JEC COMPOSITES MAGAZINE - Issue #109 - November/December 2016 - 64

TECHNOLOGY certification

the FEA model which is explained later in this article. The total mass
of the Stowage unit including its contents is given also in Table 1.

vide accurate loads, stress results and importantly, confidence in the
model simulation results.

Aircraft and Patran-Nastran FEA coordinate systems

A variety of model quality verification tools built into Patran or Femap. The Quad4 element mesh for each panel, once complete should be
checked by the element verification modules. Element performance
is defined by its topology (shape) and properties. There is a quality
tool for element aspect ratios which will survey each element and report on its aspect ratio. Preferably, element aspect ratios should be a
value of 1.0. However, element aspect ratios of upto 5 will produce
acceptable results. Other element quality checks include examination
for element Duplicates, Warp, Skew, Taper, Jacobean ratio. A boundary condition check called "Free element edges", should be applied to
the completed model after all the mpc-elements are connected to the
meshed panels. This tool will visually identify if panel common node
positions have not been merged. The unbroken outline of the Stowage
model would confirm that panels remain independent of each other.

The origin of the aircraft coordinate system starts at the front of the
aircraft where the x-coordinate is assigned the value of zero. Progressing towards the aft of the aircraft the value of x increases. The origin of
the y-axis runs down the middle of the aircraft. The value of the y-coordinate increases towards the port side. The z-value increases from its
origin at the seat rail attachment in a vertical upwards direction.
The FEA Patran-Nastran software has a slightly different coordinate
system. The y-axis is positive towards the starboard side. The x and z
coordinate directions remain the same as those in the aircraft coordinate system.

Loadcases applied to the stowage unit FEA model
A flight loadcase is defined by two paramTab. 2: Loadcases
eters: its direction and its gravitational
Item
magnitude. The Stowage unit is structur- Loadcase
ally examined for six aircraft loadcases
1
9g Forward
(Table 2) and each loadcase applies to the
1.5g Aft
2
stowage unit's self weight and its contents.
3
7.3g Down
To represent these loadcases in the FEA
simulation, the self weight of the structure
4
4.2g Up
is divided by the total number of nodes of
5
3g Port
the Stowage model. This load per node is
6
3g Starboard
then applied to each node in the direction
of the loadcase.
The red arrows in Figure 4 indicate the self-weight 9g loadcase is directionally applied , and its magnitude is nine times the Stowage unit self
weight load. The coat and the baggage mass for the 9g loadcase are attached to respective central nodes along the y-axis at positions where
the items would touch the aft panel during the loadcase event (Figure
5). The magazine racks mass are attached to the forward panel. For
every loadcase the contents are applied to the appropriate panels that
would react the load.

FEA model quality and integrity
FEA model quality control checks and adjustments are essential during the FEA model build. A regular check on model quality will pro-

Fig. 4: Stowage unit structure self-weight applied
per node for 9g loadcase

Fig. 5: Coat, magazine
rack and floor baggage
position for 9g loadcase

The FEA model may comprise of composite panels and reinforcement of different materials and thickness. Patran offers colour plots of
material types or thickness that make it possible to visually assess the
consistency and continuity of the structure's quality.

Model analysis
If all model quality checks have been successfully completed the model can be linear statically analysed in the Nastran solver. The output file
requested can either be the .op2 or .xdb extension. The weight (mass)
card should also be requested as this will provide the total weight of
the Stowage unit (including contents) and the coordinates for the
combined centre of gravity.

Model results and interface loads
In the results folder, open the f06 file and examine to ensure that no
FATAL error messages are reported, the combined mass and its centre
of gravity coordinates should also be reported in the same file (Table
4). These values of mass and centre of gravity coordinates can then be
compared to those previously calculated by hand. Import either the
.op2 or .xdb results file into the Patran model. A stress contour plot
should be created first by selecting;

Fig. 6: Initial check of FEA model - Stress results

jec composites magazine / No109 November - December 2016

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #109 - November/December 2016