1. Title of the invention

Core stabilisation in composite sandwich structure by introducing an adhesive layer

2. Field of invention
It is a mechanical engineering design gives guide lines to prevent/minimise core movement and core crushing while manufacturing the composite sandwich structure.

3. Use of invention
This invented core stabilisation process used in sandwich composite structures like doors and covers of Fighter / Trainer aircrafts.

4. Prior art
In composite sandwich structure honeycomb cores are used due to their extraordinary bending stiffness to weight ratio. Based on shear strength & lowest possible density core is selected. This core selection criterion often causes serious problems like core crush & core shift/ movement, especially when the components are cured in the autoclave with pressure on the skins made of prepregs. the extent of core movement 7 core crush depends on the core density, type of core, core thickness, type of face skins, face skin thickness, consolidation pressure, vacuum applied during cure and core ramp angle, core crush & core movement are shown in figure 1 below.

b) Core shift
Figure 1. Core crush/core movement in sandwich composite parts

Signature of the Inventor

5. Draw backs of prior art

1) Limitations of core in sandwich composite parts, because of low density cores being very
complaint are highly prone to core crush and core movement. Flex cores are more prone to
core movement compared to hexagonal or ox cores for the given core density.

2) Limitations of core in sandwich composite parts, because deep cores and deep chamfer angle aggravate the core shift for given consolidation pressure and vacuum.
3) Often the ramp angle is limited to 20 degrees to minimize the core shift.

4) Component size also plays important role on extent of core movement, Long/Slender components bound to suffer larger extent of core shift compared to short edged components.

5) Core crush/ core movement is a defect in sandwich composite parts, because of this distance between fastener point to core ramp start point is difficult to maintain.

6) Weight penalty is one drawback as core is filled in ramp area with core filler.
Figure 2. Causes of Core/Crush and core movement: a) Core characteristics b) Process parameter -Consolidation Pressure and Vacuum

6. Comparison between prior art and present invention

The new design in sandwich composite part prevents core crush & core movements whereas the prior art is a defect in sandwich composite parts. This design reduces the weight of core filler in ramp area along the core edge with respect to prior art. In the process of core stabilisation production time is more as compared to prior art. In the prior art bending load & shear load carrying capacity of the structure is low compared to the present invention. The design is tested and proved on the sandwich composite structure like doors and covers of fighter/trainer aircrafts.

7. Aim of the invention
Aim of the invention is to minimise/prevent core movement and core crushing while manufacturing the composite sandwich structure, so that the dimensional stability is achieved, which is structural requirement.

8. Summary of the invention
There are several methods which may be used to stabilize core to prevent movement or crushing during processing. Aramid honeycomb core which is thicker than 12 mm or with a chamfer angle greater than nominal value of 20 degree may require core stabilization to prevent core movement or crushing during part cure. Recommended stabilization methods are described below. Selection of the most efficient stabilization method will generally be an iterative process involving the Designer, Material Technology and Manufacturing teams..

Firstly Core is to be machined to drawing dimensions and ensured that the core is free from any contamination. Then core has to be placed on to the tool with pre-laid of a layer of release film. One layer of adhesive film (Redux 319a) is laid on top surface of the machined honeycomb core without damaging the core, the set-up is vacuum bagged and oven cured. Carefully remove the bagging without incurring any damage to the core, the core with adhesive film cured on the top surface is the required stabilized core ready for using the component manufacturing. The bottom skin of the component is laid up with a layer of Redux 319 adhesive film as required, the stabilized core is appropriately positioned on the above layup. Fill the core at the hard points with appropriate core filler and allow the core filler to cure as per the standard procedure. The top skin of the component is laid over the stabilized core beginning with a fresh layer of Redux 319 adhesive film as required by the drawing. Vacuum bagged tool/part is subjected to autoclave curing with standard cure cycle.

9. Brief description of drawings
Fig. Sheet-3 shows the view of the Prior design of Core crush/core movement in sandwich composite parts
Fig.Sheet-4 shows Prior design of Causes of Core/Crush and core movement: a) Core characteristics b) Process parameter - Consolidation Pressure and Vacuum
Fig.Sheet-7 Shows the view of the Present design of Core stabilisation with adhesive film on the bag side.

10. Statement of invention
This invented Core stabilisation enables us to design the Doors/Covers of sandwich composite structure to Fulfill the Requirements without core crush & core movements.

11. Detailed description of invention
Honeycomb core Stabilization Procedure for Sandwich Components
The following are the steps involved in the core stabilization procedure for components with and without hard points / attachment zones on the core area.
a) Honeycomb core is to be machined to drawing dimensions and QA certified and ensured that the core is free from any contamination.
b) A simulated tool has to be fabricated as per simulated bottom skin of the component to take care of stabilization of core separately.
c) If the component has hard points, then mark the hard point zones on the machined core (on the top side) as required in the drawing, in order to provide opening/window on the stabilizing adhesive layer. If component does not have any hard point, skip the step.
d) The machined core is placed on the simulated bottom skin tool pre laid with a layer of perforated release film and held in position by PS tapes or any other appropriate method.
Note: a) The hard points are to be filled up with LY556 & K5 core filler mixture as per drawing. No core filler should be used in chamfer zone of the core, b) It is advisable to combine core stabilization and curing of core filler at hard points due to possible exothermic and core damage while handling after curing of the filler resin.
e) One layer of adhesive film (Redux 319a) is laid on top surface of the machined honeycomb core without damaging the core, with necessary window to pour the core filler resin at hard points, if applicable.
f) A layer of peel ply can be used over the adhesive film to have a protected surface. However, adequate care should be taken not to cause de-bonding of adhesive layers while peeling of the peel ply. it is also ensured by the QA that the peel ply is completely remove before using the stabilized core in the layup.
g) The above set-up is vacuum bagged and oven cured at 175 deg for one hour under the
application of 0.3 bar vacuum. The heating and cooling rate not to exceed 5 deg c per minute
h) Carefully remove the bagging without incurring any damage to the core, the core with adhesive film cured on the top surface is the required stabilized core ready for using the component manufacturing.
i) Inspect the stabilized core visually for possible damages and de-bonding between the adhesive layers and core.
j) Adequate care should be taken to preserve the stabilized core from on the top of the adhesive layer while vacuum bagging.
k) The bottom skin of the component is laid up with a layer of Redux 319 adhesive film as required by the drawing .the stabilized honeycomb core is appropriately positioned on the above layup.
I) Fill the core at the hard points with appropriate core filler and allow the core filler to cure as per the standard procedure. Skip this step if no hard point exists in the components.
m) The top skin of the component is laid over the stabilized core beginning with a fresh layer of Redux 319 adhesive film as required by the drawing.
n) Vacuum bagged tool/part is subjected to autoclave curing with standard cure cycle.

Claims

1. The Core stabilised sandwich composite structure will have better bending load and shear load carrying capability.

2. The Core stabilised sandwich composite structure will be lighter in weight, because of no core filled in the ramp area of the core.

3. By this process of manufacturing the composite sandwich structure, core movement and core crushing can minimise/prevent.

4. By this invention better dimensional accuracy can be achieved in the composite sandwich structure.

5. This is cost effective in the fighter aircraft as the these parts are lighter.

6. The distance between fastener point to the core ramp start point can be maintained.