Double Bipod Fitting Mitigates Thermal Stress in Engine Exhaust Joints

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Solution Overview

Problem

The interface between metallic and ceramic ducts in aircraft engine exhaust structures experiences stress concentrations due to thermal growth mismatches, leading to potential cracking, as traditional joints fail to accommodate the differing coefficients of thermal expansion between these materials.

Innovation Solution

A double bipod fitting with an arc-shaped base flange attached to the metallic duct and spring-loaded fasteners connecting to the ceramic duct, allowing for thermal expansion differences to be absorbed and maintaining hoop strength, thereby reducing stress on the joint.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Device complexity

If traditional lap or butt joints are used to connect metallic and ceramic duct portions, then the joint structure is simple, but stress concentrations occur at the interface causing ceramic duct cracking

Engineering Contradiction:

Improvejoint structureVSAvoidjoint integrity

Core Design Contradiction:

Device complexityVSReliability

Solution Approach 1:

The joint is divided into multiple discrete components: a metallic duct portion, a ceramic duct portion, and a separate bipod fitting with multiple legs. The bipod fitting segments the connection function across multiple attachment points rather than a single continuous joint, distributing thermal stresses across discrete locations and preventing stress concentration at any single interface point.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The bipod fitting serves as an intermediary component between the metallic and ceramic duct portions. This intermediate structure absorbs and accommodates the differential thermal expansion through its flexible leg members, preventing direct stress transmission from the high-expansion metallic duct to the low-expansion ceramic duct, thereby eliminating the cracking issue.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Weight of moving object

If ceramic matrix composite materials are used to substitute metallic exhaust ducts, then weight is reduced by approximately 50 lbs, but thermal expansion mismatch with metallic portions causes joint stress

Engineering Contradiction:

Improveexhaust duct weightVSAvoidjoint stress

Core Design Contradiction:

Weight of moving objectVSStress or pressure

Solution Approach 1:

The bipod fitting's leg members are designed with specific geometric parameters (length, thickness, curvature) that allow them to flex and accommodate thermal expansion differences. The fitting's material properties and structural parameters are optimized to maintain rigidity at attachment points while allowing controlled deformation in the leg members, enabling weight reduction with ceramic materials without excessive joint stress.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The joint system transitions from a static rigid connection to a dynamic flexible connection. The bipod fitting's leg members can flex and deform dynamically in response to thermal expansion forces, allowing the joint to adapt to changing thermal conditions while maintaining connection integrity between the ceramic and metallic duct portions.

Inventive Principle:

Principle #15Dynamics

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 double bipod fitting effectively mitigates thermal loading by allowing for displacement and maintaining a secure connection between metallic and ceramic ducts across the full temperature spectrum, preventing undue stress and cracking, while maintaining hoop strength to protect against buckling.

Implementation Method 1

A spring loaded fastener attaches each head to the ceramic duct portion

Methodology Applied

Scientific EffectElasticity: Elasticity

Implementation Method 2

The resultant difference in thermal growth at exhaust temperatures between the two duct portions stresses the joint joining the two duct portions

Methodology Applied

Scientific EffectThermal expansion: Thermal Expansion

Data Source

PatentUS11767770B2Double bipod fitting to mitigate thermal loading of engine exhaust structures

Publication Date: 2023.09.26 THE BOEING CO

AI summary

A system and method for mitigating thermal loading between engine exhaust structures having different coefficients of thermal expansion. The engine exhaust structure comprises a metallic duct portion, a ceramic duct portion, and a double bipod fitting joining the metallic duct portion to the ceramic duct portion. The double bipod fitting is capable of flexing and taking up the thermal expansion differences between the joined metallic and ceramic ducts across the full temperature spectrum that an engine exhaust structure will experience in service.