The Finite Element Method (FEM) permits us to investigate complicated parts as well as complete subassemblies for critical areas. In the field of plastics it is particularly important to be able to image the creep behaviour of a material. Since the elongation doesn't just depend on the strain, but also on the time and temperature, plastic parts are subject to very much higher deformations than comparable metal parts.
In the plastics sector, the imaging of a non-linear material behaviour (creepage) in a part is one of the many advantages offered by the component layout on the basis of FEM. A FEM calculation permits to image materials which allow major elongations. Apart from the classical laws on creepage, the FE systems provide also the possibility of calculating hyper-elastic components (rubber parts). The best possible material data, a.o., must be to hand so that a FEM calculation can be made. Seeing that the material data are not readily available in most cases, measurements are frequently required to determine the material parameters.
We at Plast Competence Center AG use the FEM system
Ansys for the calculations. It permits to visualize a.o. the stresses, the stress distribution and the deformations. Of particular interest are often not so much the stresses or courses of stresses, but only the deflection or excursion of a part. In such cases one can usually calculate with a relatively coarse net distribution. Relatively fine FE nets are required, on the other hand, to be able to evaluate stresses. Seeing that the computing time increases with the number of elements, the simplest possible FE nets are aimed at.
It is also possible, of course, to investigate simple types of burdens such as bending, tension, pressure or torsion with formulas from standard works. However, this procedure is only resorted to if no computer-supported calculating possibilities exist. Unsophisticated FE systems increasingly provide also the designer with the possibility of subjecting the components within a short time to a first review while design is progressing. The designer thus gets an initial overview of the part's behaviour. More exact calculation methods must be applied where the parts reach e.g. their load limits. Expert systems in the FE analysis are then called for. They offer a host of possibilities to carry out structure analyses in order to make a reliable statement on the deformations and stresses that occur.
Apart from the above-mentioned simple types of burden the Finite Element Method permits to also image complicated types quickly and reliably. A further plus is the imaging of complex geometries which can't be calculated any more with conventional means. Next to the classical general conditions such as moments, forces and pressure loads one can also simulate heat strains due to elongation under heat and screw prestresses. Special attention, furthermore, is often paid to notch strains which are more often an aspect with fibre-reinforced or amorphous plastics than with semi-crystalline materials. The influence of a notch can mean that the admissible strain is by far exceeded in a certain area.
Apart from the static structure analyses, the structure dynamics permit to investigate the behaviour of components under the influences of dynamic strains. Important aspects in this context are the determination of natural frequencies (model analyses) and the implementation of transient analyses. In a transient analysis the part is caused to vibrate by an external, time-dependent strain and the part's reaction is determined. This permits to visualize the strains and deformations occurring at the component or entire subassemblies.
An FE analysis in the form of a component simulation should be made at the earliest possible design stage so that the components can be reviewed and optimized already in the development phase. This simplifies the part design because the critical areas are shown up before the first prototype and can be considered accordingly. In many applications the part weight should be reduced while maintaining a maximum mechanical strength and rigidity. In such cases one frequently reaches the strain limits of the applied materials. Here the component simulation provides the developer with a certain degree of safety. Although the FE analysis doesn't substitute the prototype phase, it helps to produce optimized prototypes. By using the FEM simulation in a purposeful manner, therefore, the time from the design until series production can be shortened.
