1. Title of the invention

Core Stabilization Technique with the help of simulator tool for manufacturing of Sandwich Structure Composite components for Aircraft and Helicopter

2. Field of invention

It is an innovative manufacturing process for elimination of Core crush and shifting in sandwich structural composite components in Aerospace industry.

3. Introduction to Sandwich Components:

Sandwich components: Sandwich component is one where in Nomex Honeycomb core (Cured Aramid / Aluminium paper) is sandwiched between two carbon composite prepreg face sheets with core filler material (Epoxy Resin and Hardener) along chamfered edge or with stabilized honey comb core, which provides more compressive strength. The finally cured in Autoclave under pressure, vacuum and temperature.

a) Two main elements of fabrication of Sandwich component:
Top and bottom skin (Composite Prepreg material)

Honey comb core (Cured Aramid / Aluminium paper) Bare/Stabilized

b) Raw Materials used:

Composite prepreg Carbon fiber

Honey comb core (Cured Aramid / Aluminium paper) Bare/Stabilized

Core filler (Epoxy Resin and Hardener)/ Adhesive cured

Adhesive film

Glass 120 prepreg

4. Use of invention

This invented Core stabilization technique is used in manufacturing of sandwich structure composite components such as doors, covers, floors, side shells, bottom shells etc. of Aircraft and Helicopters.

5. Prior art

Patents referred : The Details of the patents referred are given in the table below.

Sandwich components are used in aircraft structure due to their extraordinary compressive strength to weight ratio. Sandwich component is made up of forming of top and bottom composite prepreg face sheets with honeycomb core (Aramid paper /Aluminium) filled with core filler material (Resin and Hardener) along the chamfered edge of core or with stabilized core (reinforced core) on layup tool and finally consolidated in Autoclave under specified pressure, vacuum and temperature, as shown in figure 1.

The core selection in sandwich components is based on shear strength requirements and lowest possible density. This core selection criterion often causes serious manufacturing problems like core crush and core shift movement, especially when the components are cured in the autoclave with consolidation pressure using skins made of prepregs. Figure 2 a & b shows the Core crushing and Core shifting during the curing process

Factor effecting Core crush & core shifting:

The extent of core movement and core crush depends on the core density, type of core, core thickness, type of face skin (Unidirectional & Bidirectional Prepreg material), consolidation pressure and vacuum applied during curing and core ramp angle.

Low density cores being very complaint are highly prone to core crush and core movement. Flex core are more prone to core movement as compared to hexagonal core or Ox core for the given core density. Deep core (high thickness) & High chamfer angle aggravate the core shift for the given consolidation pressure and vacuum, (figure 3 a)

Fabric prepreg skin gives more frictional resistance to the core movement and hence less core movement compared to the UD prepreg skins (figure 3 b). Apart from the core characteristics the component size also plays an important role on the extent of core movement. Long and slender component bound to suffer larger extent of core shift compared to short edged components.

Stabilization Process: Honey comb core gets stabilize by adhesive film on top and bottom or on top only. Cured in autoclave under vacuum and temperature as shown in figure 4.

The existing process of cores stabilization with adhesive film, has draw backs such as improper compaction of adhesive film and the limitation of use stabilized core for flat panel/ lesser contour components. The less thickness and density core can only be stabilized to use in short and less contour components.

6. Draw backs of prior art

The following are the drawbacks:

• Limitation of stabilization of core for flat parts/panels.

• Less thickness and low density stabilized cores can only be used for slight/minimum contour parts.

• Limitation of bare core with low density and less thickness in sandwich composite components, because core leads to core crush and core shifting.

• The type of core used in structure part is also major criteria for deformation as flex cores are more prone to core deformation as compared to hexagonal or OX core for the given core density.

• The selection of flex core over the other type is basically due to the contour requirements. For deeper and three dimensional contour flex core is more suitable, but the core deviation is more prone in deeper contour components with flex cores.

• The core chamfer angle aggravate the core shift for the given configuration of component. Higher the core chamfer angle more prone to core crush & shifting.

• The type of prepreg material (Unidirectional / bidirectional) and the layup orientation makes the variation in core dimensions.

• The curing of core filler material is a long process of 24 hrs to 48 hrs. at room temperature, depending upon the contour and stages of core filling process.

• The filling and cleaning (after curing) of residues of Core filler material (Resin and Hardener) is a cumbersome process which leads to rejection/rework and repair.

• Spillage of core filler material in non specified areas during filling operation leads to variation in NDT requirements and rework.

• Movement of core filler material from a specified zone to non specified zone leads to rejection of components.
• Difficulties in positioning of high thickness, density and deep contour core during fabrication as these leads to variation in core location dimension variation.

• Component size and shape also plays important role on extent of core movement. Long, round, stepped components bound to suffer larger extent of core shift compared to short, flat edged components.

• The variation in core filler material used in components leads to concession and weight penalty.

• Slight variation in curing pressure and vacuum lead to variation in core crushing and shifting.

• Slight variation in size of core leads to reduction in strength of the component and the distance between the fastener points to core ramp start point is difficult to maintain.

• The aesthetic look of the component is not good being the part of the aircraft.

7. Comparison between prior art and present invention

The new innovation in manufacturing of sandwich structure components prevents core crush, core shifting and weight variation whereas the prior arts is a defects in sandwich composite components in aerospace industry.

8. Aim of the invention

The aim of the invention is to reduce the deviation i.e core crush/movement, cycle time, rejection/rework, variation & reduction in weight and providing acceptable, reliable components.

9. Summary of the present invention Core stabilization method with simulator tool adopted to prevent movement /crushing, which leads to reduction in component weight. This process improvement results shows reduction in weight and elimination in core crushing/shifting.

10. Brief description of drawings

Fig. 1 a: Process of manufacturing of sandwich component with core filler technique.

Fig. 1 b: Process of manufacturing of sandwich component with stabilized core technique.

Figs. 2 a & b: Core crush and Core shifting.

Figs. 3 a & b: Variety of cores & load acting on sandwich component during curing process.

Fig. 4: Process of manufacturing of sandwich component with stabilized core technique.

Fig. 5: Simulator tool

Fig. 6: Drawing of Component

Fig. 7: Drawing of Component

11. Statement of invention

The invention of core stabilization with simulator tool method adopted to prevent core movement / core crushing for all different type of sandwich components. This new technique shows reduction in weight and elimination in core movement and core crush. The invention as reduce the deviation i.e core crush/movement, cycle time, rejection/rework, variation & reduction in weight and provided acceptable, reliable components.

12. Detailed description of invention

The invention has been carried out in different phases as explained below:

Phase-I

Initially during development stage flat core test coupon with low density and 20deg chamfered edges was stabilized with one layer of adhesive film on flat surface tool/plate and further cured in oven. The stabilized core was further used for fabrication of sandwich test coupon with carbon prepreg layup and cured in Autoclave as per design standard. The results of DT (Destructive Testing) and NDT (Non Destructive Testing) analyzed and found that core crushing / shifting totally eliminated and the values of tests are found satisfactorily.

Phase-ll

After achieving the successes in first stage, it is decided to implement the core stabilization process for regular part fabrication, which are having slight contour with high density core and 30 deg chamfer angle.

For the core stabilization technique, core has to be stabilized as per the contour geometry requirements of the component. Hence there was a need for tool which can provide the exact replica of the contour of the component. So it is thought off to fabricate the simulator tool by laying the prepregs layers as per bottom stack of component on regular moulding tool.

Simulator tool: Simulator tool was fabricated for slightly contour sandwich parts as per bottom stack of component by using prepregs material and the core boundary was predefined in the simulator tool. Subsequently cured in Autoclave under standard cure cycle.

Core stabilization: stabilization of machined core was carried out by positioning core as per the location provided in simulator tool with the layer of adhesive film on top surface, vacuum bagged and cured in oven.

Part fabrication: during part fabrication the stabilized core used in place of bare core over the bottom prepregs stack layup and further layup of top prepregs stack and finally cured in Autoclave as per design parameters.

Results: The part fabricated was free from core crushing / shifting and weight of component is reduced. The DT (Destructive Testing) and NDT (Non-Destructive Testing) results found satisfactorily.

Phase –III

After achieving the successes in second stage it is decided to implement the core stabilization process for regular part fabrication which are having deep contour and comparatively bigger size.

Simulator tool: for deep contour parts the Simulator tool was fabricated as per bottom stack of component by using prepregs material and the core boundary was predefined in the simulator tool. The simulator tool was found with war page because of deep contour and less thickness of bottom prepregs stack. The warped simulator tool cannot be used for core stabilization, so to avoid the war page problem in the simulator tool the thickness of simulator tool was increased by laying additional layers beyond the core boundary by maintaining the symmetry in orientation of prepregs stack and curing in autoclave.

Results: The simulator tool was free from war page and perfectly matching to the contour of component as per requirements of core stabilization

Core stabilization: stabilization of machined core was carried out by positioning core as per the location provide in simulator tool with the layer of adhesive film on top surface, vacuum bagged and cure in oven.

Part fabrication: during part fabrication the stabilized core used in place of bare core over the bottom prepregs stack layup and further top prepregs stack is laid up, and finally cured in Autoclave as per design parameters.

Results: The part fabricated was free from core crushing/ shifting and reduction in weight. The DT (Destructive Testing) and NDT (Non-Destructive Testing) results found satisfactorily.

Claims

1. The core stabilized sandwich composite structure will have better bending load and shear carrying capability.

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

3. By this process of manufacturing the composite sandwich structure, core movement and core crushing can be eliminated.

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

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

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

7. This invention is implemented in any kind of sandwich component.

8. This invention take cares of any kind of contour requirement.