Stringer Foot Reinforcement via Polygonal Tuft Distribution

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Solution Overview

Problem

The design of stringer run-outs in composite aircraft wing skins faces challenges due to high shear and peel stresses, leading to potential peeling off of the stringer from the skin, poor out-of-plane strength, and Mode-1 fracture toughness, which results in crack formation and propagation at the tip of the run-out.

Innovation Solution

A panel assembly with a stringer foot and web, featuring a flange and foot run-out bonded to the panel, reinforced by distributed reinforcement elements across the foot run-out interface in a series of rows, including an end row along a polygonal curve to evenly load and prevent crack propagation, and optionally inclined reinforcement elements to reduce stress concentrations.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Ease of manufacture

If reinforcement elements are distributed in a straight line across the foot run-out interface, then the structure is simple to manufacture, but the reinforcement elements become unequally loaded leading to successive failure

Engineering Contradiction:

Improvesimplicity of reinforcement element distributionVSAvoiduniformity of load distribution on reinforcement elements

Core Design Contradiction:

Ease of manufactureVSReliability

Solution Approach 1:

The reinforcement elements in the end row are distributed along a polygonal curve rather than a straight line. This curved arrangement follows the expected line of a curved crack front, enabling the reinforcement elements to be more equally loaded and preventing successive failure, while maintaining manufacturing feasibility through defined geometric patterns.

Inventive Principle:

Principle #14Spheroidality (Curvature)

2Strength

If metallic finger plates with bolts are used to clamp the run-out, then the stringer is secured to the skin, but leakage and structural weaknesses occur at the bolt holes

Engineering Contradiction:

Improveclamping strength of stringer to skinVSAvoidleakage and structural weakness at bolt holes

Core Design Contradiction:

StrengthVSObject-affected harmful factors

Solution Approach 1:

The invention replaces the mechanical bolted connection system with a bonded reinforcement system. Reinforcement elements pass through the foot run-out interface and are bonded to both the stringer foot and the skin, eliminating the need for bolt holes that cause leakage and structural weakness while maintaining clamping strength through adhesive bonding.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Solution Approach 2:

The reinforcement system uses composite bonding materials to join the stringer foot to the skin. The reinforcement elements are bonded through the interface using adhesive materials that provide both structural strength and sealing, avoiding the harmful effects of metallic fasteners while maintaining the composite nature of the structure.

Inventive Principle:

Principle #40Composite materials

3Weight of moving object

If composites are used for the stringer and skin, then weight is reduced, but out-of-plane strength and Mode-1 fracture toughness are poor leading to crack formation

Engineering Contradiction:

Improveweight of aircraft wingVSAvoidout-of-plane strength and fracture toughness

Core Design Contradiction:

Weight of moving objectVSStrength

Solution Approach 1:

The invention uses reinforcement elements that pass through the foot run-out interface, combining multiple materials and structural features to enhance out-of-plane strength. The reinforcement elements work together with the composite stringer and skin to provide improved fracture toughness and crack resistance while maintaining the lightweight composite structure.

Inventive Principle:

Principle #40Composite materials

Solution Approach 2:

The reinforcement elements extend through the thickness dimension of the foot run-out interface, adding structural strength in the out-of-plane direction. This multi-dimensional reinforcement approach addresses the poor out-of-plane strength of composites while preserving the weight advantages of composite materials.

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

Data Source

PatentUS11124284B2Panel assembly including stringer and reinforcement elements

Publication Date: 2021.09.21 AIRBUS OPERATIONS LTD

AI summary

A panel assembly with a panel and a stringer is disclosed. The stringer has a stringer foot and an upstanding stringer web. The stringer foot has a flange which extends in a widthwise direction between the stringer web and a lateral edge and in a lengthwise direction alongside the stringer web, and a foot run-out which extends between the flange and a tip of the stringer foot. The foot run-out is bonded to the panel at a foot run-out interface. Reinforcement elements, such as tufts, pass through the foot run-out interface into the panel. The reinforcement elements are distributed across the foot run-out interface in a series of rows including an end row nearest to the tip of the stringer foot and further rows spaced progressively further from the tip of the stringer foot. At least the end row has three or more of the reinforcement elements which are distributed along a polygonal curve.