Stiffener Run-Out Fitting for Composite Panel Peeling
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Solution Overview
Problem
In composite aircraft structures, stiffener run-outs often suffer cracking due to natural offsets between stringers and skins, leading to stress concentration and peeling issues, which traditional solutions aim to mitigate by ensuring low deformation but do not enable weight savings.
Innovation Solution
A panel assembly with a stiffener having a foot, an upstanding web, and a run-out region, where a fitting is attached to the stiffener web and panel outboard of the stiffener foot, providing sufficient bending stiffness to minimize panel bending and peeling forces, and featuring a gap or fillet to enhance strength and resist through-thickness loading.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stress or pressure
If stiffener run-out region is designed with gradual reduction in height to transition load, then stress concentration is reduced, but peeling forces at the end of stiffener foot still cause cracking
Solution Approach 1:
A fitting is introduced as an intermediary component between the stiffener web and the panel. This fitting is attached to the stiffener web in the run-out region and to the panel outboard of the stiffener foot, serving as a mediator that transfers loads and reduces peeling forces at the stiffener-foot-panel interface, thereby preventing cracking while maintaining stress transition benefits
2Reliability
If structure is designed to work at low strain to prevent cracking, then reliability is improved, but weight savings are lost
Solution Approach 1:
The fitting acts as a mediator that enables the structure to work at higher strains by transferring peeling forces away from the stiffener foot end. This allows the use of lighter, higher-strain-capable composite materials while maintaining reliability through the load-transfer mechanism provided by the fitting
3Strength
If fitting is attached to panel to resist bending loads, then panel bending is minimized, but through-thickness loading requires strong fasteners
Solution Approach 1:
The fitting is positioned outboard of the stiffener foot, utilizing the spatial dimension beyond the run-out region. This positioning allows the fitting to resist bending loads through its location and orientation, while fasteners handle through-thickness loads, distributing the structural demands across different spatial dimensions and load paths
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 solution reduces peeling forces at the stiffener foot run-out, enhancing the strength against crack initiation and allowing for weight savings by distributing bending loads effectively, while the fitting's fastening ensures sufficient strength to resist through-thickness loading.
Implementation Method 1
the stiffener foot is bonded to the panel
Implementation Method 2
the fitting may be fastened, and not bonded, to the panel and so there can be sufficient strength in the fasteners to resist the resulting through thickness loading
Data Source
AI summary
A panel assembly comprising: a panel (11); a stiffener (12) having a foot (12a), an upstanding web (12b) and a run-out region at one end; and a fitting (13), wherein the stiffener foot is bonded to the panel, and wherein the fitting is attached to the stiffener web and to the panel outboard of the end of the stiffener foot. The outboard end of the web may overhang an outboard end of the foot in the run-out region, of the stiffener.