm and width was 51 mm. Quarter point loading was applied. The supporting span length was 10 mm and the upper loading span was 61 mm. The speimens were loaded at a ross-head speed o 0.5 mm/min. The test was repeated or three speimens. Eo-Core sandwih speimens showed brittle rature at ailure. Based on Eq. 1, predited shear ailure load is 1,689 N while the experimental average ailure load is 1,064 N with a standard deviation o 916 N. The predition is about 5% larger than the test data. This dierene is within the experimental satter that is expeted or these materials. Figure 5 illustrates the ailure mode o Eo-Core sandwih speimens. The Eo-Core speimens demonstrated typial shear ailure showing a 45 rak in the ore, between the top load point and the bottom support at one end. This was ollowed by immediate rak at the other end. Figure 5 Shear ailure modes o Eo-Core sandwih beam. Four-oint Flexure Test The 4-point lexure test was onduted in aordane with ASTM standard C The speimen width was 51 mm. Third point loading was applied. The speimens were tested or ive dierent span (S) lengths, namely, 17mm, 191mm, 54mm, 394mm and 508mm. The speimens were loaded at a ross-head speed o 1.7 mm/min and the 6

7 load-displaement was ontinuously reorded. The test was repeated or three speimens. None o the speimens ailed by the possible modes shown in Fig. 3. The ailure was initiated by vertial raks at the bottom o the ore within the two upper loading points as ore tension ailure (see Fig. 6(a)). Fig. 6(b) shows the ultimate ailure whih is the shear rak ollowed by ae sheet-ore separation. Core tension ailure was not reported, heretoore, as a ailure mode in sandwih panels. Beause the Eo-Core is relatively brittle and is stronger in ompression than in tension the ore tension ailure ourred. 1 3 (a) Initial raks in the ore 4 (b) Ultimate ailure Figure 6 Crak initiation, propagation and ultimate ailure o Eo-Core sandwih panel under 4-pt lexure loading. The ore tension ailure load an be derived using the beam theory and is given by: 1Dσ b St E t = (10) t where σ t and E t are the tensile strength and modulus, respetively, o the ore material. redited ailure loads versus span/depth ratio or all possible ailure modes are plotted in Fig. 7 along with the experimental data. The test data agrees very well with the predited ore tension ailure loads. Thereore, ore tension is the major ailure mode or Eo-Core or brittle ore sandwih omposite panels. The ailure is tensile or S/d>4 and is ore shear or S/d 4. Edgewise Compression Test Edgewise ompression test was onduted in aordane with ASTM standard C The speimen ends were supported by two lateral support lamps made o retangular steel bars astened together to prevent the speimen rom slipping rom the ixture. The two ends o the speimens were mahined to be lat and parallel to eah other and perpendiular to the loading. The test was perormed at the ross-head speed o 0.5 mm/min. The speimens were tested or six dierent unsupported speimen 7

8 lengths (L) resulting in length/depth (L/d) ratios o 5, 7, 11, 13, 18 and mm thik ore was used or L/d ratios o 5 and 7 while 1.7mm thik ore was used to simulate long L/d ratios o 11, 13, 18 and 0. Eo-Core Fae Wrinkling Eq Fae Yielding Eq. Core Compression 400 r /b N/mm Core Shear Eq Core Tension Eq S/d Figure 7 Comparison o experiment and theory or 4-pt lexure loaded Eo-Core sandwih beam. The predited and experimental ailure loads are plotted versus speimen length/depth (L/d) ratio in Fig. 8. The symbols represent the experimental data while the lines represent the predited values. The experimental ailure loads ( /b) or L/d o 5, 7 and 11 were 980, 1,19, and 91N/mm, respetively. These results agreed well with the predited ae sheet ompressive mirobukling ailure load (1,015 N/mm) rom Eq. 7. The ae sheet miorbukling ailure is shown in Figs. 9(a). For L/d=5, initial and ultimate ailures were learly due to ompressive mirobukling o ae sheet whereas or L/d=7 and 11, the ailure was a ombination o ae sheet mirobukling, ae sheetore separation/debonding and bukling (See Fig. 9(b)). For L/d=13, 18 and 0, ailures were a mixture o sandwih general bukling and ae sheet separation. One speimen eah o L/d=18 and 0 showed learly general bukling o the sandwih olumn (See Fig. 9()). Based on the test results one an onlude that or L/d<7 the ailure is by ae sheet mirobukling, or 7 L/d 13 the ailure is ombination o ae sheet mirobukling, debonding and bukling, and or L/d>13 the ailure is by general bukling. 8

9 Sandwih Bukling Eq. 5 Eo-Core Wrinkling Eq. 8 r /b N/mm Wrinkling Eq. 9 Mirobukling Eq L/d Figure 8 Comparison o experiment and theory or edgewise ompression loaded Eo- Core sandwih olumn. (a) Mirobukling (b) Combination ailure () General bukling Figure 9 Edgewise ompression ailure modes o Eo-Core sandwih speimens. CONCLUDING REMARKS Eo-Core, a ire resistant ore material or sandwih omposite strutures developed under the US Navy (ONR) program, was used to study its perormane as a sandwih beam with glass/vinyl ester ae sheet. Test speimens were designed to simulate shear, lexural, and edgewise ompression loadings. Failure loads and modes were ompared with eah other and the analytial preditions. In the ase o transversely loaded (4- point) beams Eo-Core sandwih speimens ailed by ore shear or span/depth (S/d) 4 9

10 and the ailure mode hanged to ore tension or S/d>4. This is attributed to weak tensile strength o the ore material. An expression or ore tension ailure load, based 1Dσ tb on beam theory, is given by =. Under edgewise ompression, ae sheet StEt mirobukling and general bukling are the two potential ailure modes or Eo-Core sandwih omposites. For speimen length/depth ratio L/d<7 the ailure is by ae sheet mirobukling, or 7 L/d 13 the ailure is ombination o ae sheet mirobukling, debonding and bukling, and or L/d>13 the ailure is by general bukling. reditions rom the existing equations agreed well with the experiment. ACKNOWLEDGEMENTS The authors wish to thank the Oie o Naval Researh or inanial support through grants #N and #N Dr. Yapa Rajapakse was the tehnial monitor o the grants. Reerenes 1. Olson K (003) Keynote Leture, roeedings o 6 th International Conerene on Sandwih Strutures, Fort Lauderdale, FL, Marh 31-April, 003. Sorathia U, erez I (004) Improving the ire saety o omposite materials or naval appliations. SAME 004, May 16-0, Long Beah, CA 3. Shivakumar KN, Argade SD, Sadler RL et al (006) roessing and properties o a lightweight ire resistant ore material or sandwih strutures. J Ad Maters 38(1): Argade S, Shivakumar K, Sadler R et al (004) Mehanial ire resistane properties o a ore material. SAME, May 16-0, 004 Long Beah Convention Center, Long Beah, CA 5. Shivakumar KN, Sharpe M, Sorathia U (005) Modiiation o eo-ore to enhane toughness and ire resistane. SAME-005, Long Beah, CA 6. Sadler RL, Sharpe MM, Shivakumar KN (008) Water immersion o eo-ore and two other sandwih ore materials. SAME Shivakumar KN, Chen H (009) Strutural erormane o Eo-Core Sandwih anels. Speial Volume on "Major Aomplishments in Composite Materials and Sandwih Strutures - An Anthology o ONR Sponsored Researh" 8. Allen HG (1969) Analysis and design o strutural sandwih panels. ergamon ress, Oxord 9. Zenkert D (1997) An introdution to sandwih onstrution. Chameleon ress, London, UK 10. Swaminathan G, Shivakumar KN, Sharpe M (006) Material property haraterization o glass and arbon/vinyl ester omposites. Comp Si Teh 66(10): Heath WG (00) Sandwih Constrution, art : The Optimum Design o Flat Sandwih anels. Airrat Engng 33: Gdoutos EE, Daniel IM, Wang KA (00) Compression aing wrinkling o omposite sandwih strutures. Meh Mater 35: Sadler RL, Shivakumar KN, Sharpe MM (00) Interlaminar rature properties o split angle-ply omposites. SAME 00, Long Beah, CA 10