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08-25-2016

Utilizing the Potentials of Plastics for Lightweight Construction

On the Track of Complex Material Behavior

Plastics materials and lightweight construction are closely connected with each other. The favorable ratio between strength and weight, as well as efficient generation by injection molding, including the freedom of design incurred, are only some of the benefits. However, the mechanical behavior of thermoplastics is very complex. This is why reliable characteristic values are necessary to dimension components with regard to respective load, and to utilize efficiently the potentials of lightweight construction.

In order to simulate these materials, the engineer requires an integrative simulation chain that takes into account the production method. In many cases, though, this is impossible to do at the early stages of production, because component geometry and production parameters are unknown. Alternative methods are therefore required to carry out preliminary design of components. The Fraunhofer Institute for Structural Durability and System Reliability LBF in Darmstadt, Germany, developed the “reduced integrative simulation” method that allows for simplified calculation models to be established for short-fiber reinforced polymer components.

Find out more at K 2016, Hall 7, Fraunhofer Booth SC01, Düsseldorf, 19-26 Oct 2016

Customized Simulation

Reduced integrative simulation can calculate short fiber-reinforced components in a simple and efficient way, as early as during preliminary design (© BMW)

The reduced integrative simulation approach only requires standard material models from structure simulation. At the early stages of design, the method is able to consider material anisotropy in the component. Especially for designers working at medium-sized producers, the approach is useful to estimate more precisely the mechanical load-bearing capacities of components at the early stages of product development. This reduces both development times as well as costs. The institute claims that, generally speaking, the qualities of structure simulation images achieved in preliminary design are significantly improved as compared to state-of-the-art methods.

Many lightweight products in automotive, aviation and leisure industry owe their success to the application of injection molded, short glass fiber-reinforced plastics. To utilize existing potentials, design and simulation for these plastics must be well targeted. Due to the production process, fibers in such types of components are locally oriented in different directions and have different lengths. As a result, material behavior is anisotropic and differs locally different. This affects stiffness, strength and failure behavior.

Optimized Design of Short Fiber-Reinforced Plastic Components

The methods available at present to design short fiber-reinforced thermoplastics can be classified in three groups. The method of isotropization is a simplified method to determine material behavior. This approach uses different loading directions to forms mean values in order to isotropically describe the material behavior.

The second approach is the phenomenological investigation of the material, under consideration of isotropy, reproducing the behavior of the material when exposed to different loading directions. The third approach comprises the methods of micromechanics. So as to set up an isotropic material model for short fiber-reinforced plastics, information must be considered concerning fiber orientation, i.e. the degree of fiber orientation and fiber length distribution. Theoretically speaking, degrees of orientation can range from complete orientation into one single spatial direction up to entirely random orientation.
The so-called integrative simulation chain enables the user to design injection-molded short fiber-reinforced components at maximum detail. The process comprises several individual stages that are validated individually, in an ideal case. Moreover, more tests have to be carried out than are required with, for example, an approach that isotropizes material characteristic values. As a result, integrative simulation, to many companies, is only economic after material and process parameters have been defined in the preliminary design stage.

Reduced Integrative Simulation

Scientists from Fraunhofer LBF therefore developed an alternative method to achieve a complete integrative simulation chain. Their target was to obtain a simplified strategy to design short fiber-reinforced components, which merely requires standard material models from structure simulation. This strategy is due to give the designer security when carrying out the simulation, as well as an option to estimate mistakes, in addition to improving the quality of structure simulations images significantly.

Lifetime prediction for short fiber-reinforced thermoplastics (injection molding) in the form of FE-postprocessing (© Fraunhofer LBF)

Reduced integrative simulation is based on the direction of flow determined by simple CAD simulation tools for injection molding. This then serves for design with isotropic material models. Test components are submitted to fiber orientation measurements and simulation studies, leading to geometry-related matching coefficients for mechanical characteristic values and to classification according to isotropic behavior. In a systematic survey, phenomenological material models are opposed to recommendations for action referring to the respective component. Experiments are conducted to determine material characteristic values.

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