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TEXTILE WRINKLING IN COMPOSITE FORMING H. Lin, J. Wang, A. C. Long, M. J. Clifford and P. Harrison ABSTRACT During the process of forming of textile composites, wrinkling is one of the most common flaws, often leading to unexpected failures. Hence, one of the main aims of optimizing textile composite structures is to improve the forming of the material by reducing wrinkles in the final product. This paper presents experimental investigations and FE analysis concerning this problem. The wrinkling mechanism is analyzed and this leads to several suggestions to prevent wrinkle formation during composite forming, for example by means of appropriate selection of blank holding force together with taking into account the effect of anisotropic behaviour of woven composite materials. These are illustrated by FE simulations and are verified experimentally via forming of a hemispherical component. The study shows that in-plane membrane forces in the material and tow architectures can be controlled by changing blank holding forces, potentially allowing wrinkle-free forming with woven textile composite. 1. INTRODUCTION

Textile fabric possesses hardly any stiffness in bending or compression, so it is able to cover a 3D human body gracefully and can deform to a complex shape easily. However, because of its small bending rigidity, it cannot support compressive stresses. When the sheet material deforms in a compressive direction, buckling occurs and wrinkles are formed. Wrinkling of the sheet material under in-plane compression is a typical and classical problem in every day life for all sorts of clothing made of fabric. Similarly it is a problem in fabric reinforced composite forming. Wrinkling (i.e. buckling of fibres), arising during the forming of textile composites, tends to significantly degrade the performance characteristics of the final product (Prodromou and Chen, 1997). Hence, one of the main aims of optimizing textile composite structures is to improve the forming of the material by reducing wrinkles in the final product. A number of parameters could affect wrinkling. These include processing parameters such as forming rate, forming temperature, the number of plies, blank holding pressure and material orientation; and fabric parameters such as tow spacing, tow friction, tow size and weave pattern. There are two ways to prevent wrinkling in composite forming; one is to optimize processing parameters and another one is by material selection. According to Yu et al. (2005), the blank holder force can be used as an independent optimization parameter to prevent processing defects such as wrinkling. The influence of the blank holder force on wrinkling and on the mechanical properties of formed part are crucially important since in-plane tension of the material sheet can be controlled by blank holder force. In other words, different holder force conditions can cause different stress fields in the material sheet, consequently affecting tow rearrangements and wrinkling. Although this is important, little work has been done on simulation and prediction of the effect of the blank holder force on wrinkling. In this study, we focus on the effect of a variable blank holding force on the material flow through the mould flanges by means of FE simulations and experimental investigations of hemispherical forming. Hence we can predict and potentially control

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the tow pattern and wrinkling in textile composite forming. The study presented here has two major objectives. The first is to determine the effects of clamping force conditions on the wrinkling and tow patterns, and the second is to obtain a better understanding of how the material sheet would respond to variable holding forces around the periphery during forming. The study can facilitate optimization of processing parameters, provide materials characterization data, and evaluation of final products with a realistic representation of material and forming processing parameters. The structure of this paper starts with an analysis of wrinkling mechanisms, followed by a brief description of experimental methods and FE simulation, and ends with analysis and discussion of experimental and FE simulation results. 2. WRINKLING MECHANISMS The significantly large differences in the stiffness of fibres and of the polymer matrix results in two possible ways in which wrinkling could occur during textile composite forming. From the view point of micro/meso scale deformation, several researchers including Long et al. (1996) identified in-plane and inter-ply shear as the important parameters governing forming of aligned fibre composites. Tam ad Gutowski (1990) demonstrated that wrinkling will occur when the shear required to form the material to a particular geometry is too high. Woven fabrics differ from aligned fibre prepregs because of additional deformation modes; the mode that dominates is trellising, or the rotation of warp and weft yarns (i.e. in-plane shearing). Trellising results in continuously changing yarn/unit cell structure (Long, et al, 1996). If the fabric continues to be deformed, local shear and in-plane compressive forces build up. This is compensated by buckling or out-of-plane deformation (Prodromou and Chen, 1997).

On the other hand, from the viewpoint of global deformation, the dominant deformation mechanism during textile composite forming is out-of plane bending (shell-type curvature). During a forming process, a sheet material is deformed into a hemispherical part (shell); the part sustains compressive stresses as well as tensile forces produced by the blank holding force, and the force varies over the circumference of cross-section of the part due to the effect of orthotropic anisotropy of the material properties. In addition, in the deformed part, the Gaussian curvature (double curvature) is non-zero; regions of double curvature involve membrane stretching stresses and shearing stresses. Therefore, the force causing buckling is mainly from two components: the membrane stresses and the loading force due to holding pressure. As the material is being deformed, the curvature and the stresses generated in the material change, and these are functions of time, temperature, processing rate and holding force. Wrinkling would occur by buckling of the tows along the circumference of the hemisphere if the compressive force exceeds the critical buckling force. A wrinkle may also experience so much of compressive force that it snaps to another equilibrium position with curvature opposite to that of the deformed shell curvature. In this case, the matrix provides very negligible resistance to snapping. Obviously, it is of crucial importance to take into account that the material responds differently to variable holding forces for both wrinkling mechanisms. For the first case, the change in tow architecture during deformation depends strongly on the in-plane tension in the material, which is controlled by blank holder force. For the second case,

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the elastic shell buckling behaviour is a result of geometry, material stiffness and more importantly, as a result of in-plane loading. Therefore, it is expected that this will affect the shear and buckling behaviour. Also it is expected that wrinkling due to the compression force around the periphery of a hemispherical part can be avoided/minimized by changing the blank holder force and considering of the effect of orthotropic anisotropy of the material properties. 3. EXPERIMENTAL METHOD 3.1 Materials The material used in this research was 1x4 satin weave carbon/epoxy prepreg manufactured by Hexcel Composites. 3.2 Procedures Forming experiments for a hemisphere were carried out using either a rigid blank holder or a segmented blank holder which can apply variable holding pressure (Figure 1) by adjusting 8 springs around the holder. Components were formed at room temperature at a low constant forming rate. In order to evaluate the forming performance, prior to experiments white lines were drawn on the blank parallel to the fibres to form a grid, 10mm in grid size, 2mm line thickness. Shear deformation was measured and wrinkle locations were marked after forming was completed.

Figure 1 Experimental set-up for hemisphere forming

3.3 Boundary Conditions The following 4 boundary conditions were used in this study. In each case a 10N force was applied on selected nodes. The location of the nodes determined the direction of tensile stresses generated in the sheet. In case 1 experiments were carried out using the rigid blank holder and the holder was modeled using rigid body elements with a reference node, in subsequent cases experiments were performed using the segmented blank holder with 8 pressure pads (Fig.1) and the holder was modeled using membrane elements with aluminum properties (E=70Gpa and Poissons ratio = 0.3) and 8 node sets were generated to represent the 8 pressure pads. The boundary conditions are described below:

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1. Isotropic holding force. A rigid blank holder was used and a 10N force was applied on the reference node in FE simulations.

2. Equal tensile stresses generated in both bias directions, i.e. at 045± to the warp and weft directions (force applied on pressure pads 1, 3, 5 and 7); and equal tensile stresses generated along both tow directions (force applied on pressure pads 2, 4, 6, and 8) highlighted in Figure 2a;

3. Equal tensile stresses generated along tow directions but using alternative pad locations to case 3 (force applied on pads1-8, Fig. 2b);

4. Unequal tension generated along bias directions using two diagonally opposite pads e.g. pressure pads 1 and 5 (Fig.2a).

The rationale for selection of the above boundary conditions is to provide an understanding of the effect of the interaction between blank-holder force distribution and material anisotropy. Boundary condition 2 was designed to see the effect of the combined holding force in the bias directions and along both tow directions on forming performance. Condition 3 was used to see the effect of holding force along tow directions on the performance. Condition 4 allows us to see how the material responds to an unbalanced boundary force distribution.

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Figure 2 Blank-holder boundary conditions

4. FE SIMULATION FE simulations were performed using a commercial Finite Element (FE) code (Abaqus Explicit). The textile composite was modeled as a two-phase material structure with the orthotropic fibres modeled by means of non-linear truss elements and membrane elements represent the shear behaviour via a non-orthogonal model (Harrison etc. 2005) which is implemented in the FE code via a user-defined subroutine. The model is shown in Figure 3. Realistic boundary conditions such as forming rate and temperature were used, and tool-ply friction was also incorporated in the simulation. The friction coefficient is 0.3 between the blank and tools. The geometry sizes used in the simulation are exactly the same as the rig used for the forming experiments (Fig.1), i.e., a hemispherical punch with a 60 mm radius, the punch draw-depth is 80 mm and

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the die has a 20 mm filet. The blank dimensions are 295x295x0.5mm.

Figure 3 Hemisphere forming simulation set-up showing punch, blank and blank holder ring (8 node sets represent 8 pressure pads)

5. RESULTS AND DISCUSSION 5.1 Constant Holding Force A typical image of a formed hemisphere obtained using a constant holding force is given in Fig. 4a. Here the corners of the blank were cut off prior to the experiment as these would not fit within the rigid blank holder. A simulated deformed part is given in Fig.4b. A square blank was used in the simulation in order to compare with the results produced by the segmented blank holder. Both from experimental observations and from FE simulation results, the material anisotropic behaviour is a pronounced factor determining the tow patterns (Fig. 4) and the distribution of principal stress contours (Fig. 5). For the orthotropic woven fabrics, the tow pattern is symmetric as expected; the fabric deformation behaviour can be divided into three distinctive regions. First highly sheared regions in the four flanges where shear rigidity is lower (Lin etc. 2006). Second a high stretching force region at the apex, i.e. orthogonal tows with massive in-plane membrane forces and third, the regions between the flanges where the material may be in compression (Figure 5b) and where tow waviness and wrinkles are often observed. The observations reveal that the deformation of the woven fabric during a forming process is dominated by the three deformation mechanisms, intra-ply shearing, in-plane fibre bending and out-of-plane fibre bending. In addition, comparing Fig. 5a and b, it can be seen that the shear force response (Fig.5a) is not affected by the presence of these tow forces (Fig.5b), i.e., different response of the matrix and the fibres in the forming performance, which can be supported by the experimental findings of Sharma et al. (Sharma 2003), who performed bias extension measurements to characterize the shear behaviour of dry fabric during draping. The findings from the simulation confirm again that the material deformation during forming process behaves in a manner which was well described by a pin-jointed net analysis (Robertson et al. 1981), despite the presence of significant forces in the fibres. From these simulation results we can conclude that the matrix plays an important role to control shear behaviour of the part being formed, while fibres dominate the buckling behaviour of the part. Wrinkles are connected either with the mechanism of intra-ply shear and/or with tow buckling behaviour. Therefore, it is possible to avoid or minimize

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wrinkling by reducing the compressive forces in between the flanges and changing the shear behaviour in the flanges.

(a) (b) Figure 4 A deformed hemisphere at constant holding force, (a) experimental, (b)

predicted shear angle contours (degrees)

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Figure 5 Shear stress field in the matrix (a), principal stress field in the fibres (b)

5.2 Boundary Conditions 2 and 3 Comparing the deformed parts (Fig.6 a and b) produced using boundary conditions 2 and 3, the part under condition 2 has better quality (less tow waviness). The FE simulations reveal that the material responds to the two boundary conditions as expected. Fig.7 shows that the shear force response is affected by the presence of two modes of boundary conditions. Greater shear response is seen in condition 2 in both simulation (Fig.7a) and measured results (maximum measured shear angle is 580, Fig.6a). This is attributed to exerting grip forces along the bias tow direction ( 045± ), the forces assist in overcoming tow cross-over forces, hence the degree of intra-ply shear is increased within the part being deformed. Fig.7b illustrates that shear in the part formed with condition 3 is lower and more scattered, and the measured maximum shear angles is 520 for this deformed part. Fig.8 shows the corresponding in-plane force developed in the deformed components. Comparing Fig. 8a and b, it can be seen that larger forces are developed in the fabric during forming and less compressive forces exist around the spherical section of the deformed part under condition 2, consequently

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the deformed part is smoother with less waviness compared with the part under condition 3.

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Figure 6 Deformed hemispheres for boundary conditions 2 (a) and 3(b)

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Figure 7 Simulated shear angle countours (degrees) in the hemisphere for boundary conditions 2 (a) and 3 (b)

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Figure 8 Simulated stress fields in the fibres for boundary conditions 2 (a) and 3 (b) 5.3 Boundary Condition 4 In the case with two opposite flanges pressed, the highly sheared regions are in the

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areas where the load are applied, the sheared regions are less pronounced in the areas which are free during the forming process. In contrast to the constant holding force situation, there is now a higher shear angle at the hemisphere apex shown by the diamond shape of the grid squares. This is a result of material flow being affected by the holding force direction. In addition, the lines of the grid pattern at the apex remained parallel and straight, indicating uniform shear in this region. Furthermore, the non-uniform holding force causes localized deformation which results in compressive stress concentrations in between flanges (Fig. 10 b) , where tow waviness and wrinkles are observed in both the testing result and the FE simulation.

Figure 9 A deformed hemisphere for boundary condition 4

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Figure 10 Simulated hemisphere for boundary condition 4, (a) shear angle countours

(degrees) and (b) stress field in the fibres 6. CONCLUSIONS Wrinkling in textile composite forming has been analyzed by means of experimental study and FE simulation analysis. FE simulation results correlate well with the experimental results and offer an explanation for the differences between the different boundary conditions. The stress fields in the material and the tow patterns are very sensitive to holding pressure modes as expected. In-plane compressive stresses can occur locally, resulting in wrinkles, especially for unbalanced boundary conditions. Wrinkling is not homogeneous in the circumferential direction due to the effect of the

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anisotropic material properties. The experimental results and FE simulations demonstrate that the orthotropic anisotropic behaviour of textile composites allow new design possibilities of using the blank holder force profile to minimize wrinkling. A preliminary comparison of all experimental results and FE simulations suggests that boundary condition 2 should be used in hemispherical forming for woven textile composites. Further work will involve an investigation into the effects of rate and temperature dependent forming behaviour on the results of complex press forming operations including wrinkling prediction. 7. REFERENCES Harrison, P., Yu, W.R., Wang, J., Baillie, T., Long, A.C. and Clifford, M.J. “Numerical Evaluation of a Rate Dependant Model for Viscous Textile Composites”, 15th International Conference on Composite Materials, 27 June -1st July, Durban, South Africa, 2005. Lin, H., Evans, P. Harrioson, P., Wang, J., Long, A.C. and Clifford, M.J., “An experimental investigation into the factors affecting the forming performance of thermoset prepreg.”, Proc. ESAFORM 2006, Glasgow, April, 2006. Long, A.C., Rudd, C.D., Blagdon, M. and Smith, P., “Characterising the processing and performance of aligned reinforcements during preform manufacture.” Composites Part A. 27, 247-253. 1996. Prodromou, A.G and Chen, J., “On the relationship between shear angle and wrinkling of textile composite preforms.” Composites Part A, 28, 491-503, 1997. Robertson, R.E., HSIUE, E.S., Sickafus, E.N. and Yeh, G.S.Y., “Fibre rearrangements during the molding of continuous fibre composites, I. Flat cloth to hemisphere”, Polymer Composites, 2(3), 126-131, 1981. Sharma, S.B., Sutchliff, M.P.F. and Chang, S.H., “Characterisation of material properties for draping of dry woven composite material”, Composites Part A, 34, 1167-1175, 2003. Tam, A.S. and Gutowski, T.G., “The Kinematics for forming ideal alighned fibre composites into complex shapes.” Composites Manufacturing, 1( 4), 219-228, 1990. Yu, W.R., Harrison, P., and Long, A.C., “Finite Element Forming Simulation for Non-crimp Fabrics using a Non-Orthogonal Constitutive Equation.” Composites Part A, 38(6), 1079-1093, 2005. ACKNOWLEDGEMENTS We would like to thank the following organizations for their support: EPSRC, Ministry of Defence, University of Cambridge, ESI Software, Ford Motor Company Ltd., Granta Design Ltd, Hexcel Composites, MSC Software Ltd, Polynorm Plastics (UK) Ltd , Saint Gobain Vetrotex

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CORRESPONDENCE ADDRESS: H. Lin, J. Wang, A. C. Long, M. J. Clifford Polymer Composites Research Group School of Mechanical, Materials and Manufacturing Engineering, University of Nottingham University Park Nottingham NG7 2RD, UK P. Harrison Department of Mechanical Engineering Materials Engineering Group Room 509 James Watt (South) Building University of Glasgow Glasgow G12 8QQ Scotland