The FE- Calculation and the structural-mechanical strength verification of components and assemblies is increasingly finding its way into everyday applications. This vivid example of a stool is particularly interesting because it is not an abstract component from mechanical and plant engineering, but an object of daily use.
Thanks to extensive material tests in cooperation with industrial partners and the development of a corresponding calculation model, Goebel Engineering is one of the few engineering companies able to calculate 3D models from the printer made of stable plastics.
Basic material models for the FEM calculation
For the strength calculation of plastic and wood, failure criteria must first be defined. For this purpose, either corresponding standards and regulations must be used to estimate the strength, or the corresponding material data must be determined by tensile tests. DIN EN 338 was used to define failure for wood. For the 3D printed plastics used, however, the material data were determined by tensile tests according to ISO 527BA for different sample orientations in the installation space.
FEM calculation of wood
In contrast to steels, wood-based materials have different strength properties or material characteristics for different directions of stress. In the case of wood-based materials, additional material properties such as type of wood and moisture are decisive for the evaluation of strength in addition to the fibre orientation.
The different strength properties of wood as a natural material are due to growth (e.g. through branches, knotholes, etc.) and are not homogeneous. Wood exhibits the greatest stability in the longitudinal direction of the component. Radial and tangential orientation of the fibres depend on the cutting direction, which is often unknown.
FEM calculation of plastic from the 3D printer
3D- printed parts and models are becoming more and more common, even in the private sector. Until a few years ago, 3D printers were mainly used in industry or in the scientific field, as the devices often cost 10.000 € or more. Furthermore, the printed models were previously of low stability and often only suitable for demonstration purposes.
In the meantime, not only plastics for 3D printing with remarkable structural-mechanical strength properties are available, but also 3D printers at prices starting from 1000 €. The strength of the components depends mainly on the printing direction, but also on the component orientation in the workspace of the 3D printer.
Investigated load cases
For the study, the following load cases, which were assumed to be commonplace, were examined in more detail by means of FE analyses:
- Total load concentrated on the centre of the seat
- Comfortable sitting with one leg on lower crossbrace
- Total body weight on the lower crossbars
- Inclined load, tilting on the chair
Results of the FE analysis
Load case no. 3 is clearly identified as a particularly critical load case. In the following, the comparison of the results is shown in percent for the two compared materials from the 3D printer:
This case study clearly shows that new types of plastics can carry more than twice the load compared to materials previously used.
The material model of wood can only be represented here by rough estimates based on the DIN standards. Therefore, the calculation results cannot exactly reflect the load on the wood. For an exact failure assessment, extensive tests would have to be carried out, which would go beyond the scope of this case study.
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Further project examples for structural mechanical FE analyses: