Optical Fastener Preload Measurement via Strain-Sensitive Channel

Solution Overview

Problem

Commercial aircraft fasteners often require oversized and overtorqued specifications to ensure preload due to variability in torque-induced friction, leading to increased weight and operational costs.

Innovation Solution

A fastener with an axially extending filled channel on its outer surface filled with optically transmissive, strain-sensitive material that functions as a waveguide or resonator, allowing for real-time measurement of preload during installation by coupling light and measuring frequency changes, enabling precise preload achievement without oversizing or overtorquing.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If fasteners are oversized and overtorqued to ensure specified preload, then preload reliability is improved, but aircraft weight increases

Engineering Contradiction:

Improvepreload reliabilityVSAvoidaircraft weight

Core Design Contradiction:

ReliabilityVSWeight of moving object

Solution Approach 1:

The patent replaces mechanical torque measurement with optical measurement. A fiber optic sensor with Bragg gratings is embedded in the fastener shaft to directly measure axial strain and calculate preload, eliminating the need to rely on torque-friction relationships and allowing precise preload control without oversizing fasteners

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Solution Approach 2:

The patent implements real-time feedback by continuously monitoring preload through the fiber optic sensor during fastener installation and service. The measured preload data is fed back to adjust tightening procedures, ensuring specified preload is achieved without requiring oversized fasteners or excessive torque margins

Inventive Principle:

Principle #23Feedback

2Reliability

If fasteners are oversized and overtorqued to ensure specified preload, then preload reliability is improved, but operating costs increase

Engineering Contradiction:

Improvepreload reliabilityVSAvoidfuel costs

Core Design Contradiction:

ReliabilityVSLoss of energy

Solution Approach 1:

The patent implements real-time feedback by continuously monitoring preload through the fiber optic sensor during fastener installation and service. The measured preload data is fed back to adjust tightening procedures, ensuring specified preload is achieved without requiring oversized fasteners or excessive torque margins

Inventive Principle:

Principle #23Feedback

Solution Approach 2:

The patent replaces mechanical torque measurement with optical measurement. A fiber optic sensor with Bragg gratings is embedded in the fastener shaft to directly measure axial strain and calculate preload, eliminating the need to rely on torque-friction relationships and allowing precise preload control without oversizing fasteners

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

3Measurement precision

If fiber optic sensor is embedded in fastener shaft, then measurement precision is improved, but device complexity increases

Engineering Contradiction:

Improvepreload measurement precisionVSAvoidfastener structure complexity

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The patent embeds the fiber optic sensor within the hollow shaft of the fastener, nesting the sensing element inside the existing fastener structure. The sensor is positioned concentrically within the shaft, utilizing the hollow space without adding external complexity to the fastener's overall geometry

Inventive Principle:

Principle #7Nested doll (Nesting)

Solution Approach 2:

The patent combines the fastener function with the sensing function by integrating the fiber optic sensor directly into the fastener shaft. The single component serves both as the mechanical fastening element and the preload measurement device, eliminating separate sensing hardware

Inventive Principle:

Principle #5Merging (Combining)

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 solution provides accurate and noise-resistant preload measurement, reducing the need for oversized fasteners, thereby decreasing aircraft weight and associated fuel and operating costs.

Implementation Method 1

A channel extends axially along an outer surface of the shank. Optically transmissive, strain-sensitive material fills the channel. As used herein, a 'filled channel' refers to a channel that is filled with the optically transmissive, strain-sensitive material. The filled channel may function as a waveguide or a resonator.

Methodology Applied

Scientific EffectWaveguide: Waveguide (optics)

Implementation Method 2

The filled channel may function as a waveguide or a resonator. Strain of the shank may be determined by coupling light into an input of the filled channel. This will manifest as changes in resonance of the filled channel, or interference between reflections in the filled channel.

Methodology Applied

Scientific EffectResonance: Resonance

Implementation Method 3

This will manifest as changes in resonance of the filled channel, or interference between reflections in the filled channel. The strain is determined from a change in frequency of the light at an output of the filled channel.

Methodology Applied

Scientific EffectInterference: Interference

Data Source

PatentEP2899522B1Optical measurement of fastener preload

Publication Date: 2018.09.05 THE BOEING CO

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AI summary

A fastener comprises a head, and a shank having an outer surface and an axially-extending channel in the outer surface. Optically transmissive, strain-sensitive fills the channel.