Contoured Composite Stiffener Wrinkle Reduction
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Solution Overview
Problem
Contoured composite laminate stiffeners experience undesired wrinkling due to compressive strain during the contouring process, which affects their structural performance and increases costs associated with repair, rework, inspection, and certification.
Innovation Solution
The method involves increasing tension on the outside radius of a compactor with slots to counteract compression strain on the inside radius, using a contoured compactor with greater curvature than the tool surface to apply tensile strain and reduce buckling, and maintaining tension throughout the curing process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Shape
If a straight compactor is used to form the stiffener onto a curved tool surface, then the stiffener can be formed into the desired contour, but compressive strain is generated on the inside radius causing ply wrinkling
Solution Approach 1:
The compactor is designed with a curved cross-section that allows it to dynamically adapt its shape during the forming process. The compactor's cross-sectional geometry changes from straight to curved as it conforms to the tool surface, enabling it to apply distributed pressure that reduces compressive strain on the inside radius while maintaining the desired stiffener contour.
Solution Approach 2:
The invention changes the geometric parameters of the compactor by introducing a curved cross-section with specific radius of curvature. This parameter modification allows the compactor to better match the tool surface geometry, distributing the forming pressure more evenly and reducing the compressive strain that causes wrinkling on the inside radius of the stiffener.
2Shape
If atmospheric pressure is applied through a vacuum bag to form the stiffener, then the stiffener conforms to the curved tool surface, but bending strain causes local buckling and wrinkling
Solution Approach 1:
The curved compactor acts as an intermediary element between the vacuum bag and the stiffener. It distributes the atmospheric pressure from the vacuum bag more uniformly across the stiffener cross-section, preventing concentration of compressive forces that lead to buckling and wrinkling while still enabling the stiffener to conform to the curved tool surface.
Solution Approach 2:
The compactor applies different pressure distributions to different regions of the stiffener cross-section. The curved geometry allows for reduced pressure concentration on the inside radius where wrinkling occurs, while maintaining sufficient pressure on the outside radius to achieve the desired curvature, creating a localized quality improvement in pressure distribution.
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
This approach effectively reduces or eliminates ply wrinkling, enhancing the structural performance of the stiffener and lowering costs by minimizing defects and inspection needs.
Implementation Method 1
increasing tension on the outside radius of a compactor used to form the laminate onto a contoured tool surface. The tension strain on the outside radius of the compactor is increased by cutting slots into the outside radius of the compactor
Implementation Method 2
bending strain is generated within the stiffener as it conforms to the contour of the forming tool. This bending strain may cause local buckling of the composite fibers in the green laminate, producing undesired wrinkling
Implementation Method 3
The slots allow the laminate to be sealed to the compactor and allow out of plane movement of the laminate without introducing compression strain into the inside radius of the stiffener
Data Source
Figure 1~2
Figure 3
Figure 4~5
AI summary
A contoured composite laminate stiffener (20) is fabricated by assembling a substantially flat composite laminate charge (38) and forming the charge into a substantially straight stiffener having a desired cross sectional shape. A contour is formed in the stiffener (20) which has an inside radius (34) and an outside radius (36). Ply wrinkling is substantially eliminated by reducing compression strain on the inside radius as the stiffener (20) as being contoured.