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Technical paper

  1. 1. Finite Element Modelling of Low Density Thermally bonded Monocomponent Fibre NonwovensABRSTRACTDue to the manufacturing-induced composite microstructure and random orientationof fibres, nonwoven demonstrates a complex mechanical behaviour. In order tounderstand this behaviour, two micro-scale discontinuous finite element models areintroduced to determine the deformation response in the finite element environment.One of them is Machine Direction (MD) and the other is in Cross Direction (CD). Theobtained results of FE simulation were compared to the experiments performedusing the tensile tests. Further Analysis will be included by changing the MaterialProperties, Fibre Cross sectional Area, bond point thick ness and loading conditions.A comparative study for both the models with respect to these functions will bediscussed in this project.Introduction: (web) or a three dimensional (batt) proceeding to web formation [16]. WebNonwovens are polymer based bonding is the next stage, wherestructures of randomly oriented fibres polymer based fibres are thermallybonded together mechanically, bonded which is executed by hot-thermally or chemically [1]. Nonwoven calendar process. The thermallymaterials have a wide range of bonded nonwovens are manufacturedapplications spawning from household by interlocking polymeric fibres whichproducts, medical equipment to major are done by heating down the fibrestechnical equipment because of the partially by method of hot calendaringinsulation characteristics. They are [3]. A thermal bond comes intomanufactured in three major types existence when a mechanical bond isnamely dry laid, wet laid and polymer formed due to temperature fall in thelaid. The most widely used Nonwoven bonding material [4]. Industrially, themanufacturing fabric type is Polymer- thermal bonding refers to generatingbased nonwoven. It is otherwise web-like structures from individualknown as ‘spun melt’ nonwoven thermoplastic fibres which are passedmanufactured from polymer extrusion through a hot calendar that isor melting down the polymer material maintained at high temperature.[2]. A nonwoven could be sectioned During the process, the individualinto two parts, namely bond points and fibres and the bonding points arefibres. The bond points formed by plasticised firmly with their respectiveheating down the fibres and the bond points. The thermal bondingremaining fibres constitute a fibrous occurs in three steps (1) heating ofweb. filaments (2) formation of bond pointsBy melting down, these staple fibres (3) cooling and re-solidification ofare converted into two dimensional fibres[5].
  2. 2. The material properties used is 20 gsm randomly oriented fibres may be towardsthermally bonded Monocomponent the direction of the bond points but thenonwoven Polypropylene fibre (PP) which Loading condition of both the models mayis 15 mm (a) in length. The type of differ and the orientation angle remainsnonwoven used in this FE modelling is low the same.density thermally bonded nonwoven.Earlier approaches determined thedeformation behaviour for the FEmodelling in the bicomponent material [6]determining the tensile behaviour and themechanical anisotropy, hence determiningthe mechanical behaviour of theMonocomponent discontinuous FE modelis determined to be a challenge, due tothe random orientation of the fibre (b). Therandom fibre orientation and the overallmicro structure (c) show the complication (b)of modelling the nonwoven fibre in theFinite Environment. One of the models isin Machine direction (MD), whichcoincides with the direction of theconveyer belt when the nonwovens aremanufactured. (c) Fig: 1 (a) SEM image of thermally bonded nonwoven material (b) Isolated fibres and fibre bundles within the nonwoven microstructure [7], overall microstructure of the polypropylene nonwoven material (c). (a) Development of FE Model:The other is in Cross Direction (CD) whichis perpendicular to the Machine Direction A parametric modelling technique was(MD). One of the differences between the developed to generate a model withFE Model in Machine direction and the subroutine software Patran using thecross direction is that, the Discontinuous Patran Command language.
  3. 3. The model Fig 2 Was generated with the fibres (element type 9) were modelled withdimensions entered and is generated as truss elements respectively. They do notan input into the FE simulation software. It transmit the bending stiffness but carryreads the code and generates the model compression and tension [18]. The modelwith the following steps. This decreses the is subjected to 3-Dimensional Analysiswork of reformulating the model to include with its initial loads, Fixed and tensionthe actual orientation distribution of fibres setup in the boundary condition.[8]. These fibres are then arranged in theOriental distribution function (ODF).The nodes seem to enter random but laterwill form a complete structure; hence eachnode will have a fibre attached to it. Acomplete symmetric model is made whichis ready for the insertion of inputparameters, after setting up the boundaryconditions the model is ready to besimulated. This model actually ispresented after the mesh is generated.Each bond point is meshed by Bond pointfibre interface and the bond point internalin order to differentiate the fibres. Fig 3 Von Misses Stress (MD) Polypropylene (20 gsm): There is a tensile extension (Fig 3) and of 100% based on the applied input parameters. When the model is simulated 150 increments and consistent load is applied. The failure of the fibre is much seen even before the failure of the bond Fig 2: Generation of a new model point. At almost centre of the extension, itFinite element modelling is demonstrated is considered to be the area, where thefor two such models. There are total 2318 initiation of the stress tends to begin atfibres in Machine Direction (MD) and 2103 much higher level. The failure occurs infibre in (CD). For creating a bond point the the fibre, when the load applied is moreelements 139 and fibres the element 9 are than that of fibre yield strength. Beyondused during modelling the nonwovens. that the fibres or the overall model beginsThe Bond points (element type139) were to fail.modelled with shell elements and the
  4. 4. Each individual bond point begins to than that of the cross direction. At initialdislocate form its original position condition, there is constant force at thedamaging its neighbouring bond point. load 0 N, this is due to the stiffness of theThere is distortion of stress at these levels material at initial condition. It may be alsodue to the Fibre-Bond Point Collision. The due to the surrounding temperature andfibres (Fig 4) at the corner of the model other climatic conditions.exhibit higher stress, there is a neckingcurvature exhibited at the end as it is In the test, there is not much extensionresulting higher level of stress exerted and there is regular drop in theconcentration. force which results in non-uniformity of the results obtained. Therefore, the FE simulation results gave a very softer movement that that of the real fabric giving a constant elongation with respect to the applied force. Previous model [8] determines lack of inter-fibre friction and interaction, because of the current model having the same material properties as the current model. The model in cross direction, in the test, there is not much extension exerted and there is regular drop in the force which results in non-uniformity of the results obtained. Therefore, the FE simulation results gave a very softer movement that that of the real fabric giving a constant elongation with respect to the applied force. SeveralFig 4 Von Misses Stress (CD) other conditions were also considered to understand the deformation response,Results: including change in the thickness of the bond point for both loading directions,In this result, Force-Elongation of thePolypropylene fibre 20 gsm in Cross change in the fibre cross section area etc.direction both the test give 100 % the results were analysed using the Von Misses stress.elongation with respect to the appliedtensile load. The results from the test Change in fibre cross sectional areashow that the material does not exhibit at (MD)constant elongation due to the reason the Post Simulation, Various Parameters areload is at the longitudinal direction. During changed to analyse the behaviour of thethe thermal bonding process, the Polypropylene fibre 20 gsm in order topreference of the nonwoven manufactured analyse the deformation response,in the machine direction is much more
  5. 5. Task 1 was changing the cross sectional transferred to the Bond-point interface.area of the fibre (Truss) and analysing the These results in higher stress at the bond-variation of stress-strain relation observed. point interface, the thick shell bond pointThis will be discussed by a graph plotted element begins to breakdown. The figurebetween the models both MD & CD 50 below shows that the bond point failsRespectively. The reason for the selection at immediate apply of the tensile load inof fibre cross section area for the analysis the loading direction also known aswas due to its less stiffness compared to machine direction. The analysis wasthe bond point and the accurate undertaken using the Polypropylenevisualisation of the results. polymeric material fibre with all the similar material properties, boundary conditions as discussed earlier in table 4 apart from decreased bond point thickness which resulted in the change in the entire geometry of model. Fig 5 Force-Extension curve (MD & CD) Each individual bond point begins toEffect of Bond point deformation: dislocate form its original position damaging its neighbouring bond point.The bond point is considered to be stiffer There is distortion of stress at these levelsthroughout the deformation, but when due to the Fibre-Bond Point Collision.their thickness is decreased to around 10 %of the original thickness, the bond points Increase in bond point thicknesstends to disobey its geometrical property. (CD)During the FE simulations, change in thematerial properties, fibre cross sectionalarea result in stress only at the fibres and With the same material properties given toBond Point-Fibre interface. Even at high the polypropylene in table 4 but change intensile loading conditions, this is the case. the bond point thickness from 0.035 toBut when it comes to decreasing the 0.1, to understand the effect of bond pointthickness of the bond point, the load is thickness towards the overall stress
  6. 6. distribution of the model. It is clear from completely give a different result finally.the result that the fibres which join two By decreasing the thickness, severalbond points close to each other determine activities of the Bond point fibre structuremore stress. This is observed only are the has been identified. The resultant vontwo areas where the load applied is fully in misses’ stress clearly visualizes, theaffect. This is due to the fact that there is stress distributed throughout the fibreseven a change in the geometric and even some part of the bond point arearrangement of the bond point at some very high. But post increment at 150, itareas very near the stress region (i.e.) normalizes and all the stresses arefibres between two bond points. transferred to the end of the fibre, proportional to the tensile direction. Conclusion: The behaviour of the thermally bonded mono component fibre nonwoven was discussed in a finite element environment. Generation of the model was determined by using the Patran command language, further simulated using the MSC Marc software. Initially, the von misses stress and the plastic, elastic strain results were obtained and discussed further. Several input parameters were changed and simulated in order to understand the deformation characteristics of the materialVon Misses stress for bond pointthickness behaviour of polymer materials such as polypropylene, poly amide and LowDecreased Bond Point Thickness (MD): density poly ethylene were analysed byThe initial bond point thickness was including their input parameters in the0.0035 mm, it was reduced to 0.0025 mm. current FE model. The geometricThere was a sudden transformation of thestresses from maximum stress throughout properties of the bond point and fibre werethe region at initial levels of post changed, in order to understand theincrement, but later on reduced. Hence behaviour of the fibres and thewe understand that, when there is achange in fibre cross sectional area then corresponding bond-point structure. Thisthere may be stress obtained lesser included changing the fibre cross sectionthroughout the region at the initial level. area and thickness of bond point. TheBut resultant Von Misses stress would
  7. 7. results obtained were the force-elongation Nonwovens Farukh Farukh a,⇑, Emrahplot for poly propylene fibres of both Demirci a, Baris Sabuncuoglu a, Memis_ Acar a, Behnam Pourdeyhimi b,Machine Direction and cross direction. Vadim V. Silberschmidt aSome of the damage behaviour in themodels such as the bond point collision, [8] Farukh Farukh. (2012). Computational Material science. Numerical modelling offibre damage behaviour due to the less damage initiation in low0density thermallythickness of the bond point was analysed bonded nonwovens. 1 (1), 1-4.and the results were obtained.References: [9] Alvaro Ridruejo. International Journal[1] 2-Dimensional FEA of thermally of Solids and Structures.bonded material, continuous and Micromechanisms of deformation anddiscontinuous models (2009) 700-707 fracture of polypropylene nonwovenJl.46 fabrics.2011;48():153-162[2] R. Krcˇma, Nonwoven Textiles, first ed., [10] Numerical modelling of damageSNTL, Manchester, 1962. initiation in low-density thermally bonded[3] W. Albercht, H. Fuchs, W Kittelmann Nonwovens Farukh Farukh a,⇑, Emrah(2003) ‘’Nonwoven Fabrics’’ Raw Demirci a, Baris Sabuncuoglu a, Memis_materials. Manufacture, Application,Characterestics, Testing Processes Acar a, Behnam Pourdeyhimi b, Vadim V. Silberschmidt a[4] S.J. Russel, Handbook of Nonwovens,Woodhead Publishing Ltd., Cambridge,2007[5] Stephen Michelson, BehnamPourdeyhimi, Prasant Desai (2005)Review of thermally point-bondednonwovens: Materials Process andProperties[6] Emrah Demirci*, Memis Acar, Vadim V.Silberschmidt, Behnam Pourdeyhimi(2011) Finite element modelling ofthermally bounded bicomponent fibre nonwowens: Tensile behaviour[7] Numerical modelling of damageinitiation in low-density thermally bonded