# Reverse Engineering and Topology Optimization for Weight-Reduction of a Bell-Crank
## 5 Results and Discussion
### Design Validation  
#### 5.4.1 Geometry Interpretation and FEA  
Topology optimization often produces complex contours in the geometry, which may be difficult for manufacturing and/or machining [8; 33]. The results obtained from the iteration process of the topology optimization were used as guide geometries to determine where material should be withdrawn from the design domain. The guide geometries were interpreted toward the creation of optimal designs with suitable shapes and features for both machining and practical purposes. The guide geometries were exported from Abaqus into CATIA (V5R21), where their shapes and features were traced in the CAD environment. The optimized CAD design for the volume constraint models, i.e., 95%, 90%, 85%, 80%, and 75%, were then created.  
To overcome unrealistic contour issues, the geometry was simplified to produce a practical optimized part. Sharp edges obtained through the topology optimization iterations were rounded with fillets, with a minimum radius of 3 mm for ease of machining. Profiles were smoothed out, due to the checkerboard contours resulting from the removal of elements. An example of the CAD design for the 80% volume constraint model is shown in Figure 10.  
Re-analysis was carried out to assess the efficiency and structural integrity of the optimized bell crank design. The same'static-general' and frequency-finite element tests were conducted, as examined on the original model, with identical set-ups of boundary and loading conditions. The maximum von Mises, stiffness, mass and maximum principal strain for the optimal design produced in the topology optimization with different volume constraints are shown in Figure 11.  
Figure 9: Topology optimization results with associated volume constraints, side view.  
The optimization results show that the 90% volume constraint model produced higher values of stiffness, with a corresponding natural frequency of 5334 Hz. It also featured a 16.5% reduction in maximum von Mises stress value, a 4.3% reduction in maximum principal strains, and a decrease in weight by 10 g from the original bell crank. The finite element result indicated that the highest stress concentration in the optimized model was 199.76 MPa. Based on the part that was machined out of Al6082-T6, the corresponding yield stress value was 310 MPa, which indicated a safety factor of 1.55 for the optimized bell crank. Hence, through the optimization iteration processes, the 90% volume constraint model was considered as the optimal design.  
It is worth noting that the main technical challenges of converting the optimized FE analysis topological results in CAD design for further analysis and manufacturing. As highlighted previously by other investigators [29; 33] and the current study, the initial topology optimization results still required manual process to eliminate holes, shape, irregular and complex edges, for example, see Figure 10. The topology optimization results were manually reconstructed to smoothen the shape and complex edges and to fill the missing pores to improve its manufacturability to the final design before the fabrication commencing.  
Figure 11: Summary of the structural integrity of the optimized bell crank CAD design.  
Figure 10: Revised CAD model for the 80% volume constraint. (**Left**) Optimization model, (**Right**) the CAD design free of irregular and complex shapes.