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Process-related properties to predict structural behaviour of moulded parts |
Application research in detail
Process-related properties to predict structural behaviour of moulded parts
Presented by Dr. Stefan Glaser, Engineering Plastics Application Development
Trade Press Conference K 2004, June 22, 2004, Ludwigshafen, Germany
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One thing that until now has prevented engineers arriving quickly at an optimal design of moulded part – highly stressed ones in particular – has been the lack of information about process-related material properties, ie, properties – for example anisotropy due to fibre orientation that the part acquires during the moulding process.
BASF is now using a new integrative design approach that takes into account fibre orientation in the moulding, resulting in a more accurate prediction of the mechanical and thermal behaviour of the part (figure 1).
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Anisotropy makes optimal part design difficult
To reiterate, the moulding process influences the orientation of the reinforcing fibres and therefore the mechanical properties of the part - stiffness, tensile strength, resistance to heat distortion etc. This is of particular significance for parts that are subjected to high service loads and temperatures, or are prone to warpage (figure 2).
The fibre orientation within the moulded part is non-uniform, resulting in differing material properties at different places.
Anisotropy due to fibre orientation is particularly problematic when designing complex parts with the help of computer simulation. Since the local fibre orientation is usually not known, the material is generally assumed – wrongly – to behave isotropically. To compensate for error, the material’s characteristic stiffness values have to be reduced using a correct factor. Unfortunately, this factor, which is obtained by comparing computed and measured values of the material’s elastic moduli, is only valid in certain circumstances (figure 3).
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The three steps of integrative simulation
This is where integrative simulation comes into play. In a first step, the fibre orientation within the part is determined by a mould-fill simulation using MOLDLFLOW, the most widely used commercial mould-fill simulation software. The computation takes into account the properties of the moulding compound – melt viscosity, fibre content etc. – as well as the process parameters such as injection speed and holding pressure.
Secondly, the information gained about the fibre orientation in the moulded state is then used in a non-linear anisotropic material model developed by BASF. The specifically devised software module is called FIBER. With the help of this model (and module), the mechanical properties of the resin/fibre composite are calculated from the various fibre orientations and the separate mechanical properties of the resin matrix and fibres. It is thus possible to take into account the process-related material properties during the computation.
In the third and final step, a structural analysis of the part is carried out with either LS-DYNA or ABAQUS, two common commercial finite-element software packages, to which the BASF’s material model extension has been added.
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New “FIBER” software module links fill simulation and structural analysis of part
The software module – called FIBER – which processes the fibre-orientation data for use in the structural analysis was developed by BASF. FIBER transfers the fibre orientations determined from the mould-fill simulation to the finite element mesh of the part’s structural model and works out the local material parameters. Because the transfer is purely geometrical, the data can be applied to a variety of meshes. User-defined functions allow non-linearity and complex failure modes to be included in the description of the material – something that was not possible until now. Fundamental to the whole process is efficient management of the vast quantity of input data required. FIBER thus forms a link between mould-fill simulation and structural analysis of the part (figure 4).
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Precise prediction right up to failure
To demonstrate the power of integrative simulation, torsion tests were carried out on a structural beam – the LU carrier – made from an Ultramid® (nylon)/metal composite and the results compared with those of the simulation (figure 5).
The usual computation, which is based on non-linear yet isotropic material behaviour and a standard correction factor of 0.75, is unable to predict the beam’s stiffness accurately. The error is due to an inadequate description of the material. By contrast, the integrative approach with FIBER takes into account the beam’s anisotropic stiffness characteristics, resulting in a more accurate prediction of the beam’s flexural response. Simulated and experimental results agree very closely right up to the failure of the part (figure 6).
That the beam turns out to be stiffer than predicted by the conventional method is due to the anisotropic fibre orientation.
As the mould-fill simulation shows, the restricted melt flow in the ribs causes the fibres to align themselves along the axis of the ribs (figure 7).
In the torsion test, the highest tensile loads happen to occur along these ribs. Thus the beam is strongest precisely where the greatest loading occurs (figure 8).
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