Net-Area-Tension Joint for Aircraft Composite Structures
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Solution Overview
Problem
Conventional structural joints in aircraft, particularly those using composite laminates, face challenges such as low compressive load-carrying capability, non-linear behavior under load, and increased structural mass due to bearing critical joint configurations, which lead to manufacturing and assembly complexities and higher safety factors.
Innovation Solution
The implementation of a net-area-tension fastener pattern with two or more parallel rows of fastener holes, where the first row has the smallest diameter and largest spacing ratio, and the last row has the largest diameter and smallest spacing ratio, configured to distribute loads efficiently and reduce bearing stress, thereby transferring loads via tension rather than compression.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the bearing area is increased by increasing fastener size or laminate thickness, then the load-carrying capability is improved, but the structural mass increases
Solution Approach 1:
The patent changes the fundamental loading parameter from compression (bearing critical) to tension (net-area-tension). By reconfiguring the fastener pattern and loading path, the joint transitions to a tension-dominated failure mode where the net area of the laminate between fasteners becomes the critical parameter, allowing for lighter structures without sacrificing strength
Solution Approach 2:
The patent inverts the conventional bearing critical approach by using a net-area-tension fastener pattern. Instead of relying on bearing area (compression), the design uses net area (tension) as the governing parameter, effectively reversing the failure mode and enabling weight reduction
2Strength
If bearing critical joint configurations are used, then structural strength is achieved, but manufacturing and assembly complexities increase
Solution Approach 1:
By changing the governing parameter from bearing stress to net area tension, the patent simplifies manufacturing. The net-area-tension fastener pattern allows for standard fastener sizes and straightforward laminate construction without the need for thickened sections or complex bearing surface preparations
Solution Approach 2:
The patent applies local quality by positioning fasteners in specific patterns where the net area between fasteners is optimized for tension loading. This localized optimization of fastener spacing and arrangement creates efficient load paths without requiring global increases in laminate thickness or complexity
3Reliability
If higher safety factors are used to account for non-linear behavior, then reliability is improved, but structural mass increases
Solution Approach 1:
The patent changes the loading parameter from compression to tension, which fundamentally alters the material behavior characteristics. Composite laminates exhibit more linear and predictable behavior under tension loading compared to compression, reducing the need for high safety factors and enabling lighter structures with maintained reliability
Data Source
AI summary
A structure has a net-area-tension fastener pattern formed in the skin panel for receiving fasteners defining a net-area-tension joint for coupling a component attach fitting of a component to the skin panel. The net-area-tension fastener pattern includes two or more rows of fastener holes, including a first row and a last row. Each row is oriented generally perpendicular to a primary load direction of a load that the component is capable of exerting on the skin panel. The first row is located upstream of the last row relative to the primary load direction. The fastener holes in the first row and the last row are respectively the smallest and the largest in the net-area-tension fastener pattern. The rows are spaced apart at a spacing ratio of hole spacing to hole diameter. The spacing ratio of the first row is greater than the spacing ratio of the last row.