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Based on Dr.
Stefan Glaser's work on Predicting Reinforced Plastic Material Performance.
Design engineers have traditionally used Finite Element Analysis (FEA) to predict the structural performance of fiber-reinforced plastics. However, the technique presumes uniform distribution of fiber throughout the molded part. Generally, mold filling parameters and part geometry variations greatly affect the fiber distribution and orientation and, therefore, the part's resulting mechanical and thermal performance. To be sure, designers apply safety factors to their assumptions of material strength to help design against part failure. In some cases, though, the resulting designs can be too conservative – leading to added material cost – or simply marginal, requiring extra validation testing and development delays.
Barriers to a complete and accurate picture of part performance
The molding process influences the orientation of reinforcing fibers, which affects mechanical properties such as part stiffness, tensile strength and resistance to heat distortion. Fiber orientation within the molded part is non-uniform, resulting in varying material properties in different directions (anisotropy). Instead of the uniform, isotropic display of strength and properties in all directions previously assumed by FEA, more accurate FEA models should incorporate realistic non-uniform fiber orientation and the anisotropic, non-uniform properties directly related to variations in fiber orientation and concentration.
Typically, analysts use a safety factor to compensate for the gap in knowledge of how material properties vary in a fiber-reinforced plastic part. However, the safety factor neglects the part’s fiber orientation-induced anisotropy, accounting instead for its effect by degrading to some degree the strength and modulus values determined by tensile testing. Unfortunately, this approximation overestimates properties in some areas and underestimates them in others.
Evolution of Fiber Orientation in Mould Filling Process
Mechanical behaviour of anisotropic layered shells
The three steps of integrative simulation
BASF has developed an integrative simulation method to overcome the limitations of prior predictive models. In the first step, the fiber orientation within the part is determined via mold filling simulation with MOLDFLOW, the most widely used commercial mold filling simulation software. The computation takes into account the properties of the molding compound – including melt viscosity and fiber content – as well as such process parameters as injection speed and holding pressure.
In the second step, information gained about fiber orientation in the molded state is then used in a non-linear, anisotropic material model — FIBER, BASF’s proprietary software module. With the help of this model (and software module), designers can calculate mechanical properties of the resin/fiber composite from the various fiber orientations as well as the separate mechanical properties of the resin matrix and fibers.
FIBER is very flexible, allowing the use of various material/analysis models. The beauty of the software is that the designer doesn’t have to calculate the material properties; they are determined by the process.
In the third step, designers perform a structural analysis to predict the precise failure modes of the part, using either LS-DYNA or ABAQUS — two common commercial finite-element software packages, to which BASF’s material model extension has been added.
New “FIBER” software module links filling simulation
and structural analysis of part
The FIBER module factors fiber-orientation data into the structural analysis. Fiber orientations determined from the mold filling simulation transfer via the FIBER software to the finite element mesh of the part’s structural model, thereby establishing a new set of local material parameters. Because the transfer is purely geometrical, the data can apply to a variety of meshes. User-defined functions allow inclusion of non-linearity and complex failure modes in the description of the material — something previously not possible. Efficient management of the vast quantity of input data required is fundamental to the whole process.
FIBER thus forms a link between mold filling simulation, the resulting fiber orientation, and the structural analysis of the part. The results can be dramatic. In some cases, BASF’s FIBER has already helped customers entirely eliminate, or reduce the time required for, a prototype stage or stages.
To learn more about BASF’s FIBER software module, visit BASF's Plastics Portal.
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Dr. Emile Homsi