The shaping of parts made of high-temperature thermoplastic composites (e.g. PAEK, PEEK, PEKK and PPS) is a demanding process and has limitations. These issues can be modelled by numerical analysis [1,2]. From the analysis, it is possible to monitor the course of the process and the behaviour of the material during the process, reduce the time to perform technological tests, optimise the forming tools and reduce the costs of implementation of the pressing process. One of the tools used to pressing processes of model composites is Pam-Form (Pam-Composites) software [3]. To perform an analysis using Pam-Form, it is necessary to define a material model based on experimental data. To reduce the weight of aircrafts, the aviation industry replaces classic construction materials with lighter composite materials [4]. Toray Cetex® TC1200 (Toray Group) is a well-described carbon thermoplastic composite with sufficiently good properties used in aviation structures. However, due to the high melting point of PEEK (
Model of monolithic door stiffening part for the ILX-34 aircraft [11].
To perform the analysis, it was necessary to build a model reflecting the pressing process and obtain material data that depict the behaviour of the actual material. When creating the material model, one had to rely on experimental data. Due to the innovation of the Toray Cetex® TC1225 material, it was difficult to find research papers or documentation describing the material properties necessary to create a material model by Pam-Form. The construction of the model was based on experimental results carried out on the Toray Cetex® TC1200 material. This was possible due to the similarity of both materials and their properties above the melting point. It is described in CompositesWorld [12]. The results of the analysis were compared with those of the produced part. The purpose of this work was to show how to adjust material data and how to build a model by Pam-Form to correctly model the pressing process (Fig. 2).
Dimensions of the ILX-34 aircraft door in millimetres [11].
In Table 1, a cross-sectional diagram of the door part is given together with the embossing dimensions of the stiffening part.
Geometrical data of stiffening part embossing [11].
The thermoforming process uses previously consolidated composite panels cut to the right dimensions. The panels have specific layer arrangement that determines the thickness of the part. Trimmed panels are referred to as blanks in this article. Blanks are equipped with metal parts (‘cold spots’) attached with screws, which can be placed in their corners and on the side edges (Fig. 3).
Examples of spring fastenings using ‘colds pots’ zones [5].
They perform the function of local heat collection and assembly. A blank is mounted on the transport frame with springs attached to the ‘colds pots’ parts (Fig. 4).
Example of mounting the blank on a transport frame.
The frame with the mounted blank is transported between infrared radiators, where the material is heated above the melting point of the thermoplastic matrix
a) Block diagram of the thermoforming process [13], b) process chart.
In Pam-Form, the laminate is considered as independent layers of the orthotropic material connected by contact constraints (friction, described in section 5.2). The orthotropic model used assumes a plane state of stress described by the following constitutive equation:
Shear strain is defined as the relative angle of rotation in radians between the principal directions of the fibres:
In addition to values such as density, Young’s modulus (in two directions: weft and warp) or thermoplastic viscosity, it is necessary to define additional properties whose existence has a key impact on the course of the forming process (e.g. buckling or wrinkle creation [14]).
The properties are as follows:
shear modulus in the plane of the layer – bending behaviour –
Shear stiffness is temperature-dependent. This parameter can be defined as shear stress as a function of shear angle. Indirectly, it can be determined using a unidirectional sample tensile test (uniaxial bias extension test). Due to the lack of availability of the results of the tensile test for Toray Cetex® TC1225, tensile test results for Toray Cetex® TC1200 were used [15]. A diagram of the tested sample is shown in Figure 6. Initially, the sample fibres are oriented at an angle of 45° to the direction of the tensile force. The test was performed at a speed of 30 mm/min. The sample was extended by 60 mm. Such a large deformation of the sample results from the arrangement of the fibres (very low tensile forces of the fibres) and very low shear stiffness in the plane of the layer of the thermoplastic matrix at a temperature close to
Scheme of a composite sample for determining the shear angle: a) before applying force, b) after reaching the maximum elongation [16].
The theoretical change of the shear angle during stretching is a function of the elongation of the sample and its geometric dimensions (2). The theoretical angle plot is shown in Figure 8.
For the numerical analysis of the sample stretching, the following values were used: a Young’s modulus of 58 GPa for 0° and 59 GPa for 90° [9] and a thermoplastic matrix viscosity of 0.13 Pa/s [17]. Figure 7a shows the numerical model of the sample at 60 mm elongation with shear angle values. Model calibration allowed adjusting the stiffness so that the analysis results are as close as possible to the real tensile test.
a) Model of the stretched sample (shear angle in degrees), b) illustrative photo of the stretched sample [18].
The graph in Figure 8 shows the theoretical change in the shear angle when stretching the sample (4).
Graph of the theoretical shear angle change.
The results of the tensile test for selected temperatures are shown in Figure 9.
Tensile test result [15].
The obtained results allowed determining the shear stiffness in the G plane (Fig. 10). The stiffness plot was generated using the conversion file provided with Pam-Form software. The area marked with dashed lines represents the change in stiffness in the shear angle range from 0° to approx. 30°. This is a value that is generally not exceeded in process with shallow moulds used.
Shear stiffness in the plane of the layer
Calibration of shear stiffness allowed obtaining results comparable with the tensile test. Figure 11 presents a comparison of sample stretching diagrams for the real test and numerical analysis. Figure 12 presents a comparison of the theoretical shear angle and angle obtained during the analysis. The shear angle during the real tensile test can be measured using the digital image correlation (DIC) system [19].
Comparison of stretching graphs of the composite sample at 360°C and the result of numerical analysis.
Comparison of theoretical shear angle and shear angle diagrams obtained in the numerical analysis.
The nature of the change in the shear angle is similar to the results of other works on this type of issues [18,20]. The change in the nature of the shear angle curve begins around 40°. The range of changes in the shear angle of the formed material does not exceed this value.
Similar to shear stiffness
The conditions and the course of the bending test, as well as its results, are given in the study in Ref [21]. The calibration value of the end of the sample arrow was used to calibrate the model. Figure 13 presents a diagram of the test stand. The test is performed in a temperature chamber, where the sample is heated to the appropriate temperature.
Diagram of the test stand for bending the sample.
Using an accurate camera, it is possible to read the deflection value. For the bending test, samples with the dimensions were used: a length of 200 mm, a width of 50 mm and a thickness of 0.31 mm. The length of the bending part of the sample was 120 mm. The bending test was performed for a sample with a fibre orientation of 0° and 90° along the longer edge of the sample, determining the bending stiffness
Sample model during free overhanging: a) fibre direction 0°, b) displacement values in [mm] for 0° direction at T = 360°C.
Figures 15 and 16 present graphs comparing the shape of the sample measured in a bending test at 360°C, for the 0° and 90° directions, and obtained by numerical analysis for the same deflection value. The same values of sample deflection were obtained for a stiffness
Comparison of the shape of the bent sample at T = 360°C in the direction of the fibres 0°.
Comparison of the shape of the bent sample at T = 360°C in the direction of the fibres 90°.
Pam-Form v2019.0 software, version V1.9.N was used to simulate the moulding process.
Pam-Form enables modelling the processes of producing composite structures based on thermoforming, stamping, vacuum methods and automated fibre placement (AFP). It allows predicting the final orientation of the fibres, shape of the blanks and formation of wrinkles. Modelling in Pam-Form is based on two types of phenomena: the analysis of mechanical deformations and the analysis of heat transfer between parts, tools and the environment. The solver of Pam-Form uses an explicit type algorithm.
The following analysis ignores the thermal phenomena occurring in the pressing process of thermoplastic composites. For the sake of simplicity, a constant temperature of the composite was assumed, with no heat transfer between the composite and the tools and the environment. After cooling, the part hardens and retains the shape obtained during moulding.
The rigidity of forming tools in relation to the laminate being shaped is much higher, i.e.
Diagram of a numerical model.
Gravity was applied to the blank. The following boundary and initial conditions were adopted during the analysis:
die: ux, uy, uz, rz, ry, rz1 = 0; punch: ux, uy, rx, ry, rz=0; free spring end: ux, uy, uz, rx, ry, rz=0. velocity of punch: Vz
1u – displacements, r – rotations, V – velocity
The profile of punch displacement is corresponding to the velocity of the press (0.032 [mm/ms], 0.0026 [mm/ms]) and is shown in the diagram in Figure 18.
Stamp displacement profile.
Friction is another phenomenon that affects the process and the quality of components produced during thermoforming. One type resulted from the contact of forming tools and the shaped material, and the other type resulted from the mutual slip of laminate layers. Friction coefficients are determined experimentally for both cases [22]. The friction coefficient is determined for variable conditions depending on pressure force, viscosity (being a function of temperature) and sliding speed.
Due to the fact that thermoforming takes place in a specific temperature range above the melting point of the matrix,
The coefficient of friction can be described by the Hersey number. It is expressed by Eq. (5):
[22]
where
[22]
where
Elements of meshes for individual layers were oriented in accordance with the direction of the fibres of a given layer. The layers of the composite are modelled using the created orthotropic model and are modelled with shell, 3 and 4 node finite elements of a given thickness of 0.31 mm. The size of the elements is the result of applying a factor. The same finite elements are used to model flat shaping surfaces without thickness. Pam-Form allows modelling of springs as bar parts with mass. They can be subjected to compression and tension. Their rigidity is described by a linear function (7):
Graph of spring stiffness used in the analysis.
Springs were placed in the corners of the blanks at an angle of 45 to the direction of transport blanks under the press (Fig. 17).
The first observation from the analysis is the effect of gravity on the blank mounted in the transport frame and its fall by nearly 29 mm. This allows estimating the height at which the transport frame should be located in relation to the mould. The results are shown in Figure 20a. Figure 20b shows the influence of springs on the format mounted in the transport frame. Visible changes are observed in the shear angle in the 45° layer and deformations of the material before forming.
a) Blank displacements in the y direction, b) shear angles in the 45 layer.
Figure 21 shows a comparison of the shaped part with the simulation result. It can be seen that the warping of the material resulting from the pressing in the area of technological allowances coincide.
Comparison of the part shape with the result of the analysis.
To accurately reproduce the embossing and warping created on the edges of the blanks, the mesh had to be compacted in these areas. Figure 22 shows a comparison of the density of the meshes and the difference in representation of the pressed part.
Mesh comparison: a) without local mesh ‘
Figure 23 shows a comparison of shear angles in layers 45° and 0° at the end of the analysis. Shear angles do not exceed 16° in the usable area of the part.
Shear angle values for: a) 0° layer, b) 45° layer.
Figure 24 shows the value of bending moments in layers 0° and 45° during pressing. We can observe the formation of warping on the edges of the form. This can result in wrinkles in the finished item.
Values of bending moments in fibres during the process, for: a) 0° layer, b) 45° layer.
Shear angles and bending moments affect the appearance of wrinkles.
Figures 25 and 26 show a comparison of the finished part with the result of the analysis during the process. The result of the analysis showed the possible appearance of wrinkles that can be observed on the finished part. Wrinkles are the result of shear angles and bending moments occurring in the material.
Comparison of wrinkle formation as a result of the shear angle in the part with the result of the analysis.
Formation of wrinkles as a result of bending the material.
Figure 27 presents the values of the coefficient of friction in individual layers during the pressing process.
Values of friction coefficient during the process for a) 0° layer, b) 45° layer.
Modelling with Pam-Composites software can provide a lot of useful information on the technology of the thermoplastic composite pressing process such as shaping the material during the process and changing the shear angle of fibres and their arrangement, stresses and other process parameters. This allows to, for example, optimise the shape of the blankets or correct the shape of the forming tools to minimise friction in the process.
To correctly define the material model, it is necessary to have experimental data based on the analysed material. The use of experimental data made on related material characterised by similar properties and structure is allowed.
Accurate assumption of boundary and initial conditions such as height of the blank above the mould, arrangement of springs and their stiffness, correct definition of the punch speed profile, introduction of gravity force acting on the blank, adjustment of the ‘cold spots’ zones are as important as the properly defined material model and are necessary to obtain the correct results of the analysis.
To obtain more accurate results, it is necessary to compact mesh locally in places of embossing, rounding, radii, etc. In the case of laminates composed of many layers, high mesh density significantly extends the analysis time.
Despite taking into account a large number of parameters, some of the changes occurring during the process have been neglected, among others, change in temperature and values dependent on it.
Expanding knowledge about the pressing of thermoplastic composites and building accurate numerical models can significantly facilitate and accelerate the implementation of effective processes for shaping parts with complex shapes.