Slip Sheet Compensation Surface for Composite Curing Deformation
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Solution Overview
Problem
Composite skins deform during curing and cooling due to anisotropic properties, leading to gaps between the skin and support structures, which require costly shims and limit design flexibility, as existing methods struggle to compensate for these deformations effectively.
Innovation Solution
A slip sheet with a compensation surface is used, differing in thickness to provide a modified outer mold line that accounts for deformation, reducing the need for shims and allowing for more design flexibility by compensating for cure-related deformations during the manufacturing process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If composite skin is cured and cooled using conventional forming surfaces, then the composite material achieves its structural properties, but deformation occurs due to anisotropic thermal expansion and cure shrinkage, creating gaps between the skin and support structure
Solution Approach 1:
The slip sheet is prepared in advance with a compensation surface that has varying thickness designed to counteract the known deformation patterns of the composite material during curing and cooling. This preliminary compensation geometry is built into the slip sheet before the composite layup process, so that when the composite cures and deforms, the slip sheet's pre-designed thickness variations offset these deformations, maintaining the desired outer mold line accuracy.
2Manufacturing precision
If shims are positioned between composite skin and support structure to fill gaps, then the geometric accuracy is improved, but manufacturing time and cost increase
Solution Approach 1:
The invention extracts the compensation function from the assembly phase (where shims would be installed) and transfers it to the tooling phase (the slip sheet). Instead of adding shims during assembly to compensate for gaps, the slip sheet itself is designed with varying thickness that proactively compensates for deformation, eliminating the need for separate shim components and their associated installation time and labor.
3Manufacturing precision
If maximum allowable force is applied by fasteners to compensate for deformation, then the fit between skin and support structure is improved, but the risk of damaging the composite skin or support structure increases
Solution Approach 1:
The slip sheet with its pre-designed compensation surface geometry performs the deformation compensation function during the curing and cooling process itself, before the final assembly fastening occurs. This preliminary geometric compensation ensures that the composite skin maintains the correct outer mold line without requiring excessive fastener forces, thereby protecting the structural integrity of both the composite skin and support structure.
4Adaptability or versatility
If a modified outer mold line is provided through slip sheet thickness variation, then design flexibility is enhanced and shim quantity is reduced, but the complexity of the slip sheet manufacturing process increases
Solution Approach 1:
The slip sheet is designed with local quality variations in its thickness, where specific regions have different thickness values tailored to compensate for local deformation patterns of the composite material. This localized thickness variation allows the slip sheet to provide precise compensation where needed while maintaining uniform thickness in regions where deformation is minimal, thereby managing manufacturing complexity while achieving the desired design flexibility and fit accuracy.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The slip sheet system effectively reduces the quantity of shims needed and enhances design flexibility by compensating for deformations, thereby reducing manufacturing costs and time, while ensuring a better fit between composite skins and support structures.
Implementation Method 1
Deformation of the composite skin results from differing coefficients of thermal expansion with respect to the three principle axes or orientations of the composite skin due to the anisotropic nature of composite materials
Implementation Method 2
Deformation of the composite skin also results from cure shrinkage due to chemical changes during curing
Data Source
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AI summary
A method and an apparatus are presented. The method locally compensates for cure related deformations in a composite skin by laying up a composite material on a compensation surface of a slip sheet (206) designed to form a modified outer mold line (216) different from a final desired shape, wherein the slip sheet (206) is held against a forming surface (208) of a layup tool (204), wherein the forming surface (208) is designed to form an outer mold line of the final desired shape.