Finite element method structural analysis

41

Figure 2.15 - Non-design space, first review. Second view, NX

42

compare stress levels between the original and the optimized manifold (see more on chapter 7).

This analysis is performed with the original material, which is Anticorodal 6082, an Aluminium alloy. The mechanical properties that are mandatory to perform and review the analyses are the Young’s modulus E, the yield stress σY, Poisson ratio ν and the density ρ at the functioning temperature, which is 20°C. These properties are specified in chapter 2.1.

While on Inspire it was easy to apply loads directly on the surfaces by creating rigid connections, with NX the creation of RBE3s are required to add a moment on a surface. It is also required for linking the lumped masses from the centre of gravity of the components around the part (CONM1 elements) to the surfaces where they act.

On Inspire there is the possibility to see the results for both displacements and stresses with Result Envelope mode, that shows the maximum value for each result type across all load cases. Since NX doesn’t have this option, the results will be displayed divided by load case. Displacements achieved with Inspire are presented below in figure 2.16 and 2.18 for load case 1 and 2.20 and 2.22 for load case 2.

Displacements achieved on NX Nastran are instead presented in figure 2.17 and 2.19 for load case 1 and 2.21 and 2.23 for load case 2. In the different load cases the maximum value is less than three hundredth of a millimeter, which is acceptable.

43

Figure 2.16 - FEA of the original component on Inspire. Displacement load case 1. View 1

Figure 2.17 - FEA of the original component on NX Nastran. Displacement load case 1. View 1

44

Figure 2.18 - FEA of the original component on Inspire. Displacement load case 1. View 2

Figure 2.19 - FEA of the original component on NX Nastran. Displacement load case 1. View 2

45

Figure 2.20 - FEA of the original component on Inspire. Displacement load case 2. View 1

Figure 2.21 - FEA of the original component on NX Nastran. Displacement load case 2. View 1

46

Figure 2.22 - FEA of the original component on Inspire. Displacement load case 2. View 2

Figure 2.23 - FEA of the original component on NX Nastran. Displacement load case 2. View 2

47

In the pictures below (figure 2.24, 2.25 and 2.26, 2.27 for Load case 1 and figures 2.28 and 2.23 for Load case 2) Von Mises stress contour plots are displayed with both Inspire and NX Nastran software. Safety factor contour plots could also have been displayed here, but since NX Nastran does not have this option and since a comparison between the two different software could have not be made, it was decided not to include any safety factor contour plot. Safety factor, calculated as yield stress divided performed stress, should never be less than 1.

𝑠𝑎𝑓𝑒𝑡𝑦 𝑓𝑎𝑐𝑡𝑜𝑟 = 𝑦𝑖𝑒𝑙𝑑 𝑠𝑡𝑟𝑒𝑠𝑠 𝑎𝑐𝑡𝑢𝑎𝑙 𝑠𝑡𝑟𝑒𝑠𝑠 (2)

The minimum safety factor chosen for these analyses is 1,2. This means that no performed stress should be higher than the yield stress divided by 1,2 (193 MPa for Anticorodal). The minimum factor obtained is almost 1 only in some very small areas, that are negligible due to errors on minimum mesh element size. In the rest of the component, the safety factor is always higher than 1.35, which respects the minimum value defined.

Figure 2.24 - FEA of the original component on Inspire. Von Mises stress load case 1. View 1

48

Figure 2.25 - FEA of the original component on NX Nastran. Von Mises stress load case 1. View 1

Figure 2.26 - FEA of the original component on Inspire. Von Mises stress load case 1. View 2

49

Figure 2.27 - FEA of the original component on NX Nastran. Von Mises stress load case 1. View 2

Figure 2.28 - FEA of the original component on Inspire. Von Mises stress load case 2. View 1

50

Figure 2.29 - FEA of the original component on NX Nastran. Von Mises stress load case 2. View 1

Figure 2.30 - FEA of the original component on Inspire. Von Mises stress load case 2. View 2

51

Figure 2.31 - FEA of the original component on NX Nastran. Von Mises stress load case 2. View 2

As already clarified, on NX Nastran it is not possible to have a result envelope that summarizes the results and there is no option to see a safety factor contour plot. On the contrary, there are more options to display stresses and displacements, for example based on their direction, magnitude or if they are referred to a node or to an element.

The results of displacements with NX Nastran are presented below in figures 2.24 to 2.26. Von Mises stresses on NX Nastran are displayed in figures 2.27 to 2.30.

In order to make the comparison more consistent, the scale in the left was modified on NX Nastran, so that the maximum values was settled as the same on Inspire. The minimum value had been considered as zero in all the contour plots. The maximum values achieved on NX Nastran are visible in Table 5. On Nx Nastran the result was presented viewing all the elements in the mesh, but to make the comparison more adequate since on Inspire this is not possible, the mesh was hidden.

52

Comparing the results of the two software, they look different, expecially in the maximum values. But with a closer view to the colour distribution in the images, it is about the same in both cases. Stresses are hugh in the same areas.

Below a tab that displays the different maximum values.

Displacements [mm] Inspire NX Nastran

Load Case 1 0,023 0,021

Load Case 2 0,006 0,011

Table 4 - Maximum values of displacement, Inspire VS NX.

Von Mises Stress [MPa] Inspire NX Nastran

Load Case 1 123 331

Load Case 2 222 163

Table 5 - Maximum values of Von Mises stress, Inspire VS NX Nastran

The maximum values of displacements and Von Mises stress in both load cases are extremely different. This could be due to an error in the mesh, because the results would have been more accurate with a second order analysis. Anyway, the maximum values reached are just for small elements, which may be considered negligible.

53