STRUCTURE-PROPERTY RELATIONSHIPS OF COMPOSITE COIL SPRINGS PROCESSED BY RTM

Transcription

1 STRUTURE-PROPERTY RELATIONSHIPS OF OMPOSITE OIL SPRINGS PROESSED BY RTM Byung-Sun. Kim *, Joon-Hyung Byun *, and Byung-Ha Kim * * omposie Maerials Group, Korea Insiue of Machinery & Maerials(KIMM), P. O. Box 66, hangwon, Korea, SUMMARY: Texile echnology and resin ransfer molding (RTM) process hae been combined o manufacure composies coil springs. Preforms for coil springs were made by braiding process, in which axial yarns are locaed in he cenral region and braiding yarns coered hem circumferenially. The geomeric and process parameers ha are relaed wih he fiber orienaion and fiber olume fracion of he braided composies hae been idenified. The carbon fibers and aramid fibers were uilized for fabricaing he spring preforms wih he wire diameer <5mm. For he RTM process, braided preform was wrapped ino he fie-piece mold caiy designed. The resin was injeced ino he mold a 120 o for cure. In order o assess he mechanical performance of he composies spring, an analyic mehod for predicing he spring consan has been deeloped. The model predicion compared faorably wih experimenal resuls. KEYWORDS: omposie oil Spring, Braided Preform, RTM, Spring onsan. INTRODUTION omposies coil springs hae many adanages oer he conenional ones: hey are ligh and hae beer performance in faigue and dynamic response. In addiion o he srucural benefis, composie springs can be designed o render he opimal mechanical properies by ailoring he orienaions and conen of reinforcing fibers. They hae been ypically manufacured by he filamen winding (F/W) process. Alhough his process can be cos-effecie, he reinforcing fibers maybe limied in he longiudinal direcion for a small composie springs, which resuls in low srengh and siffness in he ranserse direcion. Also, due o he limiaion of reinforcing direcion, opimized srucure is hardly achieed in he F/W process. In his sudy, he exile echnology was used o produce preforms and resin ransfer molding (RTM) process was used o manufacure composies coil springs. Preforms for coil springs were made by braiding process, in which axial yarns are locaed in he cenral region and braidng yarns coered hem circumferenially. The geomeric and process parameers ha are relaed wih he fiber orienaion and V f of he braided composies hae been idenified. They include he number and he bundle size of respecie axis and braiding yarns, and he ake-up lengh of he preform. The carbon fibers and aramid fibers were used, respeciely, o braid he

2 preforms wih he wire diameer <5mm. For he RTM process, he fie-piece mold was designed, and braided preform was wrapped in he mold caiy. The resin was injeced ino he closed mold, and cured in he oen a 120 o. In order o assess he mechanical performance of he composies spring, an analyic mehod o predic he spring consan has been deeloped. The analysis includes he geomeric model of a uni cell, he coordinae ransformaion of he crimp yarns, and olumeric aeraging of siffness and compliance consans of he consiuen maerials. The model predicion compared faorably wih experimenal resuls. This analyic model is ery effecie for designing he composies coil spring which requires specific performance. Molds for oil Spring Fie - piece mold is designed o fabricae he composie coil spring (Fig. 1) whose wire hickness is less han 5mm by RTM process. Fie-piece mold(fig. 2a, b) is composed of exernal & inernal molds, resin injecion por, air en and ejec pin. To allow easier resin injecion and air flow, he resin injecion por was locaed a upper end of he spring. Inerior of he 5 piece mold was crome coaed for beer surface finish. Exernal Mold Resin Injecion Por 95 Ejec Pin Inernal Mold Air Ven Hole 285 o Fig. 1 omposie oil Fig. 2a. Schemaic drawing Fig. 2b. 5-piece mold for Spring (Uni : mm) of 5-piece mold composie coil spring PREFORM Braiding machine (Fig. 3) was used o make he carbon and kelar preforms(fig. 4) for composie coil spring, respeciely[1]. For carbon preforms, braiding and axial yarns used were Nippon arbon/300f. For kelar preforms, ke49 were used. The braiding condiions are summarized in Table 1. Table 1. Braiding ondiions Specimen Lengh, L Dia, D Angle, Braid Yarn Axial Yarn arbon ea 16 ea Kelar

3 PROESSING In he RTM process(fig. 5), he epoxy sysem used were LY564 and HY2954(iba-Gygi). Resin injecion pressure was 20 Psi and acuum was applied when oen cured for 2hrs a 120 o. The fabricaed springs are shown in Fig. 6. TESTING OF THE SPRINGS In order o obain he spring consans, he deflecion ess were carried ou as shown in Fig. 7. Fig. 3 Preforms being made by Fig. 4 arbon and Kelar Fig. 5. Processing of omposie Braiding M/ preforms oil Spring Fig. 6 Fabricaed omposie oil Springs(uni:mm) Fig. 7. Deflecion Tes Se Up GEOMETRI MODEL A 2-D braided fabric consiss of wo ses of yarns passing oer and under each oher. In addiion o he braiding yarns, axial yarns are ofen insered for dimensional sabiliy and improed mechanical properies in he longiudinal direcion. [2] X w X' θ b uni cell X' h/2cosθ b α h braider yarn Z φ d d Y r φ Fig. 8 Geomery of he braid. Fig. 9 Schemaic of he yarn secions and he yarn crimp.

4 As shown in Fig. 8, he braid geomery is characerized by he braider yarn angle,, and he pich lengh, h. Braids hae he smalles repeaing srucure ermed as a uni cell. Fig. 9 shows he schemaic of he yarn secions and he yarn crimp. To describe he way geomery of he crossing yarn, he axial yarn cross secion is assumed o be lenicular shape. The parameers gien in he braiding process or measured from he microsrucure are uilized o deermine he yarn crimp angle and he braider yarn lengh. Thus, olumes of axial yarns (V a ) and braider yarns (V b ) are deermined from he yarn secional area, he yarn lengh, and he number of axial and braider yarns wihin he uni cell. ELASTI MODEL The mechanical properies of braided exile composies hae been prediced based upon he fiber and marix properies and he fiber archiecures resuled from he geomeric model. Since he principal maerial direcion of spaially locaed yarns do no coincide wih he coordinae direcion of our ineres, ransformaion of he sress-srain relaions from one coordinae sysem o anoher is obained. [ S ] [ ] [ ][ ] I = T S T The effecie compliance marix of a crimp yarn can be obained by aeraging he ransformed compliance marix of he infiniesimal yarn segmen hrough he crimp angle. S ij 1 = φ φ 0 S I ijdφ I In he uni cell, layers of he braider yarns in q orienaions, axial yarns, and marix maerials are arranged in parallel in he longiudinal direcion. When load is applied in he x-direcion of he composies, each layer can be assumed o be in he sae of consan srain. Thus, he siffness of each layer is aeraged based upon he olume o ge he effecie siffness of he composies. a a bp b bm b m = ij + ij + ij + ij ( y) where a, bp, and bm are he inered siffness of axial and braider yarns of q orienaions, respeciely. m is he [6x6] siffness marix of he marix maerial. Then, he siffness is inered o he compliance, S ij, which finally resuls in he following engineering consans of he braided exile composies: for example, E xx = 1/S, G = 1/S 11 xy 66

5 ANALYSIS OF SPRING DEFLETION For he compression spring of closely coiled ype he spring consan can be expressed as he following equaion. (G: shear modulus, d: wire diameer, R: spring radius, n: number of coiling, a: coil angle) In he case of orhoropic maerials, he shear modulus has he following form [3] κ = F 4 δ = Gd R n α, G = cos G G G + G ORRELATION The geomeric inpu daa for he model predicions were obained from he phoomicrographs of he sample secion. Fig. 10 shows he cross-secion of he composies spring. I can be seen he braider yarns surround he axial yarns locaed in he cenral par of he composies. Table 1 summarizes he geomeric inpu daa and he comparison beween he model predicions and he es resuls for 4 ypes of composies springs. Relaiely good agreemen beween he model predicions and he es resuls can be obsered. Alhough he experimenal daa was no enough o suppor he predicions due o he limied es resuls, he mehodology proposed in his paper can be effeciely uilized in designing he braided composies springs. Fig. 10 omposies spring secion. ONLUSION Table 2. The geomeric inpu daa, he model predicions, and he es resuls Geomeric Parameer k (N/m) ode d f k V f Pred. Exp K-700 Noe) V f is he es resuls

6 (1) The carbon and kelar fibers were braided o a preform of circular cross-secions, respeciely. The RTM process was applied on preforms o manufacure small composie coil springs. (2) The geomeric model and he elasic model of 2-D braided exile composies hae been proposed o predic he spring consans of composies. From he geomeric model he crimp yarn angle, he yarn lengh, and he fiber olume fracion were obained. Using he geomeric parameers, 3-D engineering consans hae been deermined from he elasic model, which uilizes he coordinae ransformaion and he aeraging of siffness and compliance consans based upon he olume of each reinforcemen and marix maerial. (3) The prediced spring consans agreed quie well wih he esed alues. REFERENES 1. J. H. Byun e al, "Deelopmen of Braided Pulrusion Process and Srucure-Propery Relaionships for Tubular omposies," ATA Meallurgica SINIA, ISSN , V 9 No 6, P , Dec, '96 2. hou, T-W. Microsrucural design of fibrous composies, ambridge Uniersiy Press, ambridge, Lekhniskii, S. G., Theory of Elasiciy of an Anisoropic Elasic Body, Holden-Day, San Francisco, 1963.