Thermomechanical Response of a Thin Sandwich Composite Structure

Transcription

1 Thermomechanical Response of a Thin Sandwich Composie Srucure Horaiu Teodorescu-Draghicescu, Sorin Vlase, Dana Luca Mooc, and Anghel Chiru Absrac The paper presens a siffness evaluaion as well as hermal response of a sandwich srucure wih hin nonwoven polyeser ma as core. The srucure presens dissimilar skins from which one is a glass fabric reinforced polyeser. Tensile and hree-poin bend ess have been carried ou o deermine he mos imporan feaures of his srucure. The hermal expansions have been measured using a DIL 4 PC dilaomeer from NETZSCH, on boh glass fabric reinforced polyeser skin and for he whole srucure. The coefficiens of hermal expansion have been deermined only for he srucure s upper skin. For each sample, wo successive heaing sages in order o size he influence of he hermal cycling, and emperaure inerval from C o 5 C, a a heaing rae of K/min have been used. To eliminae he sysem errors, he dilaomeer has been calibraed by measuring a sandard SiO specimen under idenical condiions. The experimenal researches accomplished on specimens show a welve imes increase of he flexural rigidiy agains he upper skin one. Index Terms Core, sandwich, siffness, hermal expansion. I. INTRODUCTION In general, a sandwich srucure is manufacured of hree layers: wo cover layers called skins, ha form he carrying srucure, layers composed of siff and resisan maerial, and an inermediae layer named core, which has he main purpose o susain he skins and o give siffness o whole srucure [] [6]. This siffness is obained acually hrough hickening he composie srucure wih a low-densiy core maerial. This leads o a subsanial increase of flexural rigidiy of he srucure, on he whole, wihou a significan increasing in is enire weigh [7] []. Sandwich srucures are more and more used in various applicaions due o heir high siffness a bending. Nowadays, here are a grea variey of cores such as rigid foams, hexagonal srucures made from hermoplasics, meallic and non-meallic maerials, expandable and fireproof maerials, balsa wood, ec. In general, composie laminaes are formed by hin layers called laminae. These laminaes presen a quie Manuscrip received March 5,. This work was suppored by he Deparmen of Auomoives and Engines wihin Transilvania Universiy of Brasov, Romania. H. Teodorescu-Draghicescu is wih he Deparmen of Mechanics wihin Transilvania Universiy of Brasov, Romania (corresponding auhor o provide phone: ; fax: ; draghicescu.eodorescu@)unibv.ro). S. Vlase, is wih he Deparmen of Mechanics wihin Transilvania Universiy of Brasov, Romania ( svlase@unibv.ro). D.L. Mooc, is wih he Deparmen of Precision Mechanics wihin Transilvania Universiy of Brasov, Romania ( danaluca@unibv.ro). A. Chiru, is wih he Deparmen of Auomoives and Engines wihin Transilvania Universiy of Brasov, Romania ( achiru@unibv.ro). low siffness and flexural rigidiy. A soluion could be heir siffening using ribs. However, here are consrucive siuaions when hese ribs can no be used. Anoher soluion could be he increase of layers number ha composes he srucure bu his leads o he disadvanage of he increase of resin and reinforcemen consumpion wih economic and environmenal consequences. A beer and common soluion is o use a hin nonwoven polyeser ma as core. II. THE STRUCTURE To increase he siffness of common fibre-reinforced composie laminaes, and o avoid he previously presened disadvanages, he following srucure is presened: layer RT5 glass roving fabric; layers RT8 glass roving fabric; layer CSM45 chopped srand ma; layer nonwoven polyeser ma as core; layer CSM45 chopped srand ma; A usually used gelcoa layer. I can be noiced ha his srucure presens dissimilar skins. The core presens he mos imporan influence in he overall srucure s siffness and flexural rigidiy. The core maerial is a random oriened nonconinuous nonwoven polyeser ma conaining microspheres ha preven excessive resin consumpion. The mos imporan feaures of he whole srucure using his kind of core are: Siffness increase; Weigh saving; Resin and reinforcemen saving; Fas build of he srucure s hickness; Superior surface finish. The nonwoven polyeser ma is sof, presen excellen resin impregnaion and high drapeabiliy when i is we and herefore is suiable for complex shapes. I is mos ofen applied agains he gelcoa o creae a superior surface finish for insance on hull sides. The applying of he nonwoven polyeser ma agains he gelcoa layer is more imporan when dark gelcoas are used, o preven he appearance of he glass fibres reinforcemen. This maerial has a good compaibiliy wih he polyeser, vinyleser and epoxy resins and is suiable for hand lay-up and spray-up processes. Oher processes like vacuum bag moulding can be used also. For insance, an applicaion of his kind of srucure is an underground large spherical cap sheler showed in Fig. and formed by welve curved shells bonded ogeher. To wihsand he soil weigh, he wall srucure presen a variable hickness.this kind of srucure can be used in oudoor applicaions also.

2 d d + d + Fig.. Large underground spherical cap sheler made from hin sandwich composie srucures III. STRUCTURE S FLEXURAL RIGIDITY According o he ordinary beam heory, he flexural rigidiy, here denoed R, of a beam is he produc beween Young modulus of elasiciy E and he momen of ineria I ha depends on srucure s cross-secion. The flexural rigidiy of an open sandwich beam (Fig. ) assumed o have hin skins of equal hickness represens he sum beween he flexural rigidiies of he skins and core deermined abou he cenroidal axis of he whole cross secion []: b b d b c R = Es + Es + Ec, () 6 where E s and E c represen he Young moduli of elasiciy for skins and core respecively. If he skins presen differen maerials and unequal hickness, like our srucure wih dissimilar skins (Fig. ) and aking ino consideraion ha he local flexural rigidiies for he skins can no be negleced, which means ha []: d > 5.77, () he sandwich flexural rigidiy can be wrien as []: R ( E + E ) b d Es Es b = +. () s s ( Es + Es ) Fig.. Cross secion of an open sandwich wih equal skins b c d Fig.. Dimensions in a cross secion of an open sandwich beam wih dissimilar skins Considering he beam as a wide one, he auhors propose ha he srucure s flexural rigidiy can be compued as follows: R = b d E E s s [ E ( ) + E ( )] + υ υ s s b E s Es + + υs υ s s s, (4) where he suffixes and refer o he upper and lower skins respecively, b represen he widh of he beam cross secion, d is he disance beween cenrelines of opposie skins, is he skin hickness, c is he core hickness, υ s and υ s represen he upper respecive he lower skin Poisson raio. In case ha we consider he srucure as a sandwich panel suppored on wo sides, his panel can be seen as a wide open beam. Condiion () remains he same bu in flexural rigidiy analysis, because each skin is considered as a hin plae, he E raio beween sress and srain is υ []. IV. EXPERIMENTAL APPROACH Tensile and hree-poin bend es have been used o deermine he mos imporan feaures of his srucure. Twelve specimens have been cu from a sandwich panel and subjeced o bending unil break occurs. Some specimens characerisics are presened in able. The es feaures are presened in able. Table. Specimens feaures subjeced o bending Feaures Average dimensions Widh, b (mm) 5 Lengh (mm) 5 Sandwich hickness (mm) 8.7 Core hickness, c (mm) 4 Cross-secion area (mm ) 4.5 Upper skin s hickness, (mm). Lower skin s hickness, (mm). Disance, d (mm) 6.7 b

3 Table. Three-poin bending es characerisics Tes feaures Value Tes speed (mm/min) 4 Span (mm) The ess have been carried ou on a LR5K (5 kn maximum load) as well as on a Texure Analyser ( kn maximum load) esing machines, produced by Lloyd Insrumens. Sresses and srains have been measured applying srain gauges on a srucure s curved shell. The following feaures have been deermined using he sofware NEXYGEN Plus: Siffness; Young modulus of bending; Flexural rigidiy; Load a maximum load; bending sress a max. load; Exension a maximum load; bending srain a maximum load; Load a maximum exension; bending sress a max. exension; Exension a maximum exension; bending srain a max. exension; Load a minimum load; bending sress a min. load; Exension a minimum load; bending srain a minimum load; Load a minimum exension; bending sress a minimum exension; Exension a minimum exension; bending srain a minimum exension; Load a break; bending sress a break; Exension a break; bending srain a break. Regarding he hermal approach of his srucure, he expansions of he upper skin as well as for he whole sandwich srucure have been measured. Coefficiens of hermal expansion and he alpha feaure for he upper skin have been also deermined. The coefficiens of hermal expansion have been measured using a DIL 4 PC dilaomeer from NETZSCH (Germany), on boh glass fabric reinforced polyeser skin and he whole srucure. For each sample, wo successive heaing sages in order o size he influence of he hermal cycling, and emperaure inerval from C o 5 C, a a heaing rae of K/min ino a saic air amosphere have been used. To eliminae he sysems errors, he dilaomeer has been calibraed by measuring a sandard SiO specimen under idenical condiions. V. RESULTS Regarding he ensile ess, he mos imporan feaures of he sandwich composie srucure (e.g., Young modulus, ensile srengh and load a break) are presened saisically in form of maximum, minimum and mean values as well as he sandard deviaion (Figs. 4 6). The resuls for he load a break are more scaered han hose for Young modulus and ensile srengh showing ha he sandard deviaion is quie high. In case of he hree-poin bend ess, he mos imporan feaures of he sandwich composie srucure (e.g., Young modulus of bending, siffness, flexural rigidiy and load a break) are presened in Figs. 7. The resuls coefficiens of variance beween boh ess are presened in Figs.. Young modulus (MPa) Sandard deviaion Fig. 4. Tensile es. Saisic resuls for he Young modulus Tensile srengh (MPa) Sandard deviaion Fig. 5. Tensile es. Saisic resuls for he ensile srengh Load a break (kn) Sandard deviaion Fig. 6. Tensile es. Saisic resuls for he load a break

4 5 Young modulus of bending (MPa) Sandard deviaion Load a break (kn) Sandard deviaion Fig. 7. Three-poin bend es. Saisic resuls for he Young modulus of bending Fig.. Three-poin bend es. Saisic resuls for he load a break Flexural rigidiy (N m ) 4 Coefficien of variance (%) 5 4 Young modulus Siffness Load a break Tensile srengh Sandard deviaion Fig. 8. Three-poin bend es. Saisic resuls for he flexural rigidiy Fig.. Tensile ess. Coefficiens of variance of he mos imporan feaures of he sandwich srucure Siffness (N/m) Sandard deviaion Fig. 9. Three-poin bend es. Saisic resuls for siffness Coefficien of variance (%) 5 4 Young modulus of bending Siffness Load a break Flexural rigidiy Fig.. Three-poin bend ess. Coefficiens of variance of he mos imporan feaures of he sandwich srucure

5 Curves of hermal expansion deermined experimenally for wo heaing processes in case of he sudied sandwich srucure as well as for he upper skin are presened in Figs. and 4. In Fig., he negaive hermal expansion in he firs heaing process is due o he beginning of curing in he upper skin srucure. Regarding Fig. 4, in he second heaing sage, he significan peak is due o he high shrinkage ha ook place in he sandwich srucure. In Fig. 5, following hermal feaures have been deermined on he srucure s upper skin in he firs heaing process: The hermal expansion (coninuous line); The coefficien of hermal expansion (also known as echnical alpha dashed line); The alpha feaure (doed line). Fig.. Upper skin hermal expansions deermined in wo heaing processes Fig. 4. Sandwich srucure s hermal expansions deermined in wo heaing processes

6 Fig. 5. Srucure s upper skin hermal expansion, alpha feaure and coefficien of hermal expansion deermined in firs heaing process VI. CONCLUSION The sandwich srucure wih hin nonwoven polyeser ma as core presens an excellen bond beween skins and core. This has been noiced during he hree-poin bend ess. The sandwich srucure s flexural rigidiy deermined experimenally is welve imes graer han he upper skin s one (Fig. 6). The % difference in srucure s flexural rigidiy deermined heoreically and he experimenal approach can be reduced by a beer esimaion of he upper and lower skin s Poisson raios. Flexural rigidiy (N m ) 4. Upper skin.5 Core. Lower skin.85 Sandwich.7 REFERENCES [] D. Zenker, The Handbook of Sandwich Consrucion. Engineering Maerials Advisory Services Ld., 997. [] D.B. Miracle, R.L. Donaldson, ASM Handbook Volume : Composies. ASM Inernaional,. [] I.M. Daniel, O. Ishai, Engineering of Composie Maerials. Oxford Universiy Press, nd ed., 5. [4] J.R. Vinson, The Behavior of Sandwich Srucures of Isoropic and Composie Maerials. CRC Press, s ed., 999. [5] A. Vaurin, Mechanics of Sandwich Srucures. Springer, s ed., 998. [6] J.R. Vinson, Plae and Panel Srucures of Isoropic, Composie and Piezoelecric Maerials, Including Sandwich Consrucion. Springer, s ed., 5. [7] L.P. Kollar, G.S. Springer, Mechanics of Composie Srucures. Cambridge Universiy Press,. [8] L.C. Bank, Composies for Consrucion: Srucural Design wih FRP Maerials. Wiley, 6. [9] D.G. Lee, N.P. Suh, Axiomaic Design and Fabricaion of Composie Srucures: Applicaions in Robos, Machine Tools, and Auomobiles. Oxford Universiy Press, 5. [] A.B. Srong, Fundamenals of Composies Manufacuring: Maerials, Mehods and Applicaions. Sociey of Manufacuring Engineers, nd ed., 7. [] B.F. Backman, Composie Srucures, Design, Safey and Innovaion. Elsevier Science, 5. [] L. Librescu, O. Song, Thin-Walled Composie Beams: Theory and Applicaion. Springer, s ed., 6. [] J.R. Vinson, R.L. Sierakovski, The Behavior of Srucures Composed of Composie Maerials. Springer, 8. Experimenal approach Theoreical approach Fig. 6. Sandwich srucure s flexural rigidiy evaluaion