International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) V...
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) V...
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) V...
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) V...
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) V...
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) V...
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) V...
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Energy absorption behaviour of semi rigid polyurethane foam under

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Energy absorption behaviour of semi rigid polyurethane foam under

  1. 1. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 3, May - June (2013) © IAEME118ENERGY ABSORPTION BEHAVIOUR OF SEMI-RIGIDPOLYURETHANE FOAM UNDER LOW VELOCITY IMPACTH.C.Chittappa11Department of Mechanical Engineering, University Visvesvaraya College of Engineering,Bangalore, IndiaABSTRACTThe current work deals with a study on the energy absorption behaviour of semi-rigidpolyurethane (PU) foam under dynamic loading conditions. Foam samples of differentdensities are tested at different strain rates using a low velocity impact testing rig. Thesamples considered are of densities (75 ±3) kg/m3, (125 ±3) kg/m3and (175 ±3) kg/m3.Theinfluence of variation in density and impact velocity on the energy absorption capability ofPU foams has been studied under compressive loading conditions with a drop-weightimpactor. The experimental results reveal that both these parameters (i.e. density and strainrate) have a considerable effect on energy absorption attributes of PU foam. Finally, theexperimental stress-strain data is utilized to reproduce the test behaviors’ of foam underimpact loading using explicit finite element analysis.Keywords : Polyurethane foam, density, energy absorption, low velocity impact, strain rate,finite element analysis.I. INTRODUCTIONPolymeric foams such as polyurethane (PU) and polystyrene foams comprise thewidest variety of foams in terms of applications. Considering mechanical behaviour, suchfoams can again be broadly classified as: viscoelastic (i.e. recoverable) and rigid (i.e.crushable). The former category is suitable for repetitive use such as in seat cushions whilethe latter type of foam is preferred for applications requiring impact energy absorption withhigher stroke as a main performance objective. Energy absorbing PU foams are frequentlyINTERNATIONAL JOURNAL OF MECHANICAL ENGINEERINGAND TECHNOLOGY (IJMET)ISSN 0976 – 6340 (Print)ISSN 0976 – 6359 (Online)Volume 4, Issue 3, May - June (2013), pp. 118-124© IAEME: www.iaeme.com/ijmet.aspJournal Impact Factor (2013): 5.7731 (Calculated by GISI)www.jifactor.comIJMET© I A E M E
  2. 2. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 3, May - June (2013) © IAEME119used as vehicle occupant protection countermeasures in doors and instrument panels, or evenfor improving vehicle crashworthiness in low velocity impact as packaging in front and rearbumpers. Expanded polystyrene foams with their relatively higher strength are often found asprotective liners in motorbike crash helmets. It has been observed that visco-elastic foams areendowed with closed cells with trapped air or gas which is compressed under application ofload and which expands back on removal of load enabling the deformed cells to regain theiroriginal configuration. In crushable foams, cells are commonly of open types which allowhigher deformation of foam under applied loading leading to greater energy absorption ascompared to recoverable foams; however, damages are inflicted in cell walls causing theoverall deformation to be of permanent or irrecoverable nature. Rigid foams are actually anextreme case in which foam after crushing can be reduced to a largely amorphous state. Morecommon crushable foams can be termed as ‘semi-rigid’ as in addition to higher energyabsorption as compared to visco-elastic foams.Polymeric foams have been studied widely in literature in terms of theirmicrostructures, phenomenological behaviour, and engineering applications. A number ofinvestigators have also focused on the yielding of polymeric foam under uniaxial andmultiaxial compressive loads, structural efficiency, energy absorption attributes, and effectsof density and strain rates on engineering properties [1-5] which are of particular interest inthe study carried out by the present authors. Shaw and Sata [1] found the yield criterion for acellular material to be the maximum compressive stress when experiments were performedon foamed polystyrene under a variety of loading conditions. Saha et al. [2] consideredpolyvinyl chloride (PVC) and polyurethane (PU) foams under compressive loading at low(0.001-0.1s-1) and high (130-1750 s-1) strain rates. The authors studied the effects of foamdensity, foam microstructure and strain rate on peak stress and energy absorption. Thedensities of foam considered were relatively on a higher side (100-300 kg/m3). Kurauchi et al.[3] tried to explain the energy-absorption mechanism of polymeric foams with progressive"layer by layer" crushing of cells beginning with the layer containing the weakest cell.Hinkey and Yang [4] investigated the behaviour of polyurethane foams over a range ofdensities and strain rates, and derived empirical relations for modulus of elasticity and yieldstress as functions of density. As revealed in these published studies, polymeric foams exhibita high degree of dependence on dynamic strain rate compared to solid metallic materials.This dependence is due to the solid material properties and to the presence of a fluid,generally air, inside the foam [5]. While experimental and analytical studies provide insightinto the complexities of foam microstructures as well as the macroscopic stress-straincharacteristics of foam under loading and unloading conditions, numerical simulationtechniques can be of invaluable aid in the efficient design of foam-based countermeasures forapplications such as impact energy absorption [6-10]. To support this objective ofengineering design, samples of semi-rigid PU foam of three different densities are at firsttested here in a drop-weight testing machine with nominal impact velocities in the range of 3-5 m/s with indicative strain rates in the range of 60-100 s-1. Foam energy absorption underthese impact tests is shown to be dependent on both density and strain rate. The stress-straincurves generated from the experimental load-displacement behaviours are then used tosimulate the drop tests with an explicit LS-DYNA solver and close correlation between thecomputed load time histories and corresponding experimental responses is obtained.
  3. 3. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 3, May - June (2013) © IAEME120II. PREPARATION OF PU FOAM SAMPLESFoam specimens 0f 50 mm3are prepared with different densities. These are obtainedby mixing different quantities of methane diisocyanate (MDI) with polyol solution. In termsof density, the foam samples are categorized into three groups and designated as PU-1(density: 65 ±3 kg/m3), PU-2 (density: 75 ±3 kg/m3) and PU-3 (density: 85 ±3 kg/m3). Threespecimens are prepared for each category (i.e. PU1, PU2 or PU3) and their densities, as givenin Table 1.Fig. 1. Acrylic mould used for specimen preparationTable1. Densities of foam specimens for testingSpecimenGroupMass(10-3kg)Density(kg/m3)PU19.125 739.250 749. 500 76PU215. 250 12215. 500 12415. 875 126PU321. 500 17221. 875 17522. 000 176III. DROP TEST PROCEDUREThe setup consists of a drop mass weight about 12.5 kg along with the rectangularimpactor. The impacted mass is made to free fall under gravity through a steel guide. Theside support column is graduated in cm so that the height of fall can be noted down. Belowthere is steel rigid base plate on which the foam specimen is mounted. The high speedcamera is focused on the specimen and the load cell is placed on the impactor mass tomeasure the impact force of the specimen. Two accelerometers mounted on its either sides ofthe impactor arm for measuring the acceleration during impact. Impact Force v/s Time andAcceleration v/s Time were recorded.
  4. 4. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 3, May - June (2013) © IAEME121Fig 2: Low velocity Impact test rig Figure 3: Drop test setupIV. TEST RESULTSPolyurethane foam samples of three different densities (i.e. 75 ±3, 125 ±3 and 175 ±3kg/m3) as indicated in Table 1 have been tested under low velocity impact conditions in thedrop-weight test rig. Each of the three samples of a given foam density is subjected to adifferent impact velocity corresponding to an impactor drop height of 0.5 m or 1.0 m or 1.5m. The average test-based velocities corresponding to these drop heights in an increasingorder have been found to be 3.1 m/s, 4.4 m/s and 5.4 m/s. Truly speaking, the strain rate inany drop test varies with time. The drop tests in the current study for the three impactvelocities as mentioned have been characterized by an indicative strain rate which may beclose to the maximum strain rate experienced in any test and given as hV0, 0Vbeing thevelocity of the impact hammer just before striking a test specimen and h , the height of aspecimen which is 50 mm. For the present average impact velocities of 3.1 m/s, 4.4 m/s and5.4 m/s, the characteristic strain rates are then found to be 62.5 s-1, 88.5 s-1and 108.5 s-1respectively. As mentioned earlier, foams of a given type, viz. PU1, PU2 or PU3, are testedwith three different impact velocities, i.e. 3.1 m/s, 4.4 m/s and 5.4 m/s. The total energyabsorbed by a given foam sample is calculated by integrating the force-displacement curveobtained. Effect strain rate on energy absorption shown in fig4 and effect of density on strainrate and energy absorption shown in fig5.
  5. 5. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 3, May - June (2013) © IAEME122V. NUMERICAL SIMULATIONFinite element analysis was performed using LS-Dyna. The 50mm cube foamspecimen was meshed with 15x15x15 mesh density and the graphical view of complete FiniteElement Model as shown in Figure 6. The following boundary conditions are imposed on thenumerical model: all nodes of the bottom face of the cubic foam are restrained in bothhorizontal and vertical directions. A flat impactor moving at a defined velocity opposite face,impactor motions are constrained except the translation in vertical direction.Fig. 6. Graphical View of Assembled FE ModelFig 7 Force vs. Time comparison Curves for a foam density 172Kg/m3@ 0.5m
  6. 6. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 3, May - June (2013) © IAEME123Fig.8 Force vs. Time comparison Curves for a foam density 175Kg/m3@ 1.0mFig 9 Force vs. Time comparison Curves for a foam density 176Kg/m3@ 1.5mIn Figs 7,8 and 9 the comparison between Finite element method and experimentaldata is shown for drop-tests performed on cubic samples of the Polyurethane foam with massdensity of 175±3 Kg/m3at three strain rate of 62.5 /s, 88.6 /s and 108.5/s ( at Drop Height0.5, 1.0 and 1.5 m). The obtained result correlate well with the test and are very close to eachother, with the exception the predicted force was little higher than that seen in the tests. Thehigher predicted force can be attributed to the model not including the energy release due tothe damage (matrix cracking).VI. CONCLUSIONThe practical problems included in the numerical analysis of foams, include theselection of the material law capable of reproducing foams response under dynamic loadscausing high or at least medium deformation and the generation of finite element models thatgive a smooth representation of the complex undeformed and deformed geometries of thefoam structures.
  7. 7. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 3, May - June (2013) © IAEME124REFERENCESJournal Papers:[1] M. C. Shaw and T. Sata, “The plastic behavior of cellular materials”, Int. J. Mech.Sci. Vol. 8 (1966), pp. 469-478.[2] M.C. Saha, H. Mahfuz, U.K.Charavarty, M. Uddin, Md. E. Kabir, S. Jeelani, “Effectof density, microstructure, and strain rate on compression behaviour of polymeric foams”,Materials Science and Engineering A406, (2005) ,pp.328-336.[3] T. Kurauchi, N. Sato, O. Kamigaito and N. Komatsu. “Mechanism of high energyabsorption by foamed materials foamed rigid polyurethane and foamed glass”, J. Mater. Sci.19, (1984), pp. 871- 880.[4] W. M. Hinckley and J. C. S. Yang, “Analysis of rigid polyurethane foam as a shockmitigator”, Exp. Mech. 15, (1975), pp. 177 -183.[5] M. Avalle, G. Belingardi, R. Montanini, “Characterization of polymeric structuralfoams under compressive impact loading by means of energy-absorption diagram”,International Journal of Impact Engineering 25, (2001),pp. 455-472.[6] C. C. Chou, Y. Zhao, L. Chai, J. Co, G. G. Lim, “Development of Foam Models asApplications to Vehicle Interior”, SAE Paper No. 952733, 1995.[7] Gerhard, Gavin, Onkar Bijjargi, “Transverse Anisotropic Modelling of HoneycombExtruded Polypropylene Foam in Ls-Dyna to Optimize Energy AbsorptionCountermeasures”, SAE Paper No. 2005-01-1222, 2005.[8] C. Clifford . Chou, Y. Zhao, George G. Lim, Rasik N. Patel, Syab Shahab, Praful J.Patel “Comparative Analysis of Different Energy Absorbing Materials for Interior HeadImpact”, SAE Paper No. 950332, 1995.[9] G. S. Nusholtz, S. Bilkhu, M. Founas, “Impact Response of Foam: the Effect of theState of Stress”, SAE Paper No. 962418, 1996[10] J. Wycech, Mark C. Butler “Dynamic Reinforcing Strategies Using Structural Foam”,SAE Paper No. 2003-01-1708, 2003.[11] Piyush Chandra Verma and Ajay Gupta, “Study of Electrochemical OxidationBehaviour of High Build Epoxy, Cold Applied Poly Defined Tape and Polyurethane CoatingSystem in Saline Environment”, International Journal of Mechanical Engineering &Technology (IJMET), Volume 3, Issue 2, 2012, pp. 73 - 84, ISSN Print: 0976 – 6340, ISSNOnline: 0976 – 6359.[12] ArkanJawdat Abassa and DhaferSadeq Al-Fatal, “Experimental and Theoretical Studyof the Influence of the Addition of Alumina Powder To 7020 Aluminum Alloy Foam on theMechanical Behavior under Impact Loading”, International Journal of MechanicalEngineering & Technology (IJMET), Volume 3, Issue 3, 2012, pp. 412 - 428, ISSN Print:0976 – 6340, ISSN Online: 0976 – 6359.

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