FEA Is Often Worthless – Putting The Pieces Together

On the Simulation of Injection-Molded Short-Fiber-Reinforced Plastic Parts.

This guest contribution on Innovation Intelligence is written by Dr. Wolfgang Korte, Managing Director at PART Engineering, developers of CONVERSE. PART Engineering is a member of the Altair Partner Alliance.

Often in Finite Element Analysis (FEA) a lot of effort is put into model set-up and material modeling. Then, after completion of the simulation job, it seems as if the motivation to hit the finish line drops tremendously, as if the completion of the simulation job and obtaining the correct stresses would have any value by itself. The worth of the simulation is not determined before answering the question that was raised initially in a reliable and unambiguous manner before starting the simulation – comparable to a missing piece of a puzzle.

In most cases, the question that needs to be answered is whether the component fails under mechanical loading. That means a proper strength assessment, based on the obtained stresses, is needed. With short-fiber-reinforced plastics (SFRP), the material does not exist by itself. Rather, it is generated during the manufacturing process, hence an integrative modeling approach is necessary in order to answer that question. In figure 1, the effect of fiber orientation on the mechanical behavior of a SFRP is shown.


Figure 1: Influence of Fiber Orientation on Material Properties.

It is evident that due to the strong impact of the injection molding process on the properties of the manufactured part, the structural analyst will need to consider the effect of fiber orientation in the strength assessment.

To obtain a proper description of the stress-strain behavior of the part, a suitable material model is necessary that, in the case of SFRP, is capable to consider anisotropic elasto-plastic behavior. The proper assessment of the part failure is a question of a suitable failure limit and failure criterion.

CONVERSE by PART Engineering is an easy-to-use, reliable and fast software tool for short-fiber-reinforced components which enables the user to consider all relevant effects in order to assess stiffness and strength of SFRP parts properly. CONVERSE takes the information provided by an injection molding simulation, transfers it to a downstream mechanical simulation of the component and provides a ready-to-use Finite Element (FE) input deck with all the material properties required to run the simulation.

The easiest way to predict anisotropic failure of the SFRP part is to distinguish between allowable stresses parallel and transverse to principal fiber orientation. This possibility already exists for CONVERSE users. For a proper strength assessment of SFRP parts, CONVERSE provides a local elemental coordinate system for each element, with its 1-axis by definition always parallel to direction of the fiber axis at that particular position in the component. By comparing the simulated stresses related to that fixed elemental coordinate system, with the direction-dependent allowable stresses obtained from coupon testing, a proper strength assessment can be conducted in any post processor available.

Figure 2 shows as an application example a tensile test specimen made of SFRP containing 30 percent glass fiber.

Figure 2: Strength assessment of SFRP with CONVERSE.

Figure 2: Strength assessment of SFRP with CONVERSE.

CONVERSE was used to map the fiber orientation from the injection molding simulation model to the mechanical simulation model. It provides an anisotropic elasto-plastic material card ready-to-use in the particular mechanical solver and eventually enables the user to make a proper strength assessment with direction-dependent allowable strength values. With CONVERSE, a much better prediction of the part’s failure is achieved as with commonly used isotropic approaches. CONVERSE can be accessed through the Altair Partner Alliance.

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