Stringer Runout Fittings for Composite Panels

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Solution Overview

Problem

The connection between a composite stringer terminating end and a composite laminate panel in aerospace structures faces challenges in transferring high axial loads without inducing interlaminar stresses, leading to delamination and compromising structural integrity due to poor interlaminar behavior in existing designs.

Innovation Solution

A stringer runout configuration using a pair of metal fittings with a fitting web and base flange, where the base flange has stepped surfaces for attachment to the stringer and panel, and fastener holes positioned to counteract pull-off loads, along with a curved stringer web termination to minimize strain energy peaks and interlaminar stresses.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Device complexity

If a composite stringer terminating end is directly connected to a composite laminate panel, then the structure is simple, but high interlaminar stresses occur at the bondline causing delamination and compromising structural integrity

Engineering Contradiction:

Improveconnection structure complexityVSAvoidstructural integrity

Core Design Contradiction:

Device complexityVSReliability

Solution Approach 1:

A runout fitting made of metal (e.g., titanium) is introduced as an intermediary component between the composite stringer and the composite panel. This fitting includes a runout portion that interfaces with the stringer and a base flange that attaches to the panel, serving as a mediator that transfers loads while reducing interlaminar stresses at the bondline interfaces.

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The invention employs a hybrid material system combining metal (titanium) and composite materials. The metal runout fitting provides strength and stiffness to handle high axial loads, while the composite stringer and panel maintain their lightweight properties. This composite material approach allows effective load transfer without generating detrimental interlaminar stresses.

Inventive Principle:

Principle #40Composite materials

2Force

If high axial loads are transferred directly from stringer to panel, then load transfer efficiency is high, but interlaminar stresses increase causing bondline delamination

Engineering Contradiction:

Improveaxial load transfer capabilityVSAvoidinterlaminar stress

Core Design Contradiction:

ForceVSStress or pressure

Solution Approach 1:

The load transfer path is segmented into distinct zones within the runout fitting: a runout portion that interfaces with the stringer web and flange, and a base flange that attaches to the panel. This segmentation allows gradual load distribution, with fastener holes strategically positioned to create local moments that counteract pull-off loads and reduce peak interlaminar stresses at the bondline.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The runout fitting exhibits varying thickness and geometry along its length, with the thickness decreasing from the base flange toward the runout portion. This local variation in geometry allows optimized stress distribution, providing greater material where needed to handle high loads while gradually reducing the load transfer intensity toward the panel interface to minimize interlaminar stresses.

Inventive Principle:

Principle #3Local quality

3Ease of manufacture

If squared off stringer web and base flange are used, then manufacturing is simplified, but interlaminar behavior deteriorates reducing load capability

Engineering Contradiction:

Improvestringer fabrication simplicityVSAvoidload capability

Core Design Contradiction:

Ease of manufactureVSStrength

Solution Approach 1:

The runout fitting incorporates curved and rounded geometries at critical interfaces, particularly where the runout portion meets the stringer base flange and where the base flange contacts the panel. These curved surfaces distribute stresses more evenly compared to sharp corners, reducing stress concentration and improving interlaminar behavior while maintaining manufacturing feasibility through forming processes.

Inventive Principle:

Principle #14Spheroidality (Curvature)

Data Source

PatentUS11724791B2Enhanced design for stringer runout terminations on composite panels

Publication Date: 2023.08.15 THE BOEING CO

AI summary

A stringer runout configuration has a pair of fittings with each fitting having a fitting web that is configured for attachment to a stringer web of a stringer on opposite sides of a terminating end of the stringer. The stringer web has a terminating end with a curved configuration and base flanges with rounded terminating ends. Each fitting has a base flange that has a bottom surface with a first surface area and a second surface area positioned at different heights. The first surface area of each of the fittings is configured for attachment to a stringer base flange on opposite sides of a terminating end of the stringer. The second surface area of each of the fittings is configured for attachment to a composite panel surface adjacent the terminating end of the stringer. These and other detailed design features described herein enhance the capability of the runout configuration.