The experimental determination of energy of glass-polyester composite tubes static fracture is done. The tubes are of defined structure and known processes of fabrication. The aim was to determine the possibility of their usage as damping elements during impact. Two types of tubular samples with different diameters were tested until fracture under static (low speed) compressive loading. The applied forces and sample lengths were measured until complete destruction of samples. From the diagrams received directly from testing devices, certain energy values explained in the paper were determined.U radu je prikazano eksperimentalno određivanje statičke energije loma staklo-poliester kompozitnih cijevi poznate struktura materijala i postupka izrade. Cilj ispitivanja je bio odrediti mogućnost primjene ugradnje takvih cijevi u elemente za prigušivače udara. Pod djelovanjem spornog statičkog tlačnog opterećenja ispitivana su dva tipa cijevi različitih presjeka. Pri tom su mjerene primenjene sile i duljine uzoraka sve do potpunog loma cijevi. Na osnovu dobivenih dijagrama su izračunate vrijednosti specifičnih energija apsorpcije

J. Pavlović-Krstić

Lj. Milović

M. Stamenović

M. Zrilić

S. Putić

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M. STAMENOVI], S. PUTI], M. ZRILI], LJ. MILOVI], J. PAVLOVI]-KRSTI]SPECIFIC ENERGY ABSORPTION CAPACITYOF GLASS-POLYESTER COMPOSITETUBES UNDER STATIC COMPRESSIVE LOADINGReceived – Prispjelo: 2010-05-28Accepted – Prihva}eno: 2010-07-15Preliminary Note – Prethodno priop}enjeINTRODUCTIONComposite materials are often used to reduce theweight of structures. In the automotive industry weightreduction is important because fuel consumption is di-rectly related to vehicular weight. There is an increasedconcern for occupant safety during roadway accidents.Metals are currently used in car frames and integratedframe-body structures, and these and other metalliccomponents are designed to passively absorb energyduring accidents. However, automotive manufacturersare moving toward nontraditional materials 1 and newstructural material under consideration should be capa-ble of participating in the energy absorption process as-sociated with accidents. An important parameter whenstudying energy absorption, and one often used in theautomotive industry, is the energy absorbed per unitmass of crushed material. This is often called the spe-cific energy absorption (SEA) 2. The SEA provides ameasure of energy absorption efficiency of a structuralcomponent but, of course, says nothing about the effi-ciency of the structure in regards to other areas, such asresisting buckling, damping vibrations, or its ease ofmanufacture. It is one of several parameters that must beconsidered in automotive design.The subject of the paper 3 was investigation of theeffect of interlaminar fracture toughness on the specificenergy absorption of stitched glass/polyester compositecylindrical shells under axial compression. The laminatedcomposite cylindrical shells, used as energy absorbers,absorb large amount of impact energy during collision.Since delamination in the thin wall of axially collapsedshell is one of the major energy absorbing modes, contri-bution to SEA of tubes is significant during collision.The energy absorbing capability of fiber reinforce-ment polymer (FRP) composite cylindrical tubes usedas energy absorbers, by destroying itself progressively,depends on the way in which tube material is crushedi.e., trend of petalling [4]. This paper investigates the in-fluence of fibre orientation and stacking sequence on thepetal formation and specific energy absorption (SEA) ofglass/polyester composite cylindrical shells under axialcompression. Processing conditions effect on specificenergy absorption capacity of composite tubes was in-vestigated in 5.Tubes of circular and square cross sections were fab-ricated using orthophthalic polyester resin and plainweave E-glass fabric with fibers oriented at 0° / 90°,with respect to the tube axis. Test specimens consistingof tube segments were prepared and tested under staticcompression load.Very important factor in the study of energy absorp-tion for energy management capabilities is the shape ofMETALURGIJA 50 (2011) 3, 197-200 197The experimental determination of energy of glass-polyester composite tubes static fracture is done. The tubesare of defined structure and known processes of fabrication. The aim was to determine the possibility of theirusage as damping elements during impact. Two types of tubular samples with different diameters were testeduntil fracture under static (low speed) compressive loading. The applied forces and sample lengths were mea-sured until complete destruction of samples. From the diagrams received directly from testing devices, certainenergy values explained in the paper were determined.Key words: Polymer-matrix composites, Specific energy absorption; Low velocity compressionSpecifi~na absobcijska energija staklo-poliester kompozitne cijevi pod stati~kim tla~nim opte-re}enjem. U radu je prikazano eksperimentalno odre|ivanje stati~ke energije loma staklo-poliester kompozit-nih cijevi poznate struktura materijala i postupka izrade. Cilj ispitivanja je bio odrediti mogu}nost primjeneugradnje takvih cijevi u elemente za prigu{iva~e udara. Pod djelovanjem spornog stati~kog tla~nog optere}enjaispitivana su dva tipa cijevi razli~itih presjeka. Pri tom su mjerene primenjene sile i duljine uzoraka sve do potpu-nog loma cijevi. Na osnovu dobivenih dijagrama su izra~unate vrijednosti specifi~nih energija apsorpcije.Klju~ne rije~i: polimer matri~ni kompoziti, specifi~na energija apsorpcije, sabijanje malom brzinomISSN 0543-5846METABK 50(3) 197-200 (2011)UDC – UDK 666.112.93:666.188.2+678.674:53-083.98=111M. Stamenovi}, Belgrade Polytechnic, Belgrade, Serbia, S. Puti}, M.Zrili}, Lj. Milovi}, Faculty of Technology and Metallurgy, University ofBelgrade, Belgrade, Serbia, J. Pavlovi}-Krsti}, Institut für Fertigung-stechnik und Qualitätssicherung, Otto-von-Guericke-Universität Magde-burg, Magdeburg, Germany.the crush load vs. crush length graph. As the crushingbegins, the load quickly rises to a peak value, then dropsoff slightly and stays relatively constant. In this way theenergy absorption is maximized for the length ofcrushed material. One does not want the initial peak load(Fimax) to be much greater than the average crush load(Fa), because large loads would be needed to initiatecrushing, and the goal in energy management is to ab-sorb all the energy without imparting large forces to thepeople involved. One measure that is used to character-ize the shape of the graph is called the load ratio 2. Theload ratio, which is defined as:Load ratio =FFiamax(1)is one metric that may be used as an important parameterfor measuring crushing efficiency.EXPERIMENTAL WORKThe pipes of producer “Poliester” from Priboj weretested. The properties were given in official certificates ofcertain producers of components made of usedglass-polyester pipes. The producers of reinforced glassfibers A.D. “OHIS” and “Vidoe Smilevski-Bato” fromGostivar-Macedonia by certificate confirm “E” glasswith 1 % of alkali. Thermo-reactive polyester resin wasused as matrix, produced by “Color”-Medvode fromSlovenia. Certificate was given for “COLPOLY 7510”for the type: UP/SOM which is highly reactive, low withviscose polyester with the basis of ortoftaly acid in stan-dard glycol.The pipes were made by the method “FilamentWinding”, with structure 90°2±55°490°4. Two sam-ples of pipe were made, with outside diameter  70 mmand  50 mm.The specimens for specific energy absorption (SEA)tests (three specimens per each test) were cut from thesamples of pipes (Tables 1 and 2). The cut was done onmachine type NC-2010 (Nr 95110, Ar 001) with dia-mond tip tools and at speeds which minimized heatingof samples.Testing (SEA) was done on servo-hydraulic testingmachine INSTRON 1332 with controller INSTRONFAST TRACK 80800, with the usage of hydraulic jaws.The testing was defined by standard ASTM D 3039 6, 7.Loading was measured with a load cell of 100 kN capac-ity. Displacements were measured by double extenso-meter HOTTINGER DD1. The static specimen wasplaced on the stationary lower plate with the beveled endup. The cross-head was then lowered until the upper platewas just touching the specimen. The cross-head was thenset into motion and the load and cross-head displacementwere recorded for each test.RESULTS AND DISCUSSIONDuring the test the diagrams crush load – crushlength (F-s) were plot, Figures 1 and 2.The most important parameter that was from eachcrush test was the specific energy absorption (SEA). En-ergy is the product of the force and distance moved atthe force level. The energy absorbed during crush, E,was calculated by integrating under the crush load vs.crush distance curve,E F dss f 0(2)Where sf is the final crush length.198 METALURGIJA 50 (2011) 3, 197-200M. STAMENOVI] et al.: SPECIFIC ENERGY ABSORPTION CAPACITY OF GLASS-POLYESTER COMPOSITE TUBES...Table 1 The dimensions of tested samplesSpecimenOutside diameter/ mmInside diameter/ mmThickness/ mmT-70-1 70,9 63,8 3,6T-70-2 71,1 63,9 3,6T-70-3 70,6 63,9 3,4T-50-1 50,5 36,4 3,4T-50-2 50,3 36,1 3,6T-50-3 49,9 36,3 3,4Table 2 The dimensions of tested samplesSpecimenLength of the spe-cimen / mmVolume/ mm3Load speed/ mm/minT-70-1 128,3 4823,7 5T-70-2 126,9 5166,7T-70-3 132,1 4657,4T-50-1 129,1 3215,9 1T-50-2 131,2 3540,2T-50-3 127,3 3157,4Figure 1 Crush load vs., crush length for static test (spe-cimen 70 mm diameter (T-70))Figure 2 Crush load vs., crush length for static test (speci-men 50 mm diameter (T-50))The average crush force was calculated continuouslyas the tube was crushed, as follows:FF dssas0 (3)In order to calculate the load ratio using Equation(1), the value of the crush load at the initial peak, Fimax,was divided by the average crush load,Load ratio =FFF sF dsiaismax max0(4)To get the SEA, the energy absorbed during crushwas divided by the mass of the crushed material, mc. Thecrushed mass was found by the following:m m sc l f  (5)Where ml is the linear density or mass per unit lengthof the tube, (average value was 1250 g). The linear den-sity was determined from the length and mass measure-ments from each specimen. Specific energy absorptionis defined as:SEAEmFdsm scsl ff 0 (6)The experimental results are presented in Tables 3and 4.From Figures 1 and 2 we obtained values of energyabsorption during crash, crush length, and initial peek ofload. Load radio was calculated from Equation 4 andspecific energy absorption from Equation 6. Mass of thecrushed material was obtained from Equation 5.The valley-to-peak variations seen in the crush loadvs. crush distance graphs varied. Overall, the T-70 spec-imens showed smaller valley-to-peak variations in thecrushing load than did the T-50 specimens.Average value of energy absorbed during crush was946,2 J for the samples T-70 and 450,7 J for the samplesT-50. Average specific energy absorption (SEA) was40,1 J/g for the samples T-70 and 37,2 J/g for the sam-ples T-50 and average load ratio 1,25, for the T-70 sam-ples, and 1,28 for the T-50 samples.Several different modes of fracture can be seen inFigure 3, taken from Sample T-70. The tube wall wastorn, while the inner side wall was buckling to the inte-rior of the tubes, and the external side was buckling tothe outside.Between these two areas remained crumbled mate-rial that is likely to help the wall separate. Buckling wasdue to the stress increase in the undamaged part of thetube. After exceeding allowed stress, at the same time,cracks appeared in several places along the tube length,accompanied by acoustic effects. During the fragmenta-tion small number of fibers along the tube were dam-aged, but there has been more significant damage in ma-terial matrix. Tear away parts of the wall that were in theforms of leaves remained undamaged.Micromechanical analysis of failure can show thatthe compression load crack is initiated in all test speci-mens on the side of outer layer (Figure 4).Occurrence of the break was at the moment of reach-ing critical state of stress in the material which causes theoccurrence of critical value of crack and its progressivegrowth. The place of critical crack is related to fiber-ma-trix debonding (Figure 5) after which the fibers cracked.It is obvious that on the spot where the first break ap-peared (outer layer) exist more broken fibers which werepreviously debonded and pulled out from the matrix.Crack propagation leads through to the inner surface oftest specimen in transversal direction of test specimen.Lateral on this crack, in test specimens appearedcracks and delamination (Figure 6) as result of shearstresses in the layers. Existence of shear stresses is char-acteristic for impact and specially compression test wereinner layers were exposed to great axial stresses.METALURGIJA 50 (2011) 3, 197-200 199M. STAMENOVI] et al.: SPECIFIC ENERGY ABSORPTION CAPACITY OF GLASS-POLYESTER COMPOSITE TUBES...Table 3 Test resultsSpecimenInitial peekload / kNAveragecrush load /kNCrushlength/ mmLoad ratioFi max/FaT-70-1 78 61 52 1,27T-70-2 78 63 50 1,23T-70-3 78 62 47 1,26T-50-1 66 52 22,2 1,27T-50-2 67 51 21,8 1,31T-50-3 63 50 22,0 1,26Table 4 Test resultsSpecimenEnergyabsorbedduring crush / JMass of thecrushedmaterial / gSpecific energyabsorption / J/gT-70-1 958,3 24 39,9T-70-2 940,6 25 37,6T-70-3 939,8 22 42,7T-50-1 462,8 12,0 38,5T-50-2 447,1 12,5 35,8T-50-3 442,3 11,9 37,2Figure 3 Crush modeCONCLUSIONSThe aim of presented paper was to investigate thepossibilities of application of glass polyester compositetubes as elements for depreciation or absorption of en-ergy. The objective was to compare how the energy ab-sorption characteristics were changed by varying circu-lar cross-section area.On the basis of the result obtained, it can be notedthat as regards the energy absorbed during static loads itis better to use larger diameter pipes. Also, as mentionedin 2, a load ratio of less than 1,25 is desirable, which isthe case in the sample T-70, that confirms the previousstatement. Of course, the validity of these quantitiesmust be questioned because of the large valley-to-peakvariations and the non-distinct initial peaks. The hightow count material has proven that it can be effective inenergy absorption, so further study is warranted.The authors of this work plan to continue researchesin this field to complete the knowledge about the real pos-sibilities of absorption of energy of structures like this.There are many interesting avenues to explore as a con-tinuation of this work. First of all it is important to investi-gate dynamic impact crushing because the specimenstested statically tended to absorb more energy than thosetested dynamically. Also dynamic specimens generallyhave higher load rations and crush modes are different.Also, further researches will go in direction of additionalstandard impact tests by Sharpy method on specimens cutout of tubes. That will give us knowledge about impacttoughness of tested structures, but also we will haveknowledge about much more important datum, and thatis, what is the energy of crack initiation, and what is theenergy of crack propagation. By comparison of these val-ues with shown results of energy on static compressiveloading we will compare and confirm which energy ismore dominant for case of amortization and later, come toconclusions give recommendations about the change ofstructure to improve these properties. Certainly, as a veryuseful daum we will get the strength got by standard test-ing on static pressure loading on test tubes cut out oftubes. In the end, we plan the research in other direction,and that would be finding optimal diameter and length oftubes. We will do all done and planned new tests withlonger diameters and shorter lengths.REFERENCES1 V. P. McConnell, ed. Automotive Composites: A Designand Manufacturing Guide, Ray Publishing, Wheat Ridge,CO, 1, 1997, pp. 6-7.2 M. R. Schultz, Energy Absorption Capacity of Graphi-te-Epoxy Composite Tubes, Master of science thesis, Virgi-nia, 1998, pages 115.3 S. Solaimurugan, R. Velmurugan, Progressive crushing ofstitched glass/polyester composite cylindrical shells, Com-posites Science and Technology, 67 (2007), 422–437.4 S. Solaimurugan, R. Velmurugan, Influence of fibre orien-tation and stacking sequence on petalling of glass/polyestercomposite cylindrical shells under axial compression, Inter-national Journal of Solids and Structures, 44 (2007),6999–7020.5 J. D. D. Melo, A. L. S. Silva, J. E. N. Villena, The effect ofprocessing conditions on the energy absorption capability ofcomposite tubes, Composite Structures, 82 (2008), 622–628.6 Annual book of ASTM Standards, Vol.15.03, American So-ciety for Testing and Materials, Philadelphia, PA, USA,1999.7 ASTM D 3039-76, Standard Test Method for Tensile Pro-perties of Fiber-Resin Composites, Annual Book of ASTMStandard, v 36, 1980, p. 734-739.Note: The responsible person for English language is lecture from Bel-grade Polytechnik, Serbia.200 METALURGIJA 50 (2011) 3, 197-200M. STAMENOVI] et al.: SPECIFIC ENERGY ABSORPTION CAPACITY OF GLASS-POLYESTER COMPOSITE TUBES...Figure 4 Compression load crack initiated on the side ofouter layerFigure 5 Fiber-matrix debondingFigure 6 Cracks and delamination

Specifična absobcijska energija staklo-poliester kompozitne cijevi pod statičkim tlačnim opterećenjem

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