18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS MODELLING OF HEAT TRANSFER AND CONSOLIDATION FOR THERMOPLASTIC COMPOSITES RESISTANCE WELDING H. Shi*, I. Fernandez-Villegas, H.E.N. Bersee Design and Production of Composite Structures, Delft University of Technology, Kluyverweg 1, 2629 HS Delft, The Netherlands * Corresponding author ( H.Shi@tudelft.nl ) Keywords : Thermoplastic composites, welding, consolidation thermal, consolidation and degradation aspects of Abstract the welding process were modelled and validated. In this paper, a processing model for resistance Based on the processing model, a processing welding of glass fibre reinforced Polyetherimide window was determined by using the consolidation (GF/PEI) has been developed and evaluated. and degradation degree as constraints, and a Transient heat transfer models have been built and parameters optimization was performed. the key factors that influence the uniformity of the 2 Experimental temperature distribution at the welding interface have been discussed. A consolidation model has The GF/PEI material used in the present study was been established to predict the quality of the welds, supplied by Ten Cate Advanced Composites, the and lap shear testing has been used to validate the Netherlands. It has an 8-harness satin weave model. Model predicted results have been found to configuration and 32.4%wt resin content. Eight-ply compare reasonably well with the experimental laminates were consolidated in a hot platen press at results. A non-isothermal degradation kinetic model temperature of 320ºC with a consolidation pressure has been used to find out the upper boundary of the of 2.0 MPa for 10 min. 192mm long and 100mm heating time. Based on the present models, a wide specimens were cut from the laminate before processing window for GF/PEI has been defined, welding. A stainless steel metal mesh was used as a and optimal processing parameters have been found. heating element for the welding process, and it was cut into 250mm long and twelve wires wide (about 1 Introduction 12.7mm) strips. The mesh had a wire diameter of Resistance welding has been researched as a 0.2mm, opening of 0.858mm and thickness of 0.4mm and was impregnated with 6 layers of 60 µ m promising joining technique for thermoplastic composites. In order to fast determine and optimise thick neat Ultem PEI resin film. the welding parameters, processing models have An in-house developed resistance welding setup[4], become an important tool. Several heat transfer was used for the present study. K-type models have been developed by various groups of thermocouples were used to monitor the temperature researchers [1-5]. Most of the models were at the weld interface. Single lap shear strength tests developed for the welding process of CF/PEEK. according to ASTM D 1002 were used to evaluate Carbone et al. [5] and Stavrov et al. [4] developed the consolidation quality of the welded joints. The finite element thermal models for the resistance test specimens were cut from 192mm-wide welded welding of GF/PPS. Ageorges et al. [2] proposed a laminates, and had final dimensions of 187.3mm 3-D transient heat transfer model for CF/PEI. long, 25.4mm wide, and with an overlap length of Ageorges et al. [3] also developed a consolidation 12.7mm. At least six test samples were obtained per model for resistance welding of thermoplastic welded laminate and hence for every set of welding composites. In this paper, a processing model was parameters. developed for the resistance welding of GF/PEI with 3 Processing Model stainless steel mesh as the heating element. The
PAPER TITLE 3.1 Heat transfer model In order to get accurate simulation results, temperature dependent material properties were used Changes in the welding setup, materials and for PEI and GF/PEI. The temperature dependent dimensions can have great influence on heat transfer material properties for GF/PEI laminate were [2]. Therefore, the development and validation of a measured in DSM Resolve, the Netherlands, and for heat transfer model for the specific welding setup PEI they were provided by SABIC Innovative used in this study is a necessary step. 2D transient Plastics. The physical and thermal properties of the heat transfer models on lengthwise and crosswise materials used in this study are given in Table.1. directions were developed to get insight into the The heat generation from the heating element is temperature distribution along different directions of defined by joule heating, and a temperature the joints. dependent resistance of the heating element was Half models with symmetry boundary conditions used: ∂ ∂ = T x ( ) were used to reduce the computational / 0 = ⋅ = + α ⋅ − P I R R R T T 2 (5) time. The governing equation for heat transfer is as , (1 ( )) 0 0 shown below: Where R 0 is the initial resistance, α is the , ∂ ∂ ∂ ∂ T 2 T 2 T 2 T UI temperature coefficient of the resistivity, R is the ρ C = q ɺ + k + k + k q ɺ = 2 (1) p xx xy yy ∂ t ∂ x ∂ ∂ x y ∂ y V resistance at any temperature T. In this study, the α 2 2 value used for the metal mesh heating element was Where q ɺ is the heat generation rate of the heating 9.0e-4K -1 , which was obtained through linear element, U is the voltage, I is the current and V is regression of isothermal resistance measurements. the volume of the material. The boundary conditions of the lengthwise 2-D Table1. Material properties for the heat transfer model model are shown in Fig.1. The boundary conditions Density Specific heat Thermal conductivity** Material at the open surfaces were modelled as free ρ C p k xx, k yy k zz properties (Kg/m 3 ) (J/Kg·°C) (W/m·°C) (W/m·°C) convection to the surrounding air and surface- Stainless ambient radiation, as shown below: 7780 460 10 10 steel ⋅ ∇ = − + εσ − n k T h T T T T Wood 1242 2400 0.17 0.17 4 4 (2) - (- ) ( ) ( ) amb inf Kapton 1420 1090 0.12 0.12 With a free convection coefficient to air film = ° GF/PEI 0.49* 0.38* 1945* 1081* h = W m K T 20 C 2 5 / · [1], sink temperature , inf PEI 1270* 1248* 0.22* 0.22* ε = 0.95 surface emissivity and the Stefan- * Room temperature value **k xz is zero Boltzmann constant σ = × − W m K . 8 2 4 5.67 10 / · 3.2 Consolidation model Consolidation during resistance welding of thermoplastic composites is regarded as a sequential processes consisting of intimate contact and molecular autohesion. The degree of intimate contact evaluates the ratio of contact area to total surface area. Dara and Loos[7] first proposed an intimate contact model by assuming the surface of a unidirectional prepreg tow Fig.1. Boundary condition of 2-D heat transfer model as a periodic of rectangular elements. Mantell et The exposed area of the heating element and the al.[8] proposed a modified version of it for time- adjacent surfaces, shown in Fig.1, were modelled as dependent pressure and temperature (Fig. 2). In this free convection and surface-to-surface radiation study, Mantell’s model was used, and the degree of according to the following equations[6]: intimate contact was described hence using the ⋅ ∇ = − + ε − σ n k T h T T G T 4 - (- ) ( ) ( ) (3) following equation[8]: inf − ε = − εσ ) G J T 4 1 / 5 (1 2 P + (4) w a 5 1 0 = + app D t 0 0 (6) 1 1 ic w + b µ b b Where J 0 is the surface radiosity. mf 0 0 0 0 2
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