Composite Co-bonding Reduces Residual Stress

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Solution Overview

Problem

Conventional methods of forming stiffened composite structures result in high residual stress due to resin shrinkage and thermally-induced spring-in, leading to reduced load-carrying capability and increased structural mass, which is detrimental for weight-sensitive structures like aircraft.

Innovation Solution

A method involving assembling composite parts with a bend radius in back-to-back contact, curing on a compensated tool to allow for spring-in, and co-bonding with an uncured base laminate and skin panel to reduce residual stress, using a tool surface compensated for both cure shrinkage and thermally-induced spring-in.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Manufacturing precision

If the base laminate constrains the flanges against spring-in during curing, then the dimensional stability of the composite structure is improved, but high through-thickness residual tension is generated in the bend radii and radius filler

Engineering Contradiction:

Improvedimensional stabilityVSAvoidresidual tension

Core Design Contradiction:

Manufacturing precisionVSStress or pressure

Solution Approach 1:

The method applies preliminary action by allowing the flanges to spring in freely during curing without constraint, then subsequently applying constraint through the base laminate after curing. This reverses the conventional sequence where constraint is applied during curing, thereby preventing residual tension while achieving dimensional stability.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The invention inverts the conventional approach by removing the base laminate constraint during curing to allow free spring-in, then adding constraint after curing. This inversion eliminates the generation of residual tension that occurs when constraint is applied during the curing process.

Inventive Principle:

Principle #13The other way round (Inversion)

2Strength

If the laminate thickness is increased to compensate for residual stress, then the load-carrying capability of the skin panel is improved, but the structural mass increases which deteriorates aircraft performance

Engineering Contradiction:

Improveload-carrying capabilityVSAvoidstructural mass

Core Design Contradiction:

StrengthVSWeight of moving object

Solution Approach 1:

The method extracts the harmful residual tension from the system by eliminating the constraint that causes it during curing. By removing the base laminate constraint during the curing process, the harmful residual stresses are prevented from forming, thereby maintaining load-carrying capability without requiring additional laminate thickness and associated weight.

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The invention changes the curing process parameter by modifying when the constraint is applied - specifically, by delaying the application of base laminate constraint until after curing is complete. This parameter change in the process sequence eliminates residual tension generation, allowing thinner laminates to achieve the required load-carrying capability.

Inventive Principle:

Principle #35Parameter changes

3Shape

If the flanges are constrained against both cure shrinkage spring-in and thermally-induced spring-in, then the shape accuracy is improved, but the residual stress magnitude increases

Engineering Contradiction:

Improveshape accuracyVSAvoidresidual stress magnitude

Core Design Contradiction:

ShapeVSStress or pressure

Solution Approach 1:

The method allows preliminary spring-in to occur freely during curing without constraint, then subsequently applies constraint through the base laminate after curing to achieve the final accurate shape. This preliminary free deformation followed by post-curing constraint achieves shape accuracy without generating residual stresses during the curing process.

Inventive Principle:

Principle #10Preliminary action

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

This method effectively reduces residual stress in the bend radii and radius filler, enhancing the load-carrying capability of composite assemblies while minimizing structural mass, thus improving aircraft performance.

Implementation Method 1

resin shrinkage (e.g., chemical shrinkage) during curing of the detail assembly

Methodology Applied

Scientific EffectChemical shrinkage:

Implementation Method 2

thermally-induced spring-in which may occur as the detail assembly cools down from the cure temperature to room temperature

Methodology Applied

Scientific EffectThermal contraction: Thermal Contraction

Data Source

PatentUS9713901B2Method to reduce residual stress in an integrally-stiffened co-bonded structure

Publication Date: 2017.07.25 THE BOEING CO

AI summary

A method of reducing residual stress in a composite assembly may include assembling a first composite part to a second composite part to form a detail assembly. The first and second composite part may each have a flange and a web connected by a bend radius. The webs may be arranged back-to-back. The detail assembly may be cured on a compensated cure tool compensated for cure shrinkage spring-in predicted to occur in the first and second composite part. The method may include allowing the first and second composite part to spring in from cure shrinkage, and assembling the detail assembly to an uncured third composite. The method may also include co-bonding the detail assembly to the third composite part on an assembly cure tool to form a composite assembly having reduced cure shrinkage residual stress in the bend radii of the first and second composite part.