MOLA Plate Stress Analysis Results

Using UAI/NASTRAN

The table below gives the results of the plate analysis performed using NASTRAN element membrane forces. The values in the Von Mises Stress column were taken directly from NASTRAN results and were multiplied by the respective factors of safety (FS) to obtain the yield and ultimate stresses. The NASTRAN run used gravity loads of 20.6g in the X and Y directions; and 22.8g in the Z direction, which is the launch direction. The worst case loads for the four components of interest were taken from the results and used in the calculations.

The sixth and seventh columns are the most important. They show the Margins of Safety. These numbers need only be positive to show that the plate will not fail. As can be seen, all margins here are positive. In fact, some of these margins are extremely high, which indicates among other things that the components with high margins are over-built. Recall that MS_yield relates the actual stress to the material's yield stress, i.e., the stress it takes to permanently deform the plate.

To refresh yourself on what each of the components listed below are, jump to the FEM exploded view.

FS on yield = 1.25, FS on ultimate = 1.4

Material Limit Stress	Fy	Fu
Magnesium ZK60A-T5	22000	34000
Honeycomb Panel (Al face sheets)	35000	42000
Aluminum 6061-T651	35000	42000
Beryllium I220-B	40000	55000

The equation used to calculate the margin of safety is:

MS = [(plate material stress) / (actual plate stress)] - 1

The reason these particular four components were used in this analysis is because their finite element mesh was adequate for this type of direct analysis. Most other parts may need to be analyzed by hand calculations after recovering forces acting on an element's grid points. If the mesh of a plate is too large, i.e., not enough elements covering the surface, then the plate forces recovered will be inaccurate, particularly near corners and curves in the plate. This is because NASTRAN averages the force through the entire plate element. For example, one corner of a plate element may have a localized stress of 20,000 psi, while the rest of the plate has a stress of only 5,000 psi. The average may come out to be around 8,000 psi, which is much lower than the actual 20,000 psi. Therefore, we do direct NASTRAN analysis on the larger components with an adequate mesh.