High-performance lightweight structures with Fiber Reinforced ...

Journal of Materials Science Research; Vol. 4, No. 1; 2015 ISSN 1927-0585 E-ISSN 1927-0593 Published by Canadian Center of Science and Education

High-performance lightweight structures with Fiber Reinforced Thermoplastics and Structured Metal Thin Sheets Holger Seidlitz1, Colin Gerstenberger1 , Tomasz Osiecki1, Sylvio Simon2 & Lothar Kroll1 1

Institute of Lightweight Structures, Chemnitz University of Technology, Chemnitz, Germany

2

Professorship of Machine Tools, Brandenburg University of Technology, Senftenberg, Germany

Correspondence: Holger Seidlitz, Institute of Lightweight Structures, Chemnitz University of Technology, Chemnitz, Germany. Tel: 49-371-5313-5551. E-mail: [email protected] Received: September 30, 2014 doi:10.5539/jmsr.v4n1p28

Accepted: October 18, 2014

Online Published: November 24, 2014

URL: http://dx.doi.org/10.5539/jmsr.v4n1p28

Abstract A heavy-duty multi-material-design (MMD) can be realized through the combined use of structured sheet metal and reinforced plastics (FRP). To exploit the high lightweight potential of the various material groups within a multi-material system as efficient as possible, a material-adapted and particularly fiber adjusted joining method must be applied. The present paper primarily focuses on the manufacturing and mechanical testing of novel multi-material joints with structured sheet metals and carbon fiber reinforced thermoplastics (CFRP). For this purpose the applicability of the new Flow Drill Joining (FDJ) method, which was developed for joining of heavy-duty metal/composite hybrids, was investigated. Keywords: multi-material design, fiber reinforced plastics, structured metal thin sheets, joining 1. Introduction Load-path adapted multi material designs initiate significant savings of energy and material in automotive industry. Of all the material groups, the combined use of metal sheets and fiber-reinforced plastic (FRP) offers a very high lightweight potential (Klein, 1997; Rosato, 2005; Goede et al., 2010). With respect to this, metal components are usually applied for a uniform initiation of concentrated loads. In contrast to that FRPs are more suitable for load transmission along large distances (Schürmann, 2005; Kroll, 2009). In particular, the use of thermoplastic FRPs with high strength and stiffness properties is raised sharply within the last decade in automotive industry (Ind.Exp., 2013; Witten, 2013). Primarily this is due to comparatively short cycle times when manufacturing components and the use of inexpensive matrix systems such as polypropylene, but also higher-quality plastics such as polyamide or polyether ether ketone. Consequently they are predestined for large scale production of complex lightweight structures (Döhler et al., 2014). The use of structured metal sheets also offers a high lightweight potential for multi-material designs (Sterzing, 2005; Malikova et al., 2013). In Particular three dimensional structured sheets with embossed secondary design elements, vault structures or beadings are characterized by much higher bending stiffness-values than plane sheets. On the one hand this is caused by a higher moment of inertia and on the other by the strain hardening effect, which is evoked by the forming process (Viehweger et al., 2002). Compared to a plane sheet metal construction, the wall thickness can be consequently reduced and ultimately also the component weight. Through the combined use of thermoplastic FRPs and thin-walled structured metal sheets, weight-optimized components of a body in white can be developed for vehicle construction. For instance the described multi-material approach can contribute to increase the rigidity of a body in white structure while reducing weight. For this the required joining technology takes a central role within the manufacturing process. The joining of thin-walled structural components, made of FRP and metal sheets, is considered to be an extreme challenge. Usually - due to the strongly different thermo-mechanical behavior and the divergent material composition and morphology - primarily mechanical methods such as screwing, blind and punch riveting as well as hybrid techniques are applicable (Simon, 2005). Regarding this, high and strong local acting joining forces cause a process-related smoothing of the structured cross-section. This reduces the excellent bending stiffness of the thin-walled and structured sheet metal components considerably (Viehweger et al., 2005).

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Furthermoore the conventional placing of joining elements leads too a sectioning of load bearinng fibers at the load introductioon zone of thhe FRP. As a consequence,, high notch sstresses are innduced, whichh weaken the FRP componennt significant annd prevent thee optimal explooitation of the high anisotroppic lightweighht potential (Co olins, 2006; Seiddlitz et al., 2011). Therefore aadditional reinforcements likke cones and paatches are appllied at critical notch n stress areaas, which conseequently lead tto a strong overr-sizing of the joint, an increeased componeent weight and most of all space problems. Too fully exploit the high anisootropic materiaal behavior of tthe FRP, the jooining process must be perform med very gentlee as well as loaad path adapteed (Rotheiser, 22009). This paperr focuses on thhe qualificationn of the new fflow-drill joiniing (FDJ) concept – against the backgroun nd of thin walledd structured shheets (Seidlitzz, 2013a). The technology iss currently useed to join planne metal sheetss and thermoplastic FRPs withhin wall thicknness range of 1.5 to 4.0 mm ((Seidlitz, 2013b). In analogyy to constructio ons in nature, thee fibers can bee aligned along the force fluux lines at thee joint of the F FRP componennt. It is possib ble to achieve high joining streengths in this w way. The fiberr realigning efffect was proveen in tensile shhear tests on mixed m joints withh structured steeel and aluminuum thin sheets as well as orthhotropic carbon and glass fibber reinforced FRPs F with polyaamide 6 (PA6) matrix. Finallly the joints weere pyrolysed tto evaluate thee fiber arrangem ment at the joiint. 2. Method d 2.1 Specifi fication of Test Materials Structuredd thin sheet meetals are mosttly produced bby rolling (rollled structured sheets), emboossing (waffle-type structured sheets), or hydro h formingg (spherical-typpe structured sheets). Process-related thrrough the forming process a hhigh level of strain s hardeninng occurs. In aaddition to thee high bendingg stiffness valuues, structured d thin sheets are characterized by increased hheat absorptionn and vibrationn stiffness. In ccomparison to plane sheet metals, this phenoomenon is evooked by the laarger surface. T Therefore theyy are often ussed for heat shhielding and sound insulation.. In this papper, orthotropicc structured steeel and aluminuum thin sheetss are investigatted, which are m mainly produc ced in roll forminng processes. Table T 1 summ marizes them annd gives an ovverview referriing to mechannical propertiess, the used alloys, structure patttern, wall thicckness and the cross section height. Table 1. Prroperties of metallic joining partners material

crross secction heeight

kness thick

176 M MPa

1.88 mm

0.5 mm

110 M MPa

2.44 mm

0.5 mm

younng's moduulus

tenssile strenngth

yield strenggth

G 210 GPa

315 M MPa

73 GPa G

207 M MPa

pattern

Steel FeP04 (DIEDRIC CHS)

Aluminium m AlMg2.5 (KMT)

Thermoplaastic FRP’s in the form of so called organicc sheets are flatt semi-finishedd products, which can be prep pared by stackingg of unidirectioonal (UD) pre--impregnated llayers (prepreggs) and subsequuent pressing iin at matrix me elting temperaturre (for polyam mide 6: 220°C C). Thus therm mo formable uunidirectional and multidireectional comp posite structures with defined mechanical m prooperties can bee produced witthin short cyclle times. Furthermore, in add dition to the highh specific strenngth and stiffnness propertiess, thermoplasttic FRP’s posssess comparatively high dam mping characterisstics, good craash resistance as well as maaterial immaneent recycling pproperties. Suubject matter of o the 29

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investigations are FRP’s with orthotropic glass and carbon fiber reinforcement, embedded in a polyamide 6 (PA 6) matrix system. Table 2 gives an overview referring to the applied unidirectional (UD) prepreg systems, lamina lay-up and further specific properties. Table 2. Properties of GFRP and CFRP joining partners FRP type

prepreg

lay up

young's modulus E1 = E2

tensile strength σ1 = σ2

fiber volume ratio

thickness

GFRP

UD glass fibre/PA 6 (Ticona)

[0/90]s

16.8 GPa

368 MPa

0.6

1.2 mm

CFRP

UD carbon fibre/PA 6 (Ticona)

[(0/90)3]s

49.5 GPa

860 MPa

0.6

1.6 mm

2.2 Flow Drill Joining Concept The flow drill joining (FDJ) concept, which was developed at the Institute of Lightweight Construction at Chemnitz University of Technology, primarily serves to join comparatively thicker thermoplastic FRPs with metal sheets. The process is characterized by short processing times, high-strength joining properties appropriate for the force fluxes as well as the high degree of lightweight construction and can be utilized in various industrial sectors. The essential advantage towards typical joining technologies relates to the fact that no additional auxiliary joining elements are needed for joining the components. In order to manufacture a FDJ joint, the plastic flow properties of the metal component are exploited in a targeted way. With the aid of a rotating mandrel, a bushing is thermo-mechanically shaped from the metallic base material. While doing so, this bushing is pushed directly through the also locally plasticized thermoplastic FRP component during the shaping operation and is subsequently pulled over in a positive-locking form when the mandrel is retracted or in an additional working step (Figure 1).

Figure 1. Process scheme for manufacturing FDJ joints The rotation of the mandrel and the associated friction serve to release thermal energy which is locally introduced into the FRP component as a result of the process and thus initiates the partial melting of the thermoplastic matrix. Therefore, the continuous fibers arranged in the plastic become "moveable" and can be aligned towards the force fluxes during the shaping of the bushing (Figure 2a). The described FDJ process was carried out by an automated joining jig (Figure 2b). According to DIN EN ISO 14273, the machine serves to manufacture tensile shear test specimens with a length of 175 mm, width of 50 mm and an overlapping length of 35 mm. Joining of the comparatively thick walled GFRP and CFRP components can

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be supportted by an infrrared heating ssystem, whichh plasticizes thhe matrix compponent efficieently within a short processingg time.

aa) b) mple of a convventional Steel//GFRP (1.5mm m/3.0 mm) FD DJ joint and joining jig Figure 2. Sam FRP joints were performed. A At all specimenns, the phenom menon For the invvestigations AllMg2.5/GFRP and FeP04/CF of realigneed fibers couldd be detected viisually around the closing head-side top layyer of the FRP component. Due D to the galvanic contact corrrosion problem m that occurs whhen carbon fibbers and aluminnum get in conntact under pressence of an electtrolyte, the aluminum alloy w was not joined with CFRP. The applieed joining poinnt diameter of 5.3 mm was cchosen, since m most of the meechanical jointts in actual body in white strucctures show siimilar geometrric parameterss. Table 3 sum mmarizes the pprocessing paraameters and sh hows exemplarilly samples of the t manufacturred FDJ jointss. Table 3. M Manufactured FDJ F joints Tensile shear s specimenn

Processsing parameteers

AlM Mg2.5/GFRP (Number of specimens: 5)

Joining point diameter: 55.3 mm N Joininng force: 15 kN Flow driill speed: 45000/min Heaating time: 8 s

FeP P04/CFRP (Number of specimens: 5)

Joining point diameter: 55.3 mm N Joininng force: 20 kN Flow driill speed: 30000/min Heaating time: 14 s

2.3 Determ mination of thee Tensile Strenggth Propertiess The perforrmed specimenns of table 3 weere tested accoording to DIN E EN ISO 142733. The determinnation of the te ensile strength prroperties was carried c out witth the universaal tensile testingg machine UPM Zwick/Roell Z 250 (Figurre 3). The pre-looad was 10 N. The T span widtth was 100 mm m and the test sspeed 2 mm/m min constant.

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Figure 3. 3 Determinatioon of the tensille strength witth the UPM Zw wick/Roell Z 2250 2.4 Determ mination of thee Fiber Arranggement at the JJoint The determ mination of thhe fiber arranggement at the specimens occcurred with tthe automatic incinerator "C CEM Phoenix Standard Unit" (Figure 4). Beecause of the limited space innside the crucibles, cut outs were prepared d and incineratedd one hour at 650°C 6 (accordding to DIN EN N ISO 11667) under protecttive gas (nitroggen), to preven nt the oxidation of the carbonn fibers. Herebby the thermooplastic PA6 m matrix was coompletely detaached withoutt any residues frrom the filameents, which rem mained afterwaards loose and unbound at thhe metal compoonent.

Figgure 4. Automaatic incineratorr "CEM Phoennix Standard U Unit" and CFRP P specimen wiith crucibles 3. Results 3.1 Static T Tensile Shear Tests

Figuure 5. Damagee of failed AlM Mg2.5/GFRP annd FeP04/CFR RP FDJ joints

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In the perfformed tensile--shear test all A AlMg2.5/GFRP P and FeP04/C CFRP specimenns fail in conseequence of the high bearing strress, which occcurs at the edgee of the FRP-hholes. The robuustness of the joints is primarrily - because of o the considerabbly thinner walll thickness of the structured thin sheets - innfluenced by thhe metal compponents. The highly acting sheear stresses at the joint induuce finally bennding up the cclosing head annd consequenttly buckling of o the remaining metallic bushiing (Figure 5).. In particullar the results of the strengtth tests confirm m, that a high strength poteential is achievved by the material combinatioon FeP04/CFR RP, with a deteermined tensilee shear holdingg force of 644 N (Figure 6, bllack). In contra ast to that, the values of the AlMg2.5/GFRP A P FDJ joints arre about the half lower (Figure 6, grey). F Furthermore it is to determine,, that the stiffnness of the joint is significanttly lower. This is due to the - in comparisonn to FDJ joints with CFRP andd steel - substanntially lower yooung's moduluus of GFRP andd aluminum coomponents (cf.. Table 1 and 2 with Figure 6).

GFRP and FeP004/CFRP FDJ jjoints and assoociated exempllary Figure 6.. Tensile strenggth of structureed AlMg2.5/G force/dissplacement currves 3.2 Globall In-Plane Fiber Alignment Unlike traaditional technnologies, the F FDJ process is carried out fr from the metall side with heaat assistance. With regard to thhe FRP compoonents, this kinnd of process m management is gentler on thee material sincee fiber fracture (FF) is mostly aavoided at topp layers of thee FRP componnents. Moreoveer it is shown that, due to thhe process-ind duced realignmennt in the regionn of the joiningg zone, the reinnforcing fibers are not only apppropriate for tthe force fluxess, but also have a higher fiber volume v contennt – similar to cconstruction prrinciples in natuure (for exampple at knotholes). In the area off the joining pooint, high fiberr content is achhieved due to thhe radial dispeelling of fibers during the sha aping of the bushhing and to thee formation of the closing heead with the forming tool (Fiigure 7).

a)

b)

Figure 7. Realigned fibeers after incineeration of PA6 matrix at top layers of AlMgg2.5/GFRP (a)) and FeP04/CFRP (b)) specimens

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In comparrison to the toop plies of innner UD layerss of AlMg2.5//GFRP and FeeP04/CFRP floow drill jointss, the amount off fiber fracturess is increased aat inside lying fiber plies (Fiigure 8).

mount of FF att inner (third) ffiber layers of A AlMg2.5/GFR RP and FeP04/C CFRP FDJ join nts Figure 88. Increased am Fiber fracttures are usuallly induced by tthe formed busshing. Unlike tthe closing heaad-sided UD laayers, which will be first pierceed in the later course c of the prrocess by the rrotating mandrrel and, thus, w will be expandeed continuously y, the UD layers at the metal shheet get in conntact for the firrst time primarrily with the foormed bushingg. Once the mandrel pierces thee metal sheet, a sharp-edged bushing is form med. Howeverr, at this time, the mandrel iss situated withiin the bushing annd, therefore, does d not contribbute to a steadyy realignment oof all fibers in the joining zonne. With that, fibers f are partiallly cut by the sharp edges oof the formedd bushing until the mandrel - with increaasing load adju usted realignmennt fibers - com mpletely penetrrates the FRP ccomponent. 4. Discusssion The Flow drill joining prrocess constituutes a new typee of technologyy for joining shheet metals annd FRP compon nents on a therm moplastic basiss in a way apppropriate for thhe force fluxess. During the F FDJ process, a bushing is sh haped from the m metallic base material with the aid of a rrotating mandrrel. In this resspect, the mettallic compone ent is plasticizedd locally. Due to the contact heating, the F FRP componennt is melted loccally at the sam me time. Whille the bushing iss being shapedd by the plasticc, the then mooveable fibers are deviated aaround the impperfection in a way appropriatte for the forcee fluxes. Due tto the final deffined forming of the bushingg, the joining m members are jo oined with each other in both positive-lockinng and non poositive-lockingg forms. Moreoover, the FDJ jjoint can prov vide a defect-toleerant force intrroduction systeem for multi-m material high-pperformance sttructures, whicch can be subje ected to high loaads for applicaations in large-scale series tecchnology. The appliccability of the FDJ F process w was positively eevaluated for m multi-material jjoints with exttremely thin walled w and structuured metal sheets. It could be demonstraated that the sppecific realignnment of fiberrs – in spite of o the strongly diifferent wall thhicknesses of tthe joining parrtners – could bbe realized at tthe investigateed material pairings AlMg2.5/G GFRP and FeeP04/CFRP. T The realignmennt of fibers w was confirmedd visually at top layers and by incineratinng the polyamide 6 matrix syystem, whereinn the FRP joint could be analyyzed with resppect to fiber fra acture (FF). In paarticular these kinds of failurres mostly occuur process-relaated at inner laayers of the FR RP componentss. Through tthe load and material m adaptted design of the joints, higgh tensile shear strengths w were determined at FeP04/CFR RP specimens. Against the bbackground of tthe small standdard deviation at both experim mental series, iti can be stated that the FDJ joining process works suffficiently and sstabile. Furtheermore, as a cconsequence of the considerabbly lower requuired joining fforces against conventional techniques, thhe structures of the patterns are almost uniimpaired. Thuss, the high bendding stiffness pproperties of thhe structured m metal thin sheeets remains in a FDJ based multti-material connstruction withh thermoplasticc FRP. As part of the investiggations it was ddemonstrated that t a high beneffit in weight reeduction can bee achieved, whhen applying thhe suggested lightweight dessign approach. Acknowleedgement This workk was perform med within thee Federal Cluuster of Excellence EXC 10075 "MERGE E Technologies for Multifuncttional Lightweeight Structurees" and supported by the G German Researrch Foundationn (DFG). Fina ancial support is gratefully ackn knowledged. The publiccation costs off this article w were funded byy the German Research Fouundation/DFG (Geschäftszeiichen INST 270//219-1) and thee Chemnitz Unniversity of Tecchnology in thhe funding proggramme Open Access Publishing. Referencees Collins, M M. W., & Brebbbia, C. A. (20006). Design aand Nature II: Comparing Deesign in Naturre with Science e and Enginneering (6th edd.). Southamptoon: WIT Press.

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