Dynamic tool-ply friction for a thermoplastic viscous textile composite has been measured using a commercial rotational rheometer as a function of rate, temperature and normal pressure. Results of this novel experimental technique have enabled a general empirical equation to be determined relating the dynamic friction force to experimental conditions. The method has been corroborated using an alternative experimental technique. Advantages of using the rheometer include significantly faster production of data and more precise measurement of experimental conditions

Akkerman, Remko

Clifford, M.J.

Harrison, P.

Lin, H.

Long, A.

Long, A.C.

van de Haar, K.

English

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Investigation of tool-py friction of viscous textile composites (CD-rom)

University of Twente Research Information

8th International Conference on Textile Composites (TEXCOMP-8)Nottingham, UK, 16-18 Oct 2006T01-1INVESTIGATION OF TOOL-PLY FRICTION OF VISCOUS TEXTILECOMPOSITESLin, H. 1, Harrison, P. 2, Van de Haar, K. 3, Long, A.C. 1, Akkerman, R. 3 and Clifford, M.J. 11School of M3, University of Nottingham, University Park, Nottingham NG7 2RD, UK2 University of Glasgow, 3 University of TwenteABSTRACTDynamic tool-ply friction for a thermoplastic viscous textile composite has been measuredusing a commercial rotational rheometer as a function of rate, temperature and normalpressure. Results of this novel experimental technique have enabled a general empiricalequation to be determined relating the dynamic friction force to experimental conditions. Themethod has been corroborated using an alternative experimental technique. Advantages ofusing the rheometer include significantly faster production of data and more precisemeasurement of experimental conditions.INTRODUCTIONWith the proliferation of fibres, resins and manufacturing processes, composites materialsenable designers and product engineers to create tailored products for a broad range ofapplication areas. The manufacture of fibre composites involves a forming stage in which dryor impregnated reinforcement takes the required shape under the combined action of holdingpressure and forces due to contact with flexible or rigid tooling. The friction that occurs as thematerial slides relative to the tooling imparts a tensile stress on the material which helps tocounteract any compressive stresses in the material during forming that may cause plybuckling. Investigation of the tool-ply friction of viscous textile composites is relatively new.Existing methods for determining friction coefficients of fabric sheets consider effects ofnormal pressure, temperature and pull-out speed on friction (Murtagh et al 1995, Maldonado1994, Wilks 1999, and Groczyca et al, 2004). In these methods, either a thin composite sheetis pulled through two horizontal tools or a metal shim is pulled from between two compositesheets. The applied normal load and pull-out speed are predetermined and the pull-out forcesare recorded. Friction coefficients are calculated using Coulomb’s equation. There are somelimitations of this method. One is that it is time consuming, typically requiring 40-60 minutesfor one sample; the second is that the required force to pull out the ply from between twometal platens is a combination of the friction force and fabric deformation force (thedeformation of fabric is clearly visible after testing). Furthermore, test repeatability is ratherpoor. To overcome these limitations, a novel experimental technique, involving in the use of acommercial rheometer, is proposed and evaluated. A master curve is derived based on the dataproduced by the experimental programme, predicting the shear forces at the interface of heatedcomposites and tool surface as a function of rate, temperature and normal pressure.EXPERIMETNAL METHODTest equipment and procedureExperiments were performed on a Bohlin CVOR200 Rheometer with Extended TemperatureCell (ETC, oven) (Fig.1a). The rig consists of a pair of parallel stainless steel plates, with thelower plate a truncated cone with diameter 25 mm, and the upper plate a flat disk withdiameter 40 mm (Fig.1c). A specimen is put onto the upper plate and a ring (outer diameter of40mm and inner diameter of 30mm) with four bolts is put onto the specimen. The four boltssecure the specimen through the ring onto the upper plate. The specimen is then placed in the8th International Conference on Textile Composites (TEXCOMP-8)Nottingham, UK, 16-18 Oct 2006T01-2ETC (oven) and heated. After the specimen reaches the required temperature, the upper platewith the specimen is positioned in the rheometer parallel with the lower plate. A normal forceis set on the specimen by lowering the upper plate against the lower plate. The value of thenormal force is monitored by a computer.rRvv(a) (b) (c)Figure 1 (a) Bohlin Rheometer with ETC, (b) geometry of testing disk, (c) truncated cone andflat disk inside of the rigMaterialFigure 2 Test sampleDEVELOPMENT OF MASTER CURVEInput and output of the rheometerThe input data in the rheometer are normal force, N (N), shear stress (Pa) and temperature(°C). Experiments were performed for a range of normal forces 2.5%, 10%, 20%, 50% or 90%of 19.6 N, for a range of shear stresses 500Pa, 1100Pa, 2000Pa, and 5000Pa, and for a range oftemperatures 160, 180, 200, and 220°C. Each test was repeated three times and average resultswere used for data processing.The output of the rheometer is normal force, n, (normal force is also as an input) and rotationangle (rad) versus time (sec). The data from the rheometer needs to be adjusted in order tocompare the data with results from the pull out method.The normal force can be transferred to normal pressure (MPa) byAnN  (1)where N is normal pressure, and A is the testing area ( 25.12x ).The material tested was 1-layer of pre-consolidated 2 x 2twill weave of Vetrotex Twintex® commingled E-glassand Polypropylene yarn with fibre volume fraction of0.35.Specimens were cut at 40 x40 mm (Fig. 2) with squares of7 x 7mm removed from the corners to leave crosses for theattachment bolts.8th International Conference on Textile Composites (TEXCOMP-8)Nottingham, UK, 16-18 Oct 2006T01-3The angular velocity can be transferred to linear velocity (mm/sec) byrv  (2)where is the angular velocity, r is an arbitrary radius.The linear velocity varies from 0 (r = 0) to maximum (r = R) as shown in Fig. 1b. The linearvelocity at a radius of 2/3 of the maximum radius (R) is used to process the rheometer data.Output plot and curve shiftingTransferred outputs from the rheometer, namely normal pressure, N, versus linear velocity fordifferent shear stresses at temperature 180°C are plotted in Fig.3. Similar graphs were alsoproduced at temperature 160°C, 200°C and 220°C. Trend lines have been fitted through thedata for the different curves. All trend lines have a constant slope of v-1.37.There are several empirical techniques to shift several data curves to one master curve. Here,the ‘method of reduced variables’ (Macosko 1994) was used for shifting the curves to a mastercurve. It is possible to shift the curves horizontally and/or vertically as indicated by the arrowsin Fig.3. Since the fitted curves are linear on a log-log graph, horizontal or vertical shiftingcould be used. Here the vertical shifting method was used..Figure 3 Trend lines for different shear stresses at 180°CAn equation, relating shear stress with rate, temperature and normal pressure, for the mastercurve can be derived by setting reference parameters and calculating shift factors between thedifferent trend lines. Here, a temperature of 180°C and a shear stress of 500 Pa were set as thereference shifting parameters and shear stress and temperature were shift factors.From the reference curve fitting (shear stress 500Pa) (Fig. 3), we haveN = 37.16 )100.4(  v (3)Hence, a general function for normal pressure and velocity is21cT vbacN (4)Where "N is the normal pressure (MPa), v the velocity (mm/sec), b the shift factor for shearstress, Ta the shift factor for temperature, c1 is a curve fitting coefficient and c2 = -1.37.Therefore, the development of an equation for the master curve involves derivations of theshift factors, i.e., b and Ta .T=180Cy = 4.00000E-06x-1.37000E+000.00010.0010.010.110.0001 0.001 0.01 0.1 1 10rate (mm/sec) 2/3 from centrenormalpressureMPa500Pa1100Pa2000Pa5000Pa8th International Conference on Textile Composites (TEXCOMP-8)Nottingham, UK, 16-18 Oct 2006T01-4Shear stress shift factorThe shear stress shift factor b in Eq. 4 can be obtained by (van de Haar 2005). Assuming:mrefb (5)Then m in Eq.5 can be obtained byrefbmloglog (6)Alsorefccb11 or refbcc 11  (7)where, ref is the reference shear stress and c1ref is a curve fitting coefficient for the referenceshear stress (Fig.3).Substituting Eqs.5 and 7 in Eq.4, we have21cmrefTref vacN(8)where c1ref = 4.0x10-6, c2 = 1.37, m = 3 and ref = 500x10-6 MPaFig.4 presents plots for normal pressure versus velocity for different shear stresses. Analysingthe data in Fig.4 gives the required shifting factors c1 and b for different shear stresses asshown in Table 1.T=180Cy = 8E-06x-1,2342y = 3E-05x-1,426y = 0,0002x-1,3833y = 0,0067x-0,7779y = 4,256756E-05x-1,370000E+00y = 2,558050E-04x-1,370000E+00y = 3,996954E-03x-1,370000E+00y = 4,00000E-06x-1,37000E+000,00010,0010,010,110,0001 0,001 0,01 0,1 1 10rate (mm/sec) 2/3 from centrenormalpressureMPa500Pa1100Pa2000Pa5000PaMacht (500Pa)Macht (1100Pa)Macht (2000Pa)Macht (5000Pa)Macht (line 1)Macht (line 2)Macht (line 3)Macht (line 4)Figure 4 Trend lines for different shear stresses at temperature 180°C,Table 1 Shear stress shift factorsShear stress (Pa) c1ref b m500 4.0x10-6 1 1 -1100 4.25x10-5 2.2 11 32000 2.56x10-4 4 64 35000 4.0x10-3 10 1000 38th International Conference on Textile Composites (TEXCOMP-8)Nottingham, UK, 16-18 Oct 2006T01-5Temperature shift factorThe temperature shift factor aT was calculated based on the following procedure (Harrison etal. 2001):TAcN exp (9)where c and A are constants. Hence, the temperature shift factor is)()(refT TNTNa  (10)where N (T ref ) is normal pressure at reference temperature.Combining Eq.10 with 9 and rewriting, we haverefT TTAa11)log( (11)The constant A in Eq.9 is estimated using the least squares method, resulting in, A = -282.85.Hence  1801185.28210 TTa (12)Substituting Eq.12 in Eq.8, the master curve can be expressed by:21801185.2821 10cmrefTref vcN (13)Substituting the constants c1ref = 4.0x10-6, c2 = 1.37, m = 3 and ref = 500x10-6 in Eq. 13 andrearranging, shear stress can be expressed by  337..16060.410500TaENv  with  1801185.28210 TTa (14)in which shear stress  is a function of normal pressure N , temperature T and velocity v.Fig. 5 gives a plot of the shifted data points and the master curve.Figure 5 All data from the rheometer is normalised using the shift factors aT and b.1.0E-071.0E-061.0E-051.0E-041.0E-031.0E-021.0E-011.0E+001.0E+011.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00 1.0E+01rate mm/secNormalpressure/(at*b)(MPa)shifted data pointsReference8th International Conference on Textile Composites (TEXCOMP-8)Nottingham, UK, 16-18 Oct 2006T01-6COMPARISON BETWEEN THE MASTER CURVE AND RESULTS FROM PULLOUT METHOD0,0010,010,10,001 0,01 0,1 1 10Normal pressure MPaShearstressMPa0.8mm/sec Pull out0.83mm/sec Pull through0.8mm/sec Rheometer mastercurveFigure 6 Master curve from rheometer andPull-through and pull-out testCONCLUSIONA novel experimental programme dealing with the friction between the heated composite andtool surface during forming has been presented. The data from the new testing technique hasbeen used to derive a master curve for prediction of the shear stress as a function of rate,temperature and normal pressure. The comparison of the master curve with the results from apull out method shows a similar trend between the two. The advantages in using the rheometerinclude significantly faster production of data (2-3 minutes against 40-60 minutes per cycle)and more precise measurement of experimental conditions as normal pressures andtemperatures are controlled more accurately.REFERENCESHarrison, P., R.G. Gill, A.C. Long, Draping of Engineering Prepregs and ReinforceThermoplastic Progress Report 8, internal report, 2001.Macosko, C.W., Rheology Principles, Measurements and Applications, Wiley-VCH, Inc: NewYork. 1994.Murtagh A.M., Composites Manufacturing 6: 169-175,1995.Smiljko Asperger, Chemical Kinetics and Inorganic Reaction Mechanisms, 2 ed. Springer.2003Wilks, C. E., Processing Technologies for Woven Glass/Polypropylene Composites, PhDthesis, University of Nottingham: Nottingham. 1999.Van de Haar K., “Modelling resistance at the ply/tool contact interface for Twintex®”, MPhilthesis, Nottingham university, 2005.To evaluate the master curve, experiments werecarried out on the same material at atemperature of 180°C and a velocity of 0.8mm/sec, using the pull-out and pull-throughmethods described by Wilks (Wilks 1999). Forthe results at low pressure, a sheet of Twintex®is pulled from between two steel shim platens;high pressure measurements involve a steelshim being pulled from between two layers ofthe composite. Fig.6 shows that overall themaster curve predict a higher shear stress thanthat the pull-out and pull-through method. Thediscrepancy could be caused by deformation ofthe material sample during the pull-out/pull-through tests, ie. data measured by therheometer may actually be more accurate.

Investigation of tool-py friction of viscous textile composites (CD-rom)

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